October, 2002 (ERCITS)
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ELECTRIC RC IN THE SKY (ERCITS) Newsletter
October, 2002

The ELECTRIC RC IN THE SKY newsletter is published by Al MacDonald once a month(hopefully). It's comprised of a collection of articles on electric powered flight. Anyone may submit an article provided it is connected with RC electric flight. I will also publish any coming event (electric) or article for sale (electric and personal only).

 

 

C O N T E N T S

Shuttle ZXX
Building and Flying Electric Sport Scale
Helipad
Pitch, Roll and Yaw
Electric RC Discussion List
For Sale
Coming Events
Electric Links
Hobby Stores
Hobby Companies
Clubs
Past Newsletters
Al's Home Page

Shuttle ZXX

By Rob Campbell

A few years ago I bought a "30 size" Hirobo Shuttle ZXX kit for a price I couldn't refuse. It has taken me until now to get everything together and try it out.

This article has gone through several revisions as new discoveries were made over the last few weeks. First, some background...

Power in its current configuration is an old Astro 40 Sport Wind brushed DC motor with twenty 1700 SCRs. I say old because it is the older style with only small vent holes in the end bells and it was purchased in '92. I had to wrestle this motor out of my trusty Ace 4-40 - it is painful to look at that plane without a motor! I've been sharing my experiences with a fellow from Texas who has quite a different setup. There doesn't seem to be a lot of information available on how to go about converting helicopters.

Main rotor dia. is 48.63" and flying weight is estimated at almost exactly 8 lbs. Calculations using Martyn McKinney's electric heli spreadsheet and 20 cells indicate the main rotor rpm should be about 1400 rpm and it should take about 330 watts of mechanical power to hover. Current draw from the battery should be a little over 18 Amps.

The Shuttle is a pretty good conversion candidate for several reasons. It seems to be a good basic helicopter. I did some checking a while back and the Shuttle has a faster spinning tail rotor than most, so this helps with control for lower main rotor head speeds. It accommodates the Astro motor easily and has room for the batteries within its frame.

<-- Shuttle with Whisper

Martyn's spreadsheet is a very useful tool for predicting electric heli performance. It was used to generate the data for all the graphs here. It is available for download from Ken Myers website. The graphs speak louder than words. Thanks Martyn!

Interesting to note that the spreadsheet predicts better performance with 18 cells than with 20 and with blades with a smaller chord (to a point). The Astro 40 appears to operate very close to optimum efficiency at the hover current (about 18A). Hover time is calculated at about 5 1/2 minutes. It is worthy of note that the spreadsheet does not take into account the reduction in available voltage near the end of a flight and certain other losses. This will reduce the real-world hover time a bit.

The spreadsheet predicts equal hover times for 18 and 16 cells, but this may not be found in practice. The higher main blade angle of attack with 16 cells will reduce the pitch "headroom" for hovering as the batteries are depleted. With 16 cells the lower rotor speed will also provide poorer overall control and lift as the batteries are drained. I would suggest 18 to 20 cells instead. I know others have used 20 cells. It appears that if the number of cells is increased much more the velocity induced drag diminishes flight time.

I have access to a little hobby lathe so I thought I'd have a go at making a coupling from the motor to the pinion that normally accommodates the clutch. I machined the clutch bell right off and used the remaining flange. The motor coupling was drilled and the flange tapped to accept small cap screws. My biggest worry with this set up was the possibility of misalignment. I'm breaking a basic rule here - two twin bearing shafts connected with an inflexible coupling. I ended up truing up the motor coupling by mounting it in the lathe and using the motor to turn the coupling. That seems to have made it perfectly true on the motor and no misalignment problems are evident.

<--Modified Clutch

With regard to motor heat dissipation, I borrowed Tom Cimato's idea posted June 30 on the e-flight mailing list for cooling things with a couple of small DC fans wired in series. I can also cool the throttle and some of the battery pack with the same air. I used a passive bimetal "Airpax" thermostat to switch on the fans when the motor bracket gets above 40 (C. The fans are always connected to power so I can leave it after a flight and the motor gets cooled off automatically. In reality, the motor thermal characteristics are such that it would be much better to blow air through the armature. A shaft driven fan available as an electric conversion accessory for the Kalt Baron 30 would be ideal.

I tried to avoid using a standard 5V regulator for the receiver by trying a very efficient switching regulator first. The regulator barely gets warm with a 25V input. I measured the current draw at the regulator at about 0.22A quiescent (i.e. only the gyro active) and as high as 1.40A with all servos operating rapidly. If the average current draw at the receiver is, say, 1A in flight, it is interesting to note that the regulator is 80% efficient under these conditions, power dissipation is only about 1.25W and that the draw from the flight pack would only be about 0.25A!

Any potential benefits turned out to be irrelevant because the switching regulator interfered with the receiver quite noticeably during a range check. The regulator switches at a very high frequency and this was obviously causing the problem. This type of regulator also shuts down completely below 7V so it isn't good for low cell count applications but it would have been OK in this case because the Shuttle would have been on the ground long before the main battery has a chance to reach 7V. With appropriate shielding and/or filtering this type of regulator may be useable but more investigation is required. Until then, I'll take the reliable (if wasteful) performance of a linear regulator.

<--Motor with coupling

The heli is throttled with a Jeti 60. This was my first experience with one of these controllers and aside from having a narrow range from minimum to maximum throttle, it seems to be quite stable - a good thing in a heli! It has a brake but this is easily defeated with a supplied jumper.

With a lot of fiddling and calming of nerves I was thrilled when I got this puppy to hover! The spreadsheet predicted main rotor hover pitch quite closely. This is the first time I have tried this size of heli and I must say it is smooth. There seems to be a lot more ground effect with the bigger chopper. This was also my first experience with a piezo gyro. It seems to have much more precise tail control than with the little mechanical Futaba G155 on my Whisper.

The two little thermostatically controlled DC fans do a good job cooling the motor but their reaction time lags behind the to motor heating a lot. They don't switch on until after the heli has been hovering for at least a couple of minutes and they stay on for a few minutes after a flight until the motor is just warm to the touch. They use so little current that their power-on status during hover goes unnoticed. I really like this set up but as previously mentioned it would be a lot better to blow the air through the armature rather than across the motor. The thermostat TO-220 package bolted to the motor mounting bracket. You could use just one of these 12V fans on a smaller heli...

<-- Rear view showing cooling fans

I started out with the stock semi-symmetrical blades. On E-Flight there was an article on the Astro 40 powered Kalt Baron 30 with optional electric power components. I have recently acquired a pair of the "Baron Alpha Elec" blades, P/N 39012. The blades have a larger chord (2" vs. 1 3/4") but have washout and are flat bottomed. I would think this would provide greater lifting ability for a given rotor rpm. These do give me more flying time but for reasons outlined below this became hard to measure.

My guess is a flatter bottomed airfoil with washout (more pitch at the root) is the most efficient at lower head speeds for hover and normal flight. A washout blade will be a bit closer to true helical pitch. With this type of blades the heli seems to fly a bit longer and has a slightly mushier feel - a bit more like the Whisper. They are a dull grey in colour (a bit hard to see) but you can always use brightly coloured tracking tape. The blades were much better balanced out of the package than the stock blades and required much less work.

On an electric heli one can try throttling the motor to adjust for to reduce the torque transient during sudden collective pitch changes. I am far from being an expert heli pilot but I have observed (and reacted to) this yaw transient caused by a rapid collective pitch change. Keeping the rotor running at more constant speed with collective changes can significantly reduce the short-lived but unpleasant reaction torque. In reality, the benefits of attempting this may be marginal - especially if you don't have a lot of power to spare and it causes other performance penalties.

To have any success with throttling you need a speed control with a smooth transition to full throttle. Some throttles sort of jump to full throttle from about 90% throttle. I've noticed this on airplanes and I think this is happening with my Shuttle heli throttle. I was able to do this on the Whisper to a small degree because but power was at a premium. Fortunately, electric motors fight to stay near their specific speed (speed predicted by the speed constant RPM/V) and require very little compensation for load changes compared with an IC engine.

C H A R T S

Unfortunately, the old Astro 40 could not be properly timed for the anticipated current draw. It was purchased in 1992 or so as a geared motor and it only has one pair of tapped holes for the motor thru-bolts. Timing was much closer to neutral than it should have been. During this initial period I was too lazy to take apart the motor to drill and tap a new pair of holes. This delayed an important discovery...

I was always leaving a large proportion of the charge in the cells (700 - 1000mAh) after a short 2 1/2 minute flight. If I let the motor cool down it would fly again. What was the problem? Was the motor getting too hot? This problem was interfering with my hovering practice and needed to be solved! This effect was reduced with the new blades suggesting they required less current at hover.

Varying flight times, erratic power, hot motor and thirst for current have mystified me. My first guess was that I had an intermittent turn-to-turn short in the windings. Current draw was about 23A at hover but only 18A was predicted.

I was checking over the heli before going out to fly the other day and noticed that something in the motor was binding. I reluctantly decided to disassemble the motor from the airframe yet again. At this point, I thought that perhaps a brush had worn out, but when I took the motor apart I found that the brass housing in one of the brush holders had bent about 10 to 15 deg. and the housing was actually catching on the commutator! No wonder I was experiencing such weird behaviour. Wear on this brush was very asymmetrical.

In retrospect, it is a wonder that the heli flew as well as it did. The bent brush holder probably explains the higher than normal current draw and the motor heating. The direction the holder was bent would have significantly retarded the timing for the one brush. That can't be good!

I suspect the motor had this problem before I put it in the Shuttle because I had problems from the start. I think it just got worse with time.

Based on a discussion with Marc Thomson, I've started to look at the Kontronik stuff out of Germany. If you go to www.kontronik.com and look up the 3SL 40 sensorless controller for brushless motors it looks perfect for electric helis. You can set it to maintain a constant RPM. Most setups also claim to achieve close to 90% efficiency. I want one. :-)

Anyway, I think some basic repairs on the ol' Astro would allow me to combine the Shuttle's pleasant flying characteristics with some significantly longer flights for less money. On the other hand...

Happy Hovering!

Reprinted courtesy of the EMFSO newsletter.

Back to Table of Contents

Building and Flying Electric Sport Scale

Transcript of Keith Shaw's presentation to the 1992 Electric Model Fliers of Southern Ontario General Membership Meeting, March 1992

Transcribed by Martin Irvine, Kingston, Ont.

(There have been changes made to make this easier to read and more organized. Prices quoted are U.S. dollars. Keith lives in Ann Arbor, MI)

What It Takes To Be a Successful At Building and Flying Electric Airplanes

There are four parts to SUCCESS:

1) Good equipment
2) Sensible Choices
3) Craftsmanship
4) PRACTICE

GOOD EQUIPMENT:

There is an old saying, "Buy cheap, buy twice." I know of people who have tinkered with electrics for years. They have purchased the cheapest motors they could find, because they didn't want to spend the money on cobalts. They spent their money trying different motors, brushes, fiddling, tinkering, trying to make things work. If you were to ask them how much they had spent trying to get their plane flying, they'd tell you about $150.00!! Compare that with spending $80.00 on a cobalt. My advice is to bite the bullet and buy the good stuff. I have cobalts that I've had since 1978 and I'm still flying them on the original brushes. They are good investments.

The same goes for chargers. Many people are involved with seven cell airplanes. Any charger is going to run $25 to $100. An Astro Flight 112PK only runs a few dollars more and it will charge all the way up to 32 cells. For getting into larger systems, it's a good investment. It, always, has a good resale value. Some of the European charges can run $300 to $400 and I'd think twice before I bought one. Many of the Astro Flight chargers are an extremely good buy. The SR Smart Charger and TRC-6 are also good chargers.

SENSIBLE CHOICE:

A sensible choice is really important. I see a lot of failures in this category. Everyone wants the dream airplane, but they have to go through the steps to get there. At best, you can talk them into a trainer and then their second airplane is a B-17 with retracts! Always the way.

There is a point where you really need to progress and realize that the skills have to be developed and that they are not just going to magically appear. This doesn't mean that you have to stay with trainers forever. The next airplane should be a little bit more complicated and take a little more skill to fly than the first one, but be reasonable in the progression of the steps.

If you want to build a WWII fighter and you are flying trainers, the logical progression is to build some low wing tail-dragger. With this sport plane, you can get practice in taking off and landing a tail-dragger, because that's what most fighters were. A good idea is to take the power system that your dream will need but build a "trainer" for that system. Nominally the same wing area, don't bother to taper the wing if the dream plane has a mild taper. If it is a violently tapered wing, then go with a wing with a fair bit of taper to it. Make the system trainer with a small, typical sport fuselage, easy to build and easy to repair. Make it a tail-dragger and generally the same shape and size of what your dream plane would be. Fly the "trainer" for a while. Make provisions for adding ballast a bit at a time to get up to the weight that you think your scale airplane will weigh. This way you can develop the necessary skills to fly the airplane you want to build. I have, literally, an attic full of "trainers" that I've built. I'm still doing it.

If I've got a plane in mind, that is different from what I'm used to, or I have to solve some problem, I don't build the exact scale airplane. I build something that is close to it; to get all the bugs out of it. Maybe I want to play around with some strange force arrangement or it's a strange configuration that I've not flown before. I throw together one of these "trainers" in three days or a week or whatever, fly it a half a dozen times or so to learn whatever I need. Then I stick it in the attic as a radio test plane. Finally, I build the plane I really want.

I've been doing that for 35 years. These "trainers" are a very good way of picking up the skills you need, or figuring out a "different" airplane.

CRAFTSMANSHIP:

You can save a tremendous amount of weight just by making sure that every part put into the airplane does its full job. If you cut a part that doesn't fit and you use a lot of glue, or whatever, to make it work, you are adding a lot of weight that isn't doing anything better than the original part could have done with a lot less weight. If you spend some time making every part do its job, you save a lot of weight and end up with a stronger airplane.

PRACTICE:

The bottom line is just practice. Get as many hours flying as you can. Fly everything you can. Push yourself. By learning to land carefully, you can probably save half the weight of the airframe. Most the stuff that is in an airplane is to allow it to survive the "occasional" hard landing, (crash). The extra structure's weight is there just so that it can bounce off the ground once or twice. I don't mean really smashing it, just a hard landing. If you think of the model as a full size air-plane, most of our landings would have the FAA all over it - "No, you can't fly it again until we check it out!" That is why our structures are so over built. You know the typical "good" landing - good approach, beautiful flare - six feet high; the airplane stalls, drops one wing, does three cartwheels, flips, goes end over end a couple of times and ends on its back. The pilot is mad because he broke a prop! Then he blames the prop manufacturer for making fragile props!

Once you get to the point where you are making decent takeoffs and landings, the structure required to hold the airplane together, through the most strenuous aerobatics, is amazingly light. Fifty percent of the weight, of most model airplanes, is so that it can survive a hard landing. To make it survive really hard landings, the weight goes up 2 or 3 times. When you get this heavy, you have to stick a glow motor on it!

You must decide where you want to go and what kind of model you're going to end up with.

complexsimple
<------------------------------------------------------------------------------------------------------------------------------------------------->
complex fuselages patternStiks
custom canopies simple pylon racers
custom cowlings sport scale
retracts
wheel pants
(6 - 12 months)

Sport planes are really simple. You can dress them up a bit with commercial cowlings, wheel pants, canopies, etc. A few cosmetics can make the simplest airplane look good. A few curves in the tail can make a big difference. These simple changes and additions can make a decent looking airplane out of a stick, one that doesn't look like a Stik. (Ugly Stik, Sweet Stik, etc.) You have to decide where your interest is. If you're flying basic trainers, you need to ease into the more involved models.

STRUCTURES:

One of the best ways, I've found, to learn how NOT to build airplanes is to look at kit plane crashes and see how things fail. There are kits on the market that have built in failure modes. They put in excess weight and then they put a weak point where it will break.

Look at crashes and try to figure out exactly what it took to make the airplane break that way and then don't do that with your airplane.

When I was flying free flight, in the 50's, we had an old adage; look at what didn't break in a crash and then LIGHTEN that. It must have been too strong, and so too heavy, or it would have broken along with every-thing else. It sounds funny, but it's something to keep in mind. If you can look over the demolition at your club field, take a look at what survives. I don't think I have ever seen a broken tail. I know guys with walls covered in tails of broken airplanes, mounted like trophies, lined up! You can take that as a lesson. You can back off on the tail structure a bit. It will still hold together. You will often be surprised at just how far you can back off on the structure. The only reason that most planes have all that wood back there is that the kits are designed by guys who learned building kits 30 years ago! Nobody asked questions.

Every time I look at a set of plans, or look through a magazine - I find airplanes that are very simple and have some interesting structural features, some really good , some very bad. I often find some cute way of doing something that is new to me; it's lighter or it makes a part come off easier, when I want it to. I sometimes find these in the strangest places. I always read the free flight columns, especially free flight scale. There are a lot of interesting ideas in them. You have to be a little careful scaling up because we have a large battery pack parked in the middle of the structure. Despite the cautions, there is always something interesting to be found.

SPECIFICS ABOUT STRUCTURE

These are the three basic premises in looking for good structures. This doesn't just apply to electrics. It can be for 200 mph pylon racers or gliders or anything you want to think about.

1) TIE THE MOTOR, BATTERY, WING SPAR AND LANDING GEAR TOGETHER and everything else is a shell going along for the ride. These are the places where forces occur from the outside world. The motor is obvious. The wing spar supports the lifting surface during aerobatics, takeoffs and landings. There are loads induced upon the landing gear and in the landing gear system. There are forces trying to push the gear back and out during landings. Battery mounts should be added, as the battery is a great deal of weight in proportion to the rest of the airplane. The battery has to be kept in place for all normal maneuvers, but there is no way of keeping it permanently in place. If the plane crashes, the battery WILL find its way to the ground. If there is anything in front of the battery, it will be struck with the force of a sledgehammer. The battery should be held in place, but provide for it to exit the airplane with a minimal amount of structural damage. It is not a good idea to mount the speed controller right in front of the battery pack, unless you really want to support your local speed control manufacturer.

Basically, tie all these systems together and then things like the outside edges of the fuselage, the rest of the wing, the ribs, the trailing and leading edge and to a lesser extent, the tail, are "tack-ons"; the forces on them are much less. The "tack-ons" can be, in proportion, of much lighter structure. The central structure is where to invest weight in order to make the airframe stronger, not in the outside shell of the fuselage. You can home in and say, "That's the part that needs strength", and a little extra weight, say a spruce spar instead of a balsa spar, and increase the weight by a few grams but increase the strength by a factor of 3 or 4. The difference between skinning the airplane with 3/32" balsa instead of 1/16" balsa is that the airframe weight increases by 10% but the strength is only increased by .001%. It doesn't make it stronger, but it adds a lot of weight.

2) The second structural mechanics premise is: TRIANGLES ARE STRONG. Do everything possible with triangles. Rectangles are weak, but as soon as you make a triangle, then you maximize the strength.

3) The third thing is to PREVENT STRESS RISERS. A good example of a stress riser is the foot long, 1/4" dihedral braces at the main spar, the secondary spar, the leading edge and the trailing edge, all attached to 1/4" balsa spars, etc. going out from there. The first time the wing is stressed, the only point that the wing wants to bend is right next to those braces. The entire wing is bending right at that point. (see the free standing arrows in Figure 1) The center section sure won't bend! The wing will fail right where the braces stop. All that 1/4" ply didn't do a bit of good.

GLUES:

I can't use any CA glues because of really bad asthma, even UFO's. You should be careful around them because you can develop reactions and sensitivities to them.

For foam wings, the glue I have had the most consistent success with is Dave Brown's Sorghum. It's a thin, water-based, cement. I've tried a LOT, but this is the stuff I always come back to. The only exception to this is the high performance F5B type planes that need epoxy adhered sheeting.

WINGS:

How the wings are going to be used determines their structure. Structures will be very different from a light, floater type glider to a moderately aerobatic sport plane, to a full, fire-breathing aerobatic plane, to a pylon racer. There are different levels of structure needed for the various stresses and strains. (Along with pylon racers, I'd put in the F5B gliders. They are basically pylon racers that have to be thermaled.) You have to decide what the goal is; what you are looking for, and then build the structure to support it.

If you are flying a light floater type of glider, say a 2 meter glider with a 6 or 7 cell 05, probably the best wing is a multi-spar wing.

The wing structure is going to be open, keeping the sheeting to a minimum. In designing a floater type airplane, you want the absolute minimum weight. All the plane is going to do is go up and slowly descend, hoping that a thermal is going to run over it and it goes up. A typical 2 meter hasn't any penetration to speak of. Old timers are the same, they just don't have any penetration. The object is to stay up as long as possible with minimum sink. The absolute lightest structure is what is needed. Plan on never putting this airplane into a vertical dive or looping it.

The way to do this is to use a set of spars top and bottom. The best thing to do is to put shear webs BETWEEN the spars. An "I" beam is much, much stronger. If you think about a wing, as the tip flexes up, the two spars appear to slide in opposite directions. The shear webs prevent this.

The bottom spar is under tension while the top spar is under compression. All of the materials, typically used for models, balsa, spruce, carbon fibre, are usually 3 to 10 times stronger in tension than in compression. If you want to build a strong wing, you have to think about the materials. Many designers put just a spar on the bottom. That doesn't make sense; it should be on the top. One of the worst airfoils is:

It's probably the weakest wing design. Putting the spar on top helps a little, but not much. Using a top and bottom spar with shear webs and making an "I" beam jumps the strength by factor of 10 at least. The shear webs are really important.

Even light 1/16" balsa will work wonders. Make sure that the grain is vertical. It's harder to cut, but they are stronger.

Put the spars at the center of lift, which, for our airfoils, is around 25% to 35% of the chord. Even though every part of the wing is providing lift, if you add all the vectors, it all magically appears as one big arrow at the 25% to 35% point, so that's where the spar goes.

For just strictly bending loads, that's all that is needed. Just that one spar sitting in the middle. There are a couple of problems with having only ribs and an "I" beam spar. Trying to put any sort of covering on when there is no leading edge is just one. We have to stick something up at the leading edge. Everything else, other than the spar, doesn't add strength but is there to maintain the airfoil. If that was all you did, you would find the covering sagging between the ribs and the airfoil between the ribs would be nowhere near the designed foil.

In the rear half, it's not so important, but near the front of the airfoil, it needs something to shape the foil. The simplest way to do this is called the multi-spar.
Maintenance of the airfoil is the origin of the multi-spar. Although the "spars" are usually small, typically 1/8" sq. and not real spars, they are just keeping the covering out where it belongs.

There is a little advantage in that very slow airplanes can have problems with air flow separation. The multi-spar wing helps the air follow the airfoil. A very clean airfoil, flying slowly, hasn't got enough air flowing to keep the air attached to the airfoil. Somewhere, about the middle of the airfoil, the air flow is going off making turbulent air flow over the whole wing. It means that, for all the care you took with a nice wing, the air isn't following the airfoil you chose. It's forming its own airfoil. The air flow must stay "glued down". By putting appropriate bumps on the airfoil, turbulence is induced early. It is sort of like a bunch of little marbles that make a little tiny boundary layer and the air flows over them very nicely. The "spars" act as little turbulators and give a nice efficient airfoil. Once a plane gets up to 25 or 30 mph, they don't do anything. At 10 to 20 mph, they help a lot.

The trailing edge is where many people have a lot of trouble. Most kits use great big chunks of triangular wood, butt joined to the back of the ribs. No matter what is done, after about two seasons, the trailing edge is hanging up or down.

I haven't got the best solution, but what I've always done, because there isn't much strength required to hold the back of the wing straight, is to use a piece of sheet balsa, 1 1/2" wide and then, right at the back, glue on a piece of 1/8" x 1/4" spruce which I carve or sand to shape.

For light wings this works well. When dealing with cap strips, make sure the ribs are cut back so that the caps fair in with the spruce trailing edge.

Going faster and playing around with aerobatic airplanes means that, unfortunately, sheeting will have to added. This means weight, but the separation factor of a faster flying airplane, for doing aerobatics or for doing pylon racing, is not so much from bending loads, (loops or pylon turns) - the wing spars still take those -but the faster a plane goes, it sets up a chance for a thing called flutter. Flutter is caused by the turbulence going over the tips and the trailing edge. The whole wing is trying to twist. If you have ever heard it, there is a loud buzzing and, "Oh my god!", shortly followed by the wing going "BOOM!", followed by a bunch of crying.

The next level of structures are to provide torsional rigidity.

One method is to add a second set of spars and then, somehow, add some structure between them, but this adds a lot of structure and doesn't do that good of a job. The best way is to add leading edge sheeting which, with some sort of leading edge, ties everything together.

This is a "D" tube structure. It is like a completely closed tube. If you have ever tried to twist a tube, it's pretty hard to do. This is where the torsional rigidity comes from.

If you really want to get carried away, you can close in the rear to form a double "D".

The last step is to sheet the entire wing, which is the strongest. Now the whole wing is acting like a tube.

The front "D" tube is OK for up to 70 to 80 mph airplanes. When the planes start getting faster than that, or doing heavy duty aerobatics with lots of snap rolls, the secondary spar is a good idea. After that, you go to the fully sheeted wing or go to foam with high tech stuff like CF and Kevlar.

OK, that's the side view, looking at the ribs.

Backing up a bit to the multi-spar wing:

If you touch one wing tip on landing, the whole wing panel will try to parallelogram.

You may have seen airplanes that have made a "nice" landing (!), but every rib bay has a diagonal split and a broken rib at each spar and trailing edge joint. It still looks like a wing, but you might as well take the radio out and put the wing in the garbage. The way you solve that is really simple. Gusset the wing tips. Make sure that the gusset grain goes across the joint.

With the grain parallel to either side, it's not doing any good. As soon as there is any strain on it, the grain will split.

If you have any of those great shelving units for your basement, they use little short pieces of metal on the diagonals. That's about 90% of the strength of those units. That's what the gusset is doing. All the wood in the center of the gusset probably isn't doing anything. You could take out of the middle and just use a little strip of balsa for the same strength.

The other thing you can do is add a lot of 1/8” diagonals. A lot of gliders do that, the Amptique does it. Anything like that adds a fraction of an ounce to a wing but decreases parallelogram failure a lot.

If you use Monokote or Micafilm, both have a very high surface tension. The covering is giving a tremendous amount of strength, preventing the wing from twisting or fluttering. This is why so many gliders can get away with such light structures, even if you dive them a bit. There is a lot of strength in that thin film. Solarfilm, Econokote, Black Baron, things like that, those are very soft covering materials. They give very little torsional rigidity to the structure. A strong wing is required underneath. The worst case scenario is the iron on fabrics. They are 2 to 3 times as heavy as Monokote and have less torsional rigidity. It is like covering the structure in a tent.

When sheeting a wing, the sheeting can add a lot of strength if done properly. In a simple thermal glider wing there is a big strong spar and a lot of ribs. Most gliders have the center two bays sheeted so you can put on the rubber bands. The designers stop the sheeting suddenly. That's a problem because there is something nice and strong transitioning to something that's trying to flex.

If a little load is put on this wing, it breaks right on the x (see figure) . The most notorious case of this is the performance glider design by a major west coast manufacturer. I've seen many of these planes blow up. As long as they are flying without strain on the wing, they're great. Get them into a little bit of a dive, over speed them a little, so that they start to get some torsional flutter and bang. All the load ends up at the leading edge and snaps it. The wing twists and blows off. That's usually the failure mode.

How do you solve it? You have to get around the stress risers by distributing the loads over the area, by cutting the sheeting to spread the loads over a couple of rib bays.

Another solution is to taper the amount of sheeting.

If you don't like to cut curves, straight lines are OK, just don't stop the sheeting in one spot.

If it is a full "D" tube, I'm not sure how important it is to curve the rearward sheeting. It does look nice. It also prevents that funny little "pocket" that forms in the Monokote at a 90 degree corner.

When putting in dihedral braces, don't make them all the same length. Make them all different lengths so that they're not concentrating all the stress on one part, (or in one line).

If the plane is not in need of a fully sheeted airfoil, but you are concerned about loads, you can make a tapered spar. It's a lot or work, but you might want it.

Think about the loads on a wing. The tip is supporting itself and providing lift. The next panel is providing lift and also supporting itself, and the tip. The next is lifting and also supporting the end two panels and so on, until you get to the center section which is providing lift and supporting the whole rest of the wing.

If you think about the load on the wing, the spar doesn't need to be as strong at the tip as at the center. Tapering the spar is a pain and it doesn't save much weight. If you have an extremely strong wing and want to use a "D" tube and cap strips, scarf in spar doublers for 2 or 3

rib bays out. Taper the end to transfer the strength gently to the outer spar material at the tip. It only has to be 2 or 3 rib bays. The center section will be a little bit stronger, especially around the landing gear. That is where a lot of load is provided from landing. On most of my big airplanes and pattern airplanes, this is the spar system I use. If the upper and lower spars are 1/8" x 3/8" spruce then the doublers are 1/8" X 3/8" also.

SPARS:

For the best bending conditions, keep the spars thin and wide, as a cap. If you put shear webs against the face of the spar, the glue joint at "A" is in shear and

glue is not very strong in shear. There is not a lot of gluing surface. A good shear web in-between the spars is stronger, because the joint at "B" is not in shear and the glue is just there to hold it in place - not really much load on it.

If you don't have shear webs, the bottom spar doesn't fail under load. The top spar fails "out" or "in" . By putting the shear web in-between you control the breaking, to some degree.

If there is one place to spend more time on craftsmanship than anywhere else, it's on making shear webbing. If it doesn't fit, pitch it and make another. It's only sheet balsa. It only takes a few minutes.

If you want to increase the spar size, you have a choice between thicker and wider, remember wide and thin is better.

There is a caveat. Spruce, that is bought in the hobby shops, is a faint memory of what we used to get. Balsa has gotten bad; spruce is worse. The good stuff is difficult to find and cut. If you can't find any, you are better off going to a square piece. You at least have some chance of the grain going in the right direction. The grain in the end of good spruce looks like a leaf spring.

If you find shear webbing bothersome to glue between the spars, with square spars, gluing to the face is better than with flat spars. The gluing area is that much larger. The shearing effect is spread over much more of the glue joint. You can get away with it.

The strength of an “I” beam is linear to the strength of the caps, but it is the square of the distance between them. Get as much of the good stuff as far out as possible. BUT the wider the spar, the harder it is to find good wood for them. It's worthwhile, any time you are in a hobby shop, to check out the spruce rack and if you find any good stuff, even if you don't need it, BUY IT! The same holds true for 1/16" balsa. There just isn't enough out there to rely on getting it when you need it.

Box spars are used when you don't want to sheet the wing. You use wide spar caps and full webs front and back. This approximates a tube and that is rigid. A "D" tube is bigger and stiffer. On real airplanes it is used to avoid a "D" tube, (fabric covering). For models, you are better off with the "D" tube.

A note on struts: If a wing has struts, (i.e. a Cub), it's always a good idea to make them functional.

A NOTE ON TRAILING EDGES:

NACA research has shown that the perfect trailing edge is a razor sharp trailing edge. The NACA research also showed the next best was a square trailing edge, as much as 3/16" on a 12" section. The worst is a rounded off section, the way most models are done!

FOAM WINGS:

Foam wings can be very, very strong but are also often very heavy. It has nothing to do with the materials. It is the glue. People cut the core and bond the sheeting, and that's OK. When they put on the leading and trailing edges, instead of sanding the core smooth, so that a thin film of glue works, they gouge it and slap on about an ounce of epoxy and stick it on. They cut a big hole for the bellcrank and mount it on 1/4" ply, and use a pool of epoxy as a cure-all for their sins. THAT'S where all the weight in a foam wing comes from, adding all the other things. A carefully made foam wing, with balsa sheeting, can be as light as a built up aerobatic wing. If you are dealing with a light, floater type wing, it's senseless, but things like F5B gliders have to use foam. I believe that you couldn’t build a wing strong enough with conventional construction.

My little ducted fan uses a foam wing, 260 sq. in., with 1/16" sheet and weighs about 4 oz., which is about as light as a conventional wing could be and as strong. It just a matter of taking care to keep the amount of glue to just what is needed.

The foam is really the shear web. It's basically keeping the sheeting apart. The sheeting is the spar and the foam is just keeping it from going anywhere. The bond must be good, to stop the sheeting from popping loose. A spar is usually counterproductive, as it creates a stress riser. That means the sheeting fails near the spar. It also needs shear webbing which negates the purpose of using the foam in the first place, namely to try to minimize the internal structure. (Going to glass or carbon fibre could be another five hour discussion.)

Foam wings can be used with electrics. I do it all the time, especially with high performance airplanes. You do have to be careful where you add weight. The leading and trailing edges can be made from the softest material you can find. The skin is the strength. You do have to be careful. If you are the sort of person who takes the wing and throws it into the back of the car and the tool box rubs up against it on the way home and you put a good healthy crease in it - guess where it fails? That sheeting is the spar and if you damage it, you've got problems.

WING MOUNTED LANDING GEAR

The landing gear is another area where a lot of kits and magazine articles put in an inordinate amount of weight and no strength. The wing mounted trunnion block is typically made of 3/4" solid maple, while the vertical block is too often pine or spruce. All the load is in that vertical block. The big block is just there to stop the wire from sliding back and forth on the wing.

If you want to do a trunnion block set up use 1/32" ply laminated to the rib and notched to the spars. The

block, instead of maple, can be made of 3/32" ply with say 1/8" x 1/4" spruce fore and aft to stop the wire moving.

At the other end is where all the forces are going to concentrate. When you come in for a landing, the wire flexes back. The vertical block is trying to rotate out the front of the wing. If you are going to use maple anywhere, use it there.

If you insist on using spruce, don't put the grain vertical, put it horizontal, so that the wire can't split the grain. The grain should run front to back on the wing and the hole for the wire goes up through this. This vertical piece is glued to the ply rib doubler on the inner end. That' where all the strength in the airplane is, at that one joint. The rest is going along only to prevent the wheel from wandering.

RETRACTS:

It is possible to put retracts on electrics. I have 3 or 4 now. In general, the problem is not the weight of the retracts. With fixed landing gear you have a torsion bar, a piece of wood and plywood to support it, the wire and the wheel.

You want to make it into a retract. You've still got the wheel, most of the wire, some ply facing on the ribs to distribute load. You don't have those two pieces of torsion block on a retract, instead you've got the retract unit. These are pretty light. You've got a servo in the middle to run the retracts and you've got a slight difference of weight in the retracts themselves. To give you an example, the retracts on my 40 size Spitfire cost me 3.5 ounces.

The problem with retracts is not the weight factor. Real airplanes take off from grass or pavement. In proportion, we take off from hay fields. To make most scale airplanes work, the landing gear is pushed forward so that the bending action, that the gear goes through on landing, doesn't cause the plane to land on its nose.

That gave me fits with my Mew Gull. I tried a scale landing gear location and no matter how careful I was, no matter how much I flared, every time I came in, right up on its nose instantaneously. I don't think there was even any roll. I even put flaps on it to try to slow it down to see if I could come in to land better. I was trying to get away with a scale landing gear location. It wasn't even retracts.

What I did was to make a new set of wheel pants and cant them forward. If you see my plane you'll notice that the center line of the wheel isn't anywhere near the center line of the wheel pant. That's the only way I got it to land.

How to get the retract unit back up into the hole in the wing is usually a problem because when the wheel returns into the wing it is at quite an angle. If you're going to go off and play with retracts make absolutely sure you know where the wheel is supposed to be.

To figure out where to put the wheel requires the vertical center of gravity. To find this, (unfortunately the airplane needs to be virtually complete), take the whole airplane and find where you have to hold the airplane with the wings vertical to balance it.

Once you have that point, draw a picture of the airplane in flying stance (see figure) and drop a line

down through the vertical center of gravity. This is the magic angle. Wherever the wheel contacts the ground is the point we are concerned with.

If you are flying off pavement, and you are only flying off pavement, you can get away with 5-10 degrees. That's what most scale airplanes are set up for. If grass, it's more like 15-20 degrees. If you're flying off a hay field, it's up around 25-30%. A lot of airplanes get into trouble if the gear is too far back even on take off. The airplane's sitting there, you add a little throttle, the nose goes down so full elevator is applied. The airplane somehow walks away. In order to keep the plane from going over, it requires holding full power and full elevator. Guess what? The airplane floats off the ground in full stall, snap rolls and goes in. It never got into flying stance. It causes a lot of crashes because the gear is so far back that you are having to balance the airplane.

In general, with 40-60 size airplanes, you can probably put in retracts with no trouble because you can play with the geometry to get the gear up and down.

With 15 size and smaller, it's probably not such a good idea. You are dealing with a wheel so small in proportion to the grass that they have to be awfully far forward and therefore difficult to get off the ground.

In order to get the wheels to retract back and up, it's an intricate set of geometry. You end up having to tilt the retracts forward and out.

Retracts are really nice if you really have to have them. You'll spend a lot of time getting take offs and landings down right. Sometimes it adds so much trouble that you don't like the airplane.

TAIL SURFACES

Remember I said that tail surfaces are over built? When not dealing with a scale airplane, with specific rib locations, it's a classic case of where triangulated structures add a tremendous amount of strength for the same weight.

Note the above versus the following.

If you're worried about having the lumps showing through the covering, you can use a combination of these two. Put in the needed ribs and put in 1/8" x 1/16" diagonals that don't touch the covering.

This gives geodesic strength and the right appearance, using triangles.

In the tail there is probably only one piece that has to have a little strength added. Most of the plans and kits I've seen just make the tail out of heavy wood. Sometimes it is sheeted. This type of stab often fails. The reason is that the tail isn't the whole story. There is a fuselage in there.

Moving the elevator is causing loads on the tail. Most of the load is caused by deflecting the surface, not because there is an air load on it. The forces are occurring at the back of the stab, which is sitting on the narrow end of the fuselage. The stab is flexing up and down at the rear. There is a stress riser where the stab rests on the fuselage. That's where the stab fails. It cracks and the stab fails. A cure for this problem is a piece of 1/8" X 1/4"' spruce tapered out at the ends. It adds a gram or two and increases the strength by 100 - 200%, even for a fully sheeted stab, it adds a lot of strength.

Vertical stabs can be done the same way. Lots of airplanes have the tail glued on top of the fuselage.

When building the fin, continue the trailing edge of the fin down through the fuselage. Reinforce this with a small piece of spruce, tapered to avoid a stress riser. Little tiny pieces do wonders.

Make the elevator spar continuous through both sides. Make both sides as one piece and tack glue it to the stab. It's all one piece. It is carved and sanded and then the two parts are popped apart. I always set in a piece of block on both sides of the elevator.

Take a piece of wire (small airplane 1/16", medium 3/32", large 1/8") and make up a "U". Slot it into the two halves while they are still attached to each other. When you are done, you can cut it apart. Glue the stab in and finish the sheeting. You can still fish the wire through the fuselage when you're finished. Glue on the elevators with epoxy when everything is lined up nice and straight. It seems to work pretty well. Keeping wood in there makes sure that everything is straight when you make the final attachment.

Some planes require different ways of doing things. Sometimes you have to run two push rods and horns to 2 separate halves.

To make a curved tail outline with straight pieces is a pain. Making a laminated tail is easier. Use a piece of ply or foamboard, and under cut the outside dimensions by about 1/4". Cut strips of 1/16" X 1/4" balsa and stack it up with white glue between the laminations, wrap it around the ply or foamboard form and tape it in place. Let it dry, take the tape off, pin it down and fill in inner structures. You can use 1/32" strip for the really small airplanes. It is very strong, like eggs and circles. It weighs virtually nothing.


If you don't want to do all that, you can use the "core" method.

Turn it over and repeat on the other side. Then sand it to section.

A lot or people think that by cutting a bunch of lightening holes they are cutting a lot of weight. If you add it all up, you save a few grams and end up with a floppy piece of sheet. It probably isn't worth it. The same is true for ribs. It's unbelievable how little weight you save. It looks nice, but that's about it. That doesn't mean that you shouldn't lighten that great big piece of 1/4" ply, but for typical lightening of balsa, it's usually not worth it.

Small planes don't need a built up tail. Use sheet balsa. Use "C" grain. It's that speckled, very stiff stock. My little Shrike has that. It is very stiff for its weight. It gives strength and torsional rigidity. This is the place you use "C" grain. It doesn't bend very well, so of course that's what you get in kits for wing sheeting or fuselage sides!

On small airplanes a sheet balsa tail is not a bad idea. If you are using a sheet balsa tail and you're worried that it's going to warp, you can add tips of balsa with the grain going the opposite direction. This prevents the sheet from cupping. Another ancient trick is to cut a slot in the middle of the panel and let in a small piece of balsa with the grain going the other way. If you know that one, you're showing your age.

These are both ways of dealing with a sheet tail. On small airplanes, (250 to 300 sq.in.), you'd probably end up heavier with a built up tail. It would be thicker and therefore cause more drag.

FUSELAGE:

This is the single biggest piece of structure that people over build.

A lot of old timer kits and plans knew what they were doing when it came to wings and tails. The wings were beautifully designed. The tails were works of art. Unfortunately, the fuselages were built like baseball bats. The reason was that they were built to crash half a dozen times before they were trimmed out. We don't have to worry about that, ("It says here in small print").

Fuselages are one place to save a lot of weight. Unfortunately, it depends on what you want to do with the fuselage. If it's a sport plane (it doesn't matter what size it is) you can get away with sheet balsa and a few stiffeners. With 05's, use 1/16" sheet balsa sides and 1/8" sq. in the corners. For 25 to 40 use 3/32" sheet with simple cross bracing and 1/16" sheet top add bottom, cross grained, and you're done.

If you're talking about big scale fuselages, try doing the same thing and it weighs a ton. As the size goes up, the volume goes up as a cube. The fuselage becomes very heavy, so you have to look at other ways to do the realistic or scale structures.

The following is more for realistic or scale structures.

The simplest way is the old box type old timer structure.

Then you put in cross braces, jig the whole thing and cover it up. That's okay, but remember the triangles. You're better off with diagonal braces.

Even with verticals at the bulkhead position, you want to add a little strength so put in a diagonal like this:

If you think about it, as you come into land, the motor wants to continue down and this diagonal, in compression, prevents the bay from parallelogramming. The triangle is doing its job. You can also put one going the other way.

Another thing that you can add for very little weight, but a tremendous amount of strength in the front end is 1/64" plywood.

Many kits use a very poor design for fuselage construction.

With the above construction, the plane comes in, bounces and breaks. There you have it - a fishead, lying on the flying field.

You're better off using 1/64" ply like this.

If you think you need extra strength, this is the way to go. The nice thing is that every square inch of this 1/64" ply can be used. Save everything for gussets and local strengthening. Never throw any away. The smallest pieces can be used.

If you are building a truss structure style fuselage, and you want to make it bulletproof, take 1/64" ply and gusset the inside of all joints. It adds hardly anything to the weight, but adds a tremendous amount of strength.

The method above is okay for box fuselages. In a lot of old timers, boxes are fine. Then you can add some formers and stringers and it looks a little nicer and you can get some really attractive fuselages.

Sometimes the structures get more complicated and you can get carried away and park great big sheets of balsa as formers and stringers or sheeting. The basic truss structure box is redundant as the outside of the fuselage is not taking all the loads and the stress. The structure never sees any load. When the outside structure fails long before the inner sees the load, suddenly the load shifts to the inner structure and it fails instantly.

When you see huge bulkheads with a small box in the middle, it's time to redesign.

HALF-SHELL:

This is an old free flight method. I love building on a half shell, or crutch, which is similar.

First lay down a spine:

Make all bulkheads in two pieces and glue the bulkheads to the keel. Then add the stringers.

Essentially you are building a complete half of the fuselage. After finishing it, unpin it from the board, add the remaining bulkheads and the remaining stringers. If everything was done properly, it should end up straight. This is building on the half shell. The Mew Gull and the Spitfire were done this way.

A method similar to this is using a crutch. When building a fuselage that is normally a great big circle , and you don't want to build on the half shell, this method works well if the original airplane was built with the inside on some sort of tubing framework, but the outside is more streamlined. Use a datum line as the basis for a crutch. It even looks like a crutch.

Basically, it is two pieces of spruce, say 1/8" x 3/8", with a bunch of lightweight cross braces to hold the shape.

All the bulkheads are glued to this. Cut the bulkheads in half, glue all the bottom (or top) halves in place. Then you can set your wing saddle arrangement and stringers. Take it off the board, add the other side's pieces to finish it off.

This technique is really great for biplanes because the crutch is used as a reference for the cabane struts. The mounting blocks can be adjusted, the struts added, top wing mounted and everything jigged straight. Glue on the last pieces and finish the stringers. There's a nice hard surface to work on, and because you build it flat on the board, it is FLAT! The board and crutch are now good reference points to measure everything for the wing. The Gee Bee and the Stearman were done this way.

When building truss structures, spend time on the longerons. It's worthwhile making them from spruce, not so much for strength but, because sooner or later you're going to come in from a nice day of flying and you're going to put the fuselage down on something on the workbench and the balsa longerons will break. If you don't want to go to full spruce, you can go to a laminate of spruce and balsa, especially if the longerons are curved. It's a lot easier to bend two pieces and glue them together than to use one large piece. Use carpenter's glue and pin it down. Once it's built there is no stress transmitted to the other parts. The best thing, when making up structures, is to have every piece remain curved if taken out of the structure. The fuselage side and stringers should remain curved. When parts are pulled together, stressing them with great big clamps, then preloading of the structure occurs, so much that if hit lightly, it could fail because the structure is already close to breaking, due to the preloaded stress.

When working with large bulkheads, many people cut the middle out, forming a ring. No matter how the grain is arranged, somehow it's going to break. Two pieces of balsa could be glued together like balsa ply, but it's a pain.

There is a wonderful material called foamboard. It can be purchased at art stores. It's basically 3/16" foam with index card bonded to both sides. It has no apparent grain. Therefore, great big holes can be cut out of it. It weighs about the same as 3/32" balsa. It's a little thicker. Virtually every single airplane I fly has bulkheads made of foamboard. A great big sheet works out to about $3.00. That is enough to do a lot of bulkheads on a lot of airplanes. The only drawback is that you MUST use RC56 glue. I haven't cut a balsa bulkhead in many, many years. I just don't know how to cut balsa bulkheads without grain fractures.

Don't get the plastic covered foam. The plastic covered stuff suffers from some funny failures with age. Remember that foamboard is great material to play with.

Weldbond also sticks to the foamboard. If you use epoxy or typical white glues, it makes a very hard joint, causing a delamination failure. Weldbond All Purpose Adhesive, (identical to RC56), can be purchased from a hardware store. It's a milky white liquid that smells a little like vinyl. It's actually a polymer. You can use that to glue the foamboard to the balsa.

The bulkheads can be cut on a band saw. Just treat it like balsa. On great big airplanes, I've used it for ribs - I mean 14 foot wingspan.

In the forward fuselage there are usually enough stringers for strength. Use some sort of balsa block for the nose with another bulkhead just aft.

I'm a big fan of the rolled up tube of 1/64" ply that the motor is pushed into. I don't usually try to reinforce

any more than that. Remember to tie the battery pack, the motor, the spar and the landing gear together. With foamboard half shell fuselages, on every bulkhead, somewhere in the middle, like with the crutch, set up a place where there is going to be a pair of lengthwise 1/8" x 3/8" pieces of spruce. This goes back and becomes the stabilizer seat and also ties into the motor tube.

The spruce is also a strong sport for hanging the battery pack.

All of the outer structure is gong along for the ride. It is just there to make the model look like a real airplane. The inner structure is carrying the load.

Since the bulkheads are load bearing, face small areas of the bulkheads with 1/64" ply to help carry the

load. The load goes from the strips to the 1/64" ply and is transmitted across a larger surface of the foam board, tying them together, distributing the load.

Because electric motors have virtually no vibration, it really doesn't take much structure for the motor. When using a speed controller, the start and stop are smooth. Using an on/off switch limits you to about a 15. Hard starting a 25 with a gearbox and a large prop will probably break every glue joint in the airplane.

As an example - I was testing the forerunner of the Astro Flight 25 in a pattern plane about 1981. I was coming out of a dive to gain altitude, trying to do what may have been the first vertical 8 with an electric, and at the bottom of the dive, one of the tangs on the commutator popped straight up. The motor stopped in about a 1/2 a turn with a loud CLUNK! The front of the airplane was literally turned upside down. Every glue joint was broken; it was hanging on by a couple of pieces of Monokote and the motor wires. There was a lot of energy in that motor when it decelerated that fast.

A speed controller starts and stops smoothly, so it's not that much of a problem. Trying to hard start some of the larger motors is NOT a good idea.

MOTOR MOUNTS:

These depend upon the power levels you are dealing with. When using ferrite can-type motors, almost anything will work.

I prefer the rolled up 1/64" ply tube. I use Astro Flight cobalts in almost every airplane I have. The first reason is quality. The second is the price. They are about 1/2 the price of the European motors, or less.

The third being that you can get the parts locally or send it back to "Uncle Bob" and he fixes if up for you.

Make the tube the length of the motor and cut slots for the brush housings. The slots act as an anti-rotation device.

Another trick is to trap the motor with the gearbox. The motor can't move back and forth. Without the gear box, a snug fit is achieved by putting a strip of masking tape on the motor and pushing it into the motor mount for a snug friction fit.

Astro Flight makes a nice little plastic motor mount. It's like a tube mounted on a plastic backplate mounted to a firewall. They work out very well. The motor is held in with a locking screw. For smaller motors, SonicTronics makes a nice little mount that sort of clamps the motor with the wraps. The SonicTronics mount is rated for a 15 as maximum. Originally, it was designed for ferrite 05's.

Obviously, with electric motors, you don't have any vibrations. The great big mounts made like gas engine metal mounts, aren't such a good idea. I once saw an aluminum mount that looked like it was for a .60 glow on the front of an airplane. It had an 05 in it. The motor mount probably weighed 8 Oz. at least. That's too heavy.

When dealing with 250 watts and up, use tubes and other types of mounts. The commercial sport mounts are designed for relatively low power motors.

If the motor has threaded bolt holes in the front, for direct drive, you can bolt the motor directly to the plywood firewall. A different method is used for gear boxes, where you can't bolt the motor in directly. The 60 on the Mew Gull is bolted directly to the ply bulkhead. I also have a supporting bulkhead in the back. (The 60 weighs 24 oz. That's a little heavy for just the forward bolts.) The gear box and motor should be on the same side of the firewall. The motor and gear box should not be separated by plywood because the plywood compresses, which will allow the gearbox and motor to loosen up with time. When the bolts are tightened, there is a lot of compressive force applied. Even worse is a hard spot in the plywood which results in a crooked mounting which is very hard on the gears.

I have, a couple of times, with special purpose airplanes, made something out of 1/16" sheet metal and trapped it between the motor and the gearbox. That was before I figured out the 1/64" plywood tube and trapping the gearbox with it in that.

If you believe that you have to make a lot of motor thrust adjustments, either you have the angle between the wing and the tail way off (that's why downthrust is needed), or you haven't learned to fly rudder finesse (that's why side thrust is needed). Putting rudder offset in an airplane is done because you haven't learned how to use your left thumb. With a plane that is trimmed to fly perfectly straight, as soon as it gets out of that straight line, rudder correction is needed.

There are ways of minimizing things so that if you don't use rudder, you hardly see it. But, in reality, you need that rudder finesse to really fly airplanes correctly. Coordinated rudder is what you call it for level flight. Finesse is used when doing aerobatics. During a loop, even the most perfectly built airplane really should have rudder and aileron corrections all the way around the loop. When an airplane is flying fast, that disturbance is only a few inches, but in truth, the corrections still need to be done. The faster the plane flies, the less the correction that will be needed. There is no way of building an airplane, with a rotating prop, and getting it to fly dead straight through all maneuvers and speed ranges.

MOTOR COOLING:

The amount of cooling required by a cobalt is not worth considering unless you go out with 16 battery packs charged. Don't laugh. We have a guy, Dave Grife, in our club like that. He shows up with a plane, transmitter, and a backpack and it's ZOOM, ZOOM ZOOM, one flight after another. I went over and touched his motor between flights and it must have been about 400 degrees (Uh, Dave, I think you should let it cool off.) I couldn't believe how hot it got because he never let it cool off; he literally flew continuously for 2 hours, my frequency too.

If you are going to do something like that, yes, try to get a draft to the motor to help it cool off.

LANDING GEAR: (fuselage mounted)

In many construction articles and kits they try to make a nice light plane and then use a piece of 1/4" thick sheet metal landing gear with razor sharp edges. I don't believe this is a good idea. A better way is to make up two of those little trunnion blocks, like used in wing mounted landing gear, and then mount the gear to the bottom of the plane. If the bottom keel piece is gong through there, glue some pieces up around it to tie things together. Run twin wires out and make one of them the axle.

Like the trunnion block gear, this type also flexes out. Smaller diameter wire can be used than with the torsion bar landing gear of the wing. Use 1/8", 5/32", or 3/16" (for big planes), in wing torsion bar gear while 3/32" or 1/8" will work with this type of fuselage gear. There are all sorts of variations on this and different ways of doing it, but this is a pretty good landing gear.

With this type of gear, the load is mostly taken by the landing gear flexing out and up, but if you hit really hard, it will try to rip out of the fuselage. That's were the reinforcing ply around the blocks goes to work.

Tying the gear together in a triangle defeats the purpose of the landing gear shock absorption. If you do want to tie the gear together, never go straight across. Instead, lash a rubber band or springs to take some of the load. The landing gear shouldn't be completely rigid, but you don't want it to jackrabbit down the field either.

Most sheet metal gear is either too soft, and the flattens on impact, or it is too rigid and doesn't provide shock absorption.

BATTERY MOUNTS:

In small airplanes, a piece of Velcro on the balsa fuselage bottom with another on the battery works well on 6 or 7 cell packs. It is not a good idea on 32 cell packs.

A hidden advantage of electrics is that lead never has to be added to achieve proper balance. Moving the battery pack about a 1/2 inch can get almost anything to balance. About 1/3 of the weight of the airplane is the battery. It doesn't have to moved far to balance the center of gravity.

Admittedly, I play with a lot of big airplanes, you might have to modify this a bit for small ones. I take 1/8 ply and make a plate and stack my cells like cord wood on it and hold everything together with winds of strapping tape.

Remember those two horizontal spruce rails in the fuselage? In the battery area, I glue 1/4 inch spruce to them to allow the plate to slide in and be able to be moved back and forth. By putting a number of holes in these rails the plate can slide back and forth to change the center of gravity. When the correct balance is achieved the plate can be secured with screws.

FINE TUNING THE CG

As the airplane gets close to its perfect center of gravity, the drag of the airplane drops dramatically, which means it takes less power to fly. Flying an abnormally nose heavy airplane, burns an extra 20% power just to counteract the nose heaviness.

It's the old weigh/lift/thrust/drag problem. Normally,

an airfoil creates drag which we can't get away from, but it also creates a pitching movement, which, with most airfoils, tires to push the nose down. In a glide, a typical flat bottomed wing will try to do a half outside loop. Symmetrical airfoils glide beautifully. For flat bottomed wings, something is usually done with the horizontal stabilizer. A lot of gliders get carried away and stick the stabilizer on at a drastic leading edge down attitude. This acts like up elevator which lifts the nose.

That's all well and good, but in order to get that to work, the center of gravity is fairly far forward, so that the airplane has a chance of flying. It becomes like a beam balance. The wing is creating lift and drag. The tail is also creating lift and drag but the lift is all down. That's the wrong way. The wing is lifting the whole airplane, so that if there is a pound of lift pulling the tail down, the wing needs to lift an extra pound, which increases its drag. Reducing the downward lift at the tail to just a little downward lift, which you need to counteract the wing pitching moment, can get the center of gravity back further on the wing and get the beam balance equation to work more efficiently. The tail is creating less downward lift, therefore less drag. The wing doesn't have to lift as much, so its drag drops. The drag of the airplane becomes reasonable.

An airplane with a lot of negative tail incidence, and the CG well forward, will glide at only one speed. If it goes any faster, it will try to loop. When the plane comes out of a stall, it will drop quite a ways before it recovers.

Where should the CG be? First, set up the CG according to your plans. Then, there are several tests you can make, aerodynamically, to find out what your CG is like. These tests are based on the idea that the angle between the wing and the tail is reasonable. You

rarely need more than 2 degrees.

It sounds funny, but almost no matter what you do, the airplane will try to fly with the stab level. There are a few exceptions like biplanes.

A plane flying in the 30 to 50 mph range, probably needs 2 degrees difference between the wing and the tail. For a plane in the 20 mph range, it could be 3 degrees. At 100 mph, you only need 1/2 degree or even none at all. I've seen gliders with 5 to 7 degrees. Why they have it, I have no idea.

Assuming even semi-good wing and tail angles, a quick way of finding the optimal CG is to pull back to 1/2 throttle at altitude. Fly well above the minimum glide speed - cruising speed. Make several passes up and down the field, at several hundred feet, playing with the elevator trim until the airplane flies level with no transmitter inputs.

Leave the throttle alone, but force a 30 to 40 degree dive. When the plane has gained a 20% to 30% increase in speed, (say 50 ft. or so), so that it's accelerating, take your thumb off the stick. If the airplane continues on straight, (hopefully not for very long!), it's at the lateral perfect center of gravity. It is neutrally stable. The airplane doesn't change direction, it just keeps on going. Ideally, I shoot for something that is just slightly trying to pull up, slightly positively stable.

If the stick is released, and the airplane tries to do a half loop, the airplane is very NOSE HEAVY. When the airplane picks up speed, the negative incidence, (or slight up elevator trim), acts like up elevator and will try to make the plane loop. (The increased speed makes the trim have more effect.) As the CG is moved back, there is less of a downward load on the tail, so speed has little or no effect.

On the other had, if the airplane dives steeply, it's TAIL HEAVY. If the CG is well back, the tail actually has to provide positive lift to balance. When the airplane flies faster, the tail lifts more and the dive is increased.

If the airplane always does a loop on the test, or has a 6 or 7 degree differential, put the CG further back, and reduce the difference to 3 to 4 degrees. That should add quite a bit of duration to the flight because of the reduced drag on the airplane.

Old timers, with lifting stabs, often have the CG around 70%. My Zomby trims out at almost 70% of the cord from the leading edge. It's way back!

Often, many of the old designers didn't mark the CG on their plans, simply because they didn't know either! They would say, "Balance to suit and get a good glide." ("When you've got it, call us and let us know!")

Old timers are very draggy airplanes. There is nothing that can be done to clean them up. Unfortunately, many had a tremendously bad force layout because the designers didn't know a whole lot about aerodynamics. Whether it worked or didn't work depended on which guy stumbled into a thermal. Then, if his plane was green, everyone went off building green airplanes because it took a green airplane to thermal! Few people knew what they were doing back then, so a lot of the old timers had strange force arrangements. Each individual old timer needs its own evaluation and set up, then it's almost cheating, because the original airplane wasn't built that way, so it's no longer really the old timer.

It's always best to get the stab incidence right rather than fiddle with the wing. There are many kits on the market that have the center of gravity in ridiculous spots and have incredible angles of attack. To them, if the plane flies, it's a good airplane. It really depends on what you want to do and what means something. If flying overhead with transparent covering is desired, then you can do anything. If super long flight times mean something, then that means efficiency.

Designing and Building efficient airplanes:

In my articles from Model Builder (July 1987) for designing sport scale and from MAN (Dec. 1991) for twins, I go into great detail about this topic. (note: If back issues of these magazines are no longer available and you need/want them - send me proof that they aren't available - and I will provide copies. Ken) The concepts laid out in the articles apply to both sport planes and scale planes.

I have yet to see an airplane that an electric motor couldn't fly because the prop diameter was too small. In general, we can fly props so much larger than the gas fliers can use, we can come out lighter and beat the performance just on account of the props we can use. A case in point is my Gee Bee R1 which flies fine on a geared 25. Every other one that size, that I've heard of, uses a 60 or 90 to turn a big enough prop and they still crash.

If you do a lot of scratch building and draw your own plans, then you can pick any size you want and pick a power system for it. Another thing that can be done is to find a set of plans for a lightly built airplane and modify it for electric. Another way of doing it is to take a set of plans and just use the outline.

Glow kit conversion:

There was a kit manufactured in Germany, a Klemm 25. Every country has a trainer. In the U.S., it's the J3 Cub, in Britain, it's the Tiger Moth and in Germany, it's he Klemm 25, the equivalent of a low wing J3 Cub. It has a huge wing on it, a relatively short fuse, and a big tail. It is a very nice flying little airplane. I haven't built it yet. It's a case where its built for gas, but I can't think of what to lighten. It's such a nice structure and really nice design. It's perfect for electric. Later, I'll go through the parameters to choose the motor to make the airplane fly well.

Using glow plan outlines:

Another case is when you find a set of plans for the airplane you want to build, but it's obviously built for glow engines. It has a 1/4" plywood firewall, 1/4" balsa sides, and foam wing with 1/4" dowel rod. Sometimes it's still worthwhile to get a set of plans just for the outline. If you know what the wing looks like, you've got the ribs, and you've got the fuselage cross section. Then you can say, "I'll ignore their structure and build in a nice light structure that fits." All the sizes and shapes and ribs are done for you. You just have to decide on the wood size.

I've got a set of plans for a Bearcat. My interest in aviation is mostly from 1925 to 1940. Virtually everything I build is a racing plane or aerobatic plane of the Golden Age. I could care less about jets. I did the little ducted fan just as an experiment.

One of the few military airplanes I like was the Bearcat, and the Spitfire of course. I always intended to build an electric model of the Bearcat. I have plans for the Top Flite Bearcat, which is tremendously over-built. I intend to throw away everything and use just the outline.

Scratch building and drawing up the plans yourself:

Another route is to start from square one by taking a 3-view and blowing it up to the size you want. You can take a photo of the 3-view and use a projector to project the airplane onto a large sheet of paper mounted on the wall. Another way is to use a photostat and make an overhead transparency and again project it onto a wall. With the 3-view and some of the cross sections, you can then get some idea of the wing area, wing span, wheel size and prop size.

The plane's actual size may based on how big the back of your car is, how big your work bench is, or whatever. Once the "size" is decided, figure out the span, cowling diameter, prop, wheel size, length of the fuselage and cross section, and most importantly the wing area. That's the thing that's going to provide lift. How much weight you strap on that area determines how it's going to fly and its handling characteristics. The higher the wing loading, the more your thumb has to be educated and the more careful you have to be flying. Light wing loadings, in general, are pretty easy to handle. The lighter the wing loading, the better, within reason, but we don't have too worry about that as, with our power systems, we are pretty much assured that we won't be too light.

You have to guess at what kind of wing loading you'd be comfortable flying. For light planes, 15 - 18 oz./sq.ft., for a large one and or a small one, 12 - 15 oz./sq.ft. would be better for a nice gentle flier. For an aerobatic or fighter aircraft, 20 - 25 oz./sq.ft. works well. For great big airplanes you can go to 30 oz./sq.ft. The Mew Gull is almost 30 oz./sq.ft. but it works out pretty well because of it's big efficient wing. I didn't intend the wing loading to be that high, but there's a lot of balsa in that fuselage. It's a lot bigger than it looks.

Once the wing area is selected, wing loading can be figured. For sport flying, 20 oz./sq.ft. is a nice number for reasonable performance. Multiply the wing area in sq.ft. by the wing loading in oz./sq.ft. for the total weight in ounces. This tells the kind of weight the airplane should weigh in order to give the handling you're after. All of this is related to take off speed, stall speed, landing speed, and minimum speed to stay airborne. There are other factors, but wing loading is the most important.

If you don't always just want to be flying around level and want some aerobatic performance - roll, loops, etc., these mild aerobatic maneuvers need 50 to 60 watts per pound. If you want good aerobatics - pattern capabilities - you need 70 watts per lb. for outside maneuvers, knife edge, etc. Pylon racers are up over 100 watts per lb.

Multiply the performance level you want in watts per lb. times the weight of the airplane to establish the required power.

A 3 sq.ft. winged plane, at 20 oz./sq.ft., is a total of 50 oz. - just over 3 lbs. That means, at 50 watts/lb., 200 watts gives the airplane those characteristics - mild aerobatics.

How we create the watts needed. (Watt = Volts x Amps)

Our battery packs are fixed in size. If we want a red hot flight, it's a short one, because the current is high, but you get higher performance. The question is how long do you want the thing to fly at full power - this is your peak power, your vertical performance. This power level sets your peak current. For most reasonable airplanes - not biplanes or huge fuselages or 18 zillion rocket pods - with reasonable drag coefficients, and a current draw of 20 amps out of a 1200 mAh pack, you're going to get a 5 to 6 minute flight. If you run 30 amps, it's more like 3 minutes.

Watts are current times voltage. If we want 200 watts at 20 amps for a 5 minute flight, we need 200/20 - 10 volts. Because we get about 1 volt per cell at this current draw, we need 10 cells. A motor chart shows that a cobalt 15 is in about the right range. With 12 cells you could drop the current down to, say, 16 amps, but now, because you're at 16 amps, you might go to 900 mAh cells, save more weight and have the same flight time.

With a draggy airplane, the rule of thumb is to use a geared motor. Dealing with a pattern type airplane with no loading gear and a hand launch, or sleek fuselages or pylon racers, those are obviously direct drive. There is very little drag and the plane is better off with a smaller prop, getting the horsepower that way.

A lot of European motors offer different windings instead of gearing. They don't like gear boxes. They do everything with windings and change the windings more or less to change the "gear" the motor runs in. A motor can be set up for all torque and low rpm and turn a great big prop. If a different armature is put in it, the motor screams at a high rpm but can't use a big prop. Over there, they pick the armature, while we use gear boxes or direct drive.

There are other things to be considered. For a really good aerobatic airplane, leave the landing gear off. The landing gear causes a tremendous amount of drag. I've found the optimum power for a good aerobatic airplane is a 15 size. As far as vertical performance, per weight, per aerobatic, per flight time, it is very good. The bigger airplanes have more impressive vertical, but their maneuvers are bigger and it takes a lot of time for each one. Big planes give fewer maneuvers per flight compared to the 15.

For the 15 size aerobatic airplane, the wing area should be about 350 sq. in. If you want an off the shelf airplane and you don't mind re-engineering the fuselage a little, the Great Plane ElectroStreak with a cobalt 15, twelve 900SCRs and a light radio is one heck of an airplane. Talk about holding the airplane vertical to launch. You get about 3 minute flights at full throttle, maybe 5 minute flights with throttle use.

By the time you add a take off and landing, you're making a really aerobatic airplane a real challenge. You're dealing with nothing short of a cobalt 60 with 30 to 35 cells and lots of bucks, just to get the same performance you can get out of a hand launched 15.

When dealing with scale airplanes, to be able to do nice take offs and landings, touch and goes, and modest aerobatics, virtually any size motor will do it. 05's will do it if you're careful, geared 15's will do it, which is a really nice size for a lot of scale airplanes. If you're trying to get some good aggressive flight characteristics, take offs and landing, maybe retracts, you need a 40. A 60 motor is a hard motor to make good use of. It is capable of putting out 1.5 hp, but the problem is that we don't have any ni-cads that can feed if for very long. 1.5 hp out is 1500 watts in. That means that if you are using 30 cells, you’re drawing 50 amps! The motor can create it, but the battery pack can only deliver it for about a minute or so. Unfortunately, there is all this horsepower, but it's hard to feed it and keep the flight time. The best way to use this motor is to run wild amounts of horsepower for the vertical rolls, then pull the power back and use 1/4 power the rest of the time. Only when doing the vertical rolls, the figure "M"s and the outside maneuvers do you need full power. 60's are very expensive and it's hard to make good use of them. The only time I use them is when I want to turn a huge prop or when I need a lot of raw horsepower for a high airspeed. The sport 60 in the Mew Gull runs for about 4 minutes at full power at about 100 mph, way above scale speed.

I've actually found that the geared 40 is just about optimal for matching ni-cads to power to performance. A geared 40, running on 20 - 21 cells is about the best route to go. The geared 40 provides achievable power with flight time; with flight speed; with good aggressive performing scale aerobatic flight.

Motors and motor efficiency:

Most cobalt motors run about 75% efficient and the rules of thumb quoted here assume this figure. Most cobalts stay at 75% efficiency as far out as 40 to 50 amps. Ferrite motors, in particular little ferrite can motors, at a little over 20 amps, are down to 40% efficiency. Dropping 200 watts in the front end is only yielding the equivalent of about 80 watts out. That's power like a cobalt 035. The can motor is screaming its guts out, getting red hot and you're getting a 2 minute flight. The cobalt 035 will give the same power for 5 minutes. Be very careful with ferrite motors as their efficiency to power is low.

If you push a ferrite motor hard, it never comes back. The magnet is cooked, or the armature, or the commutator, or the brushes, then they fall apart. Remember, buy cheap, buy twice.

I feel that the reason cobalt motors aren't used in car racing is that if they used a lot of cobalt motors, the manufactures of the ferrites would go out of business. They are the ones supplying the events and writing the rules. That's why they don't allow cobalts. You buy one cobalt and run it for 10 years. You can't sell everybody motors 5 times a year or once a race or whatever. End of soapbox message.

Once the power needed is determined, weigh the power plant, battery, and the radio. Work backwards to see how much the structure has to weigh. Look at the airplanes you've built and weigh the structures to see if you can get some idea of the weight of the structures you build. Just a note; if you want a WWII fighter with full skin, all rivets and panel lines, you're not going to make it! The airplane will suffer in terms of performance. With that kind of detail, it will take off, it'll fly around level and look nice, but it won't have any kind of fighter-type aggressive performance.

I prefer performance rather than real detailed scale. I don't mind cheating here and there, using stringers, and building optical illusions for details. I'd rather have the performance. You can't see rivets and panel lines in the air.

Once the motor is chosen, look at the structure and figure out if you can do it. If there's no way, go back to square one and try a different size plane and see if something comes out the way you want.

After a while, you get used to this process and you can predict the motor needed for most airplanes, then you can reverse the procedure and go backwards. You can think; I have a geared 15 and I want 60 watts per pound. That means that I need this wing area for this wing loading, then you can size the airplane for them.

Until you're used to that trick, you can end up with strange results, way over or under horesepowered. It never works out right. When I'm playing around with a new airplane, I always go in the forward direction, because occasionally I get fooled on how much power I need.

Props:

In those write ups, (MB & MAN), there is a discussion about how to choose your props for test flights. I'm not going to get into that here. How to modify props is a little beyond most people. I'll say, at the least, that Rev Up props are, in general, very good. APC props work - I don't think they work as well as Rev Up, but other people rave about them. Maybe they've only used Zingers or Master Airscrew fixed blade props which don't work well for our purposes. The little Master Airscrew props, the 5" and 6" ones are great for small clean airplanes, but the big ones are not. The APC's are reasonable, the Rev Ups are my favorites, the Zingers can be reworked into decent props, but you need to do it correctly. The Master Airscrew Electric Props work well.

I sometimes spend four hours reworking a prop until I get the results I need and, if it's not right, I buy another one and try again. Once I get one working the way I want it to, I go out and buy another, make a back up, and put it in the flight box so that I have a replacement. A lot of my props are like this.

The single best thing you can do to improve the performance of an electric airplane is to play with the prop. You can change the performance by 30 to 40% with the same watts input. Many times it's just a case of buying a bunch of props and trying out each one. They could all be 10x6's. One of them will probably work a whole lot better than the others. You won't believe the difference. I can't tell you which prop to use because it depends on the airplane. A lot of times, it's just cut and try and experience. I tried to give some outlines in the he MAN article; how to get into the right size and shape prop, so that you are starting with a dozen props rather than 500.

A note on Twins:

In general, twins should be run in series for efficiency, unless you are running tiny 5 and 6 cell motors. They can be run in parallel because they probably only pull 5 or 6 amps each for an 11 amp total. If you try to run two cobalts in parallel at 20 amps each, the total draw on the battery is 40 amps. It's a very short flight time and the rpm will be lower, as the voltage drops at that draw.

Batteries:

Everything, and I mean everything, I fly is with Sanyo SCR's. The reason is that the SCR's are the only batteries that I have found that give me consistent performance and tolerate a relatively casual charge-discharge cycle. They are like a fuel tank, you put electrons in, you take electrons out.

There are several reasons I like the SCR's. They have very low output impedance. That means that when I ask for current out of the battery, in addition to getting the current I want, the battery, which starts out at 1.2 volts per cell, only drops to 1.1 volt per cell, even if drawing 40 or 50 amps. I'm losing only a very little bit, (this is what heats up the battery). Sanyo SCR's do very well in this situation - very little loss.

Something like SCE's, because of their impedance at high amperage draws drops about .6 of a volt per cell. It's like throwing half the cells out. About all they're going to do is keep the fuselage hot.

Wiring:

You get horsepower, yielding performance, with voltage and current. To keep the voltage up you can't have small diameter wiring, high resistance switches, inefficient speed controllers, or high impedance ni-cads. If you are using 10 cells and put a volt meter on the back of the motor and see 8 volts, something is very wrong. A good rule of thumb is 1 volt per cell at the motor at full power. If you don't get this absolute minimum voltage, start looking for where the problem is. Either you have wire that is too small, the wrong switch, the wrong connectors, or something.

In addition to the output impedance, every battery has a voltage profile. That is what the chemical voltage looks like over time as you discharge. At very low currents, virtually every battery curve looks like (A).

Many battery maintenance instruments, like the Ace Digipace cyclers, are set to shut off at 1.1 volts per cell. As we pull current out the battery, we are going to lose a little voltage because of the higher current. The curve will drop a little, (B). SCR's do a pretty good job. They basically stay flat all the way down to the end. It's just like turning a switch off. You know it's time to land when the plane falls out of the sky! With cells like the SCE's and some of the cheaper ni-cads, the profile looks more like (C). You may find the plane landing before you get to the "knee". Even at full power, there just isn't enough voltage times current to fly the airplane. When I fly an airplane with SCE's, the first minute I'm smiling, the second it's okay, the third it's boring, and then I'm scrambling to see how much more I can stay airborne before I have to land. Even then there's still lots of unusable power left over. This applies to high performance airplanes.

With an Amptique-type of airplane, where the current drain is 8 to 12 amps, the discharge curve isn't so bad, (D). Not quite as good as the SCR’s but okay. Because the SCE has more capacity for its weight, you do get a longer flight. If putting around, with ungodly long motor runs, slow fly bys, touch and goes, etc. is what you want, the SCE's aren't bad. They are a little finicky to charge. They aren't really as tolerant of over charging. They also have some funny characteristics.

Charging SCE's should be done carefully, at no more than 3 amps. I don't have a lot of experience with them, but that's what the people I know using them charge at. SCR's could care less about how fast you charge them. You can charge at 6 or 7 amps as long as you don't over charge.

AE's are even worse than SCE's. They droop pretty badly. They're the 1250 Magnum size. I won a pack of 7 x 1250 Magnums and took out my Amptique, which normally has 7 x 800 AR's. I did the same flight, took off, flew around, did touch and goes. With the he 800's, I was getting, typically, 30 touch and goes and 12 minutes of flight time. With the 1250's, which are supposed to have much more capacity, I could only do 25 touch and goes before I couldn't get it back into the air. If I had altitude, I could have cruised for some time, but the cells didn't have the voltage to get the airplane off the ground. It's also a little disconcerting to land and the pack is so hot I can hardly touch it. That was in an Amptique which is a low power design. I just don't have too many good things to say about the so called "extended flight" cells.

For carefree ni-cads - simple charging and go fly - it's pretty hard to beat SCR's. Plus they can deliver almost as much power as you need without really effecting the characteristics of the ni-cad.

Charging radio batteries versus power system batteries.

Radio ni-cads should be stored charged. Motor packs should be stored discharged. At the end of the day, I run my packs right down. It's just like running the fuel out of the tank. I don't wait for the prop to stop but I can clearly tell when they are down.

The problem with storing radio batteries flat is that there are micro crystalline growths. With SCR's, it's very unlikely, particularly when given a 5 amp charge. It blows out any growths and the cell acts normally.

I've never had much luck trickle charging radio batteries. I prefer to charge a couple of hours once a week rather than trickle charge.

Balancing Batteries

I never worry about cell reversal because I've never seen it in an SCR. I ONLY use SCR's, so they are all I can address. I never balance packs. I buy the cells in boxes of 20. Whenever I've done tests, there is never anymore than 5% variation. They are all the same, no real bad cells and no real good cells. I don't know where some people are getting their numbers. Maybe someone has already gone through all the cells I get, but I don't think so.

I don't worry. I have yet to replace a SCR and I've been flying them since 1986. Maybe I've had to change one or two of the earlier SC cells, but not the SCR's. When I've had hundreds of flights on high performance airplanes and I cycle my packs, I still get 1.2 amp hours. These are good cells.

Catapult Launches:

Use 10 feet of heavy surgical tubing and 10 - 20 feet of heavy fishing line and some sort of ring. Set the launch ring hook on the line between your vertical center of gravity and where a high start anchor would be. It will be way out in the nose of the airplane. It shouldn't be back where a high start hook would go or the plane will go straight up! The catapult is used just to accelerate the plane. If you pull it back too far, by the time you launch and you're back on the stick, the plane is gone and off the line. If the hook is a little far forward, the plane will drop a bit but it's not much of a problem. Launch with the prop turned off.

About a foot ahead of the hook, put a piece of cloth to make sure the ring drops. When I see it drop, I know that I'm off the line. Its a really nice way of launching almost anything. If you don't feel you can launch carefully or your arm is tired, it's the way to go.

Reprinted courtesy of the Ezone.

Back to Table of Contents

Helipad

By Fitz Walker - Email: Fitz

Basic Heli Setup

After reviewing my last several articles, I noticed that I have been somewhat unfocused in my topics (just like myself in real life). Since I've been flying for some years, it is pretty easy to forget that there are many people reading this that are new to the world of (electric) helicopters. Therefore, I will devote this month's column to the rank beginner. This will be a general guideline for setting up a helicopter and not specific to any machine. My aim is to provide a starting point to set up a machine properly so that it will only need minor tuning to suit individual flying tastes.

Helis 101

Model helicopters, whether glow or electric powered, are basically set up the same way. Basically, it means that there are common ways of configuring them. Since helicopters are rather complicated machines, it is important that they are built and set up correctly before flight is even attempted. It is always a good idea to seek the advice of other, more experienced fliers in you area if possible. There are also some very fine books and videos available that can be useful even though they generally cater to glow/gas type machines (and, of course, read the manual).

There are two basic types of helicopter designs, fixed pitch (FP) and collective pitch (CP). These terms refer to the rotor head design where in a FP rotor head, the blades are essentially over-sized propellers. To go up, you simple increase the rotor speed which increases the lift thrust; decreasing the rotor speed causes the opposite to happen. This type of control dates back the very earliest helicopters, as it is very simple to design and implement. The problem with this design is that controls tend to be sluggish since there is a delay in spooling the rotors up and down, and there is no chance of performing auto rotations (i.e. landing without power) if the engine/motor ever fails. Interestingly enough, electric fixed pitch helicopters tend to fly better than glow powered ones. The main reason is that electric motors have a faster reaction to torque demands and requirements, which means they can change the speed of the blades a bit faster. This provides better control of the helicopter while hovering. There is still a throttle delay, but it is noticeably improved with electric power. Another advantage with electric is the inherent ultra reliability. A properly equipped electric will almost never have a power failure in flight.

< < This is a fixed pitch rotor head on the LMH Corona. Blade angle is fixed and only the whole rotor head tilts for control.

Collective pitch (CP) designs solved the throttle delay problem. CP designs are much more complicated and a bit heavier, but the advantages generally outweigh the disadvantages. With CP helicopters, the rotor speed can remain constant while the blade angle of attack (pitch) changes to control altitude and attitude by instantly generating more or less thrust. CP designs allow much better control in forward flight and in windy weather, not to mention being able to do the awesome aerobatics commonly seen at flying fields.

< < Logo 10 has the more typical collective pitch head. Note the extra links compared to a fixed pitch head.

Almost all helicopters require the use of tail thrust to counteract the torque generated by the spinning of the main rotor blades. (The exceptions are helicopters with multiple main rotors or those that have tip thrusters.) This thrust comes from a second, but smaller rotor system positioned sideways that also usually has collective pitch as well.

< < The tail rotor is a smaller and simplified collective pitch head.

If you are just starting out, you may not be familiar with what the transmitter sticks do. Just like an airplane controller, the helicopter controller has two control sticks that move up, down, left, and right. The right stick makes the helicopter tilt forward, back (elevator), and tilt left and right (roll or aileron). The left stick makes the helicopter climb and descend, and it controls the tail rotor. For example, to make the helicopter climb, you move the left stick up. Alternatively, to make the helicopter bank left, you move the right stick left. See how easy it is.

< < This is a typical helicopter transmitter. The left stick controls the throttle/collective and tail rotor, while the right stick controls fore/aft and left/right banking (in most countries).

The Model

Are you still with me? Good. Now let's go on to something more practical. When building the model helicopter, it is essential to follow the instructions. (Yea, that's hard for us guys). They may not be perfect, but if you don't build it right from the beginning, you are asking for trouble, which will cost you money. Everything is used for a reason and helicopters have almost no redundancy. Therefore, if something fails, it usually will crash. (That's just part of the hobby.)

One of the general rules to live by is to use thread lock on any screw that screws into another metal part (except self locking nuts). Even in electric helicopters, vibrations can occur. Vibrations can and will cause the loosening of bolts.

< < Don't build a helicopter without thread lock!

As I mentioned in my last column, all linkages and mechanics must move freely. Of course, all new machines will be a little "sticky", so don't expect them to be perfect, but the parts will loosen up as they begin to fly. However, if the linkages are really hard to move, then there is a problem. Excessively resistive components can lead to poor control authority and put more of a load on servos, the motor, and other parts.

When adjusting the links, please note that most helicopter ball links are designed to be snapped onto the balls only one way. To check this, look closely at the plastic ball link. You should see small raised writing on them (i.e. company name or some such stuff). The writing always faces out from the ball mount. If it is snapped on the wrong way, you could have overly stiff links or even damage them. Feel free to squeeze lightly any tight links (while they are connected to the balls) with a pair of pliers in order to loosen them up.

< < Many helicopters have links that require the raised lettering to be placed facing outwards.

< < Stiff links can be loosed by carefully by squeezing them with pliers, or...

< < ...gadget freaks can use this nifty ball link sizer to loosen those tight links. Simply pop the link over it and give it a few twists of the hand.

While it is a good idea to apply light oil to bearings and sliders, do not use oil or grease on the gears! This will only attract dirt and sand, and will quickly wear out those gears. Run them dry!

Once the machine is built, the electronics and related items will be installed next. The obvious and first items are usually the servos, so we'll start with them.

In general, you should try to use servos with ball bearings (BB) supporting the output shafts. BB type servos have more precision in their movement and make the helicopter easier to fly. Whether to use plastic or metal-geared servos is up for debate, but normal plastic geared servos are perfectly fine. In fact, many metal-geared servos still have a sacrificial plastic gear in them anyways.

Choosing the right type of servo is also very important. Smaller helicopters (but bigger than micro types) up to the standard .30 class size should use micro or mini servos with ratings of at least 30 to 40 oz-in (2.5 kg-cm), such as Hitec HS-85, Futaba S3101/2, JR341, etc. Larger helicopters should use standard sized servos that start at 50+ oz-in. (3.5 kg-cm), with more power being better (except for the tail).

It is also important to mount the swash plate servos in the helicopter with the same linkage geometry. These days, most of the models on the market use the Cyclic Collective Pitch Mixing (CCPM) type of servo control. What this means is that the three servos that control the collective, elevator, and aileron (climb/descend, forward/backward, and left/right roll, respectively) are more or less directly connected to the swashplate. In the "old days", these functions were mechanically mixed using levers and moving servo trays.

< < Here are two different implementations of CCPM. Please note the servo arms and swashplate orientation in the picture on the left. This should be set to zero degrees of pitch.

A good starting point is to set all the servos to their neutral (centered) position and adjust the links to have the main blade pitch of zero degrees. To do this, you will need to use a blade pitch gauge. These can be found at any good hobby store. You will also want all the servo arms to be at 90 degrees to the control arms at that position as well.

< < It is a little hard to see, but this Concept EP is an older design that uses only one servo (center foreground) to control pitch instead of three. This setup does not require a (now common) CCPM capable radio controller.

Now, let's talk about the pitch setup. With everything set to neutral (blade pitch), use your radio's pitch curve functions to set the maximum and minimum pitch range. To get the required pitch range, you will usually need to use servo arms that are 18 to 20 mm long (depending on the model). Using the pitch curve function, set the collective pitch range according to the table below.

Main blade pitch setup (in degrees)

Mode Switch Low Stick Mid Stick High Stick
Norm -1 or -2 +5 +10
Stunt Mode 1 -5 0 +9
Stunt Mode 2 -9 0 +9
Throttle Hold -5 +5 +12

This table is a starting point only and will have to be adjusted once the helicopter is flying and/or according to the manual.

< < Before you can even attempt to fly your helicopter, you must set up the proper pitch curves. A pitch gauge like the one shown is an essential tool to any helicopter flier.

Norm mode is for basic hovering only, as -2 degrees of pitch will not be too good for bringing the helicopter down very quickly from high up. In norm mode, you should also be able to hover at mid-stick or so. Stunt mode 1 is for flying around in forward flight and basic aerobatics (loops, rolls, etc). Stunt mode 2 is generally meant for aggressive and heavy aerobatics normally called 3-D. Lastly, the throttlehold is generally used for practicing auto rotations. If your radio only has two stunt modes, chose the setup that best fits your planned flying style.

Tail

Next, we'll move on to the tail. Tail servos need to be fast, not strong. Generally, a speed of 0.12 seconds for 60 degrees of rotation is desirable. However, faster is better, especially for heading hold gyros where speeds of approximately 0.08 seconds are the norm. You can almost never have a servo that is too fast. While is also very important that there is very little play or slop in the tail system, the tail mechanism must move very freely.

< < Futaba's GY240 makes a great entry level heading hold gyro. For high-end performance, their GY401 gyro and 9253 servo are hard to beat.

When connecting the tail servo to the tail pushrod, start with the outer hole of the servo arm (15-20mm). You can always turn the gyro gain down electronically if necessary to prevent excessive tail wagging. It is also generally best to place the gyro as close to the main shaft as you can, but today's gyros are so sensitive that is it not as important as it is to place the gyro away from radio interference sources (RF) such as the motor and speed controller. Be sure to follow the instructions for proper gyro setup. Double check to ensure the gyro moves the tail in the correct direction! You can test it by moving the tail side to side with your hand. The gyro should move the blade pitch in the opposite direction that you move the helicopter. Check this carefully, as I've seen this set wrong many times before.

< < It is usually better to use the outer-most hole in the tail control servo, and then adjust the gain down on the gyro.

Set the tail rotor pitch to about 15 degrees with the tail servo neutral. Make sure the blade angle is set so it provides thrust in the correct direction. (You don't want your helicopter to spin in the wrong direction on startup.)

Power

Since electric helicopters generally have adjustable motor mounts, it is important to have the gear mesh (teeth) set correctly. If the mesh is too tight, it will rob power and may overheat the motor. If it is too loose, it will probably strip the gears. A good way of setting gear mesh is to insert some very thin paper or tissue paper between the large spur gear and motor pinion as you tighten the motor mount screws. You want just a tiny bit of play between the teeth of the gears, and the paper method makes setting this easy.

< < A thin piece of paper between the pinion and spur gears is a good way to set the proper gear spacing. For really small sized gears, I'd suggest using tissue paper.

For the electronic speed controller (ESC), follow the instructions for proper setup. Most helicopter speed controllers run in either a "normal" mode, not unlike an airplane ESC, or a "governor mode" where the rotor RPM is maintained (more or less) regardless of the load placed on it due to blade pitch changes. In general, for governor mode to work, you will want to set the throttle to around 80% so that the ESC has headroom to adjust.

(Note: You can still use a governed ESC with a throttle curve).

If you use a non-governing ESC, you will need to set up throttle curves just like the glow powered helicopters use. Again using the radio, start with the throttle curves in the table below.

Throttle Point

Mode Switch 1 2 3 4 5
Norm 0 40 75 90 100
Stunt Mode 1 100 90 90 90 100
Stunt Mode 2 100 90 90 90 100
Throttle Hold 0 0 0 0 0

This is for a five-point throttle curve. Not all radios have five points, but you should get the idea. Point 1 is collective/throttle stick all the way down, while point five is collective/throttle stick all the way up (climb). The other numbers represent the throttle percentage (0 - 100%). This table is for CP helicopters only. FP helicopters should us a linear throttle just like an airplane. If you are just learning to hover on a CP machine, I would suggest you use the normal throttle curve instead, even on a "governed" ESC, as it allows you to quickly cut the throttle by moving the throttle/collective stick down in case of a crash or tip-over.

Balancing

Having well balanced machines is essential for good flight performance. An unbalanced helicopter will wear faster, not run efficiently, and could cause premature equipment failure.

Obviously, the main blades should be balanced, as they are the largest rotating mass on the helicopter. The easiest way to do this is to use a blade balancer like the one shown in the picture. Simply bolt the blades onto the balancer and add weight to the lighter blade as necessary until both blades float level. For weight, I usually add material like self-adhesive trim tape.

< < To balance the rotor blades, simply bolt the two blades onto this balancer and add trim tape the lighter blade.

The next item you should balance is the whole rotor head. This is often forgotten, but it can just as easily lead to an out of balance helicopter as forgetting to balance the blades. Balancing the rotor requires a high-point type balancer and head. This should be done with the flybar attached, but without the blades.


< < High point balancer (found at most hobby shops) is the best way to balance the main rotor head.

While not wholly inclusive, I hope these setup tips make it easier for some to get their helicopters flight ready with little trouble. I eventually hope to have a part two to supplement this article and cover the finer parts of setting up and flying a helicopter.

Website of the month: Colin Mills Practical Theories on Model Helicopter Flight (scroll to bottom of page):

This article reprinted courtesy of the Ezone.

Back to Table of Contents

Pitch, Roll and Yaw

By Steve Horney - Email: Steve

Browse Articles by Steve Horney - Articles

Takeoffs and ground handling

Last time I discussed the need to land well, but the opposite side of the equation is nearly as important. Getting your plane off the ground smoothly and in one piece is imperative. There are really two aspects to this, the first being a properly trimmed and setup airplane, and the second is good pilot technique. I'll discuss both aspects for hand-launched and ground-launched wheeled airplanes.

Hand-launched aircraft

While this may seem self explanatory, it is important to have your model properly trimmed for an easy hand launch. It's a little easier to get a handle on things if someone else is launching your plane for you, but if you're doing it yourself and you try to launch a plane that immediately wants to turn, dive, etc., things will get very interesting very fast. Some planes, like certain flying wings and deltas, need extra reflex in the elevons for launch, followed by a reduction in the reflex for smooth flight.

When launching your plane by hand, be certain to avoid two common problems, twisting the plane on launch (producing an immediate turn) and launching the plane upward with insufficient energy for flight (producing a stall and a crash). Twisting the plane on launch is a normal phenomenon caused by the natural rotation of the hand as you bring it over your head. Some people avoid this by using a straight-forward shove starting at their shoulder and moving forward. I usually make a mental note to counter the twist of my hand by giving it a slight outward rotation as I throw the plane.

In order to avoid the dreaded stall/snap/crunch, most planes should be launched into the wind with a solid, level throw, somewhat like throwing a javelin. Many beginners make the mistake of launching their planes upward, rather than level. With a level launch, even an underpowered plane can be allowed to glide a short distance while it picks up speed. If however the launch is directed upward, a low-powered plane will run out of airspeed and stall before it is able to fly on its own. Some very high powered planes, like my Jerry sailplane, can pretty much pull themselves out of your hand, but it's still easier to get things smoothed out and under control with a solid, level, straight-forward launch.

Low-winged airplanes present their own challenges when hand launching. The best way is probably to provide some kind of keel to grab, or if that can't be done, by grabbing the plane behind the wing with fingers on either side of the fuselage. You then launch by thrusting the plane forward from the shoulder and keeping it level. Of course, if you're really an ace and your model has enough performance, you can always launch inverted! Another creative launching method comes from the flying wing fraternity. Grab a wing tip and throw it discus-style. This is probably best performed with EPP planes!

Ground-launched aircraft (using wheels)

When it comes to getting a plane with wheels off the ground, the difficulty can vary from almost nothing to nearly impossible. Several factors come into play, including gear configuration (tricycle gear or tail dragger), wind speed and direction, stability of the airplane, motor torque, prop blast on the tail surfaces, amount of control throw, alignment and springiness of the gear, and the surface from which takeoff is being attempted (particularly related to tailskid versus tail wheel). Between the two gear types, tricycle-geared planes are generally much easier to keep aligned on takeoff. Tail draggers tend to want to ground-loop (swing the tail around) on takeoff. This is because the center of gravity of the plane is behind the center of pressure (the wheels), creating an unstable condition. It's sort of like a pendulum; once the CG begins to move one way or the other, it tends to want to continue in that direction. Tricycle-geared planes have the CG ahead of the main gear, and tend to be self-stabilizing. A tailskid in the grass or a tail wheel on pavement will help, but they tend to have relatively little area to restrain the other forces at work. The best approach to taming a tail dragger is to set up the main wheels with toe-in. (When looking down on top of the plane, the front of the wheels point inward towards the airplane.) Toe-in will help stabilize the plane by countering the tail swing. If, for instance, the tail swings towards the right (nose to the left), the right wheel will be cocked at a greater angle relative to the direction of the airplane, causing more drag on that side and pulling it back in line. At the same time, the wheel on the left will have better alignment with the direction of the plane, resulting in less drag on that side, and swinging the tail back left (nose towards the right). If you have toe-out in your gear, the result will be a wildly unstable airplane on the ground, since the forces will work to destabilize the plane. If you don't believe me, try both approaches on a familiar plane. It will be an interesting experiment!

I've found another factor will assist some planes in making straight takeoffs, and that is sufficient airflow over the tail surfaces. (Generous tail surfaces also help.) I've had planes that I changed from smaller-propped direct-drive systems to geared systems with much larger props. (The Kyosho Cessna 180 comes to mind.) Takeoffs were sometimes dicey with the direct-drive system, but the geared system seemed to tame it down immediately with no other changes. Of course, the extra airflow can also be offset by the additional torque reaction from the larger prop trying to swing the airplane left, but I usually find it beneficial.

Now that I've discussed a little about setting up the gear on your plane, let's consider the actual mechanics of getting your plane off the ground. With trike-geared setups, you can pretty much let the plane roll along until it reaches takeoff speed, pull back lightly on the elevator, and watch it climb. With lower-powered planes, be sure to keep the climb angles modest until the plane gets "on step". One mistake many newcomers make is trying to horse their planes off the ground. With enough prop flow, many planes will takeoff before they're ready to fly, leaving the control surfaces ineffective and usually resulting in a stall/snap/crash. Be sure to let your plane build up sufficient speed before trying to lift off the ground.

When a tail dragger takes off from the ground, a little more technique is required. As the plane begins to roll, hold full up elevator to keep the tail firmly planted on the ground and give the necessary amount of rudder to hold a straight line. As the plane approaches takeoff speed, released the elevator gradually, allowing the tail to rise, yet still keeping the plane on the ground. When takeoff speed is reached, apply some up elevator again to depart the earth.

Use the throttle efficiently

Have you ever noticed how novice flyers usually have very short full-bore flights, while better pilots tend to use a lot of part throttle and fly much longer, while still performing great aerobatics? Watch a masterful pilot like Keith Shaw sometime and you'll see he is very deliberate with his throttle usage. It seems to me that back when 1200SCR's were the "big" batteries (those same cans are now rated at 2400 mAh!), Keith was able to run his big Stearman biplane through the pattern twice when other pilots were having a hard time managing a single run. Of course, it's sometimes fun to run a fast plane full-bore the whole flight, but more often than not, newer pilots simply leave the stick in the full forward position and forget about it. Pulling back on the stick will result in longer flight times, an easier-to-control airplane, and probably cooler batteries, although the speed control may get warmer. The trick is to efficiently manage your plane's energy. Most airplanes, especially "draggier" models, will have diminishing returns with added power. That is, the return in increased speed decreases with additional power. (The drag curve climbs dramatically with increased speed.) You'll probably get more climb performance, but for straight and level flying you're burning a lot of watts for very little in return.

To help manage power, look at your flight routine and think about where you need additional power and where it's wasted. You'll probably want full or nearly full power taking off, but once your plane is flying solidly, back off to a good cruise setting. Power up as it enters a loop, and then back off as it crosses the top. (This will help smooth out the loop as well.) Making use of a small dive can reduce power requirements as well. When the winds are blowing you'll probably need a little more power, but you can still adjust as you fly upwind and downwind. I find it a good idea to carry some extra power when turning downwind (otherwise the sudden drop in relative airspeed can make things dicey), but then I back off as the plane settles in. Have fun experimenting, and learn to "read" your plane. It will make you a better pilot in the long run.

Stall and spin recovery

Some planes have very nice stall manners. I almost can't stall my Kavan Partenavia. Cutting the power and holding full up elevator just results in small up and down oscillations as it loses altitude. On the other hand, planes like my long departed ElectriCub can bite if you aren't aware of their stall characteristics. My Cub was a lot of fun and a very sweet flyer, but if it got a little too slow, it would suddenly drop a wing and head for the ground! So, what do you do if that happens, assuming of course your plane has room to recover? The two secrets here are increasing the airspeed and avoiding the tendency to tighten up the spiral by pulling up elevator. Our natural tendency when the plane drops its nose is to pull up, but if it's in a stall or a spin, that will only make things worse. If your plane has entered a spin, pulling up elevator will tighten up the spin until it augers into the ground. Full scale pilots are taught to do the opposite. They give the plane down elevator and opposite rudder until the plane straightens out, and then pull out of the dive. It's the same for our models as well. Airspeed is our friend, so if your plane is in a stall or spin, immediately put it into a dive to get some air flowing over the control surfaces. Most models will straighten out on their own once they get some airspeed, but with one that does not, reverse the rudder and ailerons until it's flying straight again, and only then pull out of the dive.

Of course, keeping the airspeed up a little will help prevent a stall in the first place, but you can modify the wing to help as well. Built-up wings can often be given a little twist in the outboard panels to tame the stall characteristics. Called washout, the wing is "warped' such that the tips have a negative incidence in relation to the rest of the wing (trailing edge up/leading edge down). This keeps the outer panels of the wing flying when the inner panels have stalled, preventing the tips from dropping and keeping the stall straight ahead.

Motor of the month

Our featured motor this month will be the AstroFlight cobalt 035. AstroFlight was a pioneering company in electric flight, and probably the company most responsible for where we are today. The introduction of their cobalt 05 in 1982 brought a significant performance improvement to electric flight while maintaining a reasonable price. I still consider AstroFlight cobalt motors to be the performance standard against which I judge all others. Today brushless motors are coming on strong, offering competitive prices and often even better efficiencies, but the cobalt brushed motor isn't dead yet. These motors offer high efficiency and performance while using light, reliable, and low-cost brush controllers (or even on-off switches). This makes it easy to substantially improve the performance of many "can" motored airplanes without having to buy a new speed control. You can also run multiple motors setups on one controller and with very little hassle.

AstroFlight motors are fine pieces of machinery, looking almost jewel-like in comparison to low-cost "can" motors. It can be frustrating to buy a motor, only to discover you need to invest quite a bit more in the prop adapter, connectors, screws, etc. AstroFlight saves you this hassle. One of the nice features of any AstroFlight motor is the completeness of the system. When you buy an AstroFlight motor, it comes with connectors, prop adapter, mounting screws, and sometimes even a propeller as is the case with the brushless AstroFlight 010. One additional "nicety" is that AstroFlight brushed motors come with the brushes already broken-in at the factory. You can generally just install the motor and go fly.

Long overshadowed by its bigger sibling, AstroFlight's 05, the 035 is a superb motor in its own right. I have two versions of this motor in my collection, a standard geared sport motor with the 2.38:1 gearbox, and a 5-turn FAI 035 motor equipped with a 4.4:1 planetary gearbox. Efficiencies for these motors typically run around 70% to 80%, depending on cell count, current, etc.

AstroFlight 035

Direct-Drive Sport 035

* Model: Geared Sport Motor - 603G (direct-drive sport motor specs are the same, except for gearbox)
* Winding: 7 turns #20 wire
* Magnets: Samarium Cobalt
* Resistance: 0.040 Ohm
* Kv: 2765 RPM/volt
* Io: 2.5 Amps
* Max Current: 30 Amps
* Cell Count: 6 - 10
* Length: 3.25 in. (2.25 w/out gearbox)
* Diameter: 1.32 in.
* Weight: 7 oz. (6 oz. w/out gearbox)
* Motor Shaft: 1/8 in.
* Output Shaft: 1/4 in.
* Prop Hub: 1/4 in.
* Gearing: 2.38:1
* Features: Open endbells, adjustable timing, replaceable brushes, seven slot armatures, 0.440 in. commutators
* Available from: AstroFlight, Inc.

Planetary Geared FAI 035

* Planetary Geared FAI Motor - 604P
* Winding: 5 turns #18 wire
* Magnets: Samarium Cobalt
* Resistance: 0.017 Ohm
* Kv: 4285 RPM/volt
* Io: 5 Amps
* Max. Current: 50 Amps
* Cell Count: 7 - 8
* Length: 3.25 in. (with gearbox)
* Diameter: 1.32 in.
* Weight: 6.3 oz (w/ gearbox)
* Motor Shaft: 1/8 in.
* Output Shaft: 5/32 in. (4 mm)
* Gearing: 4.4:1
* Features: Open endbells, adjustable timing, replaceable brushes, seven slot armatures, 0.440 inch commutators, elliptical field rings, armatures are tied with Kevlar and wedged
* Available from: AstroFlight, Inc.

Although the AstroFlight 05 is often considered the replacement for 540/550/600 size ferrite motors, the 035 is really more of a direct replacement. The 1/8 in. shaft of the 035 utilizes the same prop adapters as the ferrite can motors, and the intended cell counts are in the same ball park of six to eight cells. The big difference lies in the lighter weight (usually 1.5 to 2 ounces less) and increased efficiency of the cobalt motor (typically 10% to 15% greater efficiency). Other advantages of the AstroFlight motor include very robust construction, adjustable timing, and replaceable everything. It's pretty rare to have to replace brushes, etc. for wear, and any damaged component of an AstroFlight motor can generally be replaced, and usually pretty easily. Current versions feature open aluminum endbells, providing much better cooling (FAI motors have been equipped this way for several years) and lighter weight. One of these motors could conceivably last as long as the modeler is still able to fly!

Most 035 motors end up operating with some manner of gearing. The Kv of 2765 (sport motor) is too high to run more than fairly small props at reasonable current levels. With a gearbox, the 035 really becomes a great seven to ten cell motor. The standard 2.38:1 gearing pretty much "right sizes" the 035 to spin props that match most sport airframes well. If you run eight cells on a direct 05, you'll probably be limited to an eight inch prop. Adding a gearbox bumps the prop size up to around twelve inches. The geared 035, however, will be quite happy spinning something like a 10x8, which is small enough to avoid ground clearance issues with most planes, yet big enough and spinning fast enough to have some decent zip. Even at ten cells, the 035 can generally spin a prop in the nine inch range. If you need more prop, AstroFlight offers a few other ratios, including 2.82:1 with a different pinion and the standard box, and a 3.69:1 and 4.36:1 with the Superbox. FAI motors bump things up to the competitive category, with elliptical housings, Kevlar-tied armatures, and special brushes that allow these motors to pull up to 55 amps or so. Adding a 4.4:1 gearbox to the 5-turn FAI 035 creates an outstanding sailplane motor capable of spinning a 14x9.5 prop on seven cells at over 5000 RPM.

Flight experience

I have never used the 035 direct drive, but I do know a number of them are doing great work hauling Pondside seaplanes off the water. One friend tried the direct-drive motor in his LMH Corona helicopter, but found it was far enough off its efficiency range that it tended to run hot without offering a large performance increase. Most of my experience (actually, all of it) has been with the 2.38:1 geared seven-turn 035 and the 4.4:1 planetary-geared five-turn FAI 035. I took a Kyosho Cessna 180, adding the geared sport motor and an APC 10x8 prop, and transformed a marginal flyer into very capable aerobatic fun flyer on seven and eight cells. It would leap off the ground, and loop and roll like crazy, and then throttling back would provide long flight times. This was an outstanding combination. Going a little more upscale, I've seen Ken Myers power ten cell planes using this motor and a 10x6 prop, with very impressive performance.

My Chip sailplane experienced a similar power boost from the planetary-geared FAI 035. The direct-drive ancient AstroFlight 05 that was originally in the plane when I received it pulled it up fairly well, but the geared FAI 035 added after-burner, giving near-vertical climbs and an increase in speed. It was hard to believe that such a light, small motor and gearbox could turn such a large prop with so much power!

The photo on the left shows my 604P motor it as it comes from the box, complete with AstroFlight Zero Loss connector. On the right is the motor as I used it in my Chip sailplane, now with Anderson Power Pole connectors and a 14x7 Freudenthaler folding prop. (I later switched to a 14x9.5 CAM prop.)

< < This is the standard geared sport motor, APC 10x8 prop, and AstroFlight 217D speed control.

< < This is the current version of the AstroFlight 035 sport motor, in direct-drive form. Notice the open endbells, giving better cooling and lighter weight. This was formerly a feature of FAI motors only.




< < Here are the innards of the 035. These are beautiful, precision motors that last a long time and put out some serious power.

< < The seven-turn sport 035 rotor is shown on the left. Note the twist to the laminations, reducing the cogging of the motor. The magnet housing is shown on the right. The samarium cobalt magnets are the bar sections on the inside of the housing. Samarium cobalt magnets are much stronger than ferrite magnets, and much more resistant to demagnetization due to heating. The downside is that they are brittle and expensive.

On the left is the rear endbell, complete with brushes, brush holders, capacitors, and wiring. AstroFlight supplies their motor with capacitors. The brush holders are a little awkward for some installations, but they provide a convenient means of accessing the brushes, and provide plenty of room for larger and longer brushes. On the right is the front endbell, along with mounting screws.

Technical advice

To Gear or Not to Gear

One of the more commonly asked questions related to electric flight is, "Do I want to run a geared setup, or direct drive, and what will each buy me?" The short answer usually given is to run direct-drive for faster planes, and geared setups for draggier planes. That's true to a point, but as with most things, there's much more to consider if you want the whole picture. Before looking at recommendations, it may be helpful to review exactly what gearing does and does not do, and how it accomplishes these things.

Electric motors convert electrical energy to torque, which is the measure of a motors ability to produce work. A motor with a higher torque constant (Kt, measured in in-oz/amp) will be able to turn a larger diameter and/or higher pitched prop than another motor which has a lower torque constant. Motors posses another constant, Kv (measured in RPM/volt), which is a measure of how fast a motor can turn for a given voltage. Kt and Kv are inversely proportional to each other, and related with a constant. Thus, the higher the Kt, the lower the Kv, and vice-versa. A motor with a high Kv will be able to turn the prop faster, but it will require more current to do so, or the prop will have to be smaller.

Gearing is simply a mechanical means of multiplying the torque of an electric motor by means of a gear reduction system. You could gear to reduce torque and increase speed, but that's seldom done in electric flight. If you've had much experience with a ten-speed bike, your probably know that using the smallest front gear and the largest rear gear gives you the power to climb very steep hills, but you don't go very fast. You'll spin your feet a lot for relatively few turns of the wheel. If you use the largest front gear and the smallest rear gear, you can go quite fast, but it takes a lot of effort. Now the rear wheel spins more times for each turn of the crank. It's the same with an electric motor. Here the effort expended is in the form of current, and the speed is the rotational speed of the prop. If a motor is geared 3:1, it means that the motor shaft turns three times for every one turn of the prop shaft. The motor now has more power to turn a bigger prop, but it's spinning the prop slower. So, the effort expended will be the same if the current and voltage are the same, but you're trading off rotational speed for the power to turn a bigger prop.

As an example, the AstroFlight 035 mentioned in the preceding section has a Kv of 2765 RPM/volt. Adding a 2.38:1 gearbox drops the Kv to 1162 RPM/volt, but increases the torque 2.38 times as well. On eight cells, a direct-drive 035 would be limited to a prop of approximately 6x4 to keep under its 30 amp limit. Adding a 2.38:1 gearbox, however, allows the same motor to run a 10x8 prop at about the same current level. Using a *Calc program shows that the pitch speed is much higher direct-drive, but on most planes the extra thrust of the larger prop will actually propel the plane as fast or faster, since thrust is needed to overcome the drag. The exception would be a very clean pylon-racer type of plane.

Turning a larger prop has a number of advantages. Larger props are usually more efficient, low-speed thrust is usually somewhat greater, and the large volume of air moved by the larger prop often helps ground handling. Disadvantages include the extra weight of the prop and possibly gearbox, less prop ground clearance, and usually less top end speed on faster models. Top speed is reduced due to a slower spinning prop, assuming all else is equal, although a higher pitch can offset some of the RPM losses.

The extra torque of a geared setup can be used in other ways as well. Instead of running a larger prop more slowly on the same cell count, the cell count can be increased to run a similar size or somewhat larger prop at higher RPM, since more power available to turn the prop faster. This typically results in greater efficiency and lighter weight than running an equivalent larger direct-drive motor. However, not everything is perfect, since a gearbox has its own efficiency penalties. Estimates typically run 3% to 10%, depending on the gearbox, reducing the overall system efficiency, and the smaller motor may not dissipate heat as well as the larger motor. In addition, you have to be careful not to "over-rev" the motor. (Motors generally have a maximum rated RPM.)

Recently, several companies have begun offering relatively small, light, brushless motors built for high torque. The efficiencies are sometimes a little less than other motors, but the lack of a gearbox and related losses helps even things out, or may even give these motors and advantage. Other advantages include easier mounting and less complexity, since there isn't an offset gearbox.

So, what should you do? Well, it depends on your airframe and flying style. The rule of thumb I mentioned at first is a good way to start. If you're flying a biplane or a "draggy" trainer, you'll probably want to swing a large prop for a lot of lower-speed thrust; this is a good place to use a gearbox (or a high-torque motor). The one exception would probably be if you need extra nose weight for balance, in which case it might be advantageous to use a heavier, direct-drive motor. If you're flying a pylon racer or a very clean airframe, you probably don't need that much low-speed thrust, but you really want a lot of RPM for speed. Here direct-drive is nearly always the preferred option. It's at the in-between stages where it gets interesting. Generally either approach will work. Most sport planes perform very well with a higher-torque direct-drive motor or a higher-Kv motor geared down. I've run my SIG 4 Star 40 successfully with a Jeti 45/3 geared 1.7:1 and a 14x10 prop, and with the same motor direct-drive running an 11x6 prop, both on sixteen cells. I've also run my converted Q-500 planes on ten cells using an AstroFlight 05 geared 2.38:1 (11x8 prop), direct-drive AstroFlight 15 (9x5 prop), direct-drive Aveox 1406/4Y (9x5 prop), direct-drive Aveox 1409/3y (9x7 prop), direct-drive Aveox 1412/3y (this was on 12 cells, 11x8 prop), Jeti 30/3 direct-drive (9x7 or 10x6 prop), and AXI 2820/10 (9x7, 10x6, 10x8, and 12x6 props) direct-drive. All performed well. If you look into each of these motors for the Q-500 plane, you'll find the effective Kv's range from around 900 RPM/volt to about 1500 RPM/volt. Another interesting experiment was running an ElectroStreak with a geared 05 (2.38:1). This plane is very clean and is normally flown with direct-drive motors. (I'm running my current E-Streak ARF on seven cells, Jeti 15/4, and a 9x6.5 prop). In my case, I had a geared AstroFlight 05 in need of a home, so I put it in the 'Streak, backed up by ten cells and a 10x10 prop, using the prop pitch to make up for the lower RPM. This was a very successful effort, resulting in my first truly high performance electric. In choosing a motor system, I like to compare motors of similar Kv's (for a range I know works well for the prop sizes I want to run) for weight, efficiency, mounting ease, cost, etc., and then select the motor that does the best job of meeting my requirements. When comparing Kv's, remember to divide the motor Kv by the gear ratio for geared motors to arrive at the effective Kv.

Product News

The Super Miss picks up where my "hot-rodded" Miss 2 left off after I made upgrades.

In my last column I published the results of my efforts to pump up the Miss 2. I was quite pleased with my results using first an AstroFlight 05 and eight 800AR cells, followed by a Jeti 15/4 on 8 800AR cells. Well, Scorpio has taken a similar approach, but gone a step further. Hobby Lobby International recently announced the availability of the Scorpio Super Miss, which is basically the Miss 2 with a brushless motor and an aileron wing. I have one on my table to review, and I'm really looking forward to getting this one in the air. I think this Super Miss will soon become a favorite, if it's anything close to my hopped-up Miss 2!

< < The Super Miss (on left), compared with my Miss 2 (on the right). The photo is from the Hobby Lobby web site, and used with permission.



New Charger from Great Planes

I recently received an e-mail from Hobbico announcing a new charger by Great Planes Manufacturing Company. I haven't seen or tried this charger, but the ad copy sounds good. Here's the notice as I received it.

Great Planes Mfg. ElectriFly Triton DC Peak Charger.

It is a versatile charger LOADED with features!

Computerized Charger, Discharger, and Cycler

Extremely easy to program!

It's the ultimate in charger technology, and easy to use with more functions for more applications! Like many chargers, the Triton from ElectriFly features fully adjustable current rates, and can charge NiCad and NiMH batteries with peak detection technology, as well as discharge and cycle. But unlike other chargers, this unit handles up to 24 NiCad and NiMH cells, and can also be used with Lithium-Ion and Lead-Acid batteries. Additionally, it stores memories for up to ten different battery configurations, and so much more! From park flyers to car racers, the ElectriFly Triton Computerized Peak Charger is the only charger modelers will need!

* Handles one to twenty-four NiCad or NiMH cells, one to four Lithium-Ion cells, or 6, 12 and 24V Lead-Acid batteries.
* Great for small park flyers, large scale electrics, transmitter and receiver batteries, field batteries, and even R/C car batteries.
* Up to ten different battery configurations can be stored for instant, easy recall and charger setup.
* Features adjustable peak detection sensitivity, and recalls data for up to ten full cycles.
* State-of-the-art programming combines simplicity and sophistication. It is super-simple (especially with the dial feature) to scroll through the menus.
* Optional battery temperature monitoring allows modelers to safely and accurately charge heat-sensitive Lithium-Ion and NiMH batteries.
* Comes packed with safety and protection features, including visual and audible warning messages, overload and reverse polarity protection, and more.
* The large two-line, thirty-two character LCD screen is easy to read, and displays lots of information that is designed for easy, logical access.

Sea Stylist

Many of you may have noticed the stunning seaplane photo linked with the last column. At the time of publication, I only knew it was a sharp looking plane that I photographed while at Toledo. Shortly after publication I received this e-mail from Mike Goolsbee, builder of the plane, identifying it as his design and giving me some information on it. Since then, I've heard that Mike successfully has flown the Sea Stylist. Hopefully we'll see that one kitted in the not too distant future!

It has a scratch built fiberglass fuselage and nacelles, and foam core wings with 1/16" balsa sheeting. The Sea Stylist has a wingspan of 59 inches with 404 sq. inches of wing area. It is covered with 3/4 ounces fiberglass and epoxy, and finished with Krylon paint. The ready to fly weight is 62 ounces. It uses fourteen 1400 ma NiCad cells to power two 4.35:1 Graupner Speed 480 motors turning two four bladed ground adjustable nine inch VarioProp propellers.

Planes should be seen and not heard. Pour on the Watts!

Steve

This article reprinted courtesy of the Ezone.

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This page created and maintained by Al MacDonald. Updated October 4, 2002.

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