Convair 580 (ERCITS)
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Convair 580

By Ed Putnam - Email: Ed

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Specifications:

* Builder: Ed Putnam
* Type: Scale
* Materials: Foam fuselage (hollow), .6 oz glass covering (epoxy resin and water based polyurethane) then painted. Foam core wing, 1/32" balsa sheeted, balsa cap leading edge, and Monokote. Hollow foam, glass covered and painted nacelles. Foam core, balsa sheeted stabilizers.
* Wingspan: 61 inches
* Wing area: 397 sq. inches (9-inch root chord, 4-inch tip chord)
* Length: 46.5 inch fuselage length
* Weight: Ready to fly, without battery: 43 ounces, Test flight with 8 X 800AR @ 53 ounces, Weight with 10-cell CP1700 pack or 8-cell RC2400 @ 60 ounces (using Dymond 480 motors with stator rings)
* Receiver: Hitec 555 RX
* Servos: 3 Expert mini servos (HS-81 size)
* Controls: Aileron, elevator, rudder, nose wheel steering, speed control
* Power: Graupner 2.33:1 gearboxes, Dymond 480's (long can 400), wired in parallel. Amp draw 28A static on 10 cells, 25A static on 8 cells
* Prop: VarioProp four bladed propellers, 7.7 inch length (variable pitch). Also using Aeronaut 8.5x6 two bladed props for comparing VarioProp performance

The Convair 580 is a turboprop conversion of the Consolidated/Vultee Aircraft Company (hence Con+V+Air) earlier series of piston-powered airliners. The 240, 340 and 440 series were extremely rugged ships that were first built in the late 1940's. In the late 50's and early 60's, airlines had the opportunity to breathe new life into an otherwise great but technologically lagging airframe by retrofitting them with powerful turboprop engines. The larger 340 and 440 airframes were a perfect fit, so several airlines took the plunge after the marketing value of "jet power" was being realized.

Frontier Airlines, or the "original" Frontier that went out of business in the deregulation era of the 1980's, was a pioneer in the turboprop conversion of this aircraft. As appropriate, my model is painted in the original Frontier.

Getting Ready

I did a bit of research on the aircraft by consulting several books and magazines. The general dimensions were scaled down to fit speed 400 power, and I tried to simply design each major assembly to keep building quick, light, and relatively inexpensive.

Wing

The wing has a foam core, which is sheeted with 1/32" balsa. The leading edge is capped with " balsa stock.

The wing was based on the scale outline, but I added a small amount of chord to increase the wing area slightly, as I wanted to avoid the need for flaps or "unscale" like approach speeds on this prototype.

I chose the Eppler 205 airfoil because I have a lot of experience with it, and it is a very good all around airfoil. I used 10 percent thickness in the root and 11 percent in the tip for aerodynamic washout, since the wing is high aspect and tapered. I also put in 2 degrees of washout for better slow flight handling.

Once I had the foam core out in front of me, I routed out channels for the motor wire and laminated a top and bottom carbon fiber strip in place of a conventional wood spar. I also installed a hardwood aft spar that serves as a gear mount for the ply plate, where the main landing gear attaches. In addition, I wanted functional navigation lights, so I placed super light wiring into the foam cores for later use. I laminated the balsa sheeting to the foam core using epoxy in certain stress areas, but mainly by spraying 3M Super 77 adhesive to the wing skins and cores.

The leading edge wood was glued in place and wing tip balsa was glued (both w/ odorless CA) and then sanded. Holes were drilled into the wingtips to house the LED navigation lights, and the lights were soldered to the installed wiring and press-fit mounted into the wingtip holes.

Aileron panels were cut using a razor saw, and Robart hinge points were used. I chose to use a carbon fiber hollow rod as a torque tube to control the ailerons. This allowed me to use a single servo mounted in the wing center, saving about $20. However, with this setup, there is slightly more flexing than I normally like to see, so subsequent designs would benefit from either a larger and stiffer torque tube, or individual servos mounted on the underside of each aileron station in the wing panels.

Two light ply 1/32" wing joiner spars were installed into the center joint temporarily, and they were shaped accordingly for the desired dihedral. When the fit and dihedral angle were just right, the joiners were glued into place with the two wing panels. The wing joint was laminated with two layers of .6 oz. cloth for strength.

I used Monokote and Ultracote Plus to cover the wing.


Stabilizers

The stabilizers were made of foam core and sheeted with 1/32" balsa just like the wings. The vertical stabilizer has a huge dorsal fin, so the vertical stab is a two-piece structure made of a one-piece fin and one-piece dorsal fin. The sheeting makes it seamless though, with the help of a little spackle hiding the joint and change in airfoil shape.

The horizontal stab was covered in Monokote and the vertical stab was covered in glass cloth to merge it into the fuselage seamlessly.

The elevators are attached to each other (left and right halves) using a music wire torque rod that runs across the tail cone area of the fuselage.

Fuselage

I used a combination of techniques here. The fuselage is basically hollowed foam and covered in glass. It consists of three parts: The nosecone, the center tube, and the tail cone.

The center fuselage "tube" is a hollow 2lb. density foam tube that was cut using the CNC cutting service at FlyingFoam.com. I simply told them what the dimensions were (how wide and tall the tube is) and faxed them a picture of what I wanted. I used a slightly oval shape, so I drew it out and specified the wall thickness (5/8") and a few weeks later a medium sized brown box of foam showed up at my door!

The maximum length, without upping the price of materials and shipping too much, was 28". I didn't even need that though, as with the nose and tail cone length, some of the tube was cut away to get the total length correct. They cut it out for me for about $20, plus shipping. They also cut a few extra "tubes" for that price that I am using in other scratch projects.

The front "nose cone" section and aft "tail cone" section were carved pink foam that was hollowed for lightness. I went to Home Depot and purchased a big blank of pink foam for around $10. I cut up several profile "outlines" of the nose and tail cone areas, and laminated the identical sections 3 fold to get the proper width to match the fuselage "tube." At this point, I had a heavy and "blocky" nose and tail cone. I sanded the joint area smooth, where the nose and tail cones mated with the center fuselage tube, and placed the nose and tail cones next to the fuselage tube. Using a marker, I outlined the round shape of the center fuselage tube onto the blocky "cone," to get a guide as to how much foam must go. I cut and sanded the nose and tail cones round to match the center tube.

Get shaping! For large areas of foam removal, I sometimes use a razor saw to start shaping. If you try this, be careful! You can quickly take off too much this way, so go to sanding paper sooner than later. I use 60-grit paper at first and then changed to 120-grit paper. Later I used 240-grit paper for lighter detail sanding. Using a mixture of sanding blocks and emery boards, I got the detail in the window areas that I needed. I then used a Dremel tool to hollow out the inside of the nose and tail cones. I left the forward 1/3 of the nosecone solid foam due to the need to have a place to mount the nose wheel. I did the same thing for the aft of the tail cone too. The horizontal stab mounts into this area and solid foam will keep the aft fuselage strong.

The inside neatness is not critical. However, be aware if you try this, the Dremel can pull and cut right through to the outside wall. Be careful not to wreck what you have made so far! Leave at least " to " of foam wall so you will have a semi-strong structure still to support the glass or covering later.

Before mating the front and aft cones to the tube, I installed the plywood nose wheel mounting plate into the nosecone. I also took advantage of the open working area and mounted the nose wheel steering plate onto the ply mounting plate at this point. Once satisfied with the fit, I glued the forward and aft cones to the center tube and I had a rough fuselage!

The fit will not be perfect at this point. In fact, I wanted the nose and tail cones to have a slightly oversized. I was then able to use a sanding block to merge the cones to the tube, being careful not to take too much material off the fuse tube joint, or deform the cockpit window area already carved into the nose cone.

At that point, I started on the wing saddle. After accurately locating the wing placement, I roughly cut out a bit of fuselage underside to match the wing upper surface. The wing should now rest nicely in the fuselage saddle. If it doesn't fit, fine sand the fuselage.

I mounted a forward and aft ply bulkhead into the fuselage at the leading and trailing edge of the wing center section, where it joins into the fuselage saddle.

The aft bulkhead acts as the aft wing mount. A dowel is attached to the trailing edge of the wing, and a hole is drilled in the aft bulkhead to hold the trailing edge steady. The forward bulkhead holds a "mounting plate" in place. Two nylon screws are passed through the forward section of the wing and screw into the mounting plate to hold the wing in position.

Then I used 1/32" ply to make the wing fillets. Lightweight spackle was used to "fill" in the 90 degree angles between the fuselage and the wing saddle areas. After plenty of drying time, the area was sanded smooth making nice wing fillets.

Slots were cut into the tail cone to house the horizontal stabilizer. Additionally, a vertical stabilizer slot was cut. At this point, the fit of the stabilizers was only temporary, as the permanent mounting would come later. An internal control linkage was used for the rudder and an external linkage was used for the elevator. I made a control rod exit for the elevator.

The entire fuselage was "painted" with thinned light spackle to fill larger holes in the foam, especially in the center tube, since 2lb. density foam has more holes. I used a paintbrush to get as much thinned spackle onto an area as I could. I let it dry thoroughly. Then, after lightly sanding down the entire fuselage with 220-grit to 280-grit sandpaper, it was time to glass.

I used .6-ounce glass cloth and epoxy resin for the first coat. After plenty of drying time, I very lightly sanded off any bugs and dust that fell onto the drying epoxy. I then used water-based polyurethane (WBPU) to "fill" the weave of the cloth. After several coats of the WBPU and plenty of drying time, I wet sanded the entire fuselage with 320-grit sandpaper.

When using WBPU, I skip the primer stage, as paint sticks very well to the prepped WBPU.

Finally, a battery hatch was cut into the top forward section of the fuselage for easy radio and battery access without the need to remove the wing.

I used a razor saw to cut the hatch, and I doubled some of the hatch walls with 1/32" balsa. A spring lever was used to fasten the hatch. A scale antenna is used as the hatch handle and spring lever control. After some cleaning up around the joint areas, it was time to paint.

I masked and painted the silver and white areas, and later went back and masked the fuselage stripes. I used 1/64" chart tape to make recessed panel lines. I used the panel lines to hide the hatch lines in the forward fuselage area.

Nacelles

The nacelles were made just like the nose and tail cones, except they were made in two-piece "halves" so that the four parts could be mixed and matched, to ensure identical size and proportion among right and left side nacelles.

After the exterior shape was finalized, the interior was hollowed out and gearbox fitting was ensured.

The gearboxes were attached to the wings by an internal plywood "pylon."




The internal "pylon" motor mount was used due to the extreme forward location of the propeller in relation to the wing leading edge on the actual aircraft. If the foam nacelle was used as a structural part, especially using gearboxes, I'm certain sooner or later (probably sooner) I'd loose an engine, literally speaking, in flight! The internal plywood pylon is very sturdy, and it allows some flexing for turbulence, torque, and landing loads.

After the nacelles were finish sanded, they were glassed. The same method using epoxy, WBPU, and the "no primer" finishing that was used on the fuselage was also used on the nacelles.

I cut a removable hatch into the nacelle for motor and gearbox maintenance. I hide the hatch lines using the scale panel lines that run along the surface of the finished nacelle. Again, 1/64" chart tape was laid down to simulate panel lines prior to painting. After the paint dried, I removed the panel line tape. The finished product provides recessed panel lines that glisten in the sun at certain angles - a nice touch to any scale project.

Odds & Ends

The landing gear was mounted and radio was installed fairly painlessly.

I used wheel collars as the mount for the twin wheeled landing gear assemblies on all three gear legs. I used a friend's (PlaneCrazy on E-Zone) drill press to drill and tap both sides of the wheel collar. I used Allen screws to act as wheel axles, and at the same time, they tighten down on the strut to keep the dual wheels in place. Some care was required to align them for good ground handling. LockTite was used to keep them in place for the runway abuse that was ahead.

I made my own scale struts for almost no money. Using various sized straws (McDonalds straws for the mains, what I had in the drawer for the nose) I fashioned scale struts. I did splurge and buy a bit of train modeling plastic (35 cents) for other landing gear strut pieces, but the finished struts weighed next to nothing and were almost free.

I also installed a nose wheel taxi and landing light, red anti-collision beacons (they flash) on the upper and underside of the fuselage, and a steady white position light in the tail. I wired the wing lights into a micro Deans plug for the removable wing and they all connect to a central battery housing. They look pretty cool when lit up and taxiing around - the nose light is very bright! The whole setup was bought piecemeal at Radio Shack for about 8 dollars, including the two AAA batteries and the battery holder. Initially I tried to use a 3v Lithium "coin" cell, but under the small load, the voltage was not sufficient. The weight of the AAA batteries and the holder is 1 ounce. On the test flight, I accidentally left the AAA batteries in. For a later test flight, I removed them and I couldn't tell the difference in flight at all. Therefore, I just leave them in all the time now. I just use a piece of paper placed over the end of one of the cells to "switch off" the lights when they are not needed.


Flying

I am very lucky to report the initial flights have gone very well! I have several flights so far, all of them on 8-cell packs.

The model was designed to use ten cells, which is the reason I went with the long can Speed 400 motors. As I am waiting for my 10-cell pack, I am at least pleased the 8-cell flying qualities are very good for a scale airliner.


Takeoffs are brisk. I don't know how much runway I used, but it take approximately a 5-second count after full throttle to get airborne. Lift off requires a little up elevator and is followed by a gentle lift off and a strong climb. Ground handling is very good, which is a relief.

Climb out is adequate on eight cells, and cruise at 60 to 70 percent throttle is very scale-like. Using eight 2400's, I think an easy ten minutes of flying time can be had with plenty of power for reserve. I felt that the overall performance, so far, has been best using the 2400 cell pack. Using 8x2400 packs, the model was very stable, had good gust penetration, and had excellent acceleration. The weight was 60 ounces in this configuration. I also tested 8X800AR cells. At just under 53 ounces, I expected a lot more climb and better overall performance, but the 800ARs just don't have the punch of the 2400mAh cells. The lighter weight also seemed to make the model more susceptible to gusts, but it certainly flew well. The 53-ounce model, with a similar mass CP1300 pack, would probably be a really great model for most flyers that prefer lightness to duration. I do appreciate the longer flight times of the 2400mAh pack though, and the heavier wing loading seemed to be more familiar to me.

In the future, I plan to use 10-cell CP1700 packs, which are identical in weight to the 8-cell RC2400 packs. I will likely trade off between the packs though to keep the good times rolling.

During slow flight, the model lets you know you are getting close to the stall with a fairly high body angle during level flight. There is a tendency to drop a wing during stall, a sign that I could have used a bit more washout. It is the natural tendency of a high aspect ratio wing to do this. The good news is that recovery is very automatic. Release the elevator, level the wings, and recover from the descent with power. I lost very little altitude during the stall, and had very good aileron authority immediately after the "break" from normal flight.

Landings were very straightforward. I keep in mind that the wing may drop if I stall it though, so I have not tempted fate with too slow of an approach.

Using the four bladed VarioProps, an interesting quality showed up. By reducing power to idle, the drag of the four bladed prop was very evident and the descent rate was twice as fast as the two bladed prop configuration. This, however, is viewed positively by me. I have a perfect speed brake when I need it, with no flap ballooning or additional servos and linkages. This model can do some really neat short approaches. Landing with the four bladed props seems to be smoothest with about 10 percent power, although idle can be used too, when using a closer in landing pattern to account for the descent rate.

Comparing the four bladed VarioProp to a standard two blade Aeronaut 8.5x6 gave some insights.

The two bladed prop is fixed pitch and looks a little "unscale." It allows for a good glide ratio at "idle" power. It can also be used with a variety of spinners.

The four bladed VarioProp is adjustable in pitch and looks very scale. It also gives you the option of having a free "speed brake" at idle power. However, not very many spinners are made for a 3 or 4 bladed prop. Some modifications to commercially marketed spinners will need to be made to use them. Mabey VarioProp could start a VarioSpinner line. In addition, due to the adjustable pitch, you can really fine tune the performance of your model for free (no new prop to buy), you can do it at the field, and it takes very little time. If you break a blade, each new blade is only $1. Once you get the hub for about $18, you can use a large variety of blade sizes and types, which vary between scale and performance.

On a similar battery pack, the four bladed VarioProp, set at 4.5" of pitch, accelerated and climbed better than the the 8.5x6, with only a very slight increase in amp draw. This can be adjusted, via prop pitch, to get more or less aggressive performance of course.

Conclusion


Scratch building with foam has been a lot of fun. Using foam-cutting services will give you the option of getting exactly what you want in a wing and even a fuselage, for a fairly reasonable price.

As you can see, a foamie can look like whatever you want it to - it just depends on what you are going for. In addition, a good quality fiberglass finish will not break the bank on weight, as long as you plan for equipment weight and use proven glassing techniques. I pretty much figure, for this size of model, 4 ounces will account for the glass, paint and details. With all of the new NiMH, CP NiCad cells, and micro equipment out there now, we are saving weight. You can replace it with extra cells, or extra details in this case- just be sure you do not go too far out on the details, eh?

I hope all out there that have always wanted that "certain something" would think twice about starting that scratch built project. It is fun, and can be less expensive than a kit. You will be the only one at the field with anything like it!

This article reprinted courtesy of the Ezone.

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This page created and maintained by Al MacDonald.
Updated November 14, 2002.

ERCITS1 , ERCITS2 OR ERCITS3
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