High Altitude And Electrics (ERCITS)
Make your own free website on Tripod.com

 

 

ELECTRIC RC IN THE SKY (ERCITS) Newsletter

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

Hobby Companies

Hobby Stores

Past Newsletters

Al's Home Page

Coming Events

For Sale

Links

Clubs

High Altitude and Electrics

By Bernard Cawley. Email: Bernard

One of the recurring subjects on both the Eflight mailing list and the Slow Flight list, and now here on the Ezone discussion boards, comes from a newcomer to electric flight who lives in the Rocky Mountain region of the United States or someplace else well above sea level and who has been told by some so-called experts (often in a local hobby shop) that "electrics don't work at this altitude - they're too heavy". Well, I'm here to tell you that those local "experts" are wrong! Instead, electrics work BETTER at higher altitudes than their glow- powered counterparts. In fact, the world altitude record for propeller driven aircraft, 96,500 feet is held by an electric! You can read more about that airplane here:

Helios

Not only do electrics work better than their IC powered counterparts at altitude, but all you have to do to make an electric airplane perform just about like it would at sea level is to change something that's pretty easy to change - the propeller. But I'm getting ahead of myself. First, some background.

I grew up in Colorado and New Mexico, states where the lowlands are 3000 feet above sea level, and I lived anywhere from 4500 to 7000 feet above sea level. Now I live in the Seattle, Washington area and fly at or very near sea level. But I remember as a high school student having to put larger than recommended engines on glow-powered planes to make them fly where I lived. For example, I had to put an OS .10 on a Goldberg 1/2A Skylane that was designed for an .049.

Since we've lived near Seattle, we have visited family back in the Southwest every few years and on most of my family visits back to the Southwest I've taken along a plane or two to fly. On the last three trips, they've been electrics (which are all I fly now).

I'd heard all the arguments about why electric airplanes don't work at high altitudes: But I knew that an electric motor doesn't suffer from the same disadvantage an internal combustion engine does as the air got thinner since it doesn't require oxygen to run. So it seemed to me that, if anything, electrics might have an advantage over glow or gas as the altitude increased.

Being a curious sort of fellow, and having a strange desire to test and measure things, I brought along such things as my tachometer and ammeter (and later my Astro Whattmeter). On one trip I acquired a digital fishing scale that reads up to 10 lbs. in one ounce increments for measuring static thrust. Equipped with these goodies, I made a few measurements and did some test flying, centered on trying different propellers. I soon learned a few interesting things.

First, not surprisingly, the current draw and therefore power input to the propeller, of a given motor/battery/prop combination is lower at higher altitudes, while the RPM of that combination increases. This makes sense because as the air gets thinner, the load presented to a given prop goes down. As the load goes down, the speed the motor turns goes up.

Second, static thrust drops as well for a given motor/battery/prop combination, though not very much, since the RPM's increase.

Third, you can recover the lost static thrust by either increasing prop diameter or prop pitch or both. But since the wing of the airplane needs to fly faster to generate the same lift as it would at sea level, adding pitch seems to be the right thing to do.

Flight testing supported this conclusion. I found that if I added pitch (using the same make of propeller) so that the current draw and therefore the input power were the same at the altitude where I was as that which I saw with the same motor and battery at sea level, then the airplane flew very much like it did when I was back here in the Seattle area. Of course, the airplane's landing speed was higher since it needed to fly faster to make the same amount of lift, but outside of that, takeoff performance and general flying ability, including aerobatics, were changed very little.

Note, though, that all of the airplanes on which I performed these tests (Goldberg Mirage 550, Top Flite Schoolboy, Modelair-Tech Dimwatt, Sig Kadet LT-25, Mattes Blue Foamie) are not ultra-high-performance types. I suspect that at much higher wing loadings, or on planes that run fairly high pitch-to-diameter ratio props at sea level that it won't be quite that simple, but adding pitch is still the place to start.

On the last trip to the Southwest I also tried a different approach on the LT-25. On that trip it was powered by an Aveox 1114/4Y in a Modelair-Tech H-500 belt drive. This gave me the opportunity to experiment with gear ratios as well as propellers. I found out that by decreasing the reduction ratio slightly without a propeller change you can also get the input power back up to sea level numbers, and again the performance of the airplane is similar to that at sea level (except for landing approach speeds, as before).

So when you take a plane to a higher altitude flying site, all you have to do to get the performance back is to either add pitch or decrease the gear ratio to get the same input power to the motor as you had at sea level. Since the electric motor doesn't care about the air density (at least at any altitude where you can still breathe!), it will happily deliver the same power when loaded to the same level, regardless of altitude.

But what if you don't have any sea level measurements to compare against or you didn't bring your tachometer and your Whattmeter on your Rocky Mountain vacation? Then the old adage of trying different propellers to see what works best for your airplane applies here, just as anywhere else. Here are some suggestions to get you started, though:

I did most of my flying and testing at about 5000 feet and 7000 feet above sea level, in the summer. I found that adding about 20% more pitch (say, going from an 8X4 to an 8X6 on a direct drive 7 cell airplane) did the trick. At 3000 feet, 10% seemed to be about right (go from 8X4 to 8X5).

At 7000 feet I found that reducing the gear ratio from 3.6:1 to 3.3:1 on my LT-25 also gave me sea-level current draw and close to sea-level flight performance. This is probably a better approach if you're already using a prop with a fairly high pitch to diameter ratio, too. I haven't tested this, but I expect that if you're already using, say, a 12X8, going to a 12X12 at 7000 feet won't work as well as reducing the gear ratio. At the very least takeoff performance will be poorer since the prop would likely be stalled at first. Once in the air it might be OK, though.

When this discussion came up on the Slow Flight list early this year, I was pleased to see that Don Stackhouse of DJ Aerotech, who used to design full sized airplane propellers for a living, weighed into the discussion with one of his usual wonderfully explained technical discussions. I was even more pleased when his technical explanation backed up my empirical tests. Here's some of what Don had to say in response to a question from someone having problems with a Litestik at 5500 feet (quoted with Don's kind permission):

"As others have noted, the air at 5500' MSL is thinner, but there's more to the problem than that. The answer is not quite what might first come to mind.

"Your airplane will need to fly faster in the thinner air to make the same lift, but at the higher speed that makes that same lift, it will also make about the same drag as at lower altitudes. I fly my Lite Stik on a 6-cell 600 mAh NiMH pack at about 1000' MSL. The performance isn't in the "rocket ship" category with that setup ("trainer-like" comes to mind), but it's adequate, and the 20 minute flight times are nice. Yes, shedding some weight would help (mine is definitely more spirited with a 7-cell 280 mAh NiMH pack), but that's not the real problem.

"Yes, a gas engine makes less power when it has to breathe thinner air, but you don't have a gas engine. Electric motors don't care about the altitude. That's not the problem.

"Yes, the thin air has less density but about the same viscosity, so the Reynolds numbers of your model are lower, even considering the faster airspeed. This means that your model has more drag. This also means that the wings have less maximum lift coefficient, which further increases the stall speed. That is still not the main problem.

"The main problem is your propeller. The inflow angle, angle of attack, and the required pitch angle of a propeller blade depend on the true air speed. At your altitude, if the setup of motor, prop and battery isn't changed from the low altitude arrangement, the prop is trying to pump thinner air, and has to do it with less angle of attack! The thin air also hurts the prop's efficiency as well.

"The motor's RPM is determined mostly by voltage, so the prop really doesn't turn much faster in the thinner air, so there's little compensation from that. And since the prop can't pump as large a mass of air, it doesn't put as much load on the motor, so the motor doesn't draw as much current, so the motor doesn't put as many watts into the prop. It's like trying to fly with only part throttle.

"You need to increase the pitch of the prop to match the reduced air density and the higher true airspeeds you need for flight in that thin air. Increasing voltage (adding a cell) is another approach; it makes the motor and prop turn faster, which does similar things to increasing pitch, but not as effectively. Because of the non-linear relationship between RPM and power absorbed by a prop, you won't be able to increase RPM enough from extra cells to make up for the change in inflow velocity and the lower air density. Yes, you'll get more watts, but at a "pitch speed" that might be still too slow for the airframe's needs. The best approach is to either twist more pitch into the prop you have, or find a prop with more pitch.

"Although static power and thrust measurements are a relatively poor measure of prop efficiency, you can get away with using a static watts measurement to judge approximately how much of a change you're making in power due to a prop pitch change (at least it's better than trying to use the "TLAR" method!). When the prop you're using approximately matches the static wattage (on the same batteries) of a prop OF THE SAME DIAMETER that works well at lower altitude, you should be in the ballpark.

"[At 5500 feet MSL,] the air density (at "standard day" conditions) is only about 87% of the density here at our farm. Your Lite Stik will need to fly about 7% faster than mine (lift is linear with air density, but proportional to the square of the airspeed), so you'll need about that much or a bit more prop pitch, a little over 5" of pitch. Add some more pitch (I'd guess maybe around 5.3" to 5.4" pitch [compared to the stock Lite Stik prop of 4.7 inch pitch]) to take care of the Reynolds number and mass flow effects, and you should be in business."

Don Stackhouse @ DJ Aerotech

So there you have it. Electric powered airplanes have a big advantage over internal combustion engine-powered airplanes at higher altitudes. Put on some more pitch, or lower the gear ratio, and go for it!

Aerial views of the Royal City R/C Club field and surrounding area, Santa Fe, New Mexico
(altitude 7000 feet MSL). Taken from my LT-25 on our last Southwestern visit.

Reprinted courtesy of the Ezone magazine.

Back to Top

Back to January, 2002 Newsletter

To join the "Electric RC" discussion list
(discussion of electric powered model airplane flying),
enter your email address in the box below and click on 'JOIN'.
topica
 Join Electric RC! 
       
Back to Top

This page created and maintained by Al MacDonald. Updated January 21, 2002.

ERCITS1 , ERCITS2 OR ERCITS3

Suggestions for improvements or additions to this page are always welcome.