Electric power for a 25 pound model... - RC Groups
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May 22, 2002, 02:27 AM

Electric power for a 25 pound model...

Hey guys!

Im working on a project where we hope to build a 25 pound model, capable of flying for a few HOURS on a charge! In just the little research Ive done, it most certainly looks possible! I know, I know, it would be much easier to use a glow or gas powered engine, but Id MUCH prefer to do this as an electric!

What motor(s), gearing, prop and cell count would fly such a monster for over an hour on a charge? Would it be possible to then lift an added 75 pounds of payload on this same power system (100 pound total flying weight!!)?

The design has yet to be finalized, but we expect to have a TON of wing area from current projections. With a 25 pound all up weight, the wing loading would only be 6-8 ounces per square foot! As you can see, it will have a LOT of wing area! Upped to around 100 pounds ready to fly with added payload (Added payload would most likely be more cells, run in parallel, to extend run times even more!), wing loading still remains only around 25 ounces per square foot.

I realize this is pretty vague, but where should we start? Brushless will definitely be the only choice as I can see, but whos motor will it be? Hacker, Kontronik, Actro, Aveox?

Thanks a lot guys, just looking for some basic inputs and ideas on power systems. We want the plane to ROG, and climb at a decent rate of say 200-400 feet per minute. Again, the goal is on one charge and a single pack to get at least an hour, if not more!

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May 22, 2002, 03:32 AM
high-speed freak
opualuan's Avatar
First, make sure this plane is not overweight/oversize for the AMA regulations!

David Lewis (homefly). He consults on many different research-type cutting edge planes... would be an excellent person to consult with. is this for school?? it's gonna be expensive...

you need the highest efficiencies possible if going electric, brushless motors if they can be had in large enough sizes/light enough for the cause. you may find it easier/cheaper to go to a multi-engine setup.

also, you need power densities exceeding what nimh/nicad can deliver, and given you need long flights, li-ion or li-polymer is a real option. I will be selling li-polymer cells up to 3800ma starting in two weeks. they can be tied together in any series/parallel configuration and charged with an orbit pro charger. they are spec'd for 2C+ continuous and up to 5C pulse.

say at 50 watts a pound, you need a 1250 watt-hour battery for an hour of full-throttle flight. say 30 amp draw, 12-cell li-poly (equivalent of 36 cells), a flight pack for 1 hour of full throttle will weigh 24 pounds. not a whole lot left for everything else, huh? that's why you need to consider a sailplane-type design, so that you can use a fraction of the battery weight and use throttle to climb, then glide...
May 22, 2002, 08:32 AM
jrb's Avatar

Have you looked at Steve Nue article in the July SEM (pgs 65 & 75)?

He wrote about the same AIAA project the group near him will be using the Graupner Ultra 3300/7.

Good luck!

May 22, 2002, 09:33 AM
AMA 697691 / FAA 13675
Hmmm...I'm not sure about the "few hours" part but MaxCim just came out with the MEGAMax3.7-()Y that packs over 2 kilowatts of power.
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May 22, 2002, 10:24 AM
jbourke's Avatar
An hour isn't much of a challenge.

You need to go with a multi-motor set-up so you can take advantage of off-the-shelf equipment.

The thing you need to be concerned about most is energy density (watt-hours/lb). The cells must be the majority of the aircraft weight.

In a sport plane we want about 50% of the flying weight to be taken up by the power system. You will want an even higher percentage for your plane if you want to go for maximum duration.

So for a 25 lb airplane lets imagine that you have 14 lbs of cells, 4 lbs of motor, and 7 lbs of airframe. That should be possible.

A 3000 NIMH cell weighs about 2 ounces. That means you can have 4 packs of 28 cells each. This is a nice way to do it because its easy to buy a charger for 30 cells. You should go with either four motors or two motors. If you do two motors you will need to run two packs in parallel on each motor.

The big Maxcim motor will be very efficient, but its also a bit heavy and might be better suited to other types of planes. You will need a custom wound motor to find one that is highly efficient at the 3 to 10 amps you will be pulling. I think that one of Maxcim's smaller motors might actually work better if you can get a custom wind.

Going with a single motor makes the project more difficult because it gets a lot harder to find motors/speed controls/props to match your project. I know because I spent a good deal of time working out the details for a 1/3 scale bipe that I never finished. If you go the single motor route you are going to have to do more pioneering.

The other design issue you should consider is the span loading. A nice long wing and a thin airfoil is in some ways more important than the wing loading.

If you pick the right airfoil, aspect ratio, and power system, you should be able to get 3 hours of flight or so. People have done three hours with 2 meter gliders so I don't see a reason why you can't do the same with a 25 lb plane.

May 22, 2002, 11:47 AM
Registered User


At that size/weight, could he get into some of the 12v gel cells for power? How do these cells compare in Energy deisity to the nimh's... also, what would be the safety concerns if the plane were to crash? (I.E. Is splitting open a Gell Cell as bad as splitting open an old fasioned Lead Acid Battery).

One other safety concern would be hydrogen gas accumulation.

Still, if they are safe enough to run kids toys (powerwheels) and my car (honda CRV), it should be ok, no?
May 22, 2002, 12:14 PM
Registered User
SheldonYoung's Avatar
If it's not a scale model then it's definitely doable. The Gossamar Albatross was a human powered airplane that flew for about 2.5 hours on about 300 watts. It seems it's mostly a matter of choosing what you want out of the airframe.
May 22, 2002, 12:38 PM
Registered User
member73's Avatar
Take a look at the aerovironments web site: http://www.aerovironment.com/

I believe the motors they use for Pathfinder and Helios are custom Astroflight brushless. Notice too the custom props. For long duration slow flight, your prop design will be at least as important as wing design and motors.

Good luck with your project and keep us informed.