I'm building a high altitude UAV (20,000ft) and am looking for a decent engine for it. My student budget unfortunately limits how much I can spend on it but I just need it to be extremely reliable. I'm not gonna be doing any aerobatics/inverted/3D flying just steady climb then a glide to land.

I was thinking 4.2 upwards since it'll be 3m(9ft) span and I'm designing for 60kg (120lb?) on take-off.
Right now I've got about 10kg Drag on climb at 100kph with way more than enough lift to lug it off the ground. I'm looking at a sachs dolmar 4.2 (AIr hobbies) that seems to be prettty cheap and I've heard decent things about it. But again I don't know everything about it.

Any recommendations?

Oh how I would love to reply with something useful here but the law and my employer prevents it. Oh well.

i was expecting that...

how about a 3W 106?
i think it will work?

how high do u want to get?

oh 20000 ft???

mmm. i dont know if the 3W can do that?
unlease u can reacj the carb remote control as well???
anyone can add help here?

mixture contorl is the key....a little bird told me that...

so the 106 will be good enough , still need to take care of the air/fuel ratio in the Crab? hoe to set those small neddle of H & L mixtures while the plane is at 20000ft[8D]

there's your chalenge!

Yea I'll definately need mixture control. I'll have to automate it. And I was thinking of a method for compression too. There are small motorcycle turbos that operate on electric motors. I know they don't do much for big capacity engines but for a small 100cc engine they'll do heaps at high altitude. Or even making an inlet compression duct. I've calculated this thing to reach pretty high velocities so that might be an option too.

The 3W 100 seems to be going for about a grand which is a little high for my engine budget. But I must say the specs are very attractive. Any other ideas? If you guys are interested I'll put up engineering drawings and CFD simulations up from the wings so far.

120lbs on a 9' wing is going to be a horespower hog. Redesign the wing and lower gross weight, or quadruple your engine budget.

Then why say anything?

I am not an aero engineer but I think you are going to need a much larger wing and a much larger engine to get 120 lbs to 20,000 ft. Then you will need to supercharge the engine. You are looking at least 10x the budget you seem to have

You may want to talk with RCV, see the UAV section (note the development by DARPA)

http://www.rcvengines.com/

The 9' wing works just fine. It's all been done before.

If you are serious - the engine output degredation numbers per 1000 ft are available - here at 4350 ft el-we figure on aprox 13% power loss.
your airframe is an unknown quantity-

When? How? Who?

If you obtained that position you had applied for back east you'd know.

Hi,

You need to give us a little more info for us to help a little. What is the wing area and wing loading at 120 lbs? Wing airfoil type? Flaps? slotted flaps? What percent of chord?

a 9ft wing with a 1.5 ft chord = wing loading of 145 oz per sq ft of wing. Very heavy loading! with a 2.5 ft chord, the wing loading drops to 85 oz/sq ft wing. Still very heavy!

How slick is the airframe? What flight time are you shooting for? What is your max fuel load possible?

I would think that you would need to be able to fly the plane at partial throttle at 500 ft agl before your power ratio is high enough to fly at 20,000 ft agl even with remote tuning.

Elson

Have you looked at turboprops, may be too many dollars though.

Small, high payload, and cheap. Pick two. There is not many aero engineers that can envision the finished product by looking at a short list of specifications, they do exist though. Finding one of these guys will save a lot of time.

Depends a lot on the mission type and required endurance. To simply get up to altitude and back down again is not that big of a deal. There's a few issues to overcome but they're not too difficult in the long run. To hang around at altitude for a long period of time and be able to cover some distance with a payload is another matter altogether. Cheap does not fit into either catagory, but the former is much less costly than the latter. Small works, though.

You are going to have to make so many changes to any engine you choose, other than a purpose built engine specifically made for a high altitude UAV, why even bother paying extra money for a model application engine that will have to be radically changed away from its intended purpose? You're just wasting money. Better to find a utility engine candidate and start from scratch with that. Doing this should lower the price considerably.

Other than a compression release for starting, why do you need compression control? Never heard of such a thing in a two-stroke utility engine.

Leave the turbo and super chargers alone. They are only good for competition where your displacement is limited by rules and you need to develop more horsepower. Other than that they are just toys that add multiple failure points and little horsepower increase for their weight and complexity. If you need more power, use a larger, more powerful, engine. It's cheaper and more reliable. Probably weighs about the same too.

Fuel injection would be a good thing, if controlled with the proper program riding in the computer. Beats the dickens out of carburation.


Ed Cregger

Ed,

Our general purpose RC gas engines have a lot wider performance envelope than most people think. Fuel injection or turbos are probably not required. He might fall a little short of his desired altitude being normally aspirated with a generally stock carb, but likely not by much.

If he's really interested in this kind of stuff he ought to send a letter to his Prime Minister to see if there are any research or ops positions open in his country that he could avail himself of.

The high aspect ratio is required to keep induced drag low since I'm not going to go over 300kph anytime soon with this thing. And the wing loading is high because my climb has to take place at maximum L/D which means that climb Cl is about 0.8 and I'm designing for this L=Wcos(y). The thing that's concerning me is that I need to either supercharge or turbocharge this engine, otherwise maximum power would be about 50% (probably much less) at FL20. I was planning to turbocharge it myself since it gives a better performance curve than supercharging but the already supercharged engines on the market are an attractive option too.

The airfoil is NLF so drag is pretty low (about 0.07 at maxL/D) and I'm designing the airfram quite similar to the piaggio avanti 180, beautiful plane, probably 50% laminar flow or more. I'm changing the airfoil a bit since I'm flying at lower speed but I'm planning to use a pusher prop configuration with a T tail and no canards. The big elevator and rudder surfaces are a drawback but the reduction in drag is a big plus and a requirement in my low T/W ratio.

Hopefully that gives an idea of the design, I'm sure its full of holes but I'll plug them one by one and I'd very much appreciate any criticism or advise.

The really important part is- your construction technique. (light n strong)
Historically the things that really make a difference are just two: power and weight.

Dick has it right on. The clean airframe helps a lot in cruise and fuel burn. It's hard to overcome weight in climb out especially since that's when the plane is the heaviest. You look to be on a very good track. Does it have to R.O.G. to leave the ground or is there another way it can launch? Think about possible benefits with something other than an ROG T/O.

Sorry, but I have to leave this one now.