# Is it possible to power an ultralight airplane with a single-cylinder weed-whacker engine?

Plane: Weighs 200 pounds. 16 FT wingspan, 36in. by 5 in. airfoil

Engine: 3,000 RPM, 2 HP

Propeller: 4ft with 4 blades and weighs 4 lbs.

Is there perhaps an equation that could determine this?

• – user14897
Commented Nov 26, 2019 at 16:37
• Commented Nov 26, 2019 at 16:41
• Man, 2hp is really pushing it. Last time we checked, we kind of all agreed you need about 20hp to be safe. Commented Nov 26, 2019 at 16:47
• Possible duplicate of Is it possible to fly using a small engine? Commented Nov 26, 2019 at 18:45
• Don't worry about the weight of the propeller; it doesn't make any difference. Commented Nov 26, 2019 at 19:25

2hp will give you about 10 lbs of thrust, tops (it's generally around 4-5 lbs/hp). If you have an ultralight with an all up weight of 400lbs, say, that maybe has a 10:1 L/D, you will need 40 lbs of thrust just for level flight at best L/D speed, requiring about 8-10 hp. That's with no surplus power for, you know, taking off and climbing and such.

So a weed wacker won't get you off the ground in the first place in any kind of craft that can carry a human, unless of course the wings are like those on a human powered aircraft with a span that goes off into the next county.

And your ultralight has a small wing with only 48 sqf of area that will have a wing loading of 9ish psf and will need to go 50 mph just to fly, which it ain't gonna do on 10lbs of thrust. Faggeddaboudit...

For an ultralight, you need wings big enough to get the wing loading down around 3lbs/sqf, to get a stall under 30 mph, and a thrust level equal to about 2 to 3 times the drag at max L/D. If your ultralight has 40lbs of drag at max L/D, you will want to have at least 120lbs of thrust, or about 25hp minimum. You can get by on less power if your ultralight is very clean, with long wings, like a Lazair or Mitchellwing, but not that much less.

There are methods of calculating the power required to fly a particular plane. In your case, assuming that you'll also need to carry ~18 kg of fuel and a pilot (roundly 75 kg), the answer is NO. A 2 hp (1.5 kw) power plant will be insufficient to give safe takeoff and climb performance -- with the possible exception of a high performance sailplane airframe (some self-launching sailplanes use ridiculously small engines).

• My lightweight, high-performance, self-launching glider has a ridiculously small engine: 25hp as @JohnK predicts. No way it'll happen with 2hp! Just thinking about the definition of hp, we see that a 550lb frictionless, dragless airplane could still not climb faster than 2' per second, or 120fpm. Commented Sep 25, 2023 at 12:10

"Is it possible to power an ultralight airplane with a single-cylinder weed-whacker engine?" -- I've seen someone actually try it. The engine was still attached to the weed-whacker. The string was replaced by a propeller. The unit was strapped to the pilot's waist, projecting backwards. The aircraft was a paraglider. The pilot foot-launched off a mountain and then started the contraption in the air. Unfortunately I never heard what sort of climb rate he was able to achieve once he flew away from the ridge lift.

• It would give him a flatter glide. A bit flatter. Paramotors get by on a rock bottom minimum of about 100 lbs of thrust and most are 140-160. 2hp would give you about 10lbs tops and would require a reduction drive. Commented Nov 26, 2019 at 22:00

Here is how to determine the answer (which, as pointed out by others here, is no):

We assume you know the weight of your plane, its gliding airspeed, and the glide ratio. This means you know the sink rate in feet/second which multiplied by the weight of the plane, yields the power dissipation rate during a glide which can then be converted into horsepower.

To hold altitude constant then requires that the power application rate equal the dissipation rate. To climb requires that the power application rate exceed the dissipation rate by some convenient margin (your choice).

• So power required = speed * weight / lift-to-drag-ratio (multiplied by some horrible constant for non-metric units, presumably) Commented Nov 27, 2019 at 10:54
• Not sure. the answer is yes if the lift to drag ratio is exactly equal to the ratio of the forward speed to the vertical speed but that is an assumption I do not feel qualified to justify. Peter Kaempf on the aviation stack exchange would certainly know the answer. Commented Nov 27, 2019 at 17:25
• @RobinBennett Yes! Commented Sep 28, 2023 at 0:15

For comparison, the Wright flyer had a 12 horsepower engine and the minimum requirement by the Wrights was 8 horsepower. That's not to say there can't be a smaller/lighter plane, but they definitely started small and slow, by today's standards, and the resulting numbers are very similar to the other answers of 8+ horsepower.

In theory, yes. Non powered gliders can fly with no HP just off of thermals, so a 2HP engine would be a reasonable boost for a plane like that. But to take off, that likely wouldn’t cut it.