Calculating thrust and required propeller size for a given engine power

Question

I am looking to build an Airplane, the weight of Airplane will be around 20 to 25 kg and I am going to use a 70cc motorcycle engine as an Airplane engine.

I want to know how I can calculate the engine thrust. The 70cc engine is an air-cooled, four-stroke, single cylinder, OHC petrol engine generating a maximum power of 5.19Ps at 7500 rpm and a maximum torque of 0.519kgm at 5500 rpm.

So will it work as a plane engine or not, and can you give me any rough idea about how much thrust will it produce and how big propeller do I need for this engine.

Welcome to aviation.SE. You must edit the title to reflect the question. Also consider take a tour as you are a newcomer. — Manu H, May 07 '20 at 06:20
3.8 kW sounds like way too little. Powered hang-gliders normally use 11 kW. — Jan Hudec, May 07 '20 at 07:12
Related if not a dupe: Can an ultralight aircraft fly with a 18hp engine? (it contains information needed to built such aircrafts) — Manu H, May 07 '20 at 07:55
Hint: Pushing double the volume of air at half the speed will require half the energy. This is why high-bypass ratio engines are better. You will have to balance this with weight and other losses that come from having a larger propeller. — Abdullah is not an Amalekite, May 07 '20 at 12:56
@JanHudec his airplane is almost certainly not manned at 25kg. Maybe he wants a drone? — Abdullah is not an Amalekite, May 07 '20 at 13:00
Does weight 20 to 25 kg mean take-off weight—which implies it will be unmanned—or empty weight—which would be something like a hang-glider? — Jan Hudec, May 07 '20 at 23:27

xxavier · Answer 1 · 2020-05-07T13:22:51.470

This that follows isn't an accurate calculation, but may be useful as a starting point: let's say the mass of your plane is 23kg. That's a weight of 225 newton. You have to add 830 N for the pilot, so the total weight is 1055 N. Let's assume, also, that the best L/D of your airplane is 9 at 36 km/h = 10 m/s. In a glide, that would mean a sink speed of 10/9 = 1,11 m/s. The implied 'gravitational power' would be 1055 x 1,11 = 1171 watt. That would be the minimum power required for s/l flight at best L/D and for a 100% prop efficiency. In the real world you might need at least three times that power, around 3500 watt.

Now, concerning the relationship between prop size, power, and thrust, at 100% prop efficiency, you may use this formula for the stationary case (airspeed = 0):

(From Wikipedia entry https://en.wikipedia.org/wiki/Disk_loading). Where P is the power in watts, T the thrust in newton, A the disk area of the propeller in sq. meters, and rho the air density in Kg/m3, around 1,22...

I would feel better assuming that the 23kg was the loaded weight of the (presumably unmanned) aircraft. The questioner did not indicate otherwise — Abdullah is not an Amalekite, May 07 '20 at 13:12
That's true, but the calculation can be easily repeated for any mass... — xxavier, May 07 '20 at 13:15

Calculating thrust and required propeller size for a given engine power

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