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.

  • 1
    $\begingroup$ Welcome to aviation.SE. You must edit the title to reflect the question. Also consider take a tour as you are a newcomer. $\endgroup$
    – Manu H
    May 7, 2020 at 6:20
  • $\begingroup$ 3.8 kW sounds like way too little. Powered hang-gliders normally use 11 kW. $\endgroup$
    – Jan Hudec
    May 7, 2020 at 7:12
  • $\begingroup$ Related if not a dupe: Can an ultralight aircraft fly with a 18hp engine? (it contains information needed to built such aircrafts) $\endgroup$
    – Manu H
    May 7, 2020 at 7:55
  • $\begingroup$ 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. $\endgroup$
    – Abdullah
    May 7, 2020 at 12:56
  • 1
    $\begingroup$ @JanHudec his airplane is almost certainly not manned at 25kg. Maybe he wants a drone? $\endgroup$
    – Abdullah
    May 7, 2020 at 13:00

1 Answer 1


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):

enter image description here

(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...

  • $\begingroup$ I would feel better assuming that the 23kg was the loaded weight of the (presumably unmanned) aircraft. The questioner did not indicate otherwise $\endgroup$
    – Abdullah
    May 7, 2020 at 13:12
  • $\begingroup$ That's true, but the calculation can be easily repeated for any mass... $\endgroup$
    – xxavier
    May 7, 2020 at 13:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.