# Calculating thrust and required propeller size for a given engine power

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. May 7, 2020 at 6:20
• 3.8 kW sounds like way too little. Powered hang-gliders normally use 11 kW. May 7, 2020 at 7:12
• Related if not a dupe: Can an ultralight aircraft fly with a 18hp engine? (it contains information needed to built such aircrafts) May 7, 2020 at 7:55
• 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. May 7, 2020 at 12:56
• @JanHudec his airplane is almost certainly not manned at 25kg. Maybe he wants a drone? May 7, 2020 at 13:00