I am trying to build an Aircraft, The weight of the Airplane will be around 40 Kg with engine and 8 liters of fuel and with me the total weight will be around 90 to 100 kg and I want to use a motorcycle engine in the aircraft, the aircraft will be one seater and I have a question that which type of engine is best to use, enter image description here Displacement 123 cc Engine Type 4 Stroke, Air-Cooled, SOHC Max Power 10.7 bhp @ 7500 rpm Max Torque 10.40 Nm @ 6500 rpm

enter image description here The engine produces a maximum peak output power of 11.00 HP (8.0 kW) @ 9000 RPM and a maximum torque of 9.80 Nm (1.0 kgf-m or 7.2 ft. lbs) @ 7000 RPM

So should I use 1070 or 1065 size wood propeller and if this size is not suitable so please recommend me the best size,

If this plane successfully flies so what would be speed of it and will it give me same mileage as the engine gives in Motorcycle? And for how long I can fly nonstop with these engines,

Mine Airplane design would be like this

enter image description here

Please explain me as simple as possible, and Sorry for Bad English. thanks

  • 3
    $\begingroup$ The common Cessna 172 has a weight of over 1 ton and 160 horsepower. 160/1111=around 0.15 Your aircraft: 11/100=0.11 $\endgroup$ May 9, 2020 at 10:11
  • 1
    $\begingroup$ Don't get me wrong though, the Cessna 172 carries 3 people at around 10,000 feet;you only want yourself to fly, possibly lower, where the air is thicker and planes fly easier. $\endgroup$ May 9, 2020 at 10:14
  • $\begingroup$ So, will my plane fly easily? $\endgroup$ May 9, 2020 at 10:40
  • $\begingroup$ no, it will be marginal. High span will help but you will need struts or you exceed your weight goal. $\endgroup$ May 9, 2020 at 11:05
  • 3
    $\begingroup$ When these things are on a motorcycle, the top speed is around 50-60mph. Even two of these together doesn't leave you much margin. Motorcycle engines are not a good fit because the integrated gearbox is useless and they are not built for long time full throttle operations. Get those industrial engines for generators. $\endgroup$ May 9, 2020 at 18:32

3 Answers 3


You will need a fixed landing gear and a braced wing to keep your mass at your target of 40 kg. Expect a maximum L/D of not more than 10, maybe even 8.

You say that the flight mass will be 100 kg. The weight will be 980 N. 1/10 of that is 98 N. To arrive at a power from that will need a few more heroic assumptions. Your wing loading is probably 12 to 15 kg/m² and your lift coefficient around 1.0. This equates to a flight speed of 15.5 m/s (= 56 km/h). I assume a propeller efficiency of 70%, so your power for straight flight will be 2.2 kW.

For decent take-off performance, add 1.5 m/s climb capability at an air density of 1.1 kg/m³. The extra power to climb with those 1.5 m/s at 15.5 m/s and 70% efficiency is another 2.2 kW.

The sum is 4.4 kW which is 5.9 HP. Running the engine at this power level means 54% of maximum power. Make sure you have a gearbox which reduces prop RPM to not more than 2000 in order to make those 70% realistic. The wing will need to have about 8 m² area and the tail another 1.6 m². Will that all fit within your 40 kg?

I expect that 80 kg is a more realistic empty mass. Now the wing will measure 11 m² and the power demand climbs to 3 kW for level flight and an extra 3 kW for climb. Total power will be 8 HP or 75% of maximum power. Ask yourself how long the engine does this and how big your radiator needs to be in order to avoid overheating.

  • $\begingroup$ The extra power to climb, with a mass having a weight of 980 N, at a vertical speed of 1.5 m/s with an efficiency of 70%, is 980 · 1.5/0.7 = 2100 watt, not 4.4 kW... $\endgroup$
    – xxavier
    May 9, 2020 at 19:48
  • $\begingroup$ No, I didn't. It's plain physics... You are in level flight and wish to increase your altitude. The energy you need for that increment is the difference between the gravitational potential energy of those two flight levels, and the power needed is energy/time... $\endgroup$
    – xxavier
    May 9, 2020 at 19:52
  • $\begingroup$ @xxavier: I used the total power, not the increment for climbing. Sorry! $\endgroup$ May 9, 2020 at 20:11

This has been done, but here are some things you'll need to consider.

Air-cooled motorcycle engines are designed to produce short bursts of peak power, and to be as light as possible. Lightness is an advantage for any airplane engine, but having an engine designed for brief bursts of high power is not. An aircraft engine has to operate for long periods at high power settings without overheating; if you try this with a motorcycle engine it is likely that you'll burn the exhaust valve, warp the cylinder head and score the cylinder walls- in other words, wreck the engine.

In addition, to produce peak power in a small package requires that the motorcycle engine spin very fast- 6000 to 8000RPM- and a gearbox or speed reducer of some sort will be needed so you do not drive the propeller tips supersonic.

You might be able to use the gearbox built into the engine as a speed reducer, but it adds weight.

My recommendation is to find an industrial engine of about 20HP continuous rating at 3600RPM and operate it through a 2:1 reduction, derated to 15 or 18HP to reduce stress and increase life.


The best suggestion I could give you is to abandon your project. There are no realistic consideratoins in it.

The engines you planned to use are just 10 Hp. These are moped engines and they generate their maixum power at lower RPM's. These engines are designed to operate at ground level, wherein majority part of these engines are run at low RPM's and at fixed Temperature

If the weight of the aircraft is 100Kgs, the propeller needs to displace similar KGs of air than the weight and this propel's the aircraft. To Propel such amount of air, the propellers must be able to rotate at least 1000 RPM, roughly. The engines mentioned can never be able to propel the propeller unless a gearbox is used, which complicates the things.

The pressure exerted on the propeller is huge and you need a short gearbox with short lag. Which means an automatic with less lag. Anything manual with huge gear gap might break the propeller or Engine in an instant.

There are people who mentioned about a Cessna 172. That has over 5000 CC engine which is generating a mere 100-150 HP. What does it tell you? Its a huge engine running at low RPM and generating maximum Torque.

Remember, you cannot rotate the propeller more than 2000 RPM. It breaks the speed of sound and in this process, the propeller itself might break. You need a variable pitch propeller on such small aircraft which might be expensive.

The engines you mentioned cannot operate at an altitude above 3K or 4K easily. They just choke. How do you climb? You need a much bigger engine, which can generate enough thrust.

At Ground Level, you should be able to pump up the weight of the aircraft so, you can fly. But at altitudes above 1000 Ft, the air becomes thinner drastically and your engine needs to propel 2X or 3X or maybe 4X more. Which means, it should do 3X or 4X more work at thin air lacking oxygen. Remember, Oxygen is a heavier gas and concentrates around the ground. The higher you go, the less oxygen is available.

Most of the old aeroplanes used a manual fuel adjuster, wherein the pilot adjusts the amount of fuel pumped into the engine, depending on the oxygen availabilty. With Fixed carb, that's not possible. You would need a Fuel injection.

Temperature above 3K falls by around 40% from ground level. Will the engine break when it experiences such drastic differences in temperature? There are a hundred other things which I did not mention here which might be fatal.

I am not saying it's not possible to Fly. It's possible, but will you be able to reach your destination? You will be able to fly for a few meters and technically you flew. But is it what you want?

If you really wanna fulfil your dream, look at Diesel engines, Or Rotax engines, if you can procure one. That should solve the problem in an instant. And Regarding Kmpl or MPG, don't expect it to be cheap. If it can be cheap, we would have been flying instead of riding mopeds.

Even if you try to fly with this engine, the minimum speed you might need is around 60 Knots, which is 108KMPH, which this engine can never reach. Possible if you use a 8 speed gear box and even at that speed, the power generated is so minimal to lift the aircraft.

  • 1
    $\begingroup$ The air doesn't thicken above 1000ft, it thins, and it's not some sort of drastic drop at 1000ft. The idea is realistic, people have built airplanes with light motorcycle engines before. I think the 40kg weight isn't realistic, but the concept is fine. $\endgroup$
    – GdD
    May 9, 2020 at 19:45
  • $\begingroup$ that first paragraph :(. how about we tell him to build a drone instead? $\endgroup$ May 9, 2020 at 19:48
  • $\begingroup$ Updated.. wrong word. used thickens instead of thins. Corrected it now. Even if the air thins by 10-15%, the engine must propel 10 or 15% more at an environment where the availability of air is less. $\endgroup$
    – user46196
    May 9, 2020 at 19:52
  • $\begingroup$ @ABJX, the mentioned engines cannot be used even in drones. These are moped engines which weight 40-60 pounds themselves. these have thicker shells and the gearing is huge. Often these are carb driven and the perofmrnace at heights is just not possible. $\endgroup$
    – user46196
    May 9, 2020 at 19:54

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