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I am working on my dream to build and fly an ultralight aircraft so I wouldn't like to know the following:

  1. Does adding a motorcycle automatic gear box to an engine improve on the strength for an ultralight aircraft? The engine in question is a 4 stroke, OHV, single cylinder, 440cc gasoline engine with a TCI ignition. It produces 18hp at 3600 RPM, torque 3.0 kgm/2500 RPM (21.7 lb ft), ignition TCI, gross weight 37 kg.

  2. What should be the tail, flaps and wing measurement proportions for such engine?

Thank you

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    $\begingroup$ I disagree with the close votes: while there is an overlap, the linked question does not address part one of the question. $\endgroup$
    – Hobbes
    May 16, 2020 at 12:01
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    $\begingroup$ @Hobbes given your answer, I edited the title to highlight that part of the question. Feel free to edit it again if you think I'm missing something. $\endgroup$
    – Manu H
    May 16, 2020 at 13:24
  • $\begingroup$ See also aviation.stackexchange.com/questions/8081/…. $\endgroup$
    – Jan Hudec
    May 16, 2020 at 16:43
  • $\begingroup$ @Hobbes, while I also disagree it is a duplicate of the 18 hp question, we also have a detailed discussion why aircraft don't have shifting gears. $\endgroup$
    – Jan Hudec
    May 16, 2020 at 22:22
  • $\begingroup$ Many engines such as the Rotax 582 used in ultralight and lightsport aircraft do in fact use gears to reduce the prop RPM, and the word "gearbox" is in fact used in conjunction with these gears. I'm editing the title to help make it clear that this isn't what you mean. $\endgroup$
    – quiet flyer
    May 17, 2020 at 11:17

3 Answers 3

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A gearbox is not a useful addition. Some aircraft have geared propellers, but that's a single, fixed gear. The engine has to be able to turn the propeller when it's in the highest gear you'll use, so that determines how much power is required.

In a car or motorcycle, the lower gears are used to match engine speed to wheel speed, allowing the vehicle to drive at lower speeds than 'idle rpm in top gear' without slipping the clutch or the wheels.

In an aircraft, that function is not needed: a propeller can slip freely.

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  • $\begingroup$ Not teally true, I think. Motorcycle engines (like all engines) produce more power at high RPMs, with some high-performance bikes redlining above 9,000 RPM. OTOH, typical GA propellors (and thus their engines) tend to run around 2800 RPM. So a reduction gearbox would seem to be needed to bring engine RPM in line with propellor RPM, and the simplest way to do this with a motorcycle engine would seem to be to use the transmission, which is typically contained in the same housing as the engine, anyway. $\endgroup$
    – jamesqf
    May 16, 2020 at 17:29
  • $\begingroup$ A gearbox would be much heavier than a single gearset. A gearbox also runs the risk of disengaging the selected gear, unlike a gearset that can be splined onto the axles. $\endgroup$
    – Hobbes
    May 16, 2020 at 18:22
  • $\begingroup$ That's certainly true, but doesn't take in to consideration that motorcycle engines & transmissions are usually a single unit, so the gearbox is not an addition. It might be possible to disassemble the engine, remove unnecessary parts, and lock the desired reduction in place. $\endgroup$
    – jamesqf
    May 17, 2020 at 4:41
  • $\begingroup$ Some ultralights or lightsport planes with Rotax engines have a feature where the propeller doesn't turn when the engine is at very low RPM. I don't know how the seeming extra complexity of such a feature turns out to be a significant advantage (less work for the starter if it doesn't have to fight the inertia of the prop?) but I've seen it on the Bailey-Moyes Dragonfly. Could be grounds for another ASE question. $\endgroup$
    – quiet flyer
    May 17, 2020 at 11:11
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An airplane uses propeller pitch change to achieve the effect of changing gears. Engine RPM is held constant, the pitch of the propeller is used to take a bigger bite of the air with each revolution, loading down the engine more the same as changing gears does on a car or motorcycle.

So the question becomes: Is it worth the weight to make the prop pitch changeable (on the ground only, or under pilot control in the air) with a low horsepower engine?

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The main point is to maximize the thrust that can be attained with the power available. Having a given power delivered by the engine at at given angular velocity, you should match that angular velocity to an optimum propeller. Thrust is obtained by accelerating a mass of air, and that's precisely the rôle of the propeller. Imagine you have a propeller that accelerates a mass of air X to a speed Y. In terms of energy, the cost of obtaining a given amount of thrust is halved if you double the mass of air and accelerate it to only half the speed. In other words, with a prop accelerating a mass of air 2X to a speed 0.5Y you get the same thrust for one-half of the energy delivered by the engine.

Hence, the bigger the prop, the better. Of course, you are limited by the tip speed, that has to be kept well subsonic, and by the speed to be attained by the aircraft, that should be relatively low. Helicopters use those large rotors for good reasons...

For an ultralight, slow and with little power available, you should use a propeller as big as possible, driven through a reduction gear in order to keep the tips below the speed of sound.

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