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Why would the propeller of an ultralight or lightsport plane be connected to the engine through a clutch, and not turn at low rpm?

I've often witnessed the engine start of a Bailey-Moyes Dragonfly (link to flight manual, links to cool you-tube videos but not showing engine start #1, #2) and the prop did not turn until the motor rpm was increased substantially above idle. It was clear that some sort of automatic clutch was not engaging until the engine rpm was increased above idle.

It would seem that any sort of clutch that would allow the engine to run without the prop turning would add un-necessary complexity. So why was this feature incorporated into the drive train of some versions of this aircraft?

I'm rather sure I was told the answer to this once, but I've forgotten.

I know that the aircraft in question was powered with a Rotax engine, but several different Rotax engines have been used on this type of aircraft, and at present I'm not able to identify the specific one I witnessed where the prop was not turning at idle. It seems likely that many other aircraft powered by the same engine would use the same gearbox with the same automatic clutch feature.

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  • $\begingroup$ This question could be improved by 1) identifying one or more of the specific engines exhibiting this feature, and 2) adding a link to a video showing engine start-up where it can be clearly seen that the prop is not turning until engine rpm is increased. Such videos must be widely available on the net. $\endgroup$
    – quiet flyer
    Commented May 17, 2020 at 13:05

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Centrifugal clutches are mainly used to provide a compliant connection between the engine and prop to absorb power pulse peaks, as well as allow the engine to be automatically disconnected from the drive system at idle.

The big bugaboo of reduction drives over the years has always been torsional resonance resulting from the flywheel - the prop - being driven through a springy drive path by strong pulsating power impulses. At RPMs where the pulse/feedback frequency matches the natural frequency of the drive system/prop, you get massive resonant torsional vibration, which at minimum is unpleasant and at maximum can trash the gear train.

Over years various attempts have been made to tame this phenomenon in a practical way. It's not actually a good idea to use a centrifugal clutch in a normal aircraft. If the engine quits, the prop can freewheel and this turns it into a very powerful airbrake. It's also a reliability issue, being a friction device in a critical power path.

That said, a coupling that has been used with some success, and even certified at one time, was the Flexidyne Dry Fluid Coupling, which works like a centrifugal clutch but uses steel shot encasing a wavy plate in the housing to act a bit like a fluid torque converter. The FAA certified Molt Aerocar used a Flexidyne to connect the Lycoming to the prop down a long drive shaft (which suffers from the same kind of torsional resonance problems). Problem is, Flexidynes get really hot, and you still have the windmilling problem (Molt Taylor used the Flexidyne in several of his homebuilt concepts but he never really addressed the freewheeling prop issue) and a propeller brake might be in order if you want to be able to glide (or a feathering or folding prop).

You see centrifugal clutches used in paramotors, mostly because it allows the motor to be running at idle without the prop turning, a handy feature for a paramotor. I wouldn't be surprised if the Dragonfly's engine is actually using a Flexidyne instead of a friction clutch.

The most common compliant coupling between motor and prop however, is rubber. Most commonly a rubber drive belt system, used everywhere on ultralights and some automotive aircraft conversions. Or as successfully used by Rotax, a rubber coupling in the gearbox itself. Rotax, after a lot of effort, was able to create a rubber drive isolator that would be effective in a given engine's operating range. This would have taken a massive engineering effort over the years to perfect.

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If I remember correctly, the 4-stroke Rotax engines include a sliding clutch to protect the crankshaft from shocks in case the prop accidentally contacts the ground.

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  • $\begingroup$ It would also be for torsional resonance issues as well. $\endgroup$
    – John K
    Commented May 17, 2020 at 20:20
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In addition to xxavier's answer, a clutch makes it easier to maually start the engine (without the prop load) and easier to idle (for the same reason). There is also a safety benefit as well:

If you have ever seen a video of someone desperately hanging on to a groundlooping Piper Cub that was hand started with the prop, at too high a throttle setting, you'll know what I mean. A simple centrifugal clutch, that does not engage the propeller until the engine rpm is raised significantly, is a good idea. These are common on pull-cord started chainsaws and go-carts.

Having a second person assisting you with the Piper Cub is a good idea too.

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