"the dance" is because we have a piston engine.
consider this simplified Newtons first law: Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.
In the case of our engine Rotax uses a light, high RPM, engine that has little pistons of light mass. When we need to make the propeller turn we have to consider the speed of the prop and how well it can move the air. (max is about .8 of Mach 1) To do this we have to use a gearbox so the prop turns much slower than the crankshaft. In doing so Rotax did this with gears. As with any mechanical geared parts there are some tiny gaps between the bits inside and they have backlash. So this is the design and we have to deal with Newtons first law. Once we start to spin the prop it tries to drive the gears when the engine has power pulses in the initial start and idle.
As the engine cranks, with the resultant spark and acceleration of the piston downward from the power stroke, the propeller is accelerated. once the mass of the propeller starts to move we are well aware that the prop wants to keep turning. the engine however now pushes a piston up to make compression, the result is a tortional moment introduced to the gearbox in the opposite way trying to slow the prop down. If we did not have a way to absorb this tortional moment the gears and little bits begin to fail. The good part is if we can get the engine started and over the initial "dance", or rebound going into the gearbox, the engine will run fast enough to negate any damage that would be done. We hear this initial gearbox noise as we crank the engine and as it is switched off. Once started the tortional damping system is not damaged as long as we run our engine outside the rattle range of initial start. On shutdown there is no extra forces and not an issue, the only concerns are really on starting.
Internally there are a few different configurations. The gears have a ramp and dog system. this allows for a small amount of movement to occur in the initial cranking. Once to idle these should no longer be in our hearing range. if we persist on hearing this rattle at our idle speed then we have some big problems in accelerated wear of the components as they go from a normal dance to an all out stomping internally. The gearbox has limits also, this system is dependent on not exceeding the mass inertia of the rotating bits bolted to the prop flange. Check out the very old SI-11- 91UL (from Dec 1992) bulletin if you wish to find the technical way to check your prop.
Simple bits to know. If you hear it at your idle speed, speed it up. If it is not stopping at your idle speed its time to get it re-shimmed internally at the very least.
Don’t know if that helps or confuses more people, we try to cover that in the Service Training for iRMT and the iRMT with Heavy rating are well trained on gearbox maintenance, look one up if you need help.
