Now for getting the resonance to low rpm, ideally we want the first resonance order signficantly below idle. That is what we do in cars. And we do this by putting a soft spring between the flywheel mass and the transmission input, so the geartrain is isolated from the firing pulses. Put resonance at least a half-octave below idle and you will have largely isolated the engine from everything downstream of the spring. The spring must be able to carry all of the engine torque plus the oscillation of the crank times the spring rate, which is the torque ripple that is left after the spring. And the spring rate is chosen to drive the resonance safely below idle. If the resonance is up in the operating range (even at the low end), any lash in the geartrain, splines, etc, will be exercised and be noisy, even if not destructive, and you have to go through the resonance when you shove the throttle up - some systems can get hung at the resonance point. Put the resonance down below idle and you go through it once as the engine comes up to idle after cranking/firing, and again when you turn it off. You already see this in the engine mount system going through resonance on start and on stop. The rotating parts are nicely handled this way too.
So, the system has to be designed to have a suitably soft element to get the resonance below idle. If the soft element is not soft enough, the resonance only gets moved around in the operating range... Steel springs, rubber springs, etc will all work if you can get low enough spring rate and high enough torque capacity. A number of PSRU out there have rubber donuts or other elements that seem to work pretty well. Some rubber spring designs actually become rising rate springs: At low torque (and low rpm), the spring is soft and keeps the resonance below idle; as torque comes up also rpm goes up, and the spring gets stiffer, but the resonance is still safely below operating speed. This obviously has to be DESIGNED or LABORIOUSLY DEVELOPED. Tracey Crook did it. If you want a 160 hp Mazda rotary scheme see if Pat Panzera will work with you a little. Size things improperly and you will either destroy the rubber elements or have resonance in the operating range or both. Oh, and then there is that pesky thing that can happen on a fast descent - while the power is pulled back, the torque is high (but in the opposite direction to when power is being made), but the windmilling prop is running along at half to two-thirds of cruise speed - you can get into resonance if the spring rates are a tad too high.
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There is no general. But that doesn't mean that they can't be used. You'd need to calculate the required compliance and see if there is a suitable one.I realize it's difficult, dangerous even, to make general statements in this area, but are "Giubo" AKA "Rotaflex" couplings IN GENERAL torsionally soft enough for this decoupling function?
You can not just say "any will do". Let's remember that they are not designed to do what you are planning on. We are taking advantage of some lucky coincidences. Several characteristics all have to be acceptable:I realize it's difficult, dangerous even, to make general statements in this area, but are "Giubo" AKA "Rotaflex" couplings IN GENERAL torsionally soft enough for this decoupling function?
Rotax does the kind of engineering for their products that no homebuilder has the resources for.Rotoflex or flexible couplings are used on the Rotax C gearbox.I havent seen any common information that they fail routinely. I have flown behind a C gearbox for 250 hours (Rotax 582) without any problem or maybe our engine was the one lucky exception.
That airplane is still being flown but not by me with the original rubber.
Rotax gearbox coupling has no safety backup.