Some helicopters such as the Bell 47 are fitted with a centrifugal clutch. Centrifugal clutch will engage only at certain engine RPM. If the engine fails, the engine RPM will reduce and the centrifugal clutch will disengage the rotor from the engine. So, why is an additional freewheeling unit needed?
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1$\begingroup$ You should edit your question to highlight a link with aviation. $\endgroup$– Manu HCommented May 17, 2020 at 20:40
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$\begingroup$ @ManuH - I am new to SE. The original question was in relation to Helicopters. So, I added that. $\endgroup$– Raj ArjitCommented May 18, 2020 at 3:14
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$\begingroup$ if you have an example of a model that is fitted with such device, you should put it as an example $\endgroup$– Manu HCommented May 18, 2020 at 8:31
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$\begingroup$ @ManuH: My question is specifically related to the need for a freewheeling unit in helicopter. Not as a standalone device. $\endgroup$– Raj ArjitCommented May 19, 2020 at 16:52
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$\begingroup$ Feel free to edit your question again to add precision if feedback make you think there is still blur areas. $\endgroup$– Manu HCommented May 19, 2020 at 17:11
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2 Answers
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the centrifugal clutch will only disengage after its rotating speed falls to the disengagement point. The over-running clutch disengages the engine from the rotor at the very instant that the engine's rpm falls below that of the rotor.
In this way, the rotor is never wasting kinetic energy by trying to turn over a dead engine and the vehicle stands a better chance of achieving successful autorotation.
answered May 17, 2020 at 17:42
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$\begingroup$ Thanks for the answer. One query — if the freeweeling unit is not present and the engine fails, then the output shaft of the engine will keep rotating for a few moments. For those few moments, it could contribute to the rotation of the rotor. So, that "decaying" power of the engine is getting wasted by disconnecting instantly. Where am I wrong in this thinking? $\endgroup$ Commented May 17, 2020 at 17:54
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7$\begingroup$ @Raj As long as the "downstream" inertia is still "helping" the rotor, the freewheeling unit will not disengage - the freewheeling unit will not disconnect instantly, only when the torque transfer reverses (rotor driving the engine instead of engine driving the rotor). So no energy is ever wasted by a (sprag clutch) freewheeling unit. $\endgroup$ Commented May 17, 2020 at 18:38
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1$\begingroup$ @Sanchises - understood. Thanks a lot for the clarification. $\endgroup$ Commented May 17, 2020 at 18:45
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Helicopters may or may not have a centrifugal clutch which will provide drive to the rotors at an rpm just above engine idling, but all helicopters need a freewheel unit in case of an engine failure. The implications are best understood when considering a single engine helicopter. At the point the engine fails rotor rpm decay is very rapid. Within seconds, with no action taken the rotor rpm will decay to a point where it will become irrecoverable. The autorotative force (the turning element of autorotation) will be lost. To avoid this the collective control requires to be lowered to reduce pitch and hence rotor drag and the gearbox / rotor system needs to be disconnected from the engine immediately as this extra load would again inhibit the helicopters ability to enter autorotation.
