Originally posted by Petertenthije
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However, the speed control of the helicopters works only when the engine is running.
In the variable-pitch, constant-speed propellers of the airplanes where the governor controls the pitch to keep the RPM constant as the pilot moves the throttle (to increase or reduce the power) or the airspeed increases or diminishes. If the airplane loses power, the pitch will go to a finer pitch to keep the prop turning at the same RPM up to the point where the plane reduces its speed so much that even the finest pitch will not be able to keep the RPM.
In a helicopter, the governor (or other device or technology, sometimes the pilot himself in helicopters not fitted with these devices) controls the throttle to keep the rotor RPM constant as the pilot increases or diminishes the pitch. In an event of loss of power, the pilot must lower the pitch to prevent the rotor speed to diminish. Typically it involves moving the collective all the way down to the minimum pitch position, which is already set to be the right autorotation pitch. Then will use the cyclic to establish the right autorotation speed and steer the helicopter during the "glide". The collective will be kept all the way down until the flare, where the pilot will combine the collective ("pull up") and the cyclic (pull up too) to flare and slow down before touching down. The rotor will immediately start to slow down so you only have so much energy stored in the rotor to perform this maneuver for a few seconds, and you only have one chance to get it right. Once the rotor slows down to the critical speed you better be already on the ground.
If you lose power and, instead of pushing the collective down, you pull up on it, the governor (or equivalent) will try to "throttle up" to keep up with he RPM, but it will of course fail and the rotor will slow down and eventually stop.
Now, why would a pilot do that? It's as if an airplane loses power and the pilot, instead of establishing a glide, pulls up, stalls, falls and crashes.
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