Helicopter Flight Information
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Helicopter Flight Information |
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Low Rotor RPMRotor RPM is directly controlled by the throttle when there is engine power available. When engine power is not available, the RPM is controlled by the collective through disk loading, and in some cases with the cyclic by disk G-loading. In the event of an engine failure, rotor RPM can only be controlled by the collective and/or the cyclic. The collective must be reduced to the point required to achieve rotor RPM at or near the top of the green arc. If a condition exists where full down collective still will not provide satisfactory rotor RPM, it is possible to increase the RPM by increasing the G-load on the rotor disk by accomplishing S-turns, or a spiraling descent. If there is a loss of rotor RPM while engine power is available, the corrective action will be to lower the collective while simultaneously applying throttle as necessary to recover the RPM. If a loss of rotor RPM occurs while in a hover, the proper corrective action is simply to land the helicopter. Never raise the collective to maintain hover height while trying to recover the rotor RPM in a hover (unless you are over a mine field, then you might try some creative thinking). The recognition of a low RPM condition is best by noticing a change in the tone of the rotor and/or engine noise. Avoidance requires close monitoring of the RPM so that it is maintained in the top of the green at all times. In cases where the engine power has reached its maximum, it may become necessary to control the RPM with the collective even under powered flight. This technique is common at high altitude when the engine has reached maximum power output (full throttle). Never believe the line that the rotor is more efficient in the lower limits of the green arc. Remember that lift increases at the square of the airspeed over the airfoil. This old pilots tale comes from the knowledge that an extended glide autorotation can be achieved by decreasing the rotor RPM to the lower limits of the green arc while also increasing the glide speed. It is true that the glide can be extended through that technique, however it is not true that it is because the rotor is more efficient at that reduced RPM. During an autorotation, When the RPM is reduced to the lower limits of the green arc, the aerodynamics of the autorotation are altered. The driving portion of the rotor disk is reduced (reducing the RPM), and the driven portion is increased (providing more lift extending the glide).
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