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To obtain the minimum Rate of Descent during a power-off autorotation in still air conditions, the helicopter should be flown at a speed close to the…

  • A

    Best rate of climb speed, VY.

  • B

    Maximum range speed.

  • C

    Never-exceed speed, VNE.

  • D

    Zero speed.

Refer to figure.
In general, where the Power available is less than the Power required, then the minimum Rate of Descent will be at the speed where the power deficit is smallest. This is the minimum power required speed, VMP, which is also the best rate of climb speed, VY, and the maximum endurance speed, VMAX END, lying at the bottom of the power required curve and permits to remain airborne for the maximum possible time to deal with the emergency.

The minimum rate of descent during autorotation will occur at the point where the effects of both the disc tilt and the horizontal flow are equal to the effect of the increase in the rate of descent flow.

The manufacturers usually publish a TAS to RoD graph showing the best speeds for autorotation, which is just an other way of depicting the TAS to Power required relationship. Clearly, the best endurance speed in autorotation is the speed for minimum RoD, which is also VMP.

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