Windmilling rotors and autorotation

In power-off flight the rotor blades may be in a state of true autorotation or they may be ‘windmilling’. In a true autorotation the blades are set at a pitch angle that

nominal rotor speed 35 rad/s 0 2 4


Time (s)













і I

P r


о £


Подпись: Vertical Velocity (ft/min)
Подпись: Performance Theory 49

Fig. 2.25 Effect of pilot action on autorotative flight conditions.




Fig. 2.26 Definition of key rotor speeds associated with delay time testing.


Fig. 2.27 Effect of increasing delay time on rotor speed decay.

combines with the rate of descent to produce no net torque and thereby stabilizes the rotor speed. From this condition the pilot can make minor changes in collective pitch to reduce or increase the rotor speed. If the balance of in-plane forces results in an accelerative condition the rotor blades are said to be windmilling. Figure 2.29 compares


Fig. 2.28 In-flight cross-plot – recovery NR versus minimum NR.

the radial variation of elemental thrust and torque for the example rotor in a hover (at 5000 kg) and in a true vertical autorotation. Note how the requirement to balance the accelerative and decelerative in-plane forces along the blade length changes the lift distribution. Likewise the drag associated with the blade root and the tip-loss region results in the need for negative torque in the region 0.25 < rIR < 0.85.

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