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I have another AuRo question for you. When a Helicopter autorotates (ideally) it is in the turbulent wake state and there is virtually no flow through the rotor disk (figure of Leishman attached). Now, when Vc (descent speed) is roughly -1.8 times vh (induced velocity at hover) the vehicle is considered to autorotate ideally.

What if we do not autorotate ideally? Does the vehicle still have to be in turbulent wake state? The real question: Is autorotation (and thereby maintaining of an ideal rotor rotation speed) possible in the windmill break state?

For instance, in the work of Dalamagkidis the OHA58 (Chapter 5.1.1) and the Raptor30v2 (Chapter 5.3.1) do not seem to be in turbulent wake state. As for my calculations, vc/vh was always slightly lower than -2.

However, the vehicles seem to perform well and land softly. So is the windmill break state a legitimate state for "autorotation"?

Thanks for helping me out! :)

The 4 working states as described by Leishman and their influence on induced velocity

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Autorotation in helicopters is rarely ideal, with net rotor torque = 0, because some energy is drained from the turning blades to feed the tail rotor.

There's, however, a continuum between the ideal 'autorotation' and the 'windmill brake state/moulinet frein'. The 'point of ideal autorotation' should be taken as the 'point of minimum'. For any increase of the load (the weight of the gyro, for instance), from that point on, there's a multitude of possible equilibrium points in autorotation along that curve of 'windmill brake state'. Real-word autorotation is always in one of those points, and never at the 'point of ideal autorotation'... You may find useful this illustration and text. Source included.

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Given this is the first time I’ve encountered the mathematics associated with auto rotation and helicopters I find the chart to be a little vague. What I can say is that the descent in auto rotation and the round-out and touchdown are two different phases. During stabilized autorotation descent, you’re trying to target a specific airspeed to maintain your rotor speed. There will be upward airflow through the rotor disk; there has to be to continue to provide lift to the driving regions of the rotor disk and keep the rotor spinning. I suspect that the chart indicates that the windmill brake state is a case where you do not have an effective equivalent relative wind over the blades and a large portion of the disc is stalled. This would be very bad as the rotor can no longer maintain speed, making recovery impossible. During the roundout, you are effectively exchanging rotor momentum for lift at slow airspeeds, which is why the timing and altitude of the roundout is critical to a safe touchdown.

I hope that helps.

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