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I was wondering if a rotorcraft, that descends very slowly compared to its nominal rotor rotation rate (for example an RC Helicopter vertically autorotating with 3m/s descent rate and 2200RPM on the rotor) can maintain the rotor rotation rate even though it is having a positive collective pitch.

Is this possible? If so can it be sustained or is it just for short time frames?

The following image might enhance the understanding (Source) look at the speeds and look at the lift/drag curve, if the lift/drag ratio is big enough this should work, right? enter image description here

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  • $\begingroup$ The usual pitch for autorotation in real-size rotating-wing aircraft is a positive +3º. Most gyros do happily fly with that fixed pitch in their rotors... Anyway, the angle that matters is the angle of attack. In vertical autorotation, you can have a good AoA for autorotation even with a negative pitch, provided the vertical and circumferential airspeeds have the right value at the relevant station... $\endgroup$
    – xxavier
    Aug 1, 2020 at 17:17
  • $\begingroup$ Thanks! That helps so much. I am working on a simulation for a Small Autorotating Reentry Vehicle that supposed to land on other planets (that's my Master Thesis) and I was confused that on Titan (very dense atmosphere, only 1/7 of earths gravity) I always run into positive angles of attack. It helps a lot to know that it can be possible! I am not from the field of aerodynamics, I am a control engineer, so I really appreciate your input! $\endgroup$
    – Clex
    Aug 1, 2020 at 17:42

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During autorotation there are two main angles which are important, the inflow angle created by the rate of descent flow and rotational airflow plus the pitch angle (normally minimal on initial entry to autorotation). It is these angles which produces the lift. The second angle produces the auto rotative force, or the force which maintains the rotor blades turning in auto. This is created by the forward tilt of the main lift vector (or the horizontal component of the lift vector) which is shown on your diagram. During autorotation it is important to maintain control over rotor rpm. Density altitude has a marked effect in auto, which is your consideration albeit to a greater extent. As density altitude increases rotor rpm for the same pitch angle will increase, this can’t be allowed to continue to increase and is contained by increasing blade pitch, hence the greater the DA, the more pitch is required including positive pitch if required. Another consideration which may help is, autorotation appears to be an aerodynamic effect of rotorcraft that is considered in isolation when a helicopter loses power, rather like an on/off switch. This isn’t the case, helicopters will experience a degree of autorotation in a normal descent due to the rate of descent flow but the rotor rpm will be maintained within normal limits due to the rotor governing system. It is a progressive process from the start of a descent to fully established auto when the torque to the rotors becomes zero.

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  • $\begingroup$ I can see how you may have been confused. First para rewritten for clarity $\endgroup$
    – Rob Wilkinson
    Aug 2, 2020 at 17:28
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The angle of attack ALPHAr is the key magnitude in autorotation. The following images are from Gessow & Myers 'Aerodynamics of the Helicopter'. The angle of attack ALPHAr = PHI+THETA, PHI being the inflow angle and THETA the pitch. It's easy to see that, if PHI is high enough, stable vertical autorotation may be achieved with zero (or even negative) pitch...

enter image description here

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enter image description here enter image description here

The Wells turbine, used in some wave power plants, is an example of autorotation with zero pitch:

enter image description here

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  • $\begingroup$ Thanks! I like how you say "even negative" I have only dealt with RC Helicopters and similarly sized vehicles and there the general rule of thumb is that only negative pitch shall be applied for AuRo. This gives good perspective! I found myself sitting there and think that positive should be possible too, drawing exactly the image you have provided. But then I also thought "well years and years of RC Heli Piloting experience have to be worth something" so I listen to these people. But you guys are right, positive angles are fine as well its all about alpha $\endgroup$
    – Clex
    Aug 3, 2020 at 19:36
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I don’t have the resources here to post a diagram but also explore conservation of angular momentum. In short if you are manoeuvring and increase the load factor on the disk it will flex up at the tips (similar to an aircraft wing) this in effect reduces the rotor diameter. All things being equal the smaller diameter will have a higher rpm (think of an ice skater spinning that pulls her arms and legs in and spins faster). Often when turning this increase will require a collective increase to contain RRPM within limits.

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