slowrotor,
The following is extremely simplistic, but I think that it is basically correct, and it should support and supplement what SEL is saying;
One of the latest books on rotor aerodynamic is Principals of Helicopter Aerodynamics, by J. Gordon Leishman. He still uses 'Driving region' and 'Driven region' to explain autorotation. He says "At the tip of the blade where the induced angle of attack is low, these sections consume power because, as a result of the forward inclination of the lift vector, the propulsive component is insufficient to overcome the profile drag."
The key phrase above is 'profile drag'.
On a plane, if the wings are physically consistent from root to tip, they will have a consistent; lift, induced drag, and profile drag, from root to tip.
On a hovering helicopter, if the blades have a specific twist, they will have a consistent lift and induced drag, from root to tip. But, the profile drag will not be consistent from root to tip. It will be greater at the tip because the tip is experiencing a faster airflow. Therefore the middle portion of the blade must contribute some of the forward component of its lift to help the outer portion of the blade overcome this additional drag.
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Just for the fun of add verbiage;
The phrases 'Driving region' and 'Driven region' may be a little ambiguous, since what is driving and what is being driven, the air flow, the rotor, the craft? In addition, Leishman's statement "these sections consume power" could be superficially construed as 'the rotor consuming potential energy from the elevation' or as 'the craft consuming kinetic energy from the rotor'
Helicopters will have negative blade twist at angles up to 12 or more degrees. Gyrocopters, which are designed to operate only in autorotation, have a positive twist of about 1-degree.
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If any of the above is wrong, please beat me up.
Last edited by Dave_Jackson; 9th Oct 2003 at 04:46.