What are the three primary regions of the disc in an autorotation?

The three primary regions of the disc in an autorotation are the stalled, driving, and driven.diagram of the three primary regions of the disc in an autorotation

The stalled area of the disc is the area closest to the hub.  Because the rotation of the disc is slow at this area, the angle of attack of the blade is beyond its critical angle.  As a result, it is stalled and not producing any lift.  The middle section of the blade is the driving region.  The thrust or lift from this section of the blade is slightly forward in the direction of rotation.  As a result, this lift provides the thrust needed to rotate the blades.  The driven area is the outside portion of the disc.  This area is producing a lot of drag.  As a result, its net contribution is not assisting in lift production and is being “driven” by the middle section of the disc.

Unless descending vertically in a no-wind condition, the stalled, driving, and driven regions will be a different size on the advancing and retreating side of the disc.  During autorotation, the pilot changes the size of these regions to control the speed of the rotor disc.  For example, raising the collective decreases the size of the driving region and increases the stalled and driven region.  As a result of the decreased driving region, the rotor RPM decreases.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-25
Principles of Helicopter Flight, 2nd Edition, pg. 144

Other Helicopter Flight Conditions

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