Page 22 - Volume 14 Issue 3

ASK THE EXPERT
Windmilling in Reverse
by Tom Clements
There is more than one meaning or situation that can be addressed by the term “Windmilling in Reverse” and I hope to cover all of them in this article.
First, why would the feathered propeller of a PT6 engine that has been shut down in flight rotate backward – turn counterclockwise (CCW) as viewed from the pilot’s seat – instead of being stopped or rotating in the clockwise (CW), normal, direction? It was typical in the original three-bladed PT6s used on earlier King Airs that the propeller would indeed be stationary on a shutdown engine in flight. Since there is extremely low resistance to rotation – due to the fact that the input shaft to the
20 • KING AIR MAGAZINE
engine’s Reduction Gearbox (RGB) is the free turbine that is not connected to the compressor and its accessories – even the smallest error in the feathered blade angle can cause rotation. Sometimes the misadjustment leads to normal, CW propeller rotation and sometimes it goes in the opposite direction, CCW. If the propeller blade angle doesn’t quite go far enough to streamline into the relative wind – close to a 90° blade angle, but with the actual number depending upon both the distance from the center and the twist with which the blade is designed – then the relative wind creates a force that tries to rotate the propeller in its normal direction.
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MARCH 2020