windmilling

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windmilling

[′win‚mil·iŋ]
(mechanical engineering)
The rotation of a propeller from the force of the air when the engine is not operating.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

windmilling

i. A propeller or rotor rotated only by the airflow. In an engine failure, the forward speed of the aircraft will make the propeller or rotor rotate. The wind-milling propeller causes drag, and, in a multiengine aircraft, it makes the aircraft yaw toward the failed engine. To reduce the drag as well as to prevent further damage to the engine, the failed engine's propeller is feathered.
ii. A free-turbine propeller spinning idly in the wind when the aircraft is parked on the ground. This is prevented by braking the propeller.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved