Wind milling

An aircraft engine is said to be windmilling when it is turned by aerodynamic action of air flowing over the propeller or through the compressor in a gas turbine engine. If an engine fails in flight, air flowing over the propeller or through the compressor produces enough force to keep the engine turning. When a propeller windmills, it produces so much drag it can cause difficulty in controlling the airplane. For this reason, the propellers of most multiengine airplanes are made so that they can be feathered by turning the blades parallel to the direction of flight. When a propeller is feathered, it does not windmill.