What is the defention of windmilling?

Answer 1

Windmilling is a phenomenon which can occur in aero-engines after a fan-blade has come o during flight. A fan-blade-o event is rare with only very few incidents per year and is, in most cases, no threat to the safety of the aeroplane. For an aero-engine to be certified by the aviation regulatory bodies, one requirement is that the manufacturer proves (with `fan-blade-o tests') that the fan-blade stays contained within the engine. However, internal damage to the integrity of the engine is usually such that the engine immediately stops to produce any power. For the remainder of the flight the incoming airflow will cause the engine to rotate, thus the name `windmilling imbalance' for this scenario. As one blade is missing, large out-of-balance forces of the order of a

few tonnes from the now-asymmetric fan provide an excitation for the engine-wing-aircraft structure. Clearances which are sufficient during normal operation might now be overcome by vibrating components excited by the large out-of-balance force. This leads to rotor/stator interaction, which can potentially cause a rich mixture of effects associated with rubs and impacts.