Sentence:  When he went to switch on his rotary engine again, the Le Rhone refused to pick up. Nothing happened! The propeller simply <WSD>windmilled</WSD> in the slip stream. Garros knew immediately what was wrong and cursed himself for his imbecility. 
Meaning of the word marked with <WSD> tags:  Of a rotating part of a machine, to (become disengaged and) rotate freely.