advance ratio

noun aeronautics

the ratio of wind speed along the axis of a rotor or propeller to the speed of the blade tip
the ratio of forward flight speed to the speed of the rotor tip of a helicopter

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012