How big is a lock number?

The traditional Lock number for lifting rotors is recast in terms of a different set of parameters. This new formulation, while not changing the basic definition of the Lock number, gives additional physical insight into the design variables that influence this number. The new formulation enables one to better understand why Lock numbers fall into the range they do. The fundamental basis of the Lock number is studied for 33 rotors, ranging in size from the main rotor of the CH-53E (with seven blades and a diameter of 79 ft) down to the tail rotor of the Robinson R22 Beta (with two blades and a diameter of 3.5 ft).