In helicopter aerodynamics, the Lock number is the ratio of aerodynamic forces, which act to lift the rotor blades, to inertial forces, which act to maintain the blades in the plane of rotation.
[2][3]: 267 Typical rotorcraft blades have a Lock number between 3 and 12,[4] usually approximately 8.
[4] Larger blades have a higher mass and more inertia, so tend to have a lower Lock number.
[4] A low Lock number gives good autorotation characteristics due to higher inertia, however this comes with a mass penalty.
[3]: 327 Ray Prouty writes, "The previously discussed numbers: Mach, Reynolds and Froude are used in many fields of fluid dynamic studies.