Lock number

In helicopter aerodynamics, the Lock number is the ratio of aerodynamic forces, which act to lift the blade, to inertial forces, which act to maintain the blade in the plane of rotation.[1] It is named after C.N.H. Lock, a British aerodynamicist who studied autogyros in the 1920s.[2][3]: 267 

Typical rotorcraft blades have a Lock number of between 3 and 12,[4] usually approximately 8.[5] The Lock number is typically 8 to 10 for articulated rotors and 5 to 7 for hingeless rotors.[3]: 186  High-stiffness blades may have a Lock number up to 14.[4]

Larger blades have a higher mass and more inertia, so tend to have a lower Lock number. Helicopter rotors with more than two blades can have lighter blades, so tend to have a higher Lock number.[4]

A low Lock number gives good autorotation characteristics due to higher inertia, however this comes with a mass penalty.[3]: 327 

See also

References
  1. Seidel, Cory; Peters, David A. (1 January 2021). "How Big Is a Lock Number?". Journal of the American Helicopter Society. 66 (1): 1–3. doi:10.4050/JAHS.66.012001.
  2. Prouty, Raymond W. (2009). Helicopter aerodynamics. Lebanon, Ohio: Eagle Eye Solutions. p. 222. ISBN 9780557090440.
  3. Johnson, Wayne (1994). Helicopter theory. New York: Dover Publications. ISBN 9780486131825.
  4. "European patent specification EP3345829". data.epo.org. European Patent Office. Retrieved 19 January 2022.
  5. Leishman, J. Gordon (2002). Principles of helicopter aerodynamics (1st pbk. ed.). Cambridge: Cambridge University Press. p. 136. ISBN 9780521523967.
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