Lock number

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.{{cite journal |last1=Seidel |first1=Cory |last2=Peters |first2=David A. |title=How Big Is a Lock Number? |journal=Journal of the American Helicopter Society |date=1 January 2021 |volume=66 |issue=1 |pages=1–3 |doi=10.4050/JAHS.66.012001 |s2cid=229216105 |url=https://doi.org/10.4050/JAHS.66.012001|url-access=subscription }} It is named after C. N. H. Lock, a British aerodynamicist who studied autogyros in the 1920s.{{cite book |last1=Prouty |first1=Raymond W. |title=Helicopter aerodynamics |date=2009 |publisher=Eagle Eye Solutions |location=Lebanon, Ohio |isbn=9780557090440 |page=222}}{{cite book |last1=Johnson |first1=Wayne |title=Helicopter theory |date=1994 |publisher=Dover Publications |location=New York |isbn=9780486131825}}{{rp|267}}

Typical rotorcraft blades have a Lock number between 3 and 12,{{cite web |title=European patent specification EP3345829 |url=https://data.epo.org/publication-server/document?iDocId=5960595&iFormat=0 |website=data.epo.org |publisher=European Patent Office |access-date=19 January 2022 |language=en}} usually approximately 8.{{cite book |last1=Leishman |first1=J. Gordon |title=Principles of helicopter aerodynamics |date=2002 |publisher=Cambridge University Press |isbn=9780521523967 |edition=1st pbk. |location=Cambridge |page=179}}

The Lock number is typically 8 to 10 for articulated rotors and 5 to 7 for hingeless rotors.{{rp|186}} High-stiffness blades may have a Lock number up to 14.

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.

A low Lock number gives good autorotation characteristics due to higher inertia, however this comes with a mass penalty.{{rp|327}}

Ray Prouty writes, "The previously discussed numbers: Mach, Reynolds and Froude are used in many fields of fluid dynamic studies. The Lock number is ours alone."