The Supersonic Rotor

An account is given of some theoretical studies of the operation of a helicopter rotor at tip speeds which are about twice the speed of sound. It is shown that the blade loadings of a supersonic rotor operating near its optimum are about three times those of a conventional rotor. One horsepower generates two to three pounds of lift. The power required does not increase with altitude and in forward flight the supersonic lift does not vary significantly as the blade rotates. Theoretical estimates of the noise are compared with experimental evidence and it is shown that the noise level will remain sensibly constant beyond about M tip = 1.1. There are shock-free zones above and below the rotor. The implication of these features for helicopter aerodynamic design are discussed.