Theory of open supersonic rotor noise

Abstract

A theory is presented describing the noise generated by open rotors operating at supersonic tip speeds. It is based upon a combination of the LighthiIl aerodynamic sound theory and the Whitham weak shock theory. Aerodynamic sound theory is used to describe the basic noise generation, in a manner which is essentially an extension of subsonic propeller theory. This gives a good first approximation to the radiated sound field. However, because the radiated sound levels are high, allowance must be made for the effects of non-linear acoustic propagation, and this is accomplished by an application of the weak shock theory. Non-linear propagation causes some noticeable changes in the observed acoustic field, especially in its spectral characteristics. When these changes are incorporated in the analysis agreement is obtained with known experimental results.