Investigation and prediction of helicopter rotor noise part I. Wessex whirl tower results

Abstract This report includes noise measurements taken from a three-bladed Wessex rotor of 56 ft diameter, mounted on a rotor whirl tower. The blades used for the measurements were of constant chord and made to a N.A.C.A. 0012 profile section. The range of noise measurements taken was concentrated in the speed range from 180 to 250 rev/min and at collective pitch angles (measured at the cuff) from 7° to 17°. These measurements have been compared with theoretical predictions of broad-band noise. The magnitude of the higher harmonics of the ordered noise found from the tests appears to be in good agreement with the narrow pressure pulse width acting on the blade surface. This is discussed in the report and illustrated in Figure 10. The broad-band or “vortex” noise has been found to obey a reasonable relationship with the sixth power of the blade tip velocity and also to be dependent upon CL1·66, where CL is a tip-referred lift coefficient. The limitations on the test results imposed by the use of the whirl tower are discussed in section 6 and suggestions for further work on other rotor configurations are indicated.