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The spectra of noise and blade loading caused by interference of a propeller, helicopter rotor, or fan rotor with inlet turbulence are studied experimentally and theoretically. One test with hot-wire anemometers in a static inlet and another test with pressure transducers on the blades of a fan rotor reveal inlet turbulence to be highly anisotropic. The intensity of the transverse velocity component was found to be 2.5% of the mean flow and the streamwise component was 0.9% with ambient winds of about 1 mph. The transverse integral scale of the turbulence is a fraction of an inlet diameter, while the streamwise scale is over 100 diameters. Evidence indicates the source of these disturbances is atmospheric turbulence. The associated-noise is partially coherent, with spectrum peaks which are so narrow as to be difficult to distinguish from true harmonics. Theoretical spectrum predictions, performed with methods from random pulse modulation theory, indicate for the test fan that the peak at blade passing frequency and the high-frequency broad-band noise are due to inflow turbulence. It appears that this narrow-band random mechanism explains spectrum peaks which have previously been described as harmonic noise due to fixed inflow distortion.
Donald B. Hanson (Mon,) studied this question.