Adequacy of modeling turbulence and related effects on helicopter response

The atmospheric turbulence that a blade station experiences is called blade-fixed turbulence. It can substantially differ from the conventional body-fixed turbulence such as experienced by an element of the body or fuselage because the rotational velocity moves the blade station fore and aft through turbulence. A closed-form solution of a frequency-time spectrum for the dominant vertical turbulence velocity at an arbitrary blade station is developed. This solution explains how and to what extent the rotational velocity affects the frequency and temporal characteristics of the rotor-disk turbulence, such as cyclostationarity, and spectral peaks and split peaks at 1P/2 or 1/2 per rev, 1P, 3P/2, etc. Comparison of blade flapping response to blade- and body-fixed turbulence is also presented over a range of turbulence scale length and advance ratio: the response statistics comprise frequency-time spectra, correlations, and average threshold-crossing rates. Emphasis is on use of the frequency-time spectra of the cyclostationary turbulence and blade response to predict simultaneously the temporal and frequency characteristics. For low-altitude and low-advance-ratio flights, as in nap-of-the-Earth flights, rotational velocity substantially affects turbulence modeling and, thereby, response statistics.