"Correlations of Rotor/Wake - Airframe Interactions with Flow Visualization Data".

Journal of the American Helicopter Society . Vol. 35, No. 4, October 1990, p. 4 -15.

Albert G. Brand, Howard M. McMahon, Narayanan M. Komerath.

Abstract: Interaction between the aerodynamics of the rotor and the airframe causes large unsteady pressure fluctuations on rotorcraft airframes. A two-bladed rotor and a hemisphere-cylinder airframe model are used to study these pressure fluctuations in a wind tunnel, simulating low-speed forward flight conditions. Controlled displacement of the airframe is used to obtain finely-spaced pressure data. These are correlated with azimuth-resolved, quantitative laser sheet visualization of the dynamics of the tip vortices and the inboard vortex sheets from the rotor, along the top of the airframe. Three prominent periodic interactions are observed on the airframe surface. These are the effects of blade motion, the approach and impingement of the tip vortex, and of the vortex sheet. These interactions determine, to a large degree, the overall pressure distribution on the airframe surface, and hence the airframe unsteady airloads. The pressure signatures from the blade passage and the vortex impingement are quite different, and are explained using 2-dimensional models.