MEASUREMENT AROUND A ROTOR BLADE EXCITED IN PITCH .2. UNSTEADY SURFACE PRESSURE

This paper describes measurements of chordwise distributions of unstea dy pressure at three radial locations on a stiff two-bladed teetering helicopter rotor in hover, with the blades executing simple harmonic p itch oscillations about the quarter-chord pitch axis. The effect of dy namic inflow on rotor unsteady surface pressure has been extensively d ocumented. Two means of changing dynamic inflow have been employed, fi rst by varying rotor speed and forcing frequency, and second by changi ng the mean pitch angle. The effects of wake dynamics are seen through the phase difference between vorticity shed by previous blades and by previous revolutions of the reference blade, and through wake spacing . This phase depends on the ratio of excitation frequency to blade pas sage frequency, the number of blades in the rotor, and on the type of excitation applied. The reduced frequency effect and the tip effect on the lifting perturbation pressure have been demonstrated