INSTABILITIES ASSOCIATED WITH A ROTOR BLADE STOPPED IN FLIGHT

The great interest in the development of convertible aircraft with VTOL capability as well as efficient high speed cruise has led to the consideration of compound helicopter-type configurations in which the rotor is stopped (and possibly stowed) in flight. In support of this development, the U. S. Army has been sponsoring a program of research to develop a substantiated method to predict the aerodynamic loads developed by a rotor in a stopped configuration as well as during the starting and stopping cycles. As part of this research program, a series of tests were recently conducted with a "representative" full scale stopped rotor blade configuration in the NASA-AMES 40 × 80 ft wind tunnel. During the course of the test program, violent dynamic instabilities of the rotor blade were obtained for various combinations of shaft and blade pitch angles over a range of azimuth angles from 225 degrees to 300 degrees. Two distinct boundaries for fixed shaft angles were obtained, one for positive angles-of-attack and the other for negative angles-of-attack. While analysis of the oscillograph records indicated the basic instability occurred at a frequency close to the cantilevered torsion frequency of the blade, large amounts of low frequency flapwise bending were also present. It was concluded, after all the data had been fully analyzed, that the instability that had been encountered was basically a type of stall flutter. While the data obtained at other dynamic pressures was limited, it did indicate that the instability boundaries might be a function of dynamic pressure or possibly Reynolds Number.