Effect of the Lift Coefficient on Propeller Flutter

Summary: Flutter of propellers at high angles of attack is discussed, and flutter data obtained in connection with tests of models of large wind-tunnel propellers are analyzed and results presented. It is shown that in the high angle-of-attack range flutter of a propeller invariably occurs at a speed substantially below the classical flutter speed. The angle of attack at which flutter occurs appears to be nearly constant and independent of the initial blade setting. Thus, the blade simply twists to the critical position and flutter starts. Formulas have been developed which give an operating angle in terms of the design angle and other associated parameters, and these relations are presented in the form of graphs. It is seen that the flutter speed is lowered as the initial design lift coefficient is increased. It is further shown that by use of a proper camber of the propeller section the flutter speed may approach the classical value. A camber for which the blade will not twist is found to exist, and the corresponding lift coefficient is shown to be of special significance.