Effect of Sweep Angle and Drag on the Flutter Speed of Hydrofoils

Abstract

A determination is made of the effect on flutter frequency and speed on variations in the drag coef- ficient and sweep angle. The strip method of analysis is used with wing stiffness being estimated through strength of material analyses. The drag and lift slope coefficient are estimated to compare with experimental data obtained by T. T. Huang. These estimates obtain results which are shown to provide more favorable comparisons with various experiments conducted by him, than could be obtained heretofore. Although comparisons obtained are considered favorable, trends in the data for increasing slope angles are not completely satisfactory which indicates that a further refinement in the theory is still required. Discussion of the possible source for these trends and the limits of appli- cability of this approach are considered. method by employing arbitrary spanwise distribution of the lift curve slope and c.p. location in order to gain a more satisfactory comparison with test results. Other investigators have attempted to utilize the full application of lifting surface theories in solution of the problem. Such a procedure has been proposed by Rowe. 3 His method allows some variation in the satisfaction of the Kutta condition and inherently generates data on the lift curve slope and c.p. location. Abramson and Chu4 obtained encouraging results from their analysis which is based on strip theory. The analysis involves modifications to account for the Kutta condition and variation of the location of the lift curve slope and c.p. location. The application of these conclusions were then used to obtain a conservative flutter speed for the SWRI flutter model as a case in point, and at the same time showed the profound influence on the flutter speed caused by changes in the c.p. location and by variations in the lift curve slope along the wing. Mahig,5 using the strip theory, showed that not only did the lift curve slope affect the flutter speed but that the drag forces which have heretofore been neglected in the literature had an even more profound effect. Although the sensitivity of the flutter speed to varia- tions in the drag coefficient, c.p. location, and lift curve slope have been studied, the effect of sweep back concur- rent with the effect of the drag forces has not been consid- ered analytically by anyone. The analysis presented below does this, and as a result, makes possible a more detailed comparison of theoretical calculations with the experi- mental results obtained by T. T. Huang.6