Theoretical and Experimental Study of the Snap-through Airfoil and Its Potential as a Higher Harmonic Control Device

摘要

The snap-through airfoil is evaluated as a potential helicopter harmonic control device. Incorporation of a flexible panel into the lower surface of a segment of a rotor blade changes the airfoil camber locally resulting in an increase of decrease in the local lift and pitching moment coefficient. This in turn can result in a smoothing of the spanwise lift distribution. This paper contains an investigation of the steady and unsteady lift and moment characteristics of a snap-through airfoil obtained by modifying a section from an existing rotor blade. Experimental studies were conducted at an average Reynolds number of 1.3 million, based on chord, over the angle of attack range from -16 to 20 degrees. Experimental results are compared with theoretical predictions, The effect of the increased lift and pitching moment on the elastic twist of a sample rotor blade is examined for a forward flight case. The example rotor used in this study is that from the Boeing Vertol model 145, more commonly known as CH-47D Chinook. In all cases the effect of the snapping at points along the blade is to increase the local nose down aerodynamic pitching moment which reduces the overall blade lift despite local increase in section lift at the snap panel location. It is shown that the snap-through airfoil, properly programmed about the azimuth, can reduce the magnitude of the 1P vertical shear fluctuations on a blade by approximately sixty percent.