Flight Measurements of the Directional Stability and Control of a P-51D-5-NA Airplane (AAF No. 44-13257) with a High-Aspect-Ratio Vertical Tail

Flight measurements were made of the directional stability and control of a P-51D-5 airplane (AAP No. 44-13257), which is at Langley Laboratory for high-speed dive tests, with an extended vertical tail and with the unbalancing tab on the rudder inoperative. These modifications were made before beginning the high-speed dive program because the Langley pilots reported the airplane difficult to control in high-speed dives. Ames Laboratory tests (reference 1) show the directional stability at small angles of sideslip to be low and the change in rudder trim force with speed to be high with the original airplane configuration.

The tests showed that the modifications caused a marked increase in directional stability, rudder fixed and free as measured in steady side slips. Also, the change in rudder trim force with speed was desirably reduced with the modified tail. The pilots considered the directional stability and control of the F 51D-5 airplane to be satisfactory for the high speed dive tests with the modified vertical tail.

The expected effect of the fin extension on the vertical-tail load is compared with the results of a previous investigation on the P-63 A-l airplane with a fin extension similar to that used herein. Measurements of the natural frequencies of the vertical tail indicated that its critical flutter speed was reduced approximately 10 percent by the fin extension. The calculated flutter speed, however, is considerably higher than can be obtained with the P-51D-5 airplane.