Position of a small fin on a missile body for maximum directional stability

Windtunnel measurements and CFD calculations were performed to determine the position of a small fin on a missile body that gives the highest directional stability. The model used is the configuration KROPP of the Tvärteknikprojekt, a concept for a future heavy cruise missile. For supersonic speeds, the fin creates a shock pattern behind itself on the body. The generated pressure differences contribute to the yawing moment at sideslip, so much that it is advantageous to move the fin forward on the body. Although this decreases the effect of the fin itself (because of a shorter moment arm), it increases the total moment. The maximum yawing moment is encountered when the entire shock pattern is placed on the body. A 35% increase in yawing moment due to fin was noted. For subsonic speeds this effect is smaller, because the fin influences a smaller part of the body. The yawing moment does not vary much with fin position. The best position is most probably with the trailing edge a short distance away from the base.