Fin-planform effects on lift and center of pressure for supersonic missiles

Abstract

The missile fin-body interference factor Kw(B) (ratio of the normal force on a fin in presence of the missile body to the normal force on the isolated fin) is an important factor in the equivalent angle-of-attack preliminary design method. A finite volume Euler code, ZEUS, is used to numerically determine the fin forces as a function of fin trailing-edge sweep for 2-, 3-, 4-, 5-, and 6-fin missile configurations at small angles of attack. The fin forces are used to evaluate Kw(B) as a function of fin span to body radius ratio at Mach numbers from 2.5 to 5 at various fin aspect ratios. The movement of center of pressure with changes in trailing-edge sweep and Mach number is also evaluated. KWW and center of pressure values for the 2-, 3-, and 4-fin configurations agree with slender-body theory analytical solutions; however, the KW(B) predictions for the 5and 6-fin missiles break away from slender-body theory as the fin span increases. The fin span at which breakaway occurs is shown to be related to the position at which shock and expansion waves originating from the adjacent fins strike the fin of interest; consequently, it is a function of number of fins and Mach number. In some cases the shock and expansion wave interaction cause AVcs) to be <1, which indicates a reduction in fin lift due to interference. Tables for Kw(B) and the center of pressure are presented for use in preliminary design.