Underwing Missile Aerodynamic Effects on Flight-Measured Limit-Cycle Oscillations

Abstract

F-16 testing revealed differences in limit-cycle oscillation response characteristics associated with subtle aerodynamic variations of the underwing missiles. Physical length differences between long and short missiles led to the accidental discovery of the limit-cycle oscillation sensitivity. In addition to the length difference, the long missiles were found to have builtup collar sections, to which the fins and canards were attached to the missile body. Two cases are presented where both long and short missiles were carried underwing. For the case with no wingtip missiles present, carriage of the long missiles showed significant limit-cycle oscillation response, while the short missiles showed very little response. For the case with wingtip missiles present, the limit-cycle oscillation response levels were slightly higher for carriage of the short missiles but were otherwise quite similar to the long-missile responses. Computational fluid dynamics on a rigid wing suggest that the differences between the long and short missiles alter the flowfield by changing the impinging shock strength and location on the lower wing surface outboard of the underwing missile. For the empty wingtip launcher case, the location of the impinging shock on the lower surface influences the strength of the shock on the upper surface. This influence on the upper shock strength could be a key contributor to the limit-cycle oscillation mechanism when wingtip aerodynamics do not inhibit the flowfield. Nomenclature C p= pressure coefficient f f = linear analysis flutter frequency, Hz f n = natural frequency, Hẑ M = generalized modal mass M ∞ = freestream Mach number V f = linear analysis flutter velocity, kt