Trajectory Variation due to an Unsteady Flow-Field

Abstract

This paper attempts to answer a very basic question: can the trajectory of a store be altered due to the unsteady flow-field in and around a weapons bay? Ultimately, this question does not have a simple answer since it is a function of the specific bay/suppression device/store combination under investigation. The approach taken in this work is to look at the question from the stand-point of a “bad” case scenario. If a realistic, yet small, store is gravity-dropped from a bay which is known to be unsteady, with no suppression device, how much variation is seen in its trajectory? If there is little variation, then the unsteady aircraft flow-field has not been much of a factor and there would be some justification for scaling back the level of effort currently needed for weapon certification from a weapons bay. However, if trajectory variation is observed under these conditions, then it will have been shown that the aircraft unsteady flow-field is at least capable of altering the store’s trajectory. This analysis was accomplished using time-accurate CFD simulations of a 500pound class generic store in a rectangular cavity with a length-to-depth ratio of 4.5 at Mach 0.95. A numerical approach was taken to eliminate the effects of scale and so that specific store release times could be chosen based on the knowledge of the moment history immediately after each release point. The results of the investigation show that unsteady flow-field effects alone are capable of producing variations in the trajectory of a store.