A Tail Buffet Loads Prediction Method for Aircraft at High Angles of Attack

Aircraft designers commit significant resources to the design of aircraft in meeting performance goals. Despite fulfilling traditional design requirements, many fighter aircraft have encountered buffet loads when demonstrating their high angle-of-attack maneuver capabilities. As a result, during test or initial production phases of fighter development programs, many new designs are impacted, usually in a detrimental way, by resulting in reassessing designs or limiting full mission capability. These troublesome experiences usually stem from overlooking or completely ignoring the effects of buffet during the design phase of aircraft. Perhaps additional requirements are necessary that addresses effects of buffet in achieving best aircraft performance in fulfilling mission goals. This paper describes a reliable, fairly simple, but quite general buffet loads analysis method to use in the initial design phases of fighter-aircraft development. The method is very similar to the random gust load analysis that is now commonly available in a commercial code, which this analysis capability is based, with some key modifications. The paper describes the theory and the implementation of the methodology. The method is demonstrated on a JSF prototype example problem. The demonstration also serves as a validation of the method, since, in the paper, the analysis is shown to nearly match the flight data. In addition, the paper demonstrates how the analysis method can be used to assess candidate design concepts in determining a satisfactory final aircraft configuration. 1.0 INTRODUCTION AND BACKGROUND Since the late 1960’s, a major design objective for fighter aircraft is to achieve exceptional agility through large angle-of-attack (AOA) maneuvers. At these large-angle attitudes, the aircraft encounters highly adverse flow conditions. Generally, the flow conditions that are particularly problematic involve vortices emanating from various surfaces on the forward parts of the aircraft such as engine inlets, wings, or other fuselage appendages. Modern fighter aircraft, especially with thrust-to-weight ratios of higher than one, can generate very high-energy vortices at high AOA maneuver conditions. From a watertunnel-model test in figure 1, the buffet mechanism is illustrated by the gas bubble trail that immerses the tails in buffet flow. Figure 1. Water tunnel test showing the buffet mechanism with typical vortex burst. (Courtesy of Lockheed-Martin Aeronautics) High-energy vortices can damage aircraft when they become unstable and “burst.” Initially exhibiting highly organized smooth flow with high circular velocity in a tight radius, when burst, the vortex transitions into a flow characterized by a much larger diameter, less organized, and far more turbulent. The frequency content of the vortex undergoes a transition, as