Buffet loads on aft aerodynamic surfaces pose a recurring problem on most twin-tailed fighter airplanes: During maneuvers at high angles of attack, vortices emanating from various surfaces on the forward parts of such an airplane engine inlets, wings, or other fuselage appendages) often burst, immersing the tails in their wakes. Although these vortices increase lift, the frequency contents of the burst vortices become so low as to cause the aft surfaces to vibrate destructively.

This type of buffeting has reduced the airframe fatigue lives and the system reliabilities of several legacy aircraft. This situation has been brought on largely because tail buffet loads have generally been ignored in the design of twin-tailed fighter airplanes. There are several reasons for this oversight: Fundamentally, tail buffet loads have not been identified as a major concern in designing fighter airframes. More practical reasons are that, until recently, (1) there was a lack of quantitative data on buffet forces; and (2) no readily available and cohesive analysis method could be used to predict buffet loads during the design of fighter airplanes.

Now, there exists a new analysis capability for predicting buffet loads during the earliest design phase of a fighter-aircraft program. In effect, buffet pressures are applied to mathematical models in the framework of a finite-element code, complete with aeroelastic properties and working knowledge of the spatiality of the buffet pressures for all flight conditions. The results of analysis performed by use of this capability illustrate those vibratory modes of a tail fin that are most likely to be affected by buffet loads. Hence, the results help in identifying the flight conditions during which to expect problems. Using this capability, an aircraft designer can make adjustments to the airframe and possibly the aerodynamics, leading to a more robust design.

This capability has been utilized in the design of the Joint Strike Fighter, leading to better understanding of buffet loads and to total redesign of the airframe to avoid fatigue-life issues.

This work was done by Robert W. Moses and Anthony S. Pototzky of Langley Research Center. For further information, contact the Intellectual Property Team at (757) 864-3521. LAR-16515

NASA Tech Briefs Magazine

This article first appeared in the January, 2006 issue of NASA Tech Briefs Magazine.

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