Experimental study on a novel method for vertical tail buffet suppression

The twin tails of the modern high performance fighter aircrafts with delta wing/twin vertical tails configuration generally experience significant dynamic load due to aerodynamic buffet caused by highly unsteady vortex breakdown flow. This paper proposes a new type of buffet alleviation method by solid static or oscillating bulges attached on the forebody of a delta wing model, and conducts wind tunnel experiment for the model which consists of a sharp-edged, 70-degree leading edge sweep delta wing, and twin 31° leading edge sweep vertical tails. The model has been tested at 10 m/s and 20 m/s of free stream velocity, and at angle of attack ranging from 20° to 50° with an interval of 2° in the low-speed wind tunnel at Northwestern Polytechnical University. According to the self-induced theory proposed by experts from University of Washington, the bulges attached on the forebody of a delta wing would twist and kink the vortex core, change the vortex trajectory after the bulge, delay the vortex breakdown position, and decrease the fluctuating level of the flow around the twin vertical tails. Thus the goal of buffeting suppression is achieved. This study aims at investigating the buffet alleviation effect of the static or vibrating bulges attached on the forebody surface of the model. Two pairs of half bridged strain gauges are employed for the measurement of unsteady root strain on the vertical tails, with one pair on each side. The experimental results show that both static and vibrating bulges behave positively as a novel tool to alleviate tail buffet, and the alleviation effect correlates with the vibrating frequency. Besides, the bulge on one side can only alleviate the buffeting for the tail of the same side, and it has no obvious alleviation effect for the tail on the opposite side. Results of spectral analysis prove that the bulges used in this experiment could alleviate tail buffeting, but they do not change the dominant frequency of the oscillation of the tails in the vortex breakdown flow.