Airbrake-induced fin-buffet loads on fighter aircraft

The investigations presented focus on the influence of an airbrake wake on the buffet characteristics of the vertical tail (fin) of a single-fumed high-agility aircraft The study uses a detailed wind-tunnel model of a delta-canard configuration fitted with a center-line air brake located upstream of the fin. The results include turbulent flowfield data based on hot-wire anemometry as well as unsteady fin pressures measured with surface-integrated sensors. The tests are conducted in the low-speed region at a Reynolds number of 1 x 10' and at angles of attack of 0 to 30 deg considering airbrake deflections of 45 and 60 deg. Flow separation at the airbrake side edges and upper edge results in a highly turbulent airbrake wake. The corresponding region of high turbulence intensities impinges on the fin, with the turbulence levels increasing with angle of attack. This increase is caused by the inboard movement and expansion of the wing and canard vortices compressing the airbrake wake in the fin region. At low angles of attack, the airbrake wake exhibits distinct concentrations of turbulent kinetic energy at characteristic frequencies attributed to airbrake vortex shedding. Increasing the angle of attack, the airbrake side-edge vortices roll up to form leading-edge vortices associated with a periodic breakdown wake. At high angles of attack, the fin Is also subject to quasi-periodic loads evoked by induction of the burst wing leading-edge vortices.