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 singlefinned high-agility aircraft. The study uses a detailed wind tunnel model of a delta-canard configuration fitted with a center-line airbrake located at the rear part of the canopy. The results include turbulent flowfield data based on advanced hot-wire anemometry as well as unsteady fin surface pressures measured with integrated sensors. The tests are conducted in the low-speed region at Reynolds number Re_lμ ≃ 1 × 10⁶ for an angle-of-attack range of α = 0° - 30° and two airbrake deflection angles of η_AB = 45° and 60°. The airbrake near field wake shows specific turbulent flow structures associated with the flow separation at the airbrake side edges and upper edge. Vortices are formed which are orientated vertically or axially depending on the angle of attack. The corresponding region of high turbulence intensities impinges on the fin while the turbulence levels in the center area increase with angle of attack. This increase is caused by the inboard movement and expansion of the wing and canard leading-edge vortices compressing the airbrake wake in the fin area. At low angles of attack, the airbrake wake exhibits concentrations of turbulent kinetic energy at characteristic frequencies attributed to the airbrake vortex shedding. At high angles of attack, the fin is also subject to quasiperiodic aerodynamic loads evoked by induction of the breakdown flow of the wing vortices.