Fin buffet characteristics of advanced trainer aircraft

The present study focuses on the characteristics of the vortical flow field emanating from a wing-strake configuration and associated buffet loads experienced on the vertical tail (fin). The wind tunnel configuration studied is a single fin high-agility trainer aircraft with a 45 deg swept trapezoidal wing, wing strakes at air intakes and horizontal tail plane. The results presented include turbulent flow fields based on advanced hot-wire anemometry as well as unsteady fin pressures measured with surface embedded miniature sensors. The tests are conducted in the low-speed region at a Reynolds number of Relμ ≈ 0.7 x 106 for angles of attack of α = 0 deg to α = 30 deg. The fin flow field is dominated by large-scale vortices shed already at low angles of attack at the leading-edges of wing and strakes. Bursting of the leading-edge vortices over the wing starts already at α ≈ 5 deg. The associated regions of highly turbulent flow increase in their spatial extension and intensities with increasing angle of attack. A significant rise in the fluctuations of flow velocities and fin surface pressures above a certain angle of attack is a characteristic feature of the fin buffet flow. The fluctuations show narrow-band spectral energy distributions related to the helical mode instability of the breakdown flow. The induced un-steady loads may strongly excite the fin in its structural eigenmodes. The results demonstrate that the corresponding unsteady aerodynamic loads must be carefully addressed in the design process.