Wind tunnel tests of aircraft aerodynamic characteristics at overcritical angles of attack

During air shows or competition aerobatics pilots perform aerobatic flying. Most aerobatic figures are combination of a few basic manoeuvres like loops, rolls, spins, and hammerheads. During such manoeuvres, aerobatic aircrafts often fly in the range of overcritical angles of attack. The flight in the range of higher than critical angles of attack is accompanied by a flow separation. This phenomenon is connected with significant changes of the aircrafts aerodynamic characteristics, as well as may be accompanied by strong vibrations. For these reasons, the knowledge of the aircraft overcritical aerodynamic characteristics is required for its proper design. In wind tunnels models of aircraft are usually tested in the range up to α = 20°-25°, while aircraft performing aerobatic flying usually achieve considerably higher angles of attack. To obtain the aircraft aerodynamic characteristics in the whole used angles of attack range, a special wind tunnel stand was designed and manufactured in the Institute of Aviation enabling the wind tunnel tests in range, α = 0°-360°. The paper presents the wind tunnel tests results of aerobatic aircraft “Harnaś 3” model, for a set of chosen model configurations. The studies included both balance measurements of the model basic aerodynamic characteristic, as well as flow visualization tests. Investigation were carried out for the range of angles of attack α = -90°-90° and the range of slideslipe angles β = -90°-90°. Wind tunnel tests are very rarely carried out in such a wide angles of attack range. The experimental tests were performed in the Institute of Aviation‘s low speed wind tunnel T-1 (1.5 meter diameter test section). For the tests, the model of aerobatic aircraft (manufactured in a 1:10 scale) was situated both vertically and horizontally in the wind tunnel test section. Wind tunnel tests were performed at Mach number M ≈ 0.1 (V ≅ 34 m/s), which corresponds to the Reynolds number Re = 0.22*106.