An internal stress wave force balance for use in a shock tunnel

A new short duration internal force balance for the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Center (DLR) to measure lift, pitching moment and drag has been designed, calibrated and tested. The balance is able to measure forces of short duration (milliseconds) on instrumented models from angles of attack of -40 to 20°. Experiments were performed on a 303 mm long, 10° half angle blunt cone at angles of attack from 0 to -20°. The tests were conducted at two different test conditions with a constant Mach number of 7.8 and total enthalpies of 3.0 and 3.5 MJ/kg. At 0° angle of attack and an enthalpy of 3.0 MJ/kg, the measured axial coefficient was recovered to within 6% when compared to computational fluid dynamic (CFD) simulations. A relatively constant axial (drag) coefficient of 0.10 was obtained for the conical model over dynamic pressures ranging from 30 to 70 kPa. As the angle of attack was varied from 0 to -20°, the axial coefficient increased to 0.24, the normal coefficient decreased to -0.57 and the pitching moment coefficient (about the model tip) increased to 0.38. At -10°, the axial and normal coefficients were within 6% and 9% respectively of CFD predictions while the center of pressure (based on chord length) was within 2%. The accuracy of the force balance is estimated at approximately ±5% for the axial component and ±4% for the normal and pitching moment components.