Application of a technique for reducing supersonic starting loads on internal wind tunnel balances

Abstract

Higher than anticipated supersonic starting loads in the VTI T-38 wind tunnel were a consequence of higher than designed minimum start pressures. Initially, high starting loads limited the usefulness of the facility at high supersonic Mach numbers. Over time, it was established that several procedures and techniques had to be used cooperatively to diminish the problem. One of the techniques is the establishment of additional points of contact of the model and the support sting for the duration of the starting loads, so that a part of the aerodynamic loads is transferred to the sting through the added path, relieving the sensitive force balance. Some applications of this technique require dedicated mechanisms to operate, which was impractical for the VTI T-38 wind tunnel. To the contrary, the implementation of the idea in the VTI T-38 wind tunnel relies on using suitable inserts to make the clearance between the model and the sting such that model-sting contact is forced to occur during the start and stop phenomena, but not during the measurement phase of the test. For typical model configurations, transient starting and stopping loads can be reduced by more than 50%.