Drawing of balance for 11-Inch High Speed Tunnel. The 11-Inch HST used compressed air from the Variable-Density Tunnel (when it was being depressurized). It was similar to a venturi-type wind tunnel. Without a model, air flow speeds approached Mach 1. One of the unique features of this tunnel was its automatic-recording balance used to measure the forces. John Stack supplied a general description of the balance in NACA TR No. 463 (pp. 401-403). "The balance must measure the large range of forces resulting from the wide speed range over which tests are made, and it must be automatic recording because the allowable time for observations is short. The balance measures the lift, drag, and pitching moment by multiplying and recording the deflection of steel springs (cantilever beam type) to which the forces are transmitted. The essential parts consist of a cast-iron cradle in which is mounted a yoke to which the model is attached, the linkages necessary to transmit the forces to the steel springs, and a camera for multiplying and recording the deflections of the springs.
