11-Inch High-Speed Tunnel. Public domain image of NASA aircraft.

11-Inch High Speed Tunnel (11-Inch HST) balance. 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." The silver horizontal bar in the center of the photograph is an airfoil clamped in the yoke of the balance. As Stack further notes (p. 404): "The model is mounted in the jaws at the ends of the yoke. It is located by means of dowels so that the line joining the intersections of the horizontal and vertical linkages coincides with the quarter-chord axis of the model.
NASA Identifier: L4780