FORCE MEASUREMENTS IN WIND TUNNELS

Experimental aerodynamic information of flying vehicles is obtained through force measurements on captive models in wind tunnels. The most common type of wind tunnel balance today is the strain-gauge balance, which in principle is a spring-balance with electric out-puts. The paper describes the principles of the resistance strain-gauge, and how it can be applied in balances to measure static and dynamic forces. It also discusses proper bridge-wiring to suppress temperature influences and secondary force components. Strain-gauge balances are analog transducers which must be calibrated against known forces and roments.Multi component balances are influened by interac-tions which means that an output of a balance is a function of both a primary and of secondary components. This complicates the process of calibration and a special mathematical model of the balance is used to evaluate calibrations. This model also permits evaluation of aerodynamic forces and moments from wind-tunnel measurements with strain-gauge balances.