Performance of small skin friction balances at supersonic speeds

Abstract

Some wind-tunnel tests at supersonic speeds with small commercially available skin friction meters are described. The measurements include different meters of the same type exposed to the same shear stress and comparisons with skin friction coefficients derived from local velocity profiles. The measurements suggest that, in zero pressure gradients, accuracies as high as 1% may be obtained if great care is taken. Frequent repeat tests of a datum skin friction measurement, check static calibrations, and balance inspections are recommended. Accuracies as low as 5-1% are obtained from some tests. These balances are always liable to severe damage from the starting or stopping pressure pulse in supersonic wind tunnels. To alleviate this problem, vent holes have been incorporated in the latest balances, but these introduce significant errors. Some research to optimize this modified design is recommended.