High temperature skin friction measurement

Skin friction in the hypersonic propulsion facility of the Langley Research Center is described. The sensor configuration utilized an existing balance, modified to provide thermal isolation and an increased standoff distance. Test run times were on the order of 20 s, with ambient air then used to cool the test section and the balance. Under these conditions, the modified balance performed satisfactorily. A preliminary test in which the balance was subjected to the acoustic and structural vibration environment was completed, and the balance performed satisfactorily. The balance is an inertially balanced, closed-loop servo system where the current to a moving coil motor needed to restore or null the output from the position sensor is a measure of the force or skin friction tending to displace the moving element. The accuracy of the sensor is directly affected by the position sensor in the feedback loop which, in the present device, is a linear variable differential transformer which has proven to be influenced by temperature gradients.<>