Two floating head skin friction gages have been designed and tested to directly
measure the skin friction coefficient for the undefined flow in a SCRAM-jet engine.
The skin friction gage designs contain a floating head that is supported by ball
bearings which allow it to move in any horizontal direction and restrain it from any
vertical motion. The shearing force caused by the supersonic flow deflects the
floating head parallel to the flow direction. Strain gages mounted across a small gap
between the bottom of the floating element and the bottom clamp provide the
restoring force on the floating head. These strain gages also measure the floating
head deflection caused by the flow shearing force.
The steel model design was built and tested to determine the feasibility of the
design concept. The results from the supersonic wind tunnel at Mach 2.4 were reasonable but the steel model was unable to correctly respond to the short-duration
flow of the shock tunnel. A skin friction gage made of a machinable ceramic called
Macor was then designed to increase the resonant natural frequency (3600 Hz) and
the insulating properties of the gage. Consequently, the Macor model floating head
design is capable of measuring the skin friction coefficient in short duration, high
enthalpy supersonic flows as well as long duration supersonic flows. The Macor
model design yielded skin friction coefficient values near the expected value of 0.0014
when placed in a supersonic tunnel at both Mach 2.4 and Mach 3.0 and in a Mach
3.0 shock tunnel.
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