Study of A Direct Measuring Skin Friction Gage with Rubber Compounds for Damping

A study was conducted on the measurement of skin friction, the least under-stood component of drag. Skin friction is considered the "last frontier" in drag reduction for supersonic flight, but to understand skin friction, it must be accurately measured. This study utilized the direct measuring technique for skin friction. A small de-vice, termed a skin friction gage, measures the stress on a cantilever beam topped with a movable surface piece as a shear flow passes over the flush surface. The improvement of these devices for various flow fields is ongoing. A problem that arose with many designs was leakage of a gap-filling liquid. The typical direct measuring skin friction gage uses oil in a gap between the cantilever beam and the encasement to dampen vibrations, to create an even flow over the surface, and for temperature compensation. In high speed testing the oil leaks out; therefore, a gage with rubber to fill the gap instead of oil was introduced

This study employed a finite element method model to fully understand the strains involved with the rubber and the skin friction gage. The development of a calibration device, called the Calibration Rig, for the rubber skin friction gages was constructed. The Calibration Rig was successful, but deemed to be more cumbersome than initially expected. This led to the development of a thin rubber sheet to cover the face of the gage instead of rubber filling the entire gap. More finite element method modeling was done to finalize the design of a gage with a rubber sheet. The design consisted of a plastic skin friction gage with an approximately 0.015 in. thick rubber sheet, a 0.0625 in. wide gap between the floating head on the cantilever beam and the encasement to be filled with oil, and semi-conductor strain gages to measure the beam deflection.

Vibration tests were performed to determine if the rubber sheet produced the required damping. These tests were successful, and so much so, that the oil for damping was not necessary. However, supersonic wind tunnel tests at Mach 2.4 which were done at Virginia Polytechnic Institute and State University, initially yielded unfavorable results. The rubber sheet failed during the violent process of starting and unstarting of the tunnel. More study on the adhesive mounting of the rubber sheet to the skin friction gage face is needed.