One focus of NASA and Air Force aerodynamics research is enabling energy efficient flight through drag reduction technologies. A variety of drag reduction techniques have shown promise and are under investigation, including both active flow control and surface microstructure concepts. Experimental verification of the performance of any drag reduction technique, however, can be challenging. Drag forces are generally significantly smaller than lift and side forces. Furthermore, many drag reduction techniques are operating on components of the model, and therefore, a model mounted drag balance is required to evaluate the performance of the drag reduction technology. Further complicating the measurement is the fact that active flow control requires that high pressure air or electrical power be passed across the model mounted balance without impacting the measurement Over the past 10 years, ISSI has developed an optical sensor for measurements of skin friction known as Surface Stress Sensitive Film (S3F). S3F has demonstrate good sensitivity to skin friction while maintaining very high common mode rejection between the pressure and skin friction forces. ISSI has recently designed and built several prototype drag balances based on this sensor. The fundamental balance design is structurally similar to a traditional balance, employing four S3F pillars as the active elements. Rather than monitoring strain in the pillars, as is done with a traditional balance, the vertical and horizontal deformation of the pillars are monitored and these displacements are converted to forces. Preliminary results indicate that forces smaller than a mili-Newton may be resolved, and there is high common mode rejection between the drag force and the normal or side forces. Finally, the balance has been evaluated for resolving dynamic forces. Unsteady loads resulting from vortex shedding from a cylinder in cross-flow at frequencies of up to 940 Hz have been resolved in both the drag and side force response.
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