Description: One focus of NASA 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, 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 through 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 a prototype drag balance based on this sensor. The balance design is structurally similar to a traditional balance, employing four pillars S3F 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 is monitored and these displacements are converted to forces and moments. Preliminary results on the prototype balance indicate that forces smaller than a mili-Newton may be resolved, and there is no measureable coupling between the drag force and the normal or side forces. Development of a force balance technology that can be integrated into a model and measure small changes in drag would be of significant value for the development of energy efficient flight.

Benefits: The objective of the Phase I and Phase II program is to develop an experimental tool that can be used to measure aerodynamic forces in a variety of bench-top settings, on model components in wind tunnels, and eventually into flight testing. The development of this balance is viewed as an enabling technology for the development of drag reduction technologies, an area of active research at NASA. Specifically, one current focus of NASA research is the Environmentally Responsible Aviation (ERA) program and evaluation of drag reduction technologies is a key component of this program. A successful program will enable the design, construction, and deployment of custom balances that can be used in for this research. Reduction of noise using acoustic liners is a goal of the Subsonic Fixed Wing project at NASA. Evaluation of drag induced by these liners would benefit from the proposed balance. Finally, ongoing research between NASA and Boeing on the ecoDemonstrator seeks to evaluate drag reduction panels in flight. A balance design that could be deployed for flight testing on small samples of such a material would facilitate early stage evaluation of these drag reduction technologies.

The objective of the Phase I and Phase II program is to refine the elastomeric balance design, characterize the performance of the balance, and evolve the model mounted balance concept into a productive research tool. The result should be an experimental tool that can be used to evaluate drag reduction technologies in a variety of bench-top settings, on model components in wind tunnels, and eventually into flight testing. This device should be a valuable tool for the evaluation of a variety of drag reduction technologies. It is noted that this balance design may have applications outside of the aerodynamics community. Balances for hydrodynamics research into issues such as drag reduction of ship models, sedimentation and erosion around bridges, and biomedical research on insect locomotion have many of the same challenges as aerodynamics research. A balance design that allows high common mode rejection between channels and can be easily tuned for a particular application would be of value in those applications. ISSI is already working to develop a skin friction sensor for biomedical research, and integration of this balance design into that product is underway. We are also in discussion with several small wind tunnel manufactures as to the marketability of a six component balance for small academic wind tunnels.