Force Measurement Technique Using Laser Interference Patterns (aka “Optical Balance”) in Clean Air Hypersonic Tunnel

The measurement of aerodynamic forces on models in hypersonic tunnel facilities has been restricted in the past due to the time it takes for the test model to reach a state of force equilibrium with its support mechanisms. Conventional force balances rely on damping mechanisms and/or filters to reduce the effects of vibrations associated with the sudden application of forces when the flow starts or when the model is injected into the flow. In impulse facilities, particularly high-enthalpy facilities, the duration of the test flow may be insufficient for such vibrations to be damped before the end of the test. The duration of the flow in high enthalpy systems also prohibits the test article reaching thermal equilibrium. Matching the enthalpy to the hypersonic conditions via an innovative method to make clean, synthetic air has been achieved, which also can be sustained for long run times. And additional enthalpy from another innovation into a heavy oxidizer for combustion heating can be harnessed for creating an aggressive thermal environment to Mach 8 and higher. These attributes are challenging to conventional force and moment force measurement balances. Yet having an accurate force balance capable of operating at high temperatures and resistant to tunnel vibrations can yield important information. This paper covers this new test capability, called the Hypersonic Synthetic Environment Test Tunnel (HySETT), which is demonstrating, clean hypersonic air with long run times and matched Mach enthalpy conditions, and is requiring a fresh look at force measurement balances.