The in-situ load system (ILS) provides the ground-testing community with a comprehensive tool that permits system-level calibration and validation of force measurement systems in a test-like environment. It was developed to improve testing accuracy, repeatability, time in tunnel, and many aspects of the calibration process. The key innovations are that ILS enables a system calibration (rather than independent subsystem and component calibrations) by using the one-force-vector calibration approach and a statistically defensible estimate total force measurement uncertainty. ILS may be applied in any wind tunnel facility, private or government.
During wind-tunnel testing, a balance is used to obtain high-precision measurements of the aerodynamic loads on an aircraft model. Most balance calibrations are conducted in a laboratory environment where most of the nuisance variables — such as temperature, electrical noise, and vibrations — can be controlled. When the instrumentation is transferred to the test environment, the nuisance variables change. To ensure that the calibration of the balance is still valid for the change in environment, validation checks are conducted in the wind tunnel. Currently, multi-component test environment validation checks are mechanically complex, introduce uncertainties in the applied loads, and are time-consuming.
This technology is designed to address the challenge of evaluating wind-tunnel model system performance during test preparation activities. ILS is based on the force-vector concept where a single deadweight load is used to apply up to six loads simultaneously through changing the orientation of the wind tunnel model system relative to gravity. As the orientation of the force balance changes relative to gravity, the applied load vector that is produced imparts varying load combinations and magnitudes. During typical force-balance checkout, multiple-component loads are not applied even though researchers and wind tunnel customers expect these types of complex loadings during testing. In addition, axial force (aerodynamic drag), which is the aerodynamic component of highest interest, is rarely checked during the checkout process.
ILS permits a more robust evaluation of the laboratory calibration during checkout as opposed to current approaches that are used. Furthermore, becase the ILS uses a single load and the design is mechanically simpler than the current checkout hardware, many sources of systematic error are removed from the process.