Innovators at NASA’s Armstrong Flight Research Center have developed a method of using aircraft lift airbags to support test aircraft during wing strain gage load calibration. In calibration testing, a set of known loads is applied to the wing, and the loads and strain gage responses are then recorded. Multigage shear, bending, and torque load equations can then be derived from linear regression analysis of the data. The method aims to address challenging strain measurement conditions present in certain aircraft with particular landing gear geometries. For example, the Gulfstream III test aircraft has main landing gear attached to the inboard wing at the rear spar. In aircraft with this type of landing gear geometry, the main gear load reaction into the wing changes as the applied test load changes, corrupting the test data and compromising the accuracy of the load equations.
Alternative solutions to this challenge involve isolating the main gear during testing, such as through use of a crane with slings around the fuselage or even via a custom-built fuselage cradle. These solutions, however, require the time and expense of engineering or procuring new equipment for fundamental testing.
By contrast, Armstrong’s method makes use of aircraft lift airbags, which are typically used for recovery of aircraft with collapsed landing gear. In testing, researchers determined that this solution was not only more cost-effective but also safer than using alternative equipment. The airbags were used to support the center of gravity of the aircraft such that the main gear was kept a few inches from touching the floor. Data derived from the strain gage tests reflected the strain gage response to the intended test loads, yielding quality load equations at relatively minimal cost. Armstrong’s method could be used in other wing strain gage load calibration tests for any number of aircraft in which the main landing gear is attached to the wings.