Researchers at NASA’s Armstrong Flight Research Center have been interested in using the Dynamic Inertia Measurement (DIM) method on full-scale aerospace test vehicles, given its advantages over traditional methods for determining the mass properties of such vehicles. Developed at the University of Cincinnati, the DIM method uses a ground vibration test setup to determine mass properties using data from frequency-response functions. The method has been successfully tested on a number of small-scale test articles — including automobile brake rotors, steel blocks, and custom fixtures — but until now, has had limited success being tested in larger applications. Armstrong’s recent efforts, in conjunction with ground vibration tests, represent a step forward in applying the DIM method successfully to full-scale aerospace vehicles.
Mass properties of an aerospace vehicle are necessary to obtain in order to understand the flight dynamics of the vehicle. These properties include mass, center of gravity, moments of inertia, and products of inertia. While mass and center of gravity can typically be determined with a weight-and-balance procedure, determining moments/products of inertia requires dynamic testing. Traditional methods include spin tables or pendulum-based swing tests. How ever, these methods are time-consuming and expensive for large aerospace vehicles. They also require significant amounts of labor, materials, and time, putting project timetables and budgets at risk.
Because of the testing needs of full-scale aerospace vehicles and the shortcomings of traditional methods, the DIM method has become an attractive option to researchers. Armstrong researchers conducted mass properties testing on an iron bird test article composed of two I-beams that were comparable in mass and geometry to a fighter jet. They conducted traditional swing testing in conjunction with the DIM testing to compare the level of effort needed for each testing type as well as to validate the quality of data obtained by the DIM method. The DIM testing showed favorable results for the center of gravity and moments of inertia; products of inertia showed disagreements with analytical predictions (compared to the larger moment of inertia values, the smaller product of inertia values are more sensitive to uncertainties using the DIM test set-up and algorithm).