Modal Investigation of Lightweight Aircraft Structures Using Digital Techniques

Digital impact response test techniques were used to measure the dynamic properties of lightweight aircraft structures to include natural frequencies, mode shapes and modal damping. Two different types of structures were tested, honey-comb and skin-stiffened panels. The digital impact response method used consisted of applying a transient force pulse to the structure, measuring the structures response at various points, digitizing, calculating the transfer functions using fast Fourier transforms, and determining the dynamic properties from these data. A second method was used on these same structures for direct comparison purposes. This method was an analog technique using sine sweep tests and accelerometer mapping. The comparison indicated close agreement in the results. The use of the digital technique has resulted in a considerable savings in the manhours required to obtain the dynamic properties of the structures.