Ground-Vibration Tests

The purpose of structural dynamics experiments on the ground is to validate the fre­quencies and mode shapes of a clean airplane or important airplane configurations. To accomplish this, the airplane is equipped with strain gages and accelerometers at the roots and tips of the wings, of the horizontal and vertical tails, and of the air­plane nose. The airplane is placed on soft supports to mimic the airplanes free-free structural dynamics. Vertical actuators (i. e., shakers) are used at the tips of the wing and horizontal tail; both vertical and side shakers may be used at the tips of the nose and vertical tail. There is a variety of signal analysis methods to identify natural fre­quencies, mode shapes, and structural damping from the measurements. Generally, the actuators have a bandwidth up to 30 Hz, and a sweep of actuation frequencies is first conducted from 0.1 to 30 Hz to identify the symmetric and antisymmetric modes in this range. Classical techniques, such as Fast Fourier Transformation (FFT) and Power Spectral Density (PSD), are used for spectral analysis of unsteady elastic deformation signals to identify the natural frequencies. At this point, we must con­tinue to study details of the dynamic response at the natural bending and torsional frequencies of interest for flutter or other aeroelastic phenomena. For each mode, this entails the following:

1. Induce oscillatory motion of the mode at a certain natural frequency, measure the response, and perform an FFT analysis to identify the resonance frequency and structural damping of that mode.

2. Induce a step-function command from oscillatory motion to zero, measure the decay rate, and infer the structural damping of that mode.

3. Induce an impulsive function, measure the decay rate, and infer structural damp­ing of the mode.

Now we are ready to compare experimentally measured frequencies and mode shapes with detailed finite-element predictions. Using well-established techniques, we tune our finite element model to yield frequencies and mode shapes that fit the ground-vibration test data.