Wind Tunnel Flutter Experiments

The design of wind tunnel models that accurately represent the flutter situations of a real airplane is a complex task—quite possibly an art! The main challenge is to compose a small-scale version of the aircraft with tuned structural dynamics and a sufficiently detailed geometry that correctly reflects the aircraft’s static and unsteady aerodynamic behavior. For these tests, the model is supported by soft cables and equipped with strain gages and accelerometers at the roots and tips of wings, at the tips of horizontal and vertical tails, and on the nose. Flow turbulence is used to excite the model aeroelastic modes. FFT and PSD analysis of the various measured model deformations are used to estimate the flutter speed, frequency and severity.

The wind tunnel tests provide essential insight into the possible modes of flutter of an airplane. The tests help verify computed results as well as identify unknown aeroelastic phenomena related to the airplane configuration. However, it should be recognized that unsteady flow phenomena are strongly governed by scale, so that reduced frequency, flow Reynolds number, separation between vortices, and interac­tions between shockwaves and boundary layers may not be correctly represented by small-scale models. In such experiments, results may lead to an inaccurate prediction of flutter occurrence in the full-scale airplane. In addition, testing of small-scale mod­els in the wind tunnel provides benchmark cases for improving the computational models and tuning the unsteady aerodynamics analysis codes.