The Aerostructures Test Wing (ATW) was an apparatus used in a flight experiment during a program of research on aeroelastic instabilities. The ATW experiment was performed to study a specific instability known as flutter. Flutter is a destructive phenomenon caused by adverse coupling of structural dynamics and aerodynamics. The process of determining a flight envelope within which an aircraft will not experience flutter, known as flight flutter testing, is very dangerous and expensive because predictions of the instability are often unreliable.
The ATW was a small-scale airplane wing that comprised an airfoil and boom (see upper part of Figure 1). For flight tests, the ATW was mounted on the F-15B/FTF-II testbed, which is a second-generation flight-test fixture described in "Flight-Test Fixture for Aerodynamic Research" (DRC-95-27), NASA Tech Briefs, Vol. 19, No. 9, September 1995, page 84. The ATW was mounted horizontally on this fixture, and the entire assembly was attached to the undercarriage of the F-15B airplane (see lower part of Figure 1).
The primary objective of the ATW project was to investigate traditional and advanced methodologies for predicting the onset of flutter. In particular, the ATW generated data that were used to evaluate a flutterometer. This particular flutterometer is an on-line computer program that uses μ-method analysis to estimate worst-case flight conditions associated with flutter. This software was described in "A Flutterometer Flight Test Tool" NASA Tech Briefs, Vol. 23, No. 1, January 1999, page 52.
Flutter predictions can be evaluated only by comparison with measured flight conditions at which flutter was encountered. Therefore, the ATW was designed to enable the safe observation of the flutter instability. It was essential to ensure that the destruction of the ATW did not cause any damage to the host F-15B airplane. The ATW was constructed out of fiberglass, composite, and foam. Hence, the entire ATW was lightweight and frangible, so any pieces that struck the F-15 airplane would exert only minimal effects and would cause no damage.
It was also important to ensure that the ATW could generate data that are sufficient for the flutterometer and other means of flutter prediction. This requirement was satisfied by incorporating an excitation and measurement system into the ATW. The excitation was provided by commanding frequency-varying sweeps of energy through a set of piezoelectric patches on the surface of the wing. The measurements were provided by strain gauges throughout the wing and accelerometers in the boom.