High-altitude limit cycle flutter of transonic wings

In flutter testing, it is often implicitly assumed that the most critical case at any given Mach number will occur at sea level conditions. This is a reasonable expectation, as the highest dynamic pressure at a fixed Mach number will be encountered at sea level, where the air density is highest. In the present paper, a counterexample involving a generic swept wing representative of transport aircraft is presented in which transonic limit cycle flutter is predicted to occur at altitude rather than at sea level. The calculations are based on a time-accurate Eulerian-Lagrangian finite element scheme that fully accounts for both structural and aerodynamic nonlinearities arising from large deflections and shock motion. Flutter onset is around Mach 0.84 and the limit cycle amplitudes grow until Mach 0.95 is reached, at which point they start decreasing, vanishing abruptly around Mach 0.97. The limit cycle flutter persists down to very low air densities, corresponding to altitudes above 75,000 ft, and the maximum limit-cycle-oscillation flutter amplitude does not occur at sea level, but at altitude. If sufficient structural damping is added, the wing will be flutter-free at lower altitudes but not at higher (cruise) altitudes. At Mach 0.95, the wing is stable below about 15,000 ft, even in the absence of structural damping, but experiences strong limit cycle flutter at higher altitudes.