THE deterministic, constant coefficient, two-dimensional, airfoil with structural nonlinearities in attached incompressible ow has been, and still is, a rich source of unexplored nonlinear dynamical behavior. A good review of potential sources of such structural nonlinearities is given by Breitbach. 1 Research on this problem is currently being undertaken at various institutions, 2 5 where a number of interesting phenomena have been predicted. For example, regions of limit cycle oscillations, LCOs, below the main utter boundary have been predicted, and for certain airfoil parameters the existence of chaotic oscillations was suggested and recently confi rmed via calculation of the Lyapunov exponents. 4 However, the system under consideration in these investigations is an idealization of the real problem. One particular idealization is that the airfoil is assumed to be free of any sources of perturbation, which in reality, are always present. From both a practical and theoretical point of view, this raises the question of the presence of random noise.

In the past two decades, physicists have studied the problem of stochastic uctuations in light of the recent interest and understanding of chaotic behavior and related nonlinear dynamics. They have revised the classical, intuitive reasoning that considers randomness in a system as a secondary effect. In fact, it can no longer be argued that stochastic uctuations should be averaged, or filtered, out of signals arising from nonlinear deterministic systems. Furthermore, it is now realized that a nonlinear system excited by parametric random noise can produce bifurcation phenomena and organized behavior that have no analog in their deterministic counterparts. These phenomena have been designated as noise-induced tran-