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In this paper, the effect of a cubic structural restoring force on the flutter characteristics of a twodimensional airfoil placed in an incompressible flow is investigated. The aeroelastic equations of motion are written as a system of eight first-order ordinary differential equations. Given the initial values of plunge and pitch displacements and their velocities, the system of equations is integrated numerically using a 4 th order Runge-Kutta scheme. Results for soft-and hard-springs are presented for a pitch degree-of-freedom nonlinearity. The study shows the dependence of the divergence flutter boundary on initial conditions for a soft spring. For a hard spring, the nonlinear flutter boundary is independent of initial conditions for the spring constants considered. The flutter speed is identical to that for a linear spring. Divergent flutter is not encountered, but instead limit cycle oscillation occurs for velocities greater than the flutter speed. The behaviour of the airfoil is also analyzed using analytical techniques developed for nonlinear dynamical systems. The Hopf-bifurcation point is determined analytically and the amplitude of the limit cycle oscillation in postHopf-bifurcation for a hard spring is predicted using an asymptotic theory. The frequency of the limit cycle *Principal Research Officer and Head, Experimental Aerodynamics and Aeroelasticity Group. Also adjunct professor,

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