Equations for large amplitude coupled flap-lag motion of hingeless elastic helicopter blades are reduced to a system of coupled nonlinear ordinary differential equations with periodic coefficients, using Galerkin's method. The resulting system of nonlinear equations are linearized about a suitable equilibrium position. With using a system of linearized coupled flap-lag equations, aeroelastic stability boundaries in hover can be shown.
Next, the effect of forward flight is obtained under the requirements of trimmed flight at fixed values of the thrust coefficient. And the multivariable Floquet-Liapunov theory has been used to determine the stability of solution to systems of linearized ordinary differential equations having periodic coefficients in forward flight.