The problem of the stability of the transient motion in the flapping plane of a flexible helicopter rotor blade in forward flight is considered. The formulation of the problem considers two degrees of freedom—rigid flapping and elastic bending. A new method of determining the transient stability of the motion is developed, using the concept of Stability Number. It is shown that the results obtained by inclusion of the elastic degree of freedom agree with experimental results to a much larger extent than was achieved by analyses neglecting blade flexibility. Numerical solutions of the equations of motion using the M.I.T. "Whirlwind'' high-speed computer are given for a number of cases, for both the rigid and the flexible blades.