A SOLUTION for the flapping stability of a helicopter blade, including the influence of a periodic coefficients due to the rotor forward velocity, is obtained using the techniques of perturbation theory. Contents The dynamics of a helicopter rotor in forward flight are described by a set of linear, ordinary differential equations with periodic coefficients. Sophisticated and wellknown techniques are available for the analysis of constant coefficient equations, including stability determination and control system design. Even the determination of the stability of periodic coefficient equations requires, however, considerable numerical calculation (specifically, for one frequently used technique, numerical integration of the equations of motion is required), and other topics are considerably more difficult than the corresponding constant coefficient system analysis. An alternate mathematical approach to periodic coefficient equations is the use of small perturbation techniques. The base paper considers the application of these techniques to the calculation of the flapping stability of a single blade of a helicopter rotor. The dynamics of the flapping motion of a single blade of a helicopter rotor are governed by the following equation.