Low altitude flight involving a descent presents rather specific flying conditions among other helicopter operational conditions. Its most peculiar feature lies in considerable unsteadiness of the aerodynamic characteristics. The paper presents results of analytical and experimental investigations of single and coaxial rotor aerodynamic performance in conditions of descent. The materials pertaining the main rotor flapping motion are presented. It is found that at helicopter descent or deceleration the blade flapping motion becomes unsteady and each blade follows an individual trajectory in its flapping motion. The trajectories can differ considerably and it becomes very important to take this factor into account when finalizing the helicopter configuration. A mathematical model was developed in order to study the main rotor aerodynamic characteristics. The blade aerodynamic performance is modeled based on the nonlinear vortex theory of a lifting surface in unsteady conditions. The main elements of the mathematical model are presented. The modeling allowed to obtain aerodynamic characteristic and flapping motion curves and to establish their relationships with unsteady vortex wake behind the main rotor system.
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