Unsteady rotor aerodynamics at low inflow and its effect on flutter.

A comprehensive flutter analysis has been developed utilizing compressible unsteady aerodynamics for a helicopter rotor at a low inflow condition. The rotor is treated as a nonuniform rotating beam with finite bending and torsional stiffness. The unsteady aerodynamic representation incorporates the effects of spanwise variations in Mach number and reduced frequency by applying available two-dimensional theories in a strip theory fashion. The results are characterized by an oscillation of the apparent aerodynamic damping with decreasing frequency ratio (oscillation frequency/rotational frequency) which suggests a new flutter criteria for rotors. These results are correlated with available experimental data.