Calculated and measured values of helicopter rotor flapping angles in forward flight are compared for a model rotor in a wind tunnel and an autogiro in gliding flight. The lateral flapping angles can be accurately predicted when a calculation of the nonuniform wake-induced velocity is used. At low advance ratios, it is also necessary to use a free wake geometry calculation. For the cases considered, the tip vortices in the rotor wake remain very close to the tip-path plane, so the calculated values of the flapping motion are sensitive to the fine details of the wake structure, specifically the viscous core radius of the tip vortices. Johnson, W. Ames Research Center NASA-TM-81213, AVRADCOM-TR-80-A-11, A-8239 RTOP 505-42-21

Theoretically derived charts showing steady-state and first- and second-harmonic flapping coefficients are presented for helicopter rotors having hinged rectangular blades with a linear twist of 0, -8, and -16 degrees. The charts, showing the rotor flapping coefficients for combinations of inflow ratio and blade pitch angle, are presented for tip-speed ratios ranging from 0.05 to 0.50.