ROTOR BLADE FLAP-LAG STABILITY AND RESPONSE IN FORWARD FLIGHT IN TURBULENT FLOWS

The effect of random air turbulence on the coupled flap-lag motion of a helicopter rotor blade in forward flight is investigated. By assuming white noise turbulence and applying a special case of the stochastic averaging procedure, equations are developed which describe the stochastic first and second moments of the perturbations in flap and lead-lag angles due to turbulence.;The stability of multibladed rotors in the presence of turbulence is investigated using the example of a three-bladed rotor. The constant coefficient approximation using multiblade coordinates is shown to be accurate for moderate advance ratio in the case of a trimmed rotor. For an untrimmed rotor the approximation is less accurate and provides very conservative stability boundaries.;Numerical results for stability in forward flight illustrate that turbulence exhibits a stabilizing effect, compared to the deterministic (no turbulence) case. Results for the steady-state responses of the perturbations in forward flight indicate that the average responses of the perturbations in forward flight indicate that the average response of the blade is not significantly affected by turbulence. In contrast, the random fluctuations away from the average response exhibit significantly large values due to the effects of turbulence. The effects of various blade parameters on the steady-state response are also investigated.