Investigation of Aeroelastically Adaptive Rotor Systems

Presented are the basic principles which are being followed in an investigation of aeroelastically adaptive rotor systems, and a review of related wind tunnel test research. In these rotor systems, dynamic blade twist is enhanced and managed to increase helicopter productivity by extending rotor operating limits. Rotor operating limits, whatever their source – stall and reverse flow which limit capability directly, or the associated fatigue loads, vibration, noise, and power requirements which normally restrict operation by imposing increasing "costs" essentially result from unfavorable airload distributions, both spanwise and azimuthal. Dynamic blade twist is a powerful means of improving these distributions. Thus, aeroelastically adaptive rotors are being designed to have favorable dynamic twist which adapts to the operating condition. Candidate design features are also being examined for their value in enabling the rotor to "adapt" at high advance ratio to other desirable design changes: increased static twist, which improves hover performance but normally increases fatigue loads in forward flight; and 2/rev root pitch, which is expected to extend propulsive force limits but also increase blade fatigue loads. In testing to date, dynamic twist has been induced in blades of reduced torsional sti ffness by adj usting the aerodynamic pitching moment coefficient, CMo, and by sweepback of the outer 35 percent of the blade plan-form. Two per rev root pitch control of the torsionally softened blades has also been examined. Test results show that flap bending at high advance ratios can be reduced significantly by proper choice of CMo, so that static twist can certainly be increased to improve hover performance. Significant amounts of 2/rev dynamic twist have been induced in varying azimuthal phases, pointing to combinations of design features which should unload the retreating blade at high advance ratio, with potential gains in rotor limits of all types. Finally, the operation of planform sweep as a physical feedbacktype bending load attenuator has been demonstrated, showing the value of this feature in attaining rotor adaptability.