An Exploratory Study of Motion and Loads on Large Flap-Hinged Rotor Blades

Abstract

Large downwind rotors having flap-hinged blades are the subject of an exploratory study of blade motions and moments plus rotor loads in a variety of non-uniform winds. The focus is on generic two blade rotors in variable speed operation with rated powers of 500 kW, 1.5 MW, and 5.0 MW. The potential for reducing rotor and turbine loads by eliminating blade root out-of-plane moments with the flap hinge during rotor operation may be substantial. With small flap angles this potential can be achieved without loss of energy capture which is consistent with the possibility of reducing COE. The offshore siting option with the potential for higher rotational speeds without creating unacceptable noise problems may figure importantly in achieving this possibility. Increasing rotor speed inertially stiffens free flapping blades with the end result that mean and cyclic fluctuations in flap angles are reduced. Decreasing blade weight has the opposite effect on mean angle and hence energy capture. Reductions in cyclic fluctuations of flap angle reduce blade out-of-phase asymmetries and cyclic fluctuations of rotor hub loads transmitted through the main shaft to the drive train, yaw drive, and tower. Reductions in cyclic fluctuations also reduce the risk of tower strikes. Consistent with this goal is providing a reliable, fail safe flap angle control system which is automatically activated in all turbine startup and shut down modes. The additional safety feature of closed loop control of blade pitch angle to maintain flap angles within a safe tower clearance envelope may be advantageous in sustained operation in abnormally non-uniform winds. Resort to higher rotor speeds invokes the requirement for lower solidity rotors to maintain high energy capture, with the result that the blades will be more slender and prone to dynamic instabilities.