Effects of Blade Bending on Aerodynamic Control of Fluctuating Loads on Teetered HAWT Rotors

Abstract

Active aerodynamic control, in the form of closed-loop actuation of blade-tip ailerons or all-moveable blades, is investigated as a means of increasing the structural fatigue life of HAWT rotors. The rotor considered is upwind and teetered, with two blades of diameter 29.2 m., fiberglass construction and other properties representative of modern light-weight construction. The paper begins with a review of prior work which studied the problem for an essentially rigid structure. For that and the present research, two loading conditions were invoked: exposure to a Rayleigh distribution of operating winds with vertical shear and a 15 percent superimposed spectrum of turbulence; and occasional exposure to 62 m/s hurricanes. Accounted for herein is the effect of flatwise bending flexibility on the loads spectra of root flatwise bending moment, thrust, and torque (both open loop and closed loop). Using Miner’s rule, the moments are converted to fatigue lives. With aerodynamic control, RMS flatwise moments for the flexible blade in turbulence are found to be less than 1/2 of those without control. At a fixed blade weight of 540 kg when hurricane loads are added, the aileron-controlled blade is “designed” by that limit-load condition. In contrast, the all-moveable blade can be feather controlled in the high wind so that its life is dominated by turbulent loads. Simplified fatigue analysis permits weight reductions to be estimated which yield controlled blades capable of 30 years’ operation with a safety factor of 11. The resulting weights are about 400 kg for the aileron-controlled blade, and 230 kg for the all-moveable blade. However, such light-weight rotors require attention to other design considerations, such as start-stop cycles. Apart from limit loads, the methods of analysis in this paper are linearized (locally for aerodynamic loads). It follows that the results are more likely to be meaningful in terms of comparative, rather than absolute, values of fatigue life and weight.