Effects of blade bending on aerodynamic control of fluctuating loads on teetered HAWT rotors
- RANN, Inc., Palo Alto, CA (United States)
Active aerodynamic control, in the form of closed-loop actuation of blade-tip ailerons or all-movable blades, is investigated as a means of increasing the structural fatigue life of HAWT rotors. The rotor considered is 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 a 15% superimposed spectrum of turbulence; and occasional exposure to 120-kt hurricanes. Accounted for herein is the effect of flatwise bending flexibility on the resulting spectra, open-loop vs. closed-loop, of root flatwise bending moment, thrust and torque. By means of Miner`s rule, the moments are converted to fatigue lives. With aerodynamic control, RMS 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-movable blade can be feathered in the high wind so that its life is determined by turbulent loads. Simplified fatigue analysis permits weight reductions to be estimated which yield controlled blades capable of thirty years` operation with the safety factor of 11. The resulting weights are about 400 kg for aileron control and 230 kg for the all-movable blade. However, such light-weight rotors require attention to other design considerations, such as start-stop cycles.
- OSTI ID:
- 372113
- Report Number(s):
- CONF-960154--; ISBN 0-9648731-8-4
- Country of Publication:
- United States
- Language:
- English
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