EFFECTS OF BLADE BENDING ON AERODYNAMIC CONTROL OF FLUCTUATING LOADS ON TEETERED HAWT ROTORS

Active aerodynamic control, in the form of closed-loop actuation of bl ade-tip ailerons or all-moveable blades, is investigated as a means of increasing the structural fatigue life of HAWT rotors. The rotor cons idered is upwind and teetered, with two blades of diameter 29.2 m., fi berglass construction and other properties representative of modern li ght-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: exposu re to a Rayleigh distribution of operating winds with vertical shear a nd a 15 percent superimposed spectrum of turbulence; and occasional ex posure to 62 m/s hurricanes. Accounted for herein is the effect of fla twise bending flexibility on the loads spectra of root flatwise bendin g moment, thrust, and torque (both open loop and closed loop). Using M iner's rule, the moments are converted to fatigue lives. With aerodyna mic 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 bl ade weight of 540 kg when hurricane loads are added the aileron-contro lled blade is ''designed'' by that limit-load condition. In contrast, the all-moveable blade can be feather controlled in the high wind so t hat its life is dominated by turbulent loads. Simplified fatigue analy sis permits weight reductions to be estimated which yield controlled b lades capable of 30 years' operation with a safety factor of II. The r esulting weights are about 400 kg for the aileron-controlled blade, an d 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 lends, the methods of analysis in this paper are linearized (locally for aerodynamic loads). It follows that the re sults are more likely to be meaningful in terms of comparative, rather than absolute, values of fatigue life and weight.