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Determination of Cq for low values o Determination of Cq for low values of λ KD 97

In report KD 35 (ref. 1) a method is given for the design of a windmill rotor. In chapter 6 of KD 35 a simple procedure is given for the determination of the Cp-λ and Cq-λ curves if only the starting torque coefficient, Cq start, the maximum power coefficient, Cp max at the optimum tip speed ratio λopt and the unloaded tip speed ratio λunl are known. The first part of the Cq-λ curve is found by drawing a S-shaped curve in between the point for Cq start and Cq max. The direction of this curve at λ = 0, so at Cq start, must be taken horizontal. However this procedure gives only a very rough impression of the shape of this part of the Cq-λ curve. The real curve depends on the kind of rotor which is designed and of the kind of airfoil which is used. The shape of the real Cq-λ curve for low values of λ is important for the determination of the starting behaviour of the windmill rotor with a load like a PM-generator which has a certain “sticking torque”. In this report (KD 97) a method is given to determine Cq for values of λ in between λ = 0 and about λ = ½ λopt




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