The tip speed ratio λ (lambda) or TSR for wind turbines is the ratio between the rotational speed of the tip of a blade and the actual velocity of the wind. If the velocity of the tip is exactly the same as the wind speed the tip speed ratio is 1. A tip speed ratio above 1. means some amount of lift, below 1. usually indicates drag. A high tip speed ratio indicates mostly a better efficiency but is usually also related to a higher noise level and a heavier design.
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- <math>\mbox{Tip speed ratio} = \frac{\mbox{Tip speed of blade}}{\mbox{Wind speed}}</math>
It has been shown empirically that the optimum tip speed ratio for maximum power output occurs at
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- <math>\lambda_\mbox{max power} = \frac{4\pi}{n}</math>
where n is the number of blades.
