Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
Variable speed wind turbine formula_24 formula_31 "For further details, see Blade Element Momentum Theory" Consider the following figure: This is the depiction of the apparent wind speed, as seen by a blade (left of figure). The apparent wind speed is influenced by both the free-stream velocity of the air, and the rotor speed. From this figure, we can see that both the angle formula_32 and the apparent wind speed formula_33 are functions of the rotor speed, formula_2. By extension, the lift and drag forces will also be functions of formula_2. This means that the axial and tangential forces that act on the blade vary