Knot (unit) fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG) (ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots. Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the
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