Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power. A propeller of a ship uses this effect to control the ship's speed without changing the rotation of the shaft and to increase the efficiency of streaming fluids. In aircraft, blade pitch is usually described as "coarse" for a high angle of attack, and "fine" for a low angle of attack. Blade pitch is normally described in units of distance/rotation assuming no slip. Blade pitch acts much like the gearing of the final drive of a car. Low pitch yields good low speed acceleration (or climb rate in an aircraft) while high pitch optimizes high speed performance and economy. Because the velocity of a propeller blade varies from the hub to the tip, they must be of twisted form in order for the pitch to remain constant along the length of the blade. This is typical of all but the crudest propellers. It is quite common in aircraft for the propeller to be designed to vary pitch in flight, optimizing both cruise and takeoff performance.
