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.
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Aircraft
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.
Feathering
Feathering the blades of a propeller means to minimize their angle of attack by turning the blades to be parallel to the airflow. This minimizes drag from a stopped propeller following an engine failure in flight.
Wind turbines
Blade pitch control is a feature of nearly all large modern horizontal-axis wind turbines. While operating, a wind turbine's control system adjusts the blade pitch to keep the rotor speed within operating limits as the wind speed changes. Feathering the blades stops the rotor during emergency shutdowns, or whenever the wind speed exceeds the maximum rated speed. During construction and maintenance of wind turbines, the blades are usually feathered to reduce unwanted rotational torque in the event of wind gusts.
Boating
Some Composite propeller have interchangeable blades, which enables the blade pitch to be adjusted. Changing the blade pitch in different elevations can be very beneficial to a boater. Typically, using a lower pitch will provide better hole shot. In contrast, a higher pitch enables better top end speed for the motor. In higher elevations, a lower pitch propeller will provide better performance.
External links
- Understanding your Propeller - includes a description of blade pitch
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