Blade pitch
Blade pitch
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Blade pitch

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Blade pitch

Blade pitch or simply pitch refers to the angle of a blade in a fluid. The term has applications in aeronautics, shipping, and other fields.

In aeronautics, blade pitch refers to the angle of the blades of an aircraft propeller or helicopter rotor. Blade pitch is measured relative to the aircraft body. It is usually described as "fine" or "low" for a more vertical blade angle, and "coarse" or "high" for a more horizontal blade angle.

Blade pitch is normally described as a ratio of forward distance per 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 (and climb rate in an aircraft) while high pitch optimizes high speed performance and fuel economy.

It is quite common for an aircraft to be designed with a variable-pitch propeller, to give maximum thrust over a larger speed range. A fine pitch would be used during take-off and landing, whereas a coarser pitch is used for high-speed cruise flight. This is because the effective angle of attack of the propeller blade decreases as airspeed increases. To maintain the optimum effective angle of attack, the pitch must be increased. Blade pitch angle is not the same as blade angle of attack. As speed increases, blade pitch is increased to keep blade angle of attack constant.

A propeller blade's "lift", or its thrust, depends on the angle of attack combined with its speed. Because the velocity of a propeller blade varies from the hub to the tip, it is of twisted form in order for the thrust to remain approximately constant along the length of the blade; this is called "blade twist". This is typical of all but the crudest propellers.

In helicopters, pitch control changes the angle of incidence of the rotor blades, which in turn affects the blades' angle of attack. Main rotor pitch is controlled by both collective and cyclic, whereas tail rotor pitch is altered using pedals.

Feathering the blades of a propeller means to increase their angle of pitch by turning the blades to be parallel to the airflow. This minimizes drag from a stopped propeller following an engine failure in flight.

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