Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor 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 greater horizontal blade angle, and "fine" for a more vertical blade angle. For an aircraft that is stationary the angle of pitch of each blade of a propeller fitted to that aircraft equates to the 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 (and climb rate in an aircraft) while high pitch optimizes high speed performance and economy.
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 washout. 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, to give optimum thrust over the maximum amount of the aircraft's speed range, from takeoff and climb to cruise.
In helicopters the pitch control changes the angle of attack of the rotor blades and thus the vertical acceleration or climb rate of the vehicle. This control is also called collective as distinct from the cyclic control for lateral movement. The collective blade setting is mostly achieved through vertical movement of the swashplate.
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.
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.
Blade pitch control is preferred over rotor brakes, as brakes are subject to failure or overload by the wind force on the turbine. This can lead to runaway turbines. However, for blade pitch control, once the blades are feathered, wind speed does not affect the stress on the control mechanism.
Pitch control can be implemented via hydraulic or electric mechanisms. Hydraulic mechanisms have longer life, faster response time due to higher driving force, and a lower maintenance backup spring. However, hydraulics tend to require more power to keep the system at a high pressure, and can leak. Electric systems consume and waste less power, and do not leak. However, they require costly fail safe batteries and capacitors in the event of power failure.
Pitch control does not need to be active (reliant on actuators). Passive (stall-controlled) wind turbines rely on the fact that angle of attack increases with wind speed. Blades can be designed to stop functioning past a certain speed. This is part of the reason behind twisted blades— the twist allows for a gradual stall as each portion of the blade has a different angle of attack and will stop at a different time.
Blade pitch control typically accounts for less than 3% of a wind turbine's expense while blade pitch malfunctions account for 23% of all wind turbine production downtime, and account for 21% of all component failures.
In boating, blade pitch is measured in the number of inches of forward propulsion through the water for one complete revolution of the propeller. For example, a propeller with a 12" pitch when rotated once, will propel the vessel 12" ahead. Note that this is the theoretical maximum distance; in reality, due to "slip" between the propeller and the water, the actual distance propelled will invariably be less.
Some composite propellers 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 acceleration and a higher pitch enables better top end speed.
In rowing, blade pitch is the inclination of the blade towards the stern of the boat during the drive phase of the rowing stroke. Without correct blade pitch, a blade would have a tendency to dive too deep, or pop out of the water and/or cause difficulties with balancing on the recovery phase of the rowing stroke.
- "Wind turbine blades that change pitch boost wind power efficiency". Machine Design. 2011-08-11. Retrieved 2019-05-10.
- "Power Control of Wind Turbines". xn—drmstrre-64ad.dk. Retrieved 2019-05-10.
- "Pitch Control Critical for Wind Power". Machine Design. 2018-03-02. Retrieved 2019-05-10.