In aeronautics, a variable-pitch propeller is a type of propeller (airscrew) with blades that can be rotated around their long axis to change the blade pitch. A controllable-pitch propeller is one where the pitch is controlled manually by the pilot. Alternatively, a constant-speed propeller is one where the pilot sets the desired engine speed (typically expressed as RPM), and the blade pitch is controlled automatically without the pilot's intervention so that the rotational speed remains constant. The device which controls the propeller pitch and thus speed is called a propeller governor or constant speed unit. Reversible propellers are those where the pitch can be set to negative values. This creates reverse thrust for braking or going backwards without the need to change the direction of shaft revolution. While some aircraft have ground-adjustable propellers, these are not considered variable-pitch. These are typically found only on light aircraft and microlights.
Purpose When an aircraft is stationary with the propeller spinning (in calm air), the relative wind vector for each propeller blade is from the side. However, as the aircraft starts to move forward, the relative wind vector comes increasingly from the front. The propeller blade pitch must be increased to maintain optimum angle of attack to the relative wind. The first propellers were fixed-pitch, but these propellers are not efficient over a range of conditions. If the propeller blade angle is set to give good takeoff and climb performance, the propeller will be inefficient in cruising flight because the blade will be at too low an angle of attack. In contrast, a propeller set for good cruise performance may stall at low speeds, because the angle of attack is too high. A propeller with adjustable blade angle is more efficient over a range of conditions. A propeller with variable pitch can have a nearly constant efficiency over a range of airspeeds. A shallower angle of attack requires the least torque, but the highest RPM, because the propeller is not moving very much air with each revolution. This is similar to a car operating in low gear. When the motorist reaches cruising speed, they will slow down the engine by shifting into a higher gear, while still producing enough power to keep the vehicle moving. This is accomplished in an airplane by increasing the angle of attack of the propeller. This means that the propeller moves more air per revolution and allows the engine to spin slower while moving an equivalent volume of air, thus maintaining velocity. Another use of variable-pitch propellers is to feather the blades of the propeller, in order to reduce drag. This means to rotate the blades so that their leading edges face directly forwards. In a multi-engine aircraft, if one engine fails, it can be feathered to reduce drag so that the aircraft can continue flying using the other engine(s). In a single-engine aircraft, if the engine fails, feathering the propeller will reduce drag and increase glide distance, providing the pilot with more options for the location of a forced landing.
Mechanisms
Three methods are used to vary the pitch: oil pressure, centrifugal weights, or electro-mechanical control. Engine oil pressure is the usual mechanism used in commercial propeller aircraft and the Continental and Lycoming engines fitted to light aircraft. In aircraft without a constant speed unit (CSU), the pilot controls the propeller blade pitch manually, using oil pressure. Alternatively, or additionally, centrifugal weights may be attached directly to the propeller as in the Yakovlev Yak-52. The first attempts at constant-speed propellers were called counterweight propellers, which were driven by mechanisms that operated on centrifugal force. Their operation is identical to the centrifugal governor used by James Watt to limit the speed of steam engines. Eccentric weights were set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centrifugal force would cause the weights to swing outwards, which would drive a mechanism that twisted the propeller into a steeper pitch. When the propeller slowed, the RPM would decrease enough for the spring to push the weights back in, realigning the propeller to the shallower pitch. Small, modern engines with constant speed units (CSUs), such as the Rotax 912, may use either the conventional hydraulic method or an electrical pitch control mechanism. Hydraulic operation can be too expensive and bulky for microlights. Instead, these may use propellers that are activated mechanically or electrically.
Constant-speed propellers
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