Constant speed propeller
A constant speed propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...

is a type of propeller that can change its blade pitch to take better advantage of the power supplied by an engine in much the same way that a transmission in a car takes better advantage of its power source. The mechanism varies depending on the aircraft, but the effect is to change the angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

 of the propeller blades to take a smaller or larger "bite" of air as it rotates.


An airplane propeller operates as the source of thrust that moves the plane forward.

When an airplane is stationary with the propeller spinning (in calm air), air flows past the narrow leading edge
Leading edge
The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....

 of the propeller. This is the most efficient configuration as the drag forces on the propeller are the lowest. As the airplane starts moving forward, the airflow begins to push against the front, wider cross section of the propeller, creating greater drag.

A constant-speed propeller is able to rotate along the longest axis of the blade to take a sharper bite of air with respect to the airplane, allowing the propeller to maintain the most efficient orientation to the airflow around it. This balances the tradeoff that fixed-pitch propellers must make between high take-off performance and high cruise performance.

A shallower angle of attack requires the least horsepower, 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 you get up to speed you want to slow down the engine 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.

The first attempts at constant-speed propellers were called counterweight propellers which were driven by mechanisms which operated on centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

. A counterbalance was set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centrifugal force would cause these counterbalances to swing outwards, which would drive a mechanism that twisted the propeller into a steeper pitch. When the airplane slowed down, the RPM would decrease enough for the spring to push the counterweights back in, realigning the propeller to the shallower pitch.

In newer models of constant-speed propellers, oil is pumped through the propeller shaft to push on a piston which drives the mechanism to change pitch. The flow of oil and the pitch is controlled by a governor
Centrifugal governor
A centrifugal governor is a specific type of governor that controls the speed of an engine by regulating the amount of fuel admitted, so as to maintain a near constant speed whatever the load or fuel supply conditions...

, consisting of a speeder spring, fly weights, and a pilot valve
Pilot valve
A pilot valve is a small valve that controls a limited-flow control feed to a separate piloted valve. Typically, this valve controls a high pressure or high flow feed...

. The tension of the speeder spring is set by the prop control lever, which sets the RPMs. The governor will maintain that RPM setting until an overspeed or underspeed condition exists. When an overspeed condition occurs, the propeller begins to rotate faster than the desired RPM setting. This would occur as the plane descends and airspeed increases. The fly weights begin to pull outward due to centrifugal force which further compresses the speeder spring. As that happens, the piston moves forward allowing the pilot valve to open and oil to flow from the oil sump
A sump is a low space that collects any often-undesirable liquids such as water or chemicals. A sump can also be an infiltration basin used to manage surface runoff water and recharge underground aquifers....

 into the hub. This increase in oil pressure will increase the pitch of the propeller angle causing it to slow back down to the desired RPM setting. When an underspeed condition occurs, as in a climb with loss of airspeed, just the opposite takes place. The airspeed decreases causing the propeller to slow down. This will cause the fly weights to move inward due to a lack in centrifugal force and tension will be released from the speeder spring. As this happens, the piston will move in the opposite direction causing the pilot valve to allow oil to flow from the hub back to the oil sump. The propeller blade angle will now decrease to a lower pitch allowing the propeller to speed back up to the desired RPM setting. This process usually takes place frequently throughout flight.

All high-performance aircraft have constant-speed propellers as they vastly improve fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...

 and performance, especially at high altitude.

Constant speed units

A constant speed unit (CSU) or propeller governor is the device fitted to one of these propellers to automatically change its pitch
Blade pitch
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...

 so as to attempt to keep engine speed constant. Most engines produce their maximum power in a narrow speed band. The CSU can be said to be to an aircraft what the CVT
Continuously variable transmission
A continuously variable transmission is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios...

 is to the motor car: the engine can be kept running at its optimum speed no matter what speed the aircraft is flying through the air. The advent of the CSU had another benefit: it allowed the designers of aircraft engines to keep ignition systems simple - the automatic spark advance seen in motor vehicle engines is much simplified in aircraft engines.

Three methods are used to vary the pitch. Engine oil pressure is the usual mechanism used in commercial aircraft and the Continental and Lycoming engines fitted to light aircraft. Alternatively or additionally centrifugal
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

 weights may be attached directly to the propeller as in the Yak-52. Small modern engines such as the Rotax 912 which have a CSU may use either the traditional hydraulic method or an electrical pitch control mechanism. A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU. CSUs are not allowed to be fitted to aircraft certified under the USA Light-sport Aircraft
Light-sport Aircraft
A Light-sport aircraft, also known as light sport aircraft or LSA, is a small aircraft that is simple to fly and which meets certain regulations set by a National aviation authority restricting weight and performance...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.