|The Workings of Constant-Speed Propellers|
|Written by Jacqueline Shipe|
|Thursday, 24 February 2011 10:19|
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Most airplanes that have less than 180 hp are equipped with fixed-pitch propellers. “Pitch” refers to the blade angle between the chord line of the propeller blade and the airplane in which it rotates. Fixed-pitch propellers are made of solid aluminum (except for some of the older props, which were made of wood). The blade angle is set.
Most fixed-pitch propellers are “climb” props, “cruise” props, or a combination of the two. Climb props have a relatively flat blade angle. The low pitch angle allows the engine to turn at a high speed and produce maximum takeoff power. Airplanes that are used primarily for training purposes that perform lots of takeoffs and landings, as well as airplanes that operate off of short strips, generally do better with a climb prop. Cruise props have a high blade angle that provides for the most efficient setting when flying level at altitude. Airplanes for personal use that are flown on lots of cross country trips are usually configured with a cruise prop to get the best efficiency when flying straight and level. Some pilots don’t know what blade angle their propeller is set at. An owner of a Cessna 150 with a cruise prop would be surprised by the increase in climb performance that his or her airplane would have if the blade were changed to a climb angle. Similarly, the owner of a climb prop-equipped Cherokee Archer, who flies cross country a lot, would be surprised by the increased efficiency a cruise setting on his or her propeller would provide. The blade angles can be checked, and some propellers can be adjusted by a propeller shop.
The best of both worlds is found in the constant-speed (controllable-pitch) propellers. These propellers have blades that are able to rotate in flight, allowing a flat pitch setting for takeoff and climb and a high pitch setting for cruise, plus each intermediate setting in between. These propellers are generally found on airplanes that have 200 hp or more. They are much heavier than fixed-pitch propellers, and they also require the use of a propeller governor.
A controllable-pitch propeller has a central hub that houses the inner blade assemblies, which are mounted on ball bearings. A cylinder is mounted on the front side of the hub, and it houses an internal piston and rod mechanism, which is linked to the blade ends in the hub through link assemblies and blade actuating pins. The piston has force on it from both directions. When the piston assembly moves back and forth in the cylinder, the blades are forced to rotate in the hub. On one side of the piston is engine oil pressure regulated by the propeller governor, and on the other side is spring tension, or, sometimes, air pressure. The spring tension (or air pressure) exerts force to push the blades into a high blade angle (all the way into feather, if the airplane is equipped with feathering props). Oil pressure is used to force the blades into a flat blade angle or low pitch. Some of the specifics vary with the make and model, but the general operating principles are the same.
The propeller blade angle is controlled by a propeller governor. The governor is gear driven by the engine. It has L-shaped flyweights that spin in conjunction with the engine’s speed. The flyweights are used to open and close a pilot valve that regulates the amount of oil pressure going in and out of the propeller. The pilot valve is interconnected to a rod assembly that has the engine-driven gear on one end, the valve itself, and the flyweight assembly and “speeder spring” on the other end. The flyweights are hinged at the corner to allow them to pivot. The flyweights have to work against spring pressure exerted by the speeder spring, which presses on the valve and holds it in a down position. This down position allows governor oil pressure into the propeller to force the blades into a flat pitch.
As the governor senses increasing engine speed, the top parts of the flyweights are slung outward by centrifugal force. The toes on the L-shaped weights are under a plunger on the pilot valve. When the tops of each flyweight are forced outward, the toes on each flyweight push against the speeder spring pressure and open the valve. This allows oil pressure in the propeller hub to return to the engine, and the blade angle of the propeller increases, which reduces the propeller and engine speed. If the engine speed decreases, the flyweights lose centrifugal force, and the top parts of the weights pivot inward, while the toes pivot down. This allows the spring tension on the speeder spring to push the pilot valve closed to prevent oil pressure from leaving the propeller hub, forcing the blades into a flat pitch, which increases engine speed.
The spring tension on the speeder spring is adjusted by the propeller control in the cockpit. When the pilot selects a reduced engine-speed setting, the spring tension is reduced, and the flyweights open the valve to allow oil pressure from the hub to drain back into the engine. This allows the spring or air pressure in the propeller hub to move the blades to a higher pitch.
The controllable-pitch propeller provides good climb performance, which increases safety and allows cruising altitudes to be reached quicker, and it increases fuel efficiency and speed while cruising.
From the December 2010 issue of Cessna Owner