Fixed Pitch & Constant Speed Propellers By collin fleck

Fixed Pitch On a fixed pitch propeller, the blade angle is built into the propeller. Usually one solid piece of metal or wood. Directly connected to the engine.

Fixed Pitch Fixed pitch propellers are designed to be most efficient at one rotational and forward speed. There are three categories of fixed pitch propellers: Climb Cruise All Purpose

Fixed Pitch “Climb” propeller Small angle of attack – somewhere around 15 degrees Produces a large amount of thrust and allows for high engine RPM during take-off and climb. Efficiency and thrust are limited at high airspeeds because of the small angle of attack. Used on aircraft that operate from short, obstacled airstrips or with heavy loads where take-off performance is more important than cruise performance.

Fixed Pitch “Cruise” propeller Large blade angle / angle of attack – around 25 degrees. High efficiency and thrust at high airspeeds during cruise, but limited efficiency and thrust during take- off and climb. Used on aircraft that operate from long runways and cruise at high altitudes for long periods of time.

Fixed pitch “all purpose” propeller Blade angle / angle of attack between those of climb and cruise propellers – around 20 degrees. Performance characteristics of both climb and cruise propellers. Used on aircraft that may need high levels of performance during take-off, climb, and cruise.

Constant speed Propeller Why is it called “constant speed”? The propeller doesn’t always operate at the same speed, but the pilot can select a speed for a given situation, and the propeller will maintain that speed in revolutions per minute. Take-off: High RPM should be selected for maximum thrust. Cruise: The blade angle of the propeller can be increased in cruise. The propeller takes bigger “bites” out of the air, lowering RPM and increasing fuel efficiency.

Main Components Governor: Connected directly to the engine with gears. Maintains an RPM setting by controlling the blade angle of the propeller by sending engine oil to the propeller hub. Propeller Lever: Connected to the governor. Controlled by the pilot to choose an RPM setting. Propeller Hub: Contains an oil controlled piston connected directly to the propellers. Changes the blade angle of the propellers.

Governor Governor control lever: Attached to the blue propeller lever in the cockpit via control cables or linkages. Threaded shaft: Connected to the governor control lever. When the pilot moves the propeller lever and thus, the governor control lever, the threaded shaft moves either left and up, or right and down like a screw. Speeder spring: Sits between the threaded shaft and the flyweights. When the threaded shaft moves down, it squeezes the speeder spring and puts pressure on the flyweights, causing them to “fall” inward. When the threaded shaft moves up, it releases the tension on the speeder spring and flyweights, and the flyweights move outward.

Governor (continued) Flyweights The “L” shaped flyweights are connected to the engine with gears, so they spin in a circle. They are also connected to the pilot valve and move it up or down. When engine RPM increases, centrifugal force causes the flyweights to “fall” outward, lifting the pilot valve up. When RPM decreases, the flyweights move inward due to the pressure of the speeder spring, and push the pilot valve down. Pilot valve: The pilot valve is connected to the flyweights, and allows oil to flow into or out of the propeller hub. When the pilot valve is lifted up, oil is allowed to flow into the propeller hub. When the pilot valve is pushed down, oil is allowed to flow out of the propeller hub. When the pilot valve is neutral, the oil is “locked” in place within the propeller hub.

Governor (continued) Governor gear pump: The governor gear pump gives the oil leaving the pilot valve a boost in pressure before it enters the propeller hub for a quicker response from the propeller after you move the propeller lever in the cockpit.

Propeller hub The propeller hub contains a piston which is connected to the propeller blades. Engine oil is sent to the propeller hub through the pilot valve when the engine is running too fast. The oil fills the reservoir in front of the piston, pushing it back and twisting the propeller blade to a higher angle of attack, slowing the engine down. Engine oil is allowed out of the propeller hub and back through the pilot valve when the engine is running to slow. This allows the piston to move forward due to the tension of a spring, twisting the propeller blades to a more shallow angle of attack, allowing the engine to speed up.

Underspeed In situations where the engine has to work harder to maintain an RPM setting such as in climb, the propeller will begin to slow down, and the flyweights will fall inward due to the reduced centrifugal force. This allows the pilot valve to move downward, allowing oil to flow out of the hub and reducing the pitch of the blades.

Overspeed In an overspeed condition, the workload on the engine has been reduced, and the propeller begins to spin faster than the RPM setting. The flyweights fall outward due to increased centrifugal force, and the pilot valve is moved upward, allowing oil to flow into the hub and increasing the pitch of the blades, which decreases the speed of the engine.