Constant Speed Propellers

How they Work:

A constant speed propeller is controlled by a speed governor that automatically adjusts propeller pitch to maintain a selected engine speed. The governor senses increases and decreases in propeller RPM. If the RPM of the propeller increases, the governor responds by increasing the propeller blade angle. If the RPM of the propeller decreases, the governor responds by decreasing the propeller blade angle. An increased blade angle will cause a decrease in RPM, and a decreased blade angle will cause an increase in engine RPM. Run Ups:

When the aircraft is on the ground and you are doing the run up, the prop control should be full forward (high engine RPM, low propeller blade angle). When you pull the prop control aft, you are changing the spring tension in the prop mechanism and allowing boosted oil to flow to the governor, which in turn increases the propeller blade angle (low engine RPM, high propeller blade angle). Then when you bring the control forward, oil is allowed to go back to the engine and the blade angle is decreased again. You do this test three times. The first time you are looking for a RPM drop, the second time for oil pressure to remain constant, and the last time you are looking for NO oil spray back from the prop or engine.

In Flight:

When the aircraft is in the air that is when the constant speed propeller lives up to its name “constant speed”. When you set the prop control you are setting the spring tension to attain a desired RPM. In straight and level flight the aircraft is in equilibrium. When the aircraft is ascending or descending the governor needs to compensate for the change in the speed of the propeller. As the prop RPM increases (from the aircraft pitching down, centrifugal force also increases. This in turn forces the flyweights outward and the pilot valve is raised up. When the valve moves up, boosted oil is allowed to flow to the prop to increase the propeller blade angle. This increase in blade angle decreases RPM and in turn keeps the aircraft at a constant speed. When the aircraft pitches up, the RPM of the propeller decreases and the flyweights move inward. Since the flyweights are attached to the pilot valve, the valve moves down. Oil is then ale to flow back to the engine that in turn increases the RPM of the propeller because the blade angle is reduced. NOTE: If there is a loss of oil, the propeller blades automatically go to low pitch.

Precautions:

Always avoid high manifold and low RPM settings. There is a great danger of engine damage when manifold pressure exceeds RPM.

Ways to avoid damage –

- Keep propeller control full forward all the time (this is not always practical depending on the aircraft application)

- Reduce manifold pressure before RPM when going into cruise

- Increase RPM and the manifold pressure in climbs and go arounds

NOTE: When decreasing RPM, manifold pressure will increase just slightly. It is best to reduce manifold pressure to below what is desired.

Back to Commercial Pilot Training

Dodgen Aircraft • 740 Grand Street • Allegan, MI 49010 • Allegan: (269) 673-4157 • info@dodgenaircraft.com