How it works: Constant speed propeller

A prop for all reasons

The twist in propeller blades is called pitch. If pitch is too great, it creates drag and robs you of takeoff power. Too little, and you are forced to run the engine faster to achieve cruise speed. Most likely your trainer is not equipped with a constant-speed propeller, but one with fixed pitch. Your fixed-pitch propeller isn’t the best for takeoff but not bad, and isn’t the best for cruise—but not bad. What if you could change those “not bads” for “goods”?
How it works

The pilot uses a propeller control in the cockpit to set revolutions per minute. That setting determines how much oil will be pressurized into the propeller hub. The oil pressure moves a plate that either reduces the pitch of the blades to allow the engine to generate more power, or increases their pitch to take a bigger bite of air. A flatter pitch is used for takeoff, when more power is needed to climb.

That’s the job of the constant-speed propeller. The pilot sets a low or fine pitch (high rpm) for takeoff, and a higher or coarse pitch (low rpm) for cruise, using a propeller control knob or lever next the throttle.

What makes the concept work is the propeller governor. The governor takes in oil under pressure from the engine, usually about 50 to 70 pounds per square inch, and boosts it to 300 psi. That pressure is sent to the back side of the propeller, where its job is to push on a plunger that moves gears. The gears change the propeller blade angle. The plunger’s position is limited by a “speeder spring” that, in turn, is controlled by the pilot. When the plunger gets to a setting chosen by the pilot, oil is released back to the engine and the plunger maintains its position, keeping rpm constant at that setting.

The constant-speed propeller is dumb. It does whatever the governor tells it to do.

Alton Marsh

Alton K. Marsh

Freelance journalist
Alton K. Marsh is a former senior editor of AOPA Pilot and is now a freelance journalist specializing in aviation topics.

Related Articles