How Does A Constant Speed Prop Work?

A constant speed propeller works by varying the pitch of its blades. When the blade angle is increased it provides more thrust.

This requires more torque, which slows down the engine. When the blade angle decreases, less torque is needed, and the engine speeds back up.

How Does A Constant Speed Prop Work

The blue lever next to the throttles controls the propeller pitch. When you’re flying a plane with a constant-speed propeller, you can change the pitch to match your current conditions.

For example, if you’re taking off, you’ll increase the pitch to get more thrust. On the other hand, if you’re landing, you’ll decrease the pitch to reduce drag.

Propellers operate at different speeds depending on what you’re doing. A higher RPM allows for more power when taking off or cruising.

Lowering the RPM makes your engine more fuel-efficient. Let’s look at their functions in more detail. 

Moving The Propeller 

Propellers use hydraulic pressure to adjust the blade pitch (see also “What Color Is Hydraulic Fluid?“). Oil is used to lubricate the moving parts of the engine. A spring is used to return the propeller to a lower pitch when the engine stops. 

Most single-engine planes use stops to prevent damage to the propeller. This prevents damage to the engine when flying over rough terrain.


The governor controls the engine speed by moving oil back and forth through a propeller hub. There are five components in the governor.

The main shaft connects the engine to the props as well as the gearbox. The gearbox is connected to the propeller.

The governor control lever is attached directly to the blue prop control levers by cables or linkages.

Turning the prop levers causes the governor control lever to move as well. The speeder springs are placed between the shaft and the flywheels. 

When the shaft spins down, the spring gets compressed, and it pushes the flywheels inward. When the shaft spins up, the opposite happens, and the flywheels push outward.

Flyweights are used to control the flow of fuel into the engine. They are connected to the engine by gears. They are also connected to the pilot valves, which move up or down depending on the engine’s rpm. 

When the engine speeds increase, the flywheels spin faster and lift the pilot valves up. When the engine speeds decrease, the flywheels fall in because of the weight of the fuel flowing into the cylinders.

Using A Speed Prop In Flight 

During the take-off stage of flying a plane, the speed lever is usually pressed fully forward, this allows the plane to gain enough power to lift it off the ground and begin its ascent, when should you start pulling the lever back?

You should never pull the lever back until you reach an airspeed that allows you to maintain level flight.

So, before you pull the lever back, you need to make sure that you’re flying at a safe airspeed. Increasing the angle of attack increases the amount of thrust generated by the engine. 

This makes the plane go faster. In cruise flight, this also decreases the RPMs (revolutions per minute) of the engine. Slowing down the engine allows the engine to last longer and become less worn out.



When landing an airplane, the pilot moves the prop lever forward. This gives the plane instant power, but it also makes the engine work harder.

If you don’t use your engine controls and the airplane pitches up the engine will naturally work harder and will start to slow this causes flyweights to slow, resulting in oil flowing out of the propellers this is when underspeed occurs. Pitch also decreases and the propellers move inward. 

When the pitch of the blades drops, the engine speeds up until the governor kicks back in. At this point, the governor will take control and slow the engine down.

Once the engine slows down, the pitch of the blades increases, causing the engine to speed up again. This process repeats until the engine reaches the desired RPM.

Losing Oil 

Most single-engine planes are designed to fail forward meaning if oil is lost the propeller takes up the strain and changes settings to make up for it.

Not only does running out of engine oil cause prop control issues but when the propeller has to spin faster and higher, it causes more damage to the engine.

Looking After The Engine 

To avoid over-pressuring the cylinders, always move your propeller control forward (increase engine RPM) before increasing fuel supply, and always reduce fuel supply (decrease throttle) before decreasing engine RPM. 

Think of the throttle as controlling the amount of fuel added to the engine, and the propeller control as adjusting the volume of the balloon.

Make the balloon larger (with the throttle) by adding more fuel, and make the balloon smaller (by moving the propeller control) by taking fuel away. 

Disassembling A Speed Prop 

Speed props need to be examined regularly to ensure they are working correctly and aren’t damaged in any way.  Thankfully it’s fairly straightforward to take apart a speed prop. 

The prop can be placed on a stand. When the prop is mounted to it, it is identified by numbers. There are twelve screws holding the prop together.

The prop has two parts, the body and the head. The body is held together by four screws, while the head is held together by eight screws.

To disassemble the prop, the big nut that acts like a low-pitch stop is removed. Then, the big spring, spindles and plungers are removed. The next step is to remove the actuators that block the blade openings.

The delrin plugs that hold the blade pins into place come off. Blade removal is next, and this is the same for all 3 blades.

The blade to remove is positioned down by rotating, and the hub is secured by spreading the large split ring. 

The large pin spreader pliers are used to push the blade into the hub sufficiently that all the ball bearings can be removed. The blade is then spun to present all the balls to the technician.

Once all the balls are removed the blade simply slides out. With the blade ferule clear of the hub it can be inspected for damage or fretting and the O-ring seals checked or replaced.

The dual sealing O-rings are checked or replaced. The spacer and piston are removed and inspected.


There you have it, now you know how a constant speed prop works, maybe it’s time to try it out. 

Jacob Stern
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