What's that blue knob next to the throttle? It's the propeller control, and when you fly a plane with a constant speed propeller, it gives you the ability to select the prop and engine speed you want for any situation. But what's the benefit, and how does it all work?

Constant speed propellers work by varying the pitch of the propeller blades. As the blade angle is increased, it produces more lift (thrust). At the same time, more torque is required to spin the prop, and the engine slows down. The opposite is true when the blade angle is decreased: the torque required is decreased, and the engine speeds up.

Why Is It Called Constant Speed?

First off, the propeller doesn't always operate at the same speed. It just means that you can select the RPM you want for a given situation.

Taking off? You'll want a high RPM for max takeoff power. Cruising? You can pull the RPM back by increasing the blade angle, making your engine more fuel efficient.

How Do You Move The Propeller?

The propeller's blade pitch is changed hydraulically, using engine oil. That's right, the same oil that bounces through your cylinders, keeping them cool and lubed, is used to change the pitch of your propeller blades. We'll get into how the oil changes the blade angle in just a second.

There's also a spring on the back of the propeller hub assembly that helps the propeller return to a low pitch/high RPM setting, which we'll also get to in a bit.

For most single-engine planes, there are stops installed so the blade can't move too much in either direction, and that's a good thing, because you could find yourself in real trouble if you completely flattened or feathered the prop on your single engine plane.

The Governor (Not The One In Charge Of Your Home State)

By now, you understand that there's a blue prop lever in the cockpit that lets you adjust the prop pitch and engine speed, but there's a little more magic under the hood that makes it all happen.

The component in charge of it all is called the governor. The governor moves oil back and forth through the propeller hub to make sure the prop is at the pitch and speed that you want.

There are several parts to the governor that make everything happen. Let's take a look at what they all do:

Governor Control Lever
The governor control lever is attached to the blue prop control lever in the cockpit through cables or linkages. When you move the prop lever forward or back, the control lever moves as well.

Threaded shaft
The threaded shaft is connected to the governor control lever. It looks pretty much like a bolt, and it works the same way. When you turn it left, it moves up, and when you turn it right, it moves down. (righty tighty, lefty loosy!)

Speeder spring
The speeder spring sits between the threaded shaft and the flyweights. When the threaded shaft moves down, the spring gets squeezed (its tension increases), and it forces itself down on the flyweights, causing them to 'fall' inward. When the threaded shaft moves up, the opposite happens.

Flyweights
The "L" shaped flyweights, which spin around in a circle, are connected to the engine through gears. They're also connected to the pilot valve, which they move up and down. When the engine speeds up, the weights spin faster and fly out due to centrifugal force, lifting the pilot valve up. When the engine slows down, the weights fall in from pressure from the speeder spring, lowering the pilot valve.

Pilot Valve
The pilot valve, which we mentioned is connected to the flyweights, is moved up and down by the flyweights, allowing oil to flow into, or out of, the propeller hub. We'll get to exactly how that happens in just a second.

Governor Gear Pump
The last major part of the governor is the gear pump. The pump boosts oil pressure before it heads out of the governor and into the propeller hub. With the boosted pressure, you get better, quicker response from the propeller when you move the lever back and forth in the cockpit.

So How Do You Use A Constant Speed Prop On A Flight?

In most cases, you takeoff and land with the prop control full forward, which means your propeller is in the flat, low pitch/high RPM setting. Having your prop in that position gives you a lot of takeoff power. But once you get off the ground and closer to your cruise altitude, you want to start pulling the prop lever back.

In the above animation, every step happens separately so that you can more clearly see what's happening. But in reality, when you start pulling the prop lever back, the propeller and engine RPM start changing almost instantaneously.

By pulling the prop lever back, you adjust to prop to take a bigger 'bite' of air. What's really happening is that you're increasing the angle-of-attack of the propeller, and in turn, increasing the torque required of the engine to swing the prop around.

With that increase in torque, the engine slows down, which in cruise flight is a good thing. It means your engine isn't spinning as fast, your fuel efficiency is increased, and the engine parts won't wear down as quickly. Think of it kind of like switching to a higher gear on your bike. You need to push a little harder with your legs, but you don't need to pedal nearly as fast.

Moving The Prop Lever Forward

When you're coming in to land, you typically move the prop lever full forward.

By moving the lever forward, you give yourself more 'instantaneous' power, because the engine doesn't need to work as hard to turn the prop. That's something that's useful if you need to go-around.

What Happens When You Start To Climb Or Descend?

Once you've set your engine RPM using the prop lever, the governor will do everything it can to maintain that RPM. But what happens when you start climbing and descending?

Underspeed
Let's look at a climb first. If you don't touch your engine controls, and you pitch the airplane up, your engine has to work harder, and it will start to slow down. This is a situation called underspeed. As soon as it happens, the flyweights will start to fall inward because they slow down as well. When that happens, the pilot valve moves down and oil flows out of the propeller hub, reducing the pitch of the blades.

As soon as the blade pitch decreases, the engine is able to speed up again, and it resumes its normal set RPM.

Overspeed
When you pitch the airplane down, the governor takes over again. By pitching down and not touching the engine controls, the engine will start to speed up, and the flyweights will fly outward, due to centrifugal force. As soon as that happens, the pilot valve will raise, oil will flow into the prop hub, and the pitch of the blades will increase, slowing the engine to your set speed.

What Happens If You Lose Your Engine Oil?

Most single-engine plane propellers are designed to 'fail forward', which means that if you run out of engine oil, the propeller will automatically move into the low pitch/high RPM setting.

It happens for two reasons: 1) the spring behind the prop hub piston forces the piston forward, and 2) the natural twisting moment of the blades moving through the air causes them to return to the low pitch/high RPM takeoff and landing setting.

All of that being said, if you run out of engine oil, you are going to have bigger problems than just a lack of prop control - chances are, your engine isn't going to run for much longer.

Putting It All Together

A constant speed propeller gives you the ability to select the engine and propeller speed you want for any situation. It also makes your plane more adaptable to different phases of flight. And last off, with an extra engine control in the cockpit, it makes you look like a genius to your passengers.