It's all About Propeller Angles
What angle does your propeller blade make with the air as it chops through it? Let’s say we’re flying a Piper Arrow as an example. Just like a wing produces lift to counteract weight, the propeller produces thrust to overcome drag. And just like the wing, the propeller’s angle of attack helps determine how much thrust is produced.
A prop with a low angle of attack, where the blade is more or less perpendicular to the direction of flight, won’t produce much thrust, but, because it’s at such a low angle, it can push through the air faster using the same amount of power thus giving us more revolutions per minute. This is ideal for takeoff and climb, when we want as many RPM as we can get.
A prop with a higher angle of attack will take a bigger bite of air with each rotation, but the added drag will reduce RPM for a given power setting. This is fine for cruise, where the extra RPM doesn’t get us extra speed and so is more efficient.
Think of the different prop angles like gears on a bicycle or a car, when you’re starting from a stop or going up a big hill, you want a low gear, you’ll pedal faster and have higher RPM with less speed, while on a flat surface as you speed up you’ll want a higher gear, more power with fewer RPM.
A fixed pitch propeller like on a Cessna 172 is basically a compromise between a climb and cruise angle. But on some aircraft, like our Piper Arrow, the propeller blade angle can move during flight based on different circumstances. Because the blade angle can be varied automatically in flight by a governor, the RPM can remain at the same level, and so we call this a constant speed propeller.
How the Prop Governor Regulates RPM
How does this governor work? The propeller spins thanks to power produced by the engine, which rotates a crankshaft attached to it, pictured here in blue. On a constant speed propellor, an assembly called a governor is attached to the crankshaft through a series of gears. Attached to the governor are a series of flyweights, these red rods with weights on the ends. As the propellor, and therefore the crankshaft, are rotating, the flyweights will also spin around the governor assembly.
This is of course a stylized view of the governor. In an actual governor, the flyweights are housed within the prop spinner assembly and the flyweights look more like bars which swing inwards and outwards. They are pointed inwards, and will swing outwards as the RPM increases.
So the flyweights respond to changes in the propellor RPM. At lower RPM, the weights fall inwards towards the governor. At say 2100 RPM, the weights may be closer in to the axis. As the propellor speeds up, the flyweights spin faster, causing the weights on the end to swing outwards, so at 2500 RPM they are further out. Changes in RPM affect the angle the flyweights rotate at.
If we want a constant propeller speed, we also want a constant angle on those flyweights. We can set a desired flyweight angle, and thus a desired propeller RPM, by keeping tension on the flyweight through a spring mechanism. This is connected to a control in the cockpit, the propeller control, typically a blue handle to the right of the throttle.
If we move that handle all the way forward, in other words away from our position in the cockpit, the propellor control moves a threaded shaft attached to the spring putting more tension on it, and changing the angle of the flyweights. This “sets” our desired RPM. Let’s say it’s 2300. Now, what happens if the propeller RPM starts to change, let’s say we pitch down and it wants to go faster, due to the blade taking a smaller bite of air?
The flyweights will start to swing outwards with the faster RPM, pulling up on the bottom of the spring. Now, the bottom of the spring is attached to a pilot valve which is part of the larger oil system of the engine. The system is fed high pressure oil from the pump in the engine, and used oil is returned to the oil filter and sump. Part of this pressurized oil can be directed to the propeller assembly, the oil can apply pressure to a spring, which when moved changes the blade angle.
When those flyweights swing outward, it opens up a pilot valve, allowing oil to press in more on that spring at the propellor. This pushes the blade out so that it makes a bigger angle with the air it’s moving through. This slows the propeller down bringing us back to our original RPM, as the flyweights return to their original position, closing the pilot valve once again.
In the aircraft, this process happens instantaneously, so that the flyweights remain at their equilibrium, and the RPM doesn’t go up or down, hence the term constant speed propellor. So if we pitch back up, the governor will maintain RPM by decreasing the propeller blade angle.
Of course, pitching up and down isn’t the only way the propellor blades can be made to change their speed. In our Cessna 172, fixed pitch propellor, when we push the throttle in, we are increasing power. The increased power makes the crankshaft, and so the propellor spin faster, we see an increase in RPM with an increase in throttle.
In a constant speed propellor aircraft though, if we move the throttle forward, we don’t see the same increase in RPM. There is a gauge for power changes on a constant speed prop aircraft. It is the manifold pressure gauge. This measures the pressure of the fuel air mixture being fed into the cylinders, more pressure equals more power.
So in the constant speed unit, we have two controls for power and thrust. The blue prop control we talked about, and the throttle handle, typically to the left of the prop control, and usually in black. And so we also have the additional manifold pressure gauge, along with the RPM on the tachometer.
Two Power Controls are Better than One
Typical cruise power settings for some aircraft could be 2300 RPM, and 24 inches of manifold pressure. This is the same inches of mercury unit of measurement we use to set our altimeter, its just a measure of pressure. Let’s say we bring the throttle back to reduce power. The manifold pressure will come back to 21 inches. With less power, the propeller won’t be able to spin as fast, unless the blade angle reduces. Here’s how that will work.
As the propeller begins to slow down, and the flyweights close inwards, the pilot valve opens once again, this time causing oil to flow from the propellor assembly, back to the engine and oil filter and sump. So relieving oil pressure from the propellor causes it to fall back to a low pitch angle, allowing its speed to increase again. The speed increases swings the flyweights out, causing the pilot valve to close again, and we’re back at equilibrium. And again, this whole process happens instantaneously, so that the only affect of the throttle reduction is a decrease in manifold pressure, and a decrease in the propellor blade angle.
By now you can figure out that we can adjust our desired RPM with the blue propellor control. By moving it forward, we’re selecting a higher RPM. Instantaneously, the extra pressure on the prop governor opens the pilot valve pushing oil out of the propellor assembly, easing the pressure on it and allowing the blade angle to reduce, and RPM to increase.
Let’s say we’re at 2500 RPM, and 21 inches of pressure. This is a relatively low power setting, with a very low propellor blade angle. To achieve this, there really isn’t much oil pressure needed to push on that spring in the propellor assembly, so there’s not much stress on the system. This is considered safe, when the “prop is on top” so to speak, the prop setting is high compared to the throttle setting. When we get into how to fly a constant speed prop, in the full Commercial Ground School we’ll talk about specifically what to look for, but as a general rule, we want to avoid the opposite set up, where we have a high power setting, requiring more oil pressure and blade angle to maintain RPM, and then a low RPM setting, which requires an even bigger bite of air for the propellor to slow down. Now, there’s a lot of tension on that spring, and more importantly, the propellor is doing a whole lot of work to move through the air at such a high angle, with a great deal of engine power driving it, that the prop can become overstressed. For this reason, aircraft will often be placarded with the warning not to run below a certain RPM when at cruise power settings.
Oil Pressure Keeps it Moving
Now the whole system is dependent on oil pressure to function. The oil system runs on the engine driven fuel pump. If there is a loss of oil pressure, this will affect the propellor system, but its also of grave concern to the engine operation. With a loss of oil pressure, or a complete loss of engine power, the lack of pressure on the propeller relieves the tension on the spring and brings the blade angle to full forward, meaning its lowest angle. This isn’t always the case. Many aircraft have the oil flow we’ve illustrated completely reversed, where oil pressure pushes the blade inwards, and so with a loss of pressure, it goes instead to a very high blade angle, what’s known as feathering the prop. This is useful in a multi engine plane where we’d want to reduce the drag on flight of a lost engine on one side. But in our single engine training, we’ll focus on the prop failing to a full forward condition.
Again, in the full Commercial Pilot Ground School course, we dive into how to fly a constant speed propeller in different phases of flight, using suggested settings and proper procedures for maximum efficiency. For now, consider one of the benefits of the constant speed prop. In a fixed pitch Cessna, pitching up and down has an effect on our RPM, even without our changing the throttle setting. In our constant speed Piper Arrow though, these pitch settings have no effect on RPM, as the prop governor works to change the blade angle to maintain a constant speed. Great for engine longevity and ease of flight.