Types of Constant Speed Propellers and Governors

On the first test flight of my Bede BD-4C airplane, I discovered that I could not properly control the engine RPM. I learned that I have the wrong type of governor for my propeller. This blog post outlines the two different types of constant speed propellers and governors, how they work, and how I ended up in this situation.

Background

I purchased a used Lycoming engine with a used Hartzell constant speed propeller for my airplane. The seller disclosed that they had come off of an aerobatic airplane and that the prop had external counterweights. Since I knew that I would be replacing the propeller, I did not care about the counterweights. I should have because the presence of counterweights implies something about the way that the propeller works. It also has implications on a third component in the system, the propeller governor.

When I first tested the engine and new propeller on my airplane, I discovered that the propeller control knob was operating backwards. When I pushed it in, the prop slowed down. When I pulled the knob out, the prop sped up. It should have been the other way around.

I fixed that by rerouting the prop control cable (much like a bicycle brake cable) so that it pulled on the prop governor lever when it had been pushing, and pushed when it had been pulling. Easy peasy; problem solved. However, my A&P noted that the prop governor still did not seem to be acting quite right because sometimes the prop would go to high pitch when he expected it to go to low pitch, like when the engine was just idling. (See below for what “high pitch” and “low pitch” mean.)

During taxi tests, everything seemed fine so I continued with the test program, planning to address the prop governor issues later, when I had something more definite to go on.

The first flight provided the “something more definite.” I either had full RPM, meaning 3000 RPM, or the RPM was dropping below 1400 and still falling. I could not select any speed in between. Immediately after takeoff, I looked at the tachometer and saw 3000 RPM. I started backing the prop control out, expecting to reduce the RPM. Nothing happened. I kept moving the prop control. Still, nothing happened. I kept moving the prop control and suddenly wump the RPM dropped to 1400 (way too low) so I shoved it back in. The engine speed returned to 3000 RPM. Since the propeller governor was not working right, I had to back off the throttle to lower the engine speed below the engine red line.

Propellers and Transmissions

On an airplane, a constant speed propeller works like a transmission in a car. It allows you to pick a convenient engine RPM and hold that speed even though you will need to vary the throttle position some. This analogy is not perfect because a car transmission allows the engine RPM to vary while a constant speed propeller keeps the engine RPM set at exactly one number. In my airplane, I expect to take off at 2700 RPM and cruise at 2400 RPM. I use the propeller control to set those speeds.

In a car transmission, you select engine speed by shifting gears. In first gear, you might pull away from a stop sign at 2000-3000 RPM and then shift to second gear. Ultimately, you end up cruising down the road at the same engine RPM.

With a constant speed propeller, you change the pitch of the blades instead of changing gear. This changes how hard the propeller pulls against the air. At low pitch, it pulls (relatively) gently, allowing the engine to turn faster. You use this to for takeoff. Boldmethod drew some great pictures:

At high pitch, used for cruising, the propeller pulls harder against the air. You get the lots of thrust with lower engine RPM.

To change the pitch of the blades, you move the propeller control. It is the blue knob in the center of the instrument panel. Shove it forward for takeoff (high RPM) and pull it out for cruise (low RPM).

In case you are curious, the black knob to the left of the propeller control is the throttle. You push it forward for fast and pull backward for slow, much like a car’s accelerator pedal. The red knob to the right is the mixture, forward for lots of gas (rich mixture) and backward for less gas (lean mixture). You lean the mixture out as you climb to higher altitudes so that the engine runs efficiently.

Propeller Governors

When you move the propeller control knob, it moves a cable (think of a bicycle brake cable) which moves a lever on a device called a “propeller governor.” This is a thing about the size and shape of a small soup can, attached to the back of the engine. The governor uses the engine’s oil and oil pressure to change the pitch of the blades on the propeller. It is a hydraulic system but without the weight and complexity of a hydraulic pump and reservoir of hydraulic fluid.

The governor sounds mysterious but it isn’t. You have seen devices just like this. Here is one from the top of a steam engine. When the engine goes too fast, the balls swing out which works a valve which makes the engine slow down. Simple.

Inside a propeller governor are a pair of weights which work just the same way. When the propeller goes too fast, the weights swing out which operates a valve which changes the oil pressure in the propeller which changes the pitch of the blades to slow things down. When the propeller turns too slowly, the opposite happens; the weights swing inward which operates the valve in the opposite direction to change the oil pressure to change the pitch of the blades to speed things up.

To learn more about constant speed propellers, go read Boldmethod’s article How A Constant Speed Propeller Works. It is a well written article, even including animations.

My Problem

When I bought my engine + propeller + governor, it came with a Hartzell propeller model HC-C2YR-4CF. I replaced it with a Hartzell propeller model HC-C2YR-1BF. The last part of the model is the important part and details a critical difference. According to the Hartzell manual:

-4CF = CONSTANT SPEED, OIL PRESSURE TO LOW PITCH, COUNTERWEIGHTS TO HIGH PITCH

and

-1BF = CONSTANT SPEED, NO COUNTERWEIGHT OIL PRESSURE TO HIGH PITCH, BLADE CENTRIFUGAL FORCE TO LOW

Hartzell Propeller Owner’s Manual, number 115N

Being told about the counterweights should have tipped me off to check the propeller operation closely. The part that I should have looked up and paid attention to was “oil pressure to low pitch” versus “oil pressure to high pitch.” In plain English, that means:

• When the propeller is turning too fast, the governor that I have lowers the oil pressure which would have made the old propeller slow down but it makes the new propeller speed up.
• When the propeller is turning too slow, the governor that I have increases the oil pressure which would have made the old propeller speed up but makes the new propeller slow down.
• The old governor, paired with the new propeller, is perfectly wrong. If the prop turns too slowly then the governor makes it turn even slower. If the prop turns too fast then the governor makes it turn even faster.

The Solution

Learning the problem was the hard part. The easy part is solving it. I just had to buy a new propeller governor. The new one is constructed so that it increases oil pressure to make the blades move to high pitch (instead of low pitch). The new governor will be here in about two weeks. Installing it is pretty easy; just four bolts hold it onto the engine. Once I have it installed, I will be back in the air.