By Vic Syracuse, EAA Lifetime 180848
This piece originally ran in Vic’s Checkpoints column in the April 2023 issue of EAA Sport Aviation magazine.
While doing some recent designated airworthiness representative inspections, it was apparent there is a lack of knowledge regarding the initial setup of a constant-speed propeller. I thought I had explained this in a previous column, but in looking through my own documents, I can’t find any record of it, other than in the maintenance book.
When I am inspecting an aircraft for an initial airworthiness certificate, and it has a constant-speed propeller, I always ask if the low pitch stops have been adjusted. Most of the time I get an affirmative answer, but more often lately I have been getting a deer-in-the-headlights look, and I must explain it.
A few years back, I received an answer in the affirmative, but I had a nagging feeling the builder wasn’t exactly being truthful. I should have asked them to show me, but it was their second RV-10 aircraft build so I trusted and didn’t verify. Subsequently, on the first flight the engine over-revved to 3000 rpm! That requires a prop teardown.
So, what happened, and how do you make sure it doesn’t happen to you? Let’s start with understanding how a constant-speed propeller works. From a pilot’s point of view, the propeller lever in the cockpit controls the engine rpm, as adjusting the propeller control will do just that. However, what is really happening is that the propeller lever is connected to a device called the prop governor for a hydraulically controlled prop.
The hydraulic prop governor is basically a high-pressure oil pump. It takes oil from the engine lubrication system, which is typically running at 60-80 psi, and through its own gears it increases the oil pressure to 275-300 psi. The higher-pressure oil is now routed through the hollow crankshaft into the hub of the propeller, where it changes the pitch of the propeller blades. Increasing or coarsening the blade pitch puts more load on the engine, thereby decreasing engine rpm, while decreasing or flattening the blade pitch decreases the load on the engine, thereby increasing rpm. It’s like having a multispeed transmission for the propeller.
Notice I mentioned a hollow crank and hydraulic governor. Not all engines have a hollow crank, as some builders have come to realize when they wish to upgrade from a fixed-pitch prop to a constant-speed prop. Not all is lost, however, as there are other options, such as electrically controlled constant-speed props. These require a way to transfer electrical current from the controller in the cockpit to the propeller and are usually done using engine-mounted brushes and copper strips on the back of the spinner plate.
Some advantages of the constant-speed propeller are:
- We can get all the engine horsepower available for takeoff. Set properly, it will allow the engine to spin at rated rpm for takeoff. Instead of taking off in third gear, it’s like taking off in first gear.
- Takeoff distances are usually significantly shorter and climb rates are higher.
- Cruise speeds may not show as much gain, but aerodynamic braking during descent and landing are also much improved.
- In my flying experience, I have found that aircraft with constant-speed propellers also seem much more stable in turbulence, probably due to the heavier gyroscopic properties.
- There is no “hunting” of the engine rpm in turbulence. The rpm stays constant when the governor is working.
The disadvantages are the increased expense of the constant-speed prop and the required governor. There are also overhaul requirements for the prop and the governor, adding to the cost of ownership.
Let’s get back to understanding the initial setup, as this is important. Here’s the concept: We want the constant-speed propeller to act like a fixed-pitch propeller during the initial application of full power until we want the governor to take over. What that means is that we should set the low blade pitch stops so that the engine will only achieve 40-50 rpm less than max-rated rpm during the initial takeoff roll. With smooth application of power, we should see rated rpm just about the time the aircraft breaks ground, at which time the governor should take over and hold the rpm constant.
Here’s a real-life example. For a typical Lycoming-equipped aircraft rated at 2700 rpm, we should see about 2650-2660 rpm at full static rpm, meaning no movement of the aircraft and the propeller control in the full forward position. This could vary between seasons due to colder or hotter air, but’s it’s a good range to target. Then, just about the time the aircraft breaks ground during the takeoff roll, aerodynamic forces should allow the propeller to unload some, and the engine will reach 2690-2700 rpm. The engine should remain at that rpm until the propeller control is moved backward and decreases the engine rpm. You guessed it, we really need to make two adjustments — one prior to first flight (the prop) and one perhaps after the first flight (the governor). So how do we do that?
Well, if you look in the manual that came with the propeller, you’ll find the correct procedure. Basically, it could take you about three engine runs at full power to get it correct, so if you have a new engine, you will want to pay attention to cylinder head temperatures, being careful to not let them get more than 300 degrees on the ground. You may have to wait between runs to let the engine cool down.
Start by removing the spinner. For those of you with taildragger aircraft, I would recommend you tie the tail down and retract the flaps. You are going to have your head in the cockpit, and you don’t want to inadvertently put the aircraft on its nose. Keep the stick full back as well. It’s a simple procedure in that you are going to note the rpm at full throttle and then adjust the pitch stops accordingly. The pitch stops are located on the front of the prop and are adjusted by simply turning the nut or bolt in or out to increase or decrease the pitch of the blades. Hartzell props have a bolt that is turned using an Allen wrench, and MT props have two locking nuts. The respective manual will tell you in which direction to turn. I usually make a mark on the bolt or nut prior to moving it the first time to make it easier to see how much the adjustment affected the rpm.
You will want to check that the governor is not holding back the rpm on your first run prior to making any prop adjustments. Most of the governors come properly set up for rated engine rpm, but that can be impacted during the installation by the builder, so it’s best to check it. At full throttle, begin to slowly pull back on the propeller lever. If you immediately begin to see a decrease in rpm, then the governor is controlling the rpm. You will have to shut down and adjust the governor first by turning its adjustment screw “out” to allow more rpm before the governor is controlling the prop speed.
Once you’ve got the correct rpm at full power, don’t forget to tighten the locking nut, safety wire it if necessary, and reinstall the spinner. Now, you might have to adjust the governor after the first flight if you made any changes to the governor during the low pitch blade adjustment.
If after takeoff the rpm goes much past rated power, i.e., 2710-2720 is okay but 2700 is best, then adjust the prop control lever to 2700 while still at full throttle, and then, if practical, land without adjusting the prop lever and adjust the screw to meet the stop on the governor, and you should be all set.
If it’s a first flight, breaking in the engine is more important. Go ahead and fly that hour, and then, prior to landing at a safe altitude, go to full throttle and adjust the rpm for 2700 with the prop lever and leave it there. It will make the adjustment an easy one.
During takeoff, a nice slow application of power taking about three seconds to reach full throttle will allow a nice acceleration of the engine and prop. By the way, there’s an easy way to check for proper governor operation during your engine run-up. Make sure you are within the operating range of the governor, which can be about 2000 rpm for Hartzell governors and much lower for MT and PCU governors. Pull the prop lever back until you decrease the rpm by about 100. Then increase the throttle about 1-2 inches of manifold pressure. The rpm should not increase.
Having the low pitch blade stops set correctly will also give you a nice bit of braking in flight when the power is reduced, as the blades will go to flat pitch. It’s really helpful in formation flying.
Hopefully by now you understand the importance of properly setting up the low blade pitch stops prior to first flight. Not having the engine overspeed on that first takeoff is one way to keep the fun factor alive. That first flight should be all smiles.
Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA mechanic, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 10,000 hours in 74 different types. Vic founded Base Leg Aviation, has authored books on maintenance and prebuy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.