Going steady

A fixed-pitch propeller like those on most training airplanes is a solid piece of metal. It is optimized for cruising speed, climb performance, or a mix of the two, depending on the needs of the owner. A controllable-pitch or constant-speed propeller overcomes those limitations by altering the pitch of the blades to match the flight scenario, giving power during the climb and speed or efficiency in cruise.

Constant-speed propellers may be heavier and more mechanically complex, but those trade-offs are well worth it for most airplane owners. We say an airplane has a constant-speed propeller (singular), but it’s an entire system consisting of a central hub called surrounded by individual propeller blades. Most systems have two blades, but it’s not uncommon to find three in general aviation airplanes, and four or more in turboprops.

We call the props constant speed because, well, they maintain a constant speed. In this case, rotation speed, or rpm. Unlike a fixed-pitch prop, which will wind up as you descend, and spool down as you climb, a constant-speed prop will be rock solid regardless of the airplane’s pitch attitude.

It does this with the ingenious governor system. The governor is attached to the aircraft’s crankshaft, which is essentially a spinning piece of metal attached via rods to the engine’s pistons. Flyweights inside the hub move inward as the rpm slows down, and out as it speeds up. The flyweights are held in their respective position by a spring attached to the propeller control in the cockpit. Moving the control forward creates more tension on the spring, pushes the flyweights inward, requiring a higher rpm to maintain equilibrium. 

Engine oil is continually pumped into the system, which puts pressure on the flyweight spring. When the airplane descends and the propeller angle of attack decreases, the tendency is for rpm to increase. But as soon as that happens, the flyweights sense the difference, and through a check valve, the oil pressure at the propeller spring changes tension, adjusting the twist of the blades. The change in blade pitch slows down the rotation, and the system is in equilibrium again.

In control

Flying an airplane with a constant-speed propeller doesn’t take more skill than a fixed-pitch, but it does take more knowledge. Having the additional blue propeller control to the right of the throttle significantly ups the complexity because now instead of setting the throttle for cruise and descent, the pilot must also consider the propeller setting, the relationship between propeller and throttle, and how to optimize each one for the desired state.

To further confuse things, the propeller has largely taken over the rpm control, a job you previously asked of the throttle. That black throttle knob controls manifold pressure on a constant-speed propeller. The gauge is marked in inches of mercury, similar to the Kollsman window on the altimeter. It should read roughly the ambient pressure before starting the engine.

The engine run-up is the first operational change. There you will cycle the propeller control, checking to ensure the governor is working. When the rpm comes back, the manifold pressure goes up, and the oil pressure goes down. Most pilots do this three times on the first flight of the day in order to fully cycle warm oil through the system, but once is usually enough on subsequent checks.

On takeoff, the propeller is full forward. Then when taking the runway, slowly push the throttle forward over the course of three to five seconds. The system can lag a bit, especially when cold, and coming to full throttle slowly will give it time to adjust and not surge or overspeed.

Soon after takeoff, usually around 1,000 feet above the ground, the first decision comes. Many airplanes have a maximum power time limitation. For many years instructors taught not to fly “oversquare,” meaning not to operate the manifold pressure higher than the rpm in hundreds. So, if you pulled the rpm back from 2,600 on takeoff to a more reasonable 2,500 rpm for climb, they would first recommend pulling the manifold pressure back to 25 inches.

It turns out we were doing it all wrong. Not flying oversquare was an operating construct and had no basis in engine health or performance data. Many fixed-pitch airplanes routinely operate oversquare. We just don’t have a manifold pressure gauge to see it.

Current procedures and limitations vary based on the type of airplane and engine, and the only way to know for sure how to operate your engine is to read the manual. If the airplane and engine manual disagree, as is often the case, the airplane manual trumps. In the airplane manual of the newer Lycoming IO-540-powered 182, Cessna says that a normal climb is 23 inches of mercury and 2,400 rpm. Although, since this is the normal operation section, there’s no limitation to running the engine however you like. As you can see, this decision of where to set climb power is a decision best made long before taking the runway.

Engine expert Mike Busch told a Cessna 210 owner on AOPA’s Ask the A&Ps podcast that the best course of action is to leave the throttle fully open (forward to the stop), and not touch it until it’s time to land. This guidance may sound strange to people who have flown airplanes with constant-speed propellers for many years, but it is good advice if you consider that oversquare is perfectly acceptable in most engines. It brings down the complexity, gives operating flexibility through rpm and altitude selection, and makes best use of the engine’s potential power.

There is no single right answer on how to operate in cruise. You pick the value that is most important to you. Do you want to go fast? Are you interested in a mix of speed of efficiency, or are you trying to stretch your fuel dollars? Sometimes little tweaks can make a big difference. In the 182 at sea level, flying at 25 inches and 2,200 rpm results in about 75 percent power, giving you 133 knots on 12.9 gallons per hour. Bump that rpm up to 2,300 rpm and you’ll get two more knots, but at a cost of an additional half-a-gallon per hour of fuel. Playing with these settings and figuring out what works for you is part of the fun of more complex airplanes.

Coming in to land, you’ll notice that below a certain throttle setting the rpms begin to fluctuate again. In the pattern most pilots transition back to using rpms as the primary power setting, controlling it with throttle. On final approach the propeller will come full forward once again, prepared for a go-around.

Being an airplane system, things can and do fail. Seals go bad, which can spew oil on the windshield, and governors can fail, which will lead to a flat pitch, or high rpm situation. The manual describes how to deal with this, but it usually just involves pulling back the power. And in the event of an engine failure, you can yank that propeller control all the way back to take away as much drag as possible from the windmilling blades.

But most of the time the system will work flawlessly, keeping rpm perfectly steady far better than you ever could in a fixed-pitch airplane.

[email protected]