January 2006Features

Smooth Operator

Constant-speed props give silky rides and lots of choices


When people learn to fly, one of the simplest ideas to master in a basic training aircraft is how to increase and decrease engine power. Want to go higher or faster? Start by making the fixed-pitch propeller work harder--that is, make it spin faster. Want to come down? Slow down the propeller and let the nose come down--you will descend at your trimmed airspeed. Or, slow down the prop but keep the airplane level with back-pressure and you will decelerate while maintaining altitude. The pilot effects these changes with the throttle. When flying with a so-called fixed-pitch propeller, more power equals more propeller revolutions per minute (rpm).

Look over this fixed-pitch propeller during your next preflight. Notice that it has blades that protrude from the hub at a fixed angle, formed with a twist part way out along the blade so that the tips, which travel faster than the inner regions, remain aerodynamically efficient. Note that the prop is a single, continuous unit. This is seen most clearly on a prop with the spinner removed for maintenance, where you can inspect the prop right to the hub--where it is bolted and safety-wired onto the crankshaft.

Not all fixed-pitch props are the same. Some are more efficient for high-revolution operations or at lower rpm, giving rise to the concept of "climb props" that turn fast because of a low blade angle of attack, and "cruise props" that turn slower because of their higher pitch. Each represents a design compromise.

If only there were a way to get the best of both worlds in a single propeller....

And now you are on your way to understanding the pleasures of flying behind a propeller that can climb with authority, then give the pilot smooth, efficient cruise flight with a simple twist of the blades. The so-called constant-speed propeller, the most common breed of controllable propeller in the general aviation fleet, lets the pilot change the angle of attack of the blades, then maintain a predetermined speed of propeller rotation to achieve the best performance possible for the amount of engine power being used.

You may recall from the federal aviation regulations that a feature qualifying an airplane as a complex aircraft is a controllable propeller, along with flaps and retractable landing gear. However, not all airplanes equipped with constant-speed propellers are complex. Numerous fixed-gear airplanes use constant-speed propellers. Inspect a constant-speed propeller. It has a different appearance from the fixed-pitch-prop. The two (or more) blades rest in a hub assembly that allows them to twist from high blade pitch to low pitch, all controlled from the cockpit. There, you will see the familiar throttle and mixture controls, plus a third lever (or plunger-type knob) for controlling the prop. Often this knob is blue. On the panel the familiar rpm gauge--the one you set power with in a fixed-pitch-prop airplane--remains. But now it performs a different function. No longer a power gauge, it displays only propeller rpm. You read power output on a new gauge: an engine intake-manifold pressure gauge.

To see how this works, climb aboard for a short flight out to the practice area and back in an airplane equipped with a constant-speed prop. Compare constant-speed prop operation with fixed-pitch prop use. The airplane for this familiarization flight is a Cessna 172RG Cutlass, a typical trainer for pilots transitioning to both constant-speed propellers and retractable landing gear.

If the prospect of using a constant-speed propeller seems intimidating, relax! Turns out that the prop control comes into play for steady-state flying, such as cruise or a climb to altitude, but that's about it. For takeoff, it is simply set to maintain high rpm, forcing the blades to the low-pitch position, and is returned to that setting on final approach for landing (in case a go-around is necessary). At very low power settings, the mechanism that governs rpm is fully employed, meaning that any further power reduction cannot further reduce blade pitch. Pilots say that the prop is "off the governor"; it's essentially a fixed-pitch prop at that point.

Preparing our C172RG for flight, we run through the checklists. All is familiar until we come to item 2 under Starting Engine: Propeller--HIGH RPM. This reminds the pilot to make sure the blue knob is fully forward (set to low pitch). This is also a transitioning pilot's first encounter with a basic rule of operating the mechanically somewhat delicate constant-speed prop: when making power and prop-speed changes together, increase prop speed before increasing power; decrease power before decreasing prop speed.

The checklist remains in familiar territory as we complete the engine start and progress to the Before Takeoff checklist for the engine runup. The magneto check we have always done is as before. So is the carb heat check. The next item is new: Propeller-- CYCLE. Move the propeller control from high to the low rpm setting, then return it to high rpm (full in). This is to check that the prop control mechanism is functioning.

The next reference to the prop is in the takeoff checklist. As we add full throttle for takeoff, we are looking for 2,700 rpm on the tachometer. This may require no adjustment of prop speed.

It is during the en-route climb that the first significant change in procedure occurs. Unlike the trainer you first flew, in which you probably climbed at full throttle until reaching your cruise altitude, this complex aircraft is normally flown, after the initial takeoff climb, at an en-route climb setting of 25 inches of manifold pressure (mp) and 2,500 rpm. Reduce power first in accordance with the rule stated above! After you have throttled back to 25 inches mp, gradually rotate the outer portion of the prop control counterclockwise until 2,500 rpm shows on the tachometer. The change will be audible. This is your climb profile, stated as 25 inches and 2,500 rpm. Indicated airspeed should be 85 to 95 knots.

It is during the cruise phase that your many choices of power and rpm help you satisfy the goals of any particular flight. To get an idea of how a constant-speed prop expands your choices, compare: The cruise performance tables for a 1978 Cessna 172N, with its fixed-pitch prop, occupy a single page in the aircraft's pilot's operating handbook (POH), and give power settings and performance at altitudes from 2,000 to 12,000 feet, at three temperature intervals. The Cessna 172RG's POH has six pages of cruise performance figures for the same altitudes and temperature range. Why?

The answer is that you have so many mp/rpm combinations to choose from. This is a short flight to the practice area, at 2,000 feet. At standard temperature, the Cessna 172 has five published choices for cruise power, given in 100-rpm increments from 2,100 rpm to 2,500 rpm, each yielding different airspeeds, fuel burns, and percentages of brake horsepower (bhp) produced by the engine.

The 172RG cruise performance table for 2,000 feet is also arranged by rpm settings--but there's a difference. Each rpm increment is accompanied by four choices of manifold pressure (power), yielding a total of 20 choices for cruise power at 2,000 feet and standard temp. For example, set up 2,300 rpm at 23 inches mp. You'll get 65 percent bhp, 123 knots true airspeed, and burn 8.7 gallons of fuel per hour. Throttle back to 22 inches mp but leave the prop alone (blade pitch automatically adjusts to maintain 2,300 rpm) and you will be running 61 percent bhp with 119 kt KTAS, burning 8.2 gph.

The highest cruise speed available on the 2,000-foot-level chart, 132 KTAS, is achieved at the combination of 25 inches/2,400 rpm, at 76 percent bhp and burning 10.1 gph. Not in a hurry? Throttle back to 20/2,100, 48 percent bhp, and get 103 KTAS and an economical fuel burn of 6.7 gph. The cruise-performance tables for other altitudes give similar choices. As you go higher, available manifold pressure for the normally aspirated engine decreases.

Don't be alarmed at the abundance of choices--this is designed to give you versatility. The truth is that any mid-range combination of mp and rpm will get you there until you become familiar with the airplane. In some aircraft, certain combinations of mp and rpm are prohibited. See the pilot's operating handbook. Also, some aircraft have an operating restriction known as METO (maximum except takeoff). METO might require that a very high power/rpm setting be reduced, say, no longer than five minutes after takeoff.

Flying maneuvers or through air burdened with up- and downdrafts is smoother and more comfortable with a constant-speed prop because, as the air load changes on the prop, its governor compensates to maintain the rpm. Under similar conditions, a fixed-pitch prop speeds up and slows down erratically, transmitting vibrations to the cockpit (and its occupants), often luring inexperienced pilots into chasing the fluctuating rpm with inefficient--and ineffective--power and trim adjustments.

After we experience the flight characteristics for a while, it is time to head back home and investigate management of a constant-speed prop during descent and landing. Reducing power, perhaps to 17 inches mp, the airplane noses down to maintain trimmed airspeed and we descend comfortably to pattern altitude. No need to touch the prop control yet--the prop governor maintains constant rpm.

Leveling off in the pattern and throttling back some more to slow down, it is time to extend the landing gear, then deploy flaps as airspeed backs off into white-arc territory. A pilot used to throttling back to 1,500 rpm in a fixed-pitch trainer will have no problem remembering 15 inches mp as a reference setting for carb heat, and then for commencing descent at the end of the downwind leg.

Now comes an important prop-management step from the prelanding checklist: returning the prop control to the high-rpm setting. This will give best climb performance in case of a go-around and also avoids the undesirable combination of high power and lower prop speed, which could damage components of the prop assembly. If you learned to recite the mnemonic GUMPS before landing your first trainer, you are ahead of the game for remembering this step because now the letters U and P, for undercarriage and propeller, count. From now until shutdown on the ramp, the prop demands no more attention. Leave it in the high-rpm position for next time.

Knowing how to fly behind a constant-speed propeller is necessary for pilots seeking a commercial pilot or flight instructor certificate. A constant-speed prop will be on the agenda for the aircraft buyer who wants to fly long distances with speed and in comfort. And as a way to squeeze more performance out of an aircraft while enhancing your knowledge and experience, flying behind one is just plain old aeronautical fun. Dan Namowitz is an aviation writer and flight instructor. A pilot since 1985 and an instructor since 1990, he resides in Maine.

Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.


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