Plane & Pilot
Thursday, May 1, 2008

Understanding RPM

Whether you fly behind a fixed-pitch or constant-speed prop, a little knowledge definitely is not a dangerous thing

rpmIt was just after 6 p.m. when I turned final for runway 4R at Honolulu International Airport. My 2,160 nm crossing from Santa Barbara, Calif., into the wind had required 13 hours and 15 minutes, yielding an average speed of 163 knots. I’d maintained 8,000 feet in the new Mooney Ovation for most of the trip, climbing up to 10,000 feet for the last 500 nm into Hawaii to take max advantage of the standard trade winds.
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The throttle quadrant of a modern Piper, with the blue propeller control clearly visible.
With a fixed-pitch prop, the primary power-management tool is the throttle. Mixture comes into play as well, but to a far lesser degree than the throttle. That means you’ll be tempted to analogize rpm directly to horsepower, which would be a false analogy. As the airplane climbs into thinner air at higher altitude, the prop blades meet less air resistance and full throttle continues to develop near-redline rpm, though both horsepower and thrust decrease. Eventually, engine power is reduced so severely that rpm begins to fall.

For better or worse, you’ll probably be at full throttle all the time (assuming you cruise above 8,000 feet MSL) until you initiate descent for landing. At that point, you’ll want to decrease power to avoid exceeding the max cruise setting as the aircraft loses height.

The majority of general aviation airplanes turn their engines at 2,800 rpm or less, mostly “or less,” especially in the current atmosphere of good neighbor airports. (Remember, that’s “rpm,” not “rpms.” Obviously, “revolutions per minutes” is meaningless.)

Noise pollution is a real problem at airports all over the world, not just at the major airports that host big jets. In fact, some aircraft and engine manufacturers have taken to derating large engines to use lower redline rpm, specifically to reduce noise pollution.

Another factor governing rpm is prop efficiency. Your car can transmit its power directly to the asphalt, so the only limit on rpm is purely mechanical. In contrast, propellers lose efficiency beyond a certain point. Turn a 78-inch-diameter prop at 2,800 rpm, and you’re generating tip speeds of roughly 565 knots at sea level.

There are about a dozen variables governing the effectiveness of a propeller, but one of the most operationally critical is tip speed. Generally, the closer the prop tips come to the speed of sound, the less efficient the prop. Mach 1 is equal to about 650 knots at sea level, so a pure tip speed of 565 knots is equal to Mach 0.87. Prop efficiency begins to decrease dramatically when tip velocity approaches or exceeds Mach 0.85.

The engine page in a turbocharged Cessna (previously Columbia) 400. The two big center gauges—manifold pressure and rpm—along with TIT, CHT and EGT, are key in aiding the pilot to set the engine/propeller to an efficient setting.
That’s because portions of the prop will be operating beyond Mach 1, generating a shock wave, separating the airflow and severely dissipating propeller energy. In plain language, no matter how fast you turn the engine, the prop tips can’t exceed a certain speed.

This means most direct-drive general aviation engines are limited to about 2,700 rpm and prop diameters rarely exceed 77 inches. A few engines have been produced that turn at up to 5,500 rpm and utilize gearing to reduce prop rpm to 2,700 revolutions or less. With a fixed-pitch prop out front, there’s only one way to achieve max cruise, and that’s with max rpm, so you won’t have a lot of choice on throttle setting.

Turn the propeller slightly slower, and you’ll reduce tip speed, lower cockpit noise level and improve prop efficiency, to a point. You may also reduce vibration level, and on some airplanes, that alone may be justification for turning the prop slower.


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