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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As you reduce rpm below the sweet spot, the airplane will assume a progressively higher angle of attack, presenting more drag, but at a better trade of fuel for speed, down to about 45% power. Below 45%, most airplanes are so far behind the power curve that drag increases outweigh reduced fuel burn, and nautical mpg and absolute range begin to decrease.

Stay with me on this. It’s numerology in the extreme, but it can translate directly to dollars in your pocket. All these numbers are direct from Cessna’s respective Pilot Operating Handbooks (POHs). On a Cessna 172S Skyhawk, the engine is rated for 180 hp at sea level and 2,700 rpm. At an 8,000-foot density altitude with 2,650 rpm dialed in, the engine is down to about 72% power, worth 122 knots in exchange for 9.9 gph. That’s 12.3 nmpg. At 2,300 rpm, you’ll cruise at 48% power, true 100 knots and burn only 7.1 gph. That’s 14.1 nmpg.

Generally, the closer the prop tips come to the speed of sound, the less efficient the prop.

In other words, if you need to fly farther on the same fuel, use the lower power setting. (Duh!) Remember, however, that you’re sacrificing 22% of your speed in exchange for only 14% better fuel burn, so you obviously need to value your time less than your money.

Don’t automatically assume the rpm you’re seeing is the rpm you’re turning, by the way. Just like most people, standard analog aircraft tachometers tend to slow down as they age, so an old tach that hasn’t been overhauled in 20 years may be reading as much as 100 to 200 rpm lower than the engine’s actual revs.

Twenty years ago, a good friend with a Cherokee Six 300 complained that he couldn’t get his engine to deliver full power on takeoff, but he saw better-than-book cruise speed, if at a higher fuel burn. That sounded like a familiar problem. I put my friend in touch with a good mechanic who strobed the prop and discovered it was indicating about 125 rpm low. (Some pilots sidestep the problem altogether by purchasing a battery-powered electronic strobe and mounting it on top of the panel to monitor prop rpm.)

rpmSimilarly, don’t assume you can use any rpm you wish for cruise. Some airplanes (the Piper Arrows, most older Mooneys, the Beech Sierras, the Commander 112s, the Cessna Cardinal RGs) have restricted areas of sympathetic vibration— resonance zones marked on the tach where operation isn’t allowed. These airplanes are restricted between 2,200 and 2,350 rpm.

A constant-speed mechanism in an airplane serves a similar function to the transmission in a car, allowing the propeller to generate additional thrust at both high and low angles of attack. This helps improve both climb and cruise performance.

A constant-speed prop changes the rpm equation, because the governor allows the prop to maintain a constant rpm under all normal power settings. This provides the pilot with a choice of power settings between high rpm/low manifold pressure (MP) and low MP/high rpm. (Turbocharged airplanes again complicate the issue slightly, though turbonormalized models flying in the flight levels often can safely use oversquare settings similar to normally aspirated airplanes operating at low altitude.)


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