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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Flying oversquare is more than a little heretical to the old school of aviators. Traditional wisdom had it that you should always maintain MP in inches equal to or below rpm in hundreds. In the ’30s and ’40s, the universal advice was that you never exceed “square” settings (e.g., 24 inches and 2,400 rpm) for fear of detonation.

There were several reasons for adopting this technique. At the time, there were a number of aviation fuels available with octane ratings from 71 to 145, depending upon engine power, number of cylinders, compression ratio, etc. Pilots were often being trained relatively quickly in a succession of progressively more powerful aircraft with little transition time, and it was safer, faster and easier to teach only one, ultra-conservative operating procedure for all aircraft.

In a turboprop like the Piaggio P.180 II, pictured here, a 2,500 rpm setting can translate into jetlike speeds of 400+ knots.
Maintaining rpm equal to or “ahead of” MP was a supposedly safe procedure to teach as it would never result in detonation. In those days, many aircraft engines were radials, highly susceptible to the destructive forces of detonation. (To save money, one technique some pilots were taught was to refuel the aircraft with high-octane fuel in one tank for takeoff, climb and high-power maneuvering, then top another with cheaper, lower-octane petrol for low-power cruise.) Trouble was, traditional wisdom had it wrong. A few aircraft and engine manufacturers still cling to such antediluvian beliefs, but some do allow oversquare settings to reduce cockpit and flyover noise level.

The POH for the new Cessna 206H Stationair, for example, suggests that it’s okay to operate at 25 inches and 2,100 rpm at a 4,000-foot density altitude. That works out to 64% power and generates 129 knots on 14.5 gph. A setting of 21.5 inches and 2,500 rpm generates the identical speed, power and fuel burn, but with a considerably higher decibel count.

Pretty obviously, the oversquare argument becomes moot at higher altitude with a normally aspirated airplane, as power falls off at about one inch of MP per thousand feet. At an 8,000-foot density altitude in the Stationair, full throttle only produces 22 inches MP, so almost by definition, it’s tough to fly oversquare.

Lower rpm cruise has a number of benefits in addition to the obvious one of reduced noise level. Running a higher MP/lower rpm helps marry piston rings to the cylinder walls, resulting in better cylinder compression. It also reduces frictional losses and improves prop efficiency at lower rotational velocities. Additionally, it allows the valves to run cooler and normally results in lower EGTs and TITs.

So the next time you start the engine and taxi out in your Arrow, Mooney or Bonanza, remember that you have a greater variety of power settings than you may have been taught. Flying is a cooperative effort on the part of hundreds of mechanical components, but that rotating airfoil out on the nose is one of the most important.


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