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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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.

As I touched down and rolled off the runway toward Air Service Honolulu, I couldn’t help shaking my head in amazement at the reliability of general aviation engines. I’d just made my 179th ferry flight and 17th partial Pacific crossing, and the realities of the trip were truly staggering.

I had run the Ovation’s Continental IO-550 powerplant for 13.25 continuous hours at 2,500 rpm. That’s roughly two million revolutions with nary a miss.

I was headed for Australia, still 4,500 nm away. After the usual butt-recovery day off, I knew I’d need probably another 12 hours on the next leg to Majuro, Marshall Islands. Then, I’d log perhaps eight hours more to fly to Guadalcanal in the Solomon Islands and a final eight hours on into Brisbane. Do the math, and that works out to another four million revolutions for a total of six million.

That, folks, is a lot of revolutions.

The beneficiary of all those revolutions is the propeller. In fact, the often-ignored and neglected propeller is what keeps most general aviation airplanes in the sky. It doesn’t matter how much horsepower you have out on the nose, it’s all totally useless without a prop to translate power into thrust.

The simplest props are fixed pitch, and if you fly a basic general aviation airplane these days, chances are you fly behind one. A fixed-blade prop is less costly, less complex and sometimes lighter than a constant-speed prop, and that translates directly into lower costs for owners and reduced hourly rates for renters.

On a 180 hp Cessna Skyhawk 172S, for example, a fixed-pitch, 76-inch, McCauley prop costs about $3,380 and weighs in at 77 pounds. In contrast, a constant-speed McCauley for a 230 hp Cessna Skylane costs $7,505 ($10,601 for the three-blade version) and weighs about the same. In other words, if you’re renting a Skylane, you can expect to pay a higher rental rate just to help offset the cost of the prop.

By definition, a fixed-pitch prop can only be optimized for one aerodynamic condition because its pitch is, well, fixed. Accordingly, most are pitched for a compromise angle between the performance demands of high power and high angle of attack (climb) and the requirements of reduced power and lower angles of attack (cruise).




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