Tuesday, July 17, 2012
General-aviation piston engines are simpler and more reliable than other power plants...most of the time
First, piston aero engines operate at considerably lower rpm. Many car engines rev to as much as 7,000 revs. Aircraft direct-drive engines are designed to operate at a redline of 2,800 rpm or less, primarily a function of prop tip speed. (There are a few engines that run at slightly higher rpm—remember the old Cessna 175 and the newer Cessna 421—but those were geared engines, not well received by many in the flying public. The complexity required was often regarded as counter to an aircraft engine's simplicity.)
At tip speeds in excess of about Mach .80, propeller efficiency falls off dramatically, and a typical 78-inch prop running at 2,500 rpm for cruise is devouring sky at Mach .77—about 577 mph—at the tips. That's one reason longer propellers are sometimes less efficient, rather than more efficient, than shorter ones. A longer prop usually increases drag disproportionate to its additional thrust. In other words, you may see better climb but worse cruise.
A three-blade tractor is a sometime solution, but this has the disadvantage of adding weight, and the same rule usually applies. The extra blade often means more drag—again, better climb but worse cruise.
Accordingly, most prop manufacturers concentrate their development efforts on mid-sized, two-blade designs rather than on props of longer diameter or those fitted with an extra blade. Hartzell's new semi-scimitar props are excellent examples of prop development that doesn't demand higher rpm, larger diameter or more blades, yet delivers more thrust and higher speeds.
Aircraft power requirements offer an interesting paradox, since aircraft engines operate at high power for long periods, whereas auto engines must increase and decrease power regularly and rarely use more than 60-70% (except for the kid down the street with the new Corvette). Considering that airplanes measure engine reliability in terms of time and cars in terms of miles, aero engines are almost ridiculously reliable.
A typical Continental IO-550-powered single that cruises at 180 knots (207 mph) is rated for 2,000 hours TBO. That's 414,000 statute miles between overhauls. Can you think of any car engine that can last even half that number of miles? (Curiously, my wife's Toyota 4Runner SUV has a nearly indestructible V6 that now has 298,000 miles on the original engine.)
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