Tuesday, April 1, 2008
Airplanes, Cars—What’s the Difference?
What you need to know, on the road and in the air
What’s the difference between car (or motorcycle) engines and aircraft engines? It’s a perennial question with a series of stock answers, none of which is sufficient.
It’s as obvious as asking the difference between cars and airplanes, but it’s as technical as explaining the differences between car pistons and airplane pistons. The quick answer is, “They do different things.” The longer answer is more interesting, so let’s get right into it.
Demands differ. It takes 12 to 15 horses to run a small car 60 to 70 mph (cruise speed), and the car’s peak horsepower is 10 times that. Cars never run at full power for more than a few seconds; airplanes may use 100% all the way to cruise altitude. The car spends a lot of its life at a 10% power setting; an airplane at 70% to 80%. Also, part of the drag the airplane engine overcomes is due to a wing’s requirement to produce lift (tires produce all the lift a car needs), so the engine isn’t just moving the airplane through the air, it’s also holding it up. With a helicopter hovering OGE (out of ground effect), this is more obvious, though the principle is the same as in an airplane.
Duty cycles differ. Two thousand car hours typically represent between 70,000 and 100,000 miles, about half the car’s expected life. With a piston-airplane engine, 2,000 hours is about all we expect. On the other hand, the car will run out that mileage in five to seven years; a GA airplane will last 40. Airplane engines are used infrequently, hard and for relatively short times, all of which promote additional stress and wear.
Operating environments are different. While it’s no picnic under the hood of a car, it’s worse inside a cowling. Particularly with air-cooled aircraft engines, temperature stresses and rates of change can be enormous. A 100-degree day on the ground can be below zero in mere minutes at altitude. Rain can shock unshielded air-cooled cylinders and heads. Manual operation of aircraft cooling systems (e.g., cowl flaps) means that these systems are operated suboptimally at best and incorrectly at worst, introducing additional stress.
Operators are different. Finally, we see a factor in favor of the aircraft engine! In general, pilots are better attuned to their engines than are car drivers. Offsetting this advantage, though, is the fact that pilots face more demands from their engines, which require more attention and are fussier about fuel type and grade, throttle and mixture settings (to say nothing of prop settings!), and temperature management, all of which are irrelevant or automatic in modern cars.
Maintenance is different. Modern cars don’t need “tune ups.” Unleaded gasoline, modern electronics, improved metallurgy, and constant design and material enhancements have coalesced into today’s modern engines that require only periodic fluid changes to stay healthy above 100,000 miles. Airplane engines have few of those advantages, but they do receive a professional look at least once a year—something car engines don’t get (and usually don’t require).
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