Plane & Pilot
Tuesday, April 1, 2008

Airplanes, Cars—What’s the Difference?

What you need to know, on the road and in the air

airplanes vs carsWhat’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.
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airplanes vs cars
airplanes vs cars
A 301-cubic-inch, 500 hp V10 (top) in a BMW M5, water-cooled with a redline at 8,250 rpm. A Continental IO-550N (bottom) in a Cirrus SR22. At a 550-cubic-inch displacement, it turns out 310 hp at 2,700 rpm.
The consequences of failure are different. When a car’s engine fails, you can simply pull over; when an airplane’s engine fails, you have to land. Pulling over at an unplanned spot is much easier than landing at an unplanned spot (particularly if it’s dark). Extra safety margins are expected and required in aircraft.

So, why? A lot of the differences have to do with the fact that it’s not economically feasible to make changes to existing aircraft engine designs. While a car-engine redesign gets its expenses amortized over many hundreds of thousands (or even millions) of units in a relatively short time, aircraft engines are sold by the dozen. Any change has a greater impact on average cost.

Costs of changes, too, are different. Assuming that all changes are going to be improvements (a huge and not entirely reliable assumption), an auto-engine improvement is largely an internal affair. The manufacturer does its own evaluation, design, testing and appraisal. Expensive, certainly, but that’s the price of progress. An aviation engine builder has all those expenses, plus additional, significant considerations.

An aviation-engine manufacturer needs to explain and justify its changes through the “certification” process with the FAA and various CAAs. Not only are the processes, tests and documentation expensive, but also the time involved can’t be overestimated. These regulators are bureaucracies, accountable to essentially no one, so they work on their own schedules.

There’s also the fact that current designs work. They have proven themselves for the past 70 years or so. We know how to operate them and how to maintain them; we know what weak spots to monitor. They may not be perfect (and they aren’t), but we can live with their needs, much like an old spouse.

So, why change? We change because we have to and because we want to. As fuels change, we need to adapt. As energy costs rise, we want more economical operation. As metallurgy improves, we can save weight, repair time and money by employing better materials.

What can we learn from cars? A hundred years ago, even 60, “aircraft technology” meant “superior.” The Tucker, the most-advanced “production” car of its time, used a 300 hp water-cooled Franklin aero engine right after WWII. Recent history, though, has seen technology going the other way, toward automotive-style metallurgy, higher-rpm engines and gearboxes, automotive-style electronic engine controls, auto-style fuel injection, and adoption of auto fuels, including unleaded gasoline and diesel.

A decade and a half ago, Bob Pond shook up the Reno race crowd with his high-revving race car engines and a lightweight airframe (in his Unlimited Class Pond Racer). Frank Thielert has recently brought automotive technology to aviation, in the form of his relatively tiny (121-cubic-inch, two-liter) diesels, now in the Diamond DA42 and several other aircraft under STC. Rotax, for nearly 20 years, has offered a geared, 1.1-liter, auto-gas-burning (but still carbureted) engine that powers many of the new LSAs. Lycoming and Teledyne Continental Motors (TCM) have models that feature fuel injection, modernized ignition and one-lever operation.

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