In rec.aviation.owning Matt Whiting wrote:
: Newer technology isn't always better. Airplane engines have evolved
: very well to meet the requirements of the airplanes they are in. Sure,
: some improvements seem pretty obvious such as fuel injection, electronic
: engine management, etc., but I'm not sure what I'd change to the basic
: engine architecture. For an airplane, an air-cooled engine with
: separate cylinders makes a lot of sense. A water cooled mono-block
: design would add a lot of weight and make field overhauls much more
: difficult. And water cooling adds several more failure modes (water
: pump failure, hose failure, thermostat failure, radiator failure, etc.).
: I'd say that with respect to my automobiles over the last 30 years
: that I've had more problems with the cooling system than with any other
: part of the engine.


As I like to explain to people, traditional aircraft engines are *very* reliable in the
short-term, but very unreliable in the long term. In other words, the chances of the engine failing for
a 3-hour cross country flight is very low. The chances of the engine needing maintenance in 50-100 hours
is pretty high. The chances of it needing *expensive* maintenance in 500-1000 hours is VERY high.
Consider all the care and feeding necessary to keep them going:

- Magneto maintenance.
- Spark plug cleaning, gapping, replacing.
- Oil changes more often because of blow-by from loose-tolerances required for air-cooling, lead
contamination from requiring leaded fuel, quicker breakdown due to higher operating temperatures, etc
- Significant amount of top-end wear due to high operating CHT's.
- Sticking valves due to high top-end temperatures.

Air-cooled engines have a much higher octane requirement than would be necessary for geared,
liquid-cooled engine. What liquid-cooled engines would require 96 or 100 octane for an 8.5:1 CR? With
96 or 100, a liquid-cooled engine could easily run 10:1 or more and get 10-20% more power for the same
fuel burn.

I applaud the Rotax design, although I agree that it's a bit too high-strung for using in
high-volume certified aircraft. What's needed is a larger version utilizing the same technology that
isn't run quite so hard.

-Cory

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* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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