On Tue, 25 Nov 2008 19:29:06 -0800 (PST),
wrote:

Both the Merlin and Allison V-12s were geared. And most of the
big radials were geared. It was one of the few ways to get more
horsepower out of a given displacement.

Gearing an engine doesn't increase the horsepower; it multiplies the
torque and divides the rpm down to a more usable (by the propeller)
level. It does allow you to build a faster turning (and thus higher
horsepower) engine and still be able to use it.

In general, the way to get more horsepower out of a given displacement
is to turn the engine faster, within limits of course. With the
higher rpm's come increased wear and heat. In the "old" days (up to
the mid 1930's or so) the available materials weren't adequate for a
high revving engine, so low rpm's were the norm, and by happy
coincidence the avilable rpm's were pretty well matched to propeller
sizes convenient for the aircraft. As the technology advanced and
higher rpm's became reasonable, reduction drives began to appear,
especially on higher powered military aircraft... with some compromise
(as others have pointed out) in reliability.

Most of the small aircraft engines in use nowadays are derived from
those 1930's engines, with only minor improvements. The A-65, for
example, was redlined at 2300 rpm; the A-75, if I recall correctly,
was the same engine upgraded to turn a little faster, and today's
0-200 turns around 2600 rpm if I'm not mistaken. By contrast, modern
car engines are redlined at up to 8000 rpm.

There two main reasons we're still using the "old style" aircraft
engines. First is cost; not only does the gearing and such cost more
money, but the existing engine designs are long amortized. Developing
a new engine costs a lot of money, especially with the costs of
_certifying_ a new engine. Second is reliability and longevity; a big
slow turning engine is more reliable and lasts longer than a smaller
fast turning engine of the same horsepower.

Where we _are_ seeing reduction drive engines is in the ultralight /
light sport area, where weight is much more critical. The ultralight
movement introduced fast turning 2-stroke engines, mostly snowmobile
derived, to aviation. When your engine turns 6500 rpm you NEED a
reduction drive! For an ultralight, light weight is far more
important than a 2000 hour TBO. In the case of the Rotax 912, a
geared 4-stroke, Rotax was already used to building engines with
redrives, so it made sense for them to take that approach.

-Dana
--
The gene pool could use a little chlorine.