I am not an engineer, and my comment here is based on some reading and
observations that I have made over the years in aviation. My opinion is
worth the electrons that were used to produce this post.
In every machine that has rotating parts, torsional harmonic vibration
is present. I'll call it THV. If a vibration coincides with an operating
range of the machine, the THV can destroy the machine. If the frequency
is above or below the operating rotation, it becomes much less of a
problem. In an automobile, THV is damped by the rubber tires, and the
greater rotation mass of the flywheel, and, often, automatic
transmissions. On an airplane, the power pulses go undampened to a
propeller, which has its own harmonic vibration that can be fed back to
the engine at certain frequencies and can destroy the engine. Wooden
propellers tend to dampen this because of their composition.
Aircraft engine manufacturers take torsional harmonic vibration into
account when designing their engines. That's why some certified direct
drive engines come with a warning not to operate them at certain rpms
with certain propellers, or they have counterweights to smooth things
out. Rotax engines, for example, have a coupling that dampens THV.
Engines are certified in aircraft with designated propellers for that
reason.
Torsional harmonic vibration can snap crankshafts and destroy reduction
drives. It is similar in concept to aerodynamic flutter in flight
controls. The THV does not disappear where the power pulses are
overlapping, so that all of the forces going in same direction as they
are in 6 or 8 cylinder engines, but at least all of the force going to
the prop is in one direction. 4 or less cylinder engines have periods in
their rotation when there is a backward force on the power output. The
inertia in the engine, as well as flywheels or propellers keep the
engine turning.
There are very few companies that have been successful in experimental
aviation with reduction drives. That doesn't mean that there aren't good
ones out there, but the amount of engineering and manufacturing skill
needed to build them can be immense and costly, and they have to have a
big enough market to be profitable. It's also my opinion that as the
power of the engine goes up, the forces on any reduction drive go up
exponentially, and that's why it's easier to make a drive for an engine
developing under 100 hp (which seems to be the dividing line) than one
that has more power. It's for those reasons that I don't believe you
will see a Rotax 200 hp engine.
Years ago, there was a manufacturer of a redrive that was used on large
engine conversions. It seemed to be a good one, with few problems, but
when the drive was manufactured by someone new, the drives started
breaking. I credit the problems to manufacturing technique.
I have flown behind a Cessna 175, which has a geared engine. It was
excellent both in climb ability and cruise speed, I found it close to
the performance of a 182, and it had a fixed pitch propeller. The 175
got a bad reputation, I believe, because pilots tried to operate it like
an ungeard engine. The engine was designed to run at above the red line
of ungeared engines, and operating it at low ungeared rpms is very hard
on it. Also, it was not good practice to let the prop drive the engine.
Consider the reputation of anyone making a reduction drive, and look for
reports of people that have problems with it. Google engine problems or
engine issues, and look at what other engines they have sold in the past.
That's my 2 cents. Your mileage may vary.
Dan Branstrom
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