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In a message dated 1/22/2004 9:23:51 PM Central Standard Time,
gregory_fuess@yahoo.com writes:
<< Forgive me for beating a dead horse about torsionals. I have been reading
another list for (VAF and RV-7) ~2 years, and have anxiously anticipated
installing a 13B, and now instead a Renesis engine, in my soon-to-start
RV-7. Now, I am hearing things that cause me to question my ability to make
a sound decision in this regard, as this is the first discussion of
torsionals is the first I have heard. I am beginning to question my ability
to follow through on what I had taken to be the best alternative engine
choice.
Let me swat that horse a few times. I repeat. This is not a factor. Only a
near solid connection makes any of this a problem and there isn't one in the
Ford based reduction system. The pieces have enough slop to null out nearly all
of the offending impulses.
There are no reversals in a rotary, while there are substantial reversals in
a piston engine. If you are looking for an engine that can pitch half of a
propeller blade, buy a flat 4 cylinder (real) airplane engine.
The spring coupling used on the Ross set up, or the plastic coupling used by
Tracy's design provide more than enough of a disconnect to eliminate these
impulses as a hazard to the propeller, or the gear train and drop the remaining
offensive pulsations to the idle speed of the engine.
I believe everything your engineer friend said. It all sounds accurate to
me. However, there is an endearing quality in people that requires them to
reinvent the wheel every so often. Many competent people have been forced to cure
a failure mode in a torsionally resonant system. Any system that uses a drive
shaft to drive a propeller with a piston engine, or anything else, would have
to face down such problems. So Molt Taylor and Jim Bede and many others in
the automobile, trucking, and farm equipment industry have had to cure this
problem.
Molt Taylor used a coupler filled with steel shot to disconnect the engine
from the drive system.
Jim Bede used soft rubber donut joints in the drive line to null resonant
impulses from a very shaky cycle engine.
There are mountains of material available on how this problem has been
overcome. It is a very destructive
phenomena and a wise man thinks through each application to determine if
there is a possibility that he has designed in a fault that may show up some day
to bite him.
There are not just two basic ways to defeat the problem, but three.
The most obvious method is used by Mazda in the automobiles. A heavy
flywheel. Just that simple.
We are not interested in anything heavy in air plane use so that leaves only
a tight system that uses the gearbox and prop as the flywheel to absorb the
impulses, Or, the loose, or soft system that isolates the engine from the gear
box as in a spring system or a plastic or rubber isolation system.
The tight system requires great rigidity and close fitting, rather wide gear
assemblies. As has been proven, very expensive. Very heavy.
The loose system requires only a small amount of compliance to drop the
offending RPM close to the idle speed of the engine. The plates that mount the
plastic isolation discs or springs provide a measure of flywheel effect and the
problem is solved. Great rigidity is not required. The teeth in the gear train
can be small, and as a result lighter and cheaper., in fact, the gear train
is available at any Ford dealer, so the very conservative could change the gear
set every 500 or 1,000 hours if it makes you feel more comfortable. This
system cannot transmit high amplitude resonance to the propeller, so a wide range
of propellers can be used.
Here are two links you may like to review. The second link has to do with
the development of the BD-5 drive train.
http://home.earthlink.net/~rotaryeng/ACRE.html
http://www.prime-mover.org/Engines/Torsional/contact1/contact1.html
Lynn E. Hanover
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