From: Lehanover@aol.com
Message-ID: <155.2bfe16b4.2d42cee8@aol.com>
Date: Fri, 23 Jan 2004 14:24:24 EST
Subject: Re: [FlyRotary] FW: [VAF Mailing List] Engine Choice
To: flyrotary@lancaironline.net
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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