Thanks to Murry for finding this.

 It looks like we can just use the Ford splined part to mount the SGF
 damper and an .090" thick 4130 plate to hold the ring gear. This is
 easy to make with nothing more than a lathe and easy try.

 Paul Lamar


 Looks like the driveshaft coupling I mentioned once. How well will it
 dissipate the heat?

 Dale Davies

 Good question. On second thought we are in this for the long haul.
 It might be a mistake to depend on just one part like the SGF that might
 disappear.

 Probably better to build our own. Another advantage to that is we
 can tailor the durometer of the rubber bushings to our needs.

 Paul Lamar

 Hi, they are used in high volume production cars: Mercedes, Corvettes,
 BMWs, etc. also sold for marine use.


 Murry I Rozansky

 There are many sizes. I don't think they all use the 300 ft pound version.

 Paul Lamar

After reading about all the failures related to the damper, I started
to research into the subject of torsional resonance and found this
article on how they have solved their problem on Bede 5. The original
PDF is rather long so I just copy part of it for you to take a quick
look. You can find the original file here
http://ibis.experimentals.de/downloads/torsionalvibration.pdf

The part that I think is of most interest is reproduced here:
"THE FREEWHEEL CLUTCH
The idea for a freewheel clutch came from our machinist, Ray Johnson,
and I must give him the credit. He came in one day and told me that
when he was a kid, his dad had a thrashing machine on the farm They
would run the belt from the tractor to the thrashing machine and it
had a freewheel device on it so that any vibration coming from the old
two cylinder John Deere would be somehow taken care of. The minute he
said "freewheel clutch", it rang a bell. After previously seeing the
oscillating torque reversals of the prop I knew we needed some way to
allow a torque reverse to occur without the bounce back. I had been
looking at centrifugal clutches, manual clutches, etc. that could be
used to disengage and allow some slip. We even tried a test with a
super loose belt with idler pulleys but the slop still wasn't enough.
The torsional amplitude was just too great at the low system frequency
we were dealing with.

Immediately after Ray mentioned the freewheel clutch, I started
investigating and found a Borg Warner clutch that was used in
automatic transmissions It was a double cage, full phasing sprag
clutch. The double cage caused all the little cams inside to engage
precisely at the same time. I had previously had experience with
freewheel roller type clutches at Avian but we had problems with the
brinelling of the clutch races when one roller would engage before the
others and momentarily take the full torque, causing eventual clutch
failure. The double cage full phasing, sprag clutch solved this
problem."

From there, I wonder if Tracy's original design really works or not? I
am not sure if enough on the subject of torsional resonance has been
done. May be even a different approach is necessary?

Also, from my readings, it seems that many people recommends more
cylinders for smoother torque curve. In my simple mind, a smoother
torque curve alleviates the problem. The 2 rotors have one firing
every 180 degree, similar to a four stroke so torque curve is not
smooth. (See information on EPI Inc. on this subject)  A 3 rotor is
similar to an in-line 6 at 120 degrees so may be 20B is more
suitable/reliable for aviation? A 4 motor would be even better. Many
great airplanes in WWI are V12s and they would not have a lot less
problem with torsional vibrations.

I am not a mechanical engineer, so anyone out there please correct me
as much as they can. I am not building yet, not even a license to fly.
But sure hopes to find time to do it soon.

Henry Nee


This is to avoid having a critical "chatter RPM"..

What causes chatter RPM?

Related question:  Does the relative value of the peak "+" (positive)
torque vs the peak "-" (negative) torque change significantly with
RPM?  Or are they on different curves.  The peak "+" torque component
should be related to cylinder pressure due to combustion. The "-"
torque value should be related to how much effort is required to
complete the exhaust, intake, and compression strokes.  Compression
stroke generates the highest "-" torque?

The chatter RPM is caused by the frequency of the torque reversals
coinciding with the natural torsional resonant frequency of the
gear/propeller (acting as a spring-mass system)...

Adding a clutch allows for a piece-wise behavior - below the
engagement RPM, the prop is disconnected, not loading the gearbox/prop
spring-mass.  Above the engagement RPM, the engine does load the
system.

Downside:

- Can't use the prop to turn the engine
-- Preflight/debug - to check for even compression - requires another
method of turning the engine
-- To prime fuel into the engine - requires electric starter
-- To restart the engine while in flight - requires electric starter
-- To hand-prop the engine

Several years ago, I mentioned to the group that I could envision a
method for changing the "springiness" of the gearbox/prop.  My
thinking was that a mechanism could be devised that would change the
effective diameter of the driving "fork" on the rubber damper,
adjusting the springiness of the system.

I believe the elegant solution is to configure the gearbox/prop/damper
system so that the resonant frequency is at low enough RPM to not have
an impact on continuous operation (ie, below an acceptable idle
speed), and then use enough rubber so that the start/shutdown cycles
(where the RPM is crossing the resonant frequency) don't damage
anything.

Follow on topic/question: what's happening to the rubber damper while
not near the resonant torque range?  What are the peak loads in the
material?  How many cycles can it take?  How much heat is generated?
It's still being flexed...

Another thought...  I think it might be educational to put alignment
marks on the various parts of the gear train and then use a
strobe-style timing light to observe the relative position of each of
the components at various RPM.  It would help to see how much the
dampers are being flexed..  I realize it's probably difficult to get
your eyes to the various places while this is going on.  Periscope?  A
syncable/triggerable imaging sensor (camera/borescope) would be
ideal...


Matt-