-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Wednesday, May 05, 2004 11:00
AM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
20B - torsional vibration
Subject: [FlyRotary] Re: 20B
Al,
Same here on my 20b. Out of a 94 Cosmo. It has
the counter weight under the
accessory cover, no harmonic damper in the front or
back.
Barny
The reason I had
asked this question is because in doing some development testing on a 20B,
Powersport determined the need for a harmonic damper on the shaft at the front
to reduce a torsional vibration. The test were done with the Powersport
re-drive, which, if you are not familiar with it, is a tight tolerance spur
gear arrangement which is designed to be “stiff” in order to keep any
resonance frequencies above the top end of the operating
range.
I have been
looking (again) at the torsional vibration issue, and wondered in Mazda had
determined there may be a problem with the 3-rotor, either regarding the
engine internals, or the driven accessories. Upon further thought, I
realized that Mazda only used this engine with a torque converter coupling to
an auto trans; and the torque converter is about the best vibration damper you
could find. Putting a stiff drive on the output end changes the picture
entirely.
Tracy’s re-drive
design takes the opposite approach; i.e., a flexible coupling with some play
in the gears which lowers the resonance frequency to keep it below the low end
of the operating range. This obviously has been quite successful in
using with the 2 rotor rotary. Use with the 3-rotor is not yet proven.
Going to a 6-planet system obviously can handle more continuous torque, but if
you do have a resonance in the operating range; 6 vs 4 doesn’t really make
much difference, because you can get torque amplifications of 10 or
more.
On first cut, it
seems to me likely that picture with the RD-1B on the 3-rotor may be even
better than on the 2-rotor because of a further separation of the driving
frequency (higher because of more pulses/rev) and the natural frequency of the
re-drive system (which stays roughly the same). Unfortunately, until you
test, you don’t know, because the entire (geared) rotational system from front
pulley to prop is extremely complex analytically.
Now; if you are
not asleep yet, I will tell you that I was concerned about the amount of belt
vibration (alt and pump drive) I observed on my 20B during the dyno
runs. This could be caused by some non-uniformity in the pulleys, but
there seems to be no measurable runout. It is likely that it was the
result of torsional vibration. This could result in reduced belt life,
or a failed alternator mount, or nothing. One thing is pretty certain, a
harmonic damper on the front would reduce the risk of a
problem.
The torsional
vibration characteristics of the system will be different for the engine, with
the re-drive and the prop, than it was on the dyno. It won’t be long
(crossed fingers and toes) before I can observe what happens on the
airplane.
If I could find a
harmonic damper that will mount to the front end of the 20B e-shaft, I’d look
at putting it on (especially a viscous damper). It could reduce the
torsional vibration for the entire system. (Anybody know of
one?)
BTW; I’m not
trying to be an alarmist, it’s just that torsional vibration in pulsed drive
engines is a fact of life – auto engines, airplane engines; whatever.
Any torsional vibration resonance, which is what can get you, is something
manufactures have always had to deal with. People have in the past
wrecked dynos and broken drive shafts when unknowingly running into a
torsional vibrations resonce, and one topic-du-jour example is the MT prop on
4-cylinder Lycomings. Seems that there is mysterious cracking of prop
blades, and MT has restrictions on ceratin RPM ranges. I find evidence
(although limited and may just be my interpretation) of problems with a
3-rotor and the Ross drive. So we need to be observant when going into
uncharted territory.
Al