Date: Sun, 31 Jan 1999 23:39:32 -0800
From: Paul <rotaryeng@earthlink.net>
To: z <rotaryeng@earthlink.net>
Subject: Strange low frequency vibration.
Bill Freeman wrote:
Jim Mosur wrote:
Paul.
snip
Today I flew my RV 6 with Neil K Ivo prop on it, I am running a
13B,
plugs up, with a Ross PSRU 2.17-1. I have been flying with a
Aymar-Demuth 68 x 74 wood 2 blade prop, for 37 hrs. I have been
experiencing a continuous pulse or throb at any constant RPM flying
or
ground running, sounds like a twin with props out of sync. Running
Neils small dia. (60") 3 blade Ivo the throb has disappeared, we
checked my Aymar-Demuth for balance, pitch and run out, all were
perfect. I intend to fly tomorrow with more pitch in the Ivo as we
had
it set a little fine, bags of revs. but not much speed. I had
orignally thought the throb was caused by having my conical engine
mounts cranked up too tight, but now, any thoughts?
snip
Makes me nervous. Any gearbox driven by anything other than a
turbine is going to be suceptible to torsional vibrations, and it can
even
happen with a turbine. With a "throb" like you have described, it
would
likely be a beat frequency - i.e. the difference between two
vibrations
that are nearly the same frequency. Changing the prop changes the
mass at the end of the system and will change the resonance frequency.
If the throb had NOT gone away after changing props, it would not
likely
be a torsional resonance.
Another possiblity is an interaction between the prop pulse
freq (two bladed) and the exhaust or airframe. With a the IVO (three
blades, I think) you might change this interaction enough to get rid
of
the throb, so if that is the case, it would not be a torsonal
resonance.
The Long EZ aileron linkage would couple into the engine/prop
driving freq and we had one lose the linkage to one aileron. I pointed
this out for Rutan and they redesigned the lever in the control
system.
Pretty exciting as the aileron flopped about with the balancing tab
digging
in and flipping it up, then it would go back. The pilot managed to
land OK (was in the pattern), but when he sent me the part, it was
clear that we had a resonant vibration.
Is some part of your airframe or control system resonating with the
exhaust or the prop wash? Try touching all of the various parts of
the airframe during a ground run (WATCH the prop!) to see if
anything is vibrating. A skin panel? instrument panel, glare shield ,
baggage compartment floor? etc.
I would spend a lot of time looking for any signs of drive system
distress, on a regular basis, like every hour or two.
If it is in the drive system, I would expect something to start
tearing
itself apart. Catch it before it stops working. The system is talking
to
you and you should listen. I hope it
is something minor, but assuming that is not safe. Assume it is
serious
until you can
prove otherwise.
Look for -
* Metal in the PSRU lube (do you have a magnetic plug? Is it's
lube
separate from the engine oil system?)
* Gear tooth distress. Can you open a plug and see the
working
surfaces of the teeth?
* Wear or peening at the ends of the spring slots in the
clutch disk
You DO have a clutch disk center in your power path don't
you? this
may be a big job to inspect, but you should do it at least
every 10
hours until you are sure there is no actual torsional
resonance
going
on.
* Check prop bolt torque and look for wear, scoring or hear
damage
to the wood of the prop hub where it meets the prop flange.
These
can heat up - even char if the system is running in
resonance.
Running in resonance is not a minor problem. The sounds you are
hearing may not be a torsional resonance, but please be careful.
Years ago I was working on some large waste treatment aerators which
consisted of a 6 ft diamter impeller on the end of a 5 ft vertical
shaft,
driven by a 100 hp electric motor and a 1750 to 60 rpm gearbox. One
day
the operators called me to tell me that a couple of the aerators were
"singing". Two days later one quit running at all. Inspection
showed that
a 1/2" thick 2 " diameter key (semi-cicle type) had sheared after the
4 inch long taper on the 3 inch diam secondary shaft had slipped. The
taper should have held without the key - this took **huge** overloads
to
accomplish. Resonance. I did a detailed design review, found out
that
these
systems were running very near resonance for the second stage gear
reduction stage.
Ya'll be careful out there.
REMEMBER: The laws of physics will be strictly enforced and
ignorance of the law is no excuse.
Bill
Good article Bill. Thanks.
Paul Lamar
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