Maybe I can help out here. An acoustical design engineer I have
been
working
with on varied projects (eliminating vibration) for over 24
years now
is
a
genius in this area. His specialty is noise reduction in assembly
production
lines. He would want to know the weight you are dealing with
and the
amount
of torque the mounts would be subject to.
Doug-
Great, here are some numbers I've been coming up with (and it
got long,
so,
well, you asked P-) ).
This is based on the 2.85:1 reduction drive. This is a few
orders of
magnitude above acoustic work but the science is probably the same.
Frequency due to prop rotation at 2000-3000 rpm is 33-50 hertz.
Frequency from engine rpm at 5700-8500 is 95-142 hertz.
Prop/engine suspended mass will vary between 350-400 pounds for
a 13B. A
20B with a 3 blade prop would be more but not sure how much.
All of this goes into a dynafocal mount with its own geometry
character. The mount points are on the corners of a rectangle 9.54
inches
top to bottom and 11.92 inches side to side. The four corner
mounts are
tilted towards the center axis (thrust line or shaft centerline) 30
degrees.
Axial loading of the vibration damping mounts. Of course the 1G
loading
is
what we will see 99% of the time and where we would want the maximum
effective damping:
At 400 pounds, 4 mounting points, loads per mount:
Static load on the mounts are +/- 267 pounds
At 6G rated loads +/- 1600 pounds
Ultimate load of 9G +/- 2400 pounds
What would the thrust load be on a typical 2000# 4 place
experimental
aircraft at cruise speed,... 300 pounds? Add that to the mix.
Gyroscopic loads per Paul's earlier calculation of 600 ft*lb at
the prop
flange (metal prop) and assuming that is a 6G pull up. This
results in a
+/- 305 pound adder to the above 6G numbers for an overall maximum
operational load of +/- 1900 pounds. Since this load would
happen rarely
I
would not expect great vibration damping, only damper survival.
Now we get to the radial loads on the dampers:
Rotational vibrations:
For optimum cruise engine rpm of 5700-6600 rpm, the frequency
response
would be 2x rpm for the two rotor 13B or 190-220 Hz.
Torque load:
Assuming a cruise horsepower of 150 hp at 6000 rpm the torque is 131
ft*lb
and the radial load on the damper is (131 ft*lb/0.63 ft) = 208
lb into 4
mounts = 52 lb per mount.
Too much information? This would be much easier to digest if it
were in
a
spread sheet with a CAD drawing along side. Maybe sometime in
the next
week or two.
Thanks for asking,
Mike
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
I think you also get some axial pulsation's out of the prop. This was
determined
by using a high speed pressure sensor in the wake of the prop.
Paul Lamar
Mike, Paul
Great information. You mentioned four mounts. Correct? Equal
loading? Can
you get at little closer on the finished weight of the
assembly? Basically
my friend gets his mounts custom made per his specs. What is the
maximum
mount diameter you can work with?
Doug
Four rubber mounts?
Are we talking about a dynafocal mount or a Schertz beam? The dynafocal
mount
is of limited interest to our readers as it is too complicated and too
expensive
to implement. Only Mistral would benefit from this info. The
Scherts beam
(and Questair Venture for that matter) uses just two rubber
bushings that
take 90% of the load with a rear mount that consist of two rubber
ball joints. BTW the Questair Venture is one of the smoothest airplanes
around. Ask the man that owns one.
Paul Lamar
Yes I agree Paul. I am under the impression that we were speaking
about a
Schertz beam or a bell housing mount arrangement where the two front
bushings take most of the weight. So I can enlighten the engineer a
little
better can you refer me to the webpage that shows what type of Barry
mounts
you have in mine?
Doug
Sure Barry Controls.
http://www.barrycontrols.com/
Paul Lamar
I wasn't aware that Barry Controls already makes an extensive assortment of
motor mounts for the aircraft industry. Except for some small isolators I
wasn't able to find a picture like the one we that is shown on the Schertz
beam though. Mike, before we go down this road of reinventing the wheel
wouldn't it be wiser to contact Barry Controls and see if they can supply a
mount closer to your specs? If for some reason it is to expensive or they
don't want to bother with the project then I will be happy to pursue
the it.
Usually fine tuning rubber mounts is done in the mix as well as density of
the rubber used. Barry Controls already has reams of data in real life
testing.
Doug
Well here is the data I have from their printed catalog.
You might want to contact them for a copy of the printed catalog.
Paul Lamar
I knew about the aircraft mounts that Barry makes. My question originated
with the thought that if these were optimized for Lycoming vibrations they
could be improved upon for the rotary engine frequency range.
What got me started asking is the frequency spec that Barry lists for the
22000 series mounts. The spec is in the 10-15 hertz range. I got to
thinking the other night (usually gets me into trouble) that the rubber
vibration isolation mounts may act like a low pass filter instead of a band
damping filter. More specifically, if you take the spec from the 2200x
mount at 15 hertz, the mount will isolate the engine vibrations from the
frame at anything ABOVE 15 hz. Of course we would never see any vibrations
below that on a continuous basis, particularly on the rotary. I've likely
been thinking too much again and not getting anything built.
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
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