Michael McGee wrote:
Hi Francois: A question on your engine mount hardware:
You have a dynafocal style mount on the Mistral engine similar (or
identical) to the Lycoming mount. Did you guys have the vibration
damping
mounts made for the rotary engine or are you using mounts from a
Lycoming?
The reason I ask is the frequency response of the mounts would be
different
by a factor of 2-3. From what I've learned looking into vibration
mounts
the frequency response is part of engineering the mount. Analogous to a
capacitor in a filter, a mount tuned for a 2500 rpm engine (about 42
hertz)
would be too much for 6000 rpm (100 hertz). Perhaps the aerobatic
mounts
that are stiffer would be more appropriate.
Paul: The Barry mounts we have been looking at on here (22000
series) are
designed for 10-20 Hz. I realize the rotary is almost vibrationless
compared to the Lycoming. However, the right mount would make it
feel like
a turbine.
I have yet to talk to Barry tech support or Lord tech support. I'm
still
working on my "this is not for an airplane" dialog. Since I'm using a
standard dynafocal mount it's going to be a tough sell. I would like to
see what it would take to get the right frequency response designed into
the isolation mounts.
Open to any and all ideas...
Mike
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
Also the plain of the vibration is different in a rotary. What you
have mostly
is torsional vibration about the e-shaft center line due to the two
combustion
events per rev. At 6000 RPM that is 200 hertz. Well above the 10 hertz
resonance
of the Barry mounts. Perry Mick's engine is bolted solidly to the
airframe and
you can only feel a slight buzz.
The Mistral rotary engine is very smooth in the Piper.
All sorts of modes are happening in a piston engine.
Paul Lamar
-
Mike
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
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