Mike McGee wrote:
(Didn't mean to take so long to get back on these posts. The Pineapple
Racing Q&A are a synopsis of some things discussed at a tech session at
Pineapple Racing (rotary engine builder) in Portland, OR on Feb 22.)
P.S. This got really long...
Since it's my goal to be one of the quietest planes around, muffler
info
was high on my list. With race cars having noise limits there is some
experience with quieting the rotary at levels above 200 hp. Rob Golden
owner of Pineapple Racing has been successful at quieting the rotary's
bark. He emphasized that when you get your pipes assembled, you really
need to physically tune the assembly, on a dyno if possible, to get the
best results. This means playing with the location of the muffler with
respect to the end of the collector.
Imagine the wave train of the exhaust pulses as a standing
wave. If you
are running a muffler the exhaust pulse wave peak needs to end up
in the
first half of the muffler. According to Golden this is critical to
really
get the noise under control. Again, you have to try sliding the
muffler
back and forth about one wavelength to get the best location on the
pipe. On our test stands we need to be able to move the muffler with
respect to the collector at least one whole wavelength of the exhaust
pulse
in order to find the maximum effectiveness in the rpm band we will use
the
most. As you might imagine, the more wavelengths you get inside the
muffler the better it should work.
A tail pipe on the muffler is important for completing the setup for
maximum noise abatement. It's length will be a function of wavelength
and
will need to be tuned as well.
On muffler construction, I asked what kind they use in order to keep
them
in one piece. I didn't get a definitive answer here as the crowd was
surprised at how many mufflers Tracy has gone through. They also had
not
heard of SpinTech. I guess even during racing the engine sees enough
throttle changes that it is easier on the exhaust system than the
continuous power settings in airplanes.
On muffler design he suggested that the muffler needed a pre-chamber to
let
the pulse expand before it goes through the baffle section. This made
sense to me as I have been thinking that the rotary exhaust is
analogous
to
a leading-edge-peak saw-tooth wave. If I were trying to flatten this
out
in an electronic circuit I'd start with a capacitor branch to drain
some
of
that peak energy. Some mufflers have a chamber like this. I don't
know
if
the SpinTech does or not.
The Spin Tech muffler that Brian Trubee was looking at, the Cruiser,
gets
real close to what I think we're looking for. For my 2.85:1 motor with
1.75" primaries and a 2.5" collector, I would want 2.5" inlet and
probably
2.5" outlet although a 2.25" outlet would probably work as well. That
makes a model 733x or 732x (depending on the inlet/outlet config you
want)
measuring 2.25"x9"x16". I'm almost willing to go with the 4" thick
model
3333XL at 4x9.5x16 or if you really didn't mind the extra weight, the
5333
at 4x8.5x22. So I don't have to reprint the SpinTech model data
you can
look at the configuration chart at
http://www.spintechmufflers.com/spintech/styleimages/index.htm (Note:
the
model 5xxx is now the Sport Truck/SUV measuring 4x8.5x22). The models
are
listed with overall sizes at http://www.spintechmufflers.com/ .
Tracy, do you remember the model number on the SpinTech you had made?
It
looks like the 2.25" thick model in the pics on your web sight.
Wow, that got long winded. Any comments?
Mike
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode
All good observations Mike. Yes, the Spintech has the open chamber before
the baffles, kind of inherent in the design.
I hesitate to criticize the group buy effort but from my perspective, it is
somewhat counterproductive to establish a 'standard size' for the Spintech
and do a group buy. Aside from the excellent design, the real value of the
Spintech is that they are willing to build whatever size, inlet - outlet
configuration, etc, that you want. That feature is worth well over 10% in
price increase to me. Almost everyone has different requirements.
On the thickness, mine is 3 inchs thick and next time I think I would go
thinner in an effort to keep the muffler in the turbulent boundary layer
behind my cooling outlet. As it is, it hangs down into the clean 200+ mph
airstream a bit and costs me about 3 mph in top speed (and
cruise). Keep in
mind that mine is outside the cowl as in "Swiss muffler" installations.
There are good and bad things about doing it this way but obviously I think
the good things outweighed the bad.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
One of the "updates" I expect to do with the RV-4 is to enlarge the very
slight tunnel under the forward fuse in order to accommodate the muffler in
the way you describe. Right now the tunnel walls are about 2" high at the
cowl. Since I will be doing mods to the cowling as well, making it have
more of a chin is not significantly more work. I'm undecided if I want to
go to the scale of the 4" thick "SportsmanXL" muffler over the 2.25" thick
"Cruiser". I also think the muffler can be made much more aerodynamic once
we get the required volume dialed in.
Interesting that I found no 3" thick mufflers in SpinTech's lists. Was
yours a completely custom design? What are the overall HxWxL dimensions?
I'm tending to agree with you regarding the group buy. I'm also a nut for
flexibility. I don't want to discourage anyone from trying to save a few
bucks and I know Brian has put a good bit of work into this deal. We may
only be taking about a few percentage points difference in performance
between the Cruiser and the absolute optimum and a lot of guys just want to
get flying. I may end up trying three different mufflers just for the
data. I would expect to do the trials with carbon steel units just to save
the cash.
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode
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