Please post drawings/photos/specs...Yvon
Posted in June, 2008 (more below)
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Sunday, June
08, 2008 12:15 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Exhaust and
Muffler designs.
Exhaust and muffler design in the rotary installation is
one of the more complex of all the installation issues. There are, and have
been so many variations among the various installations that there is little
statistical proof of anything. I’ll add a few comments and opinions
that may be relevant, and describe what I did. It may trigger some ideas for
you to think about. My system on the 20B is now approaching 100 hours –
not a long term proof – but it is still solid.
The exhaust temps out of the port are very high, typically
in the neighborhood of 1600F, and sometimes maybe 1700F. Couple this with
pressure pulses and vibrational loads, and corrosive environment, and you have
a very demanding situation. When you look at material properties to
handle this, things narrow down pretty rapidly, particularly if you also want
light weight as we do in aviation. Stainless steels, like 321, can handle
the temps and be a workable exhaust – but, design for low stress levels
then becomes a must, because SS are subject to ‘stress corrosion’
at these temps. Combine the high temperatures and vibrational stress and
you get inter-granular corrosion which weakens the material and it eventually
falls apart.
On way to alleviate that is to use inconel. It gives
you higher temperature capability and corrosion resistance. And it gives
you higher cost. But is it worth it to reduce your risk a forced landing
in an unfriendly place? Compared to the total cost of your airplane
it’s a small amount. Maybe cut cost somewhere where it is less
critical to safety.
Another thing to consider is that the more quickly you can
expand the exhaust gas, the more quickly you can deal with lower
temperatures. Charles Law – temp (degree K) goes down in direct
proportion to increased volume. This becomes more complex in an exhaust
system because of other factors, but it still works in your favor. The
gas will expand down a constant diameter pipe, but expanding into a BIG pipe
can make a significant drop.
That can be one of the advantages of the tangential
muffler/manifold, or the design that Neil presented. The amount of the
temp drop of course depends on the pressure in that bigger can. These designs
have their own possible failure modes associated with welded joints and thermal
stresses, but at least there is nothing there that is going to plug up the flow
downstream. The skill of the welder and the post-weld heat treatment are
important factors.
These units are generally bolted directly to the engine
via the short header pipes, so vibration loads are a factor. Ideally
you’d like to have stress (and thermal expansion) de-coupling between the
engine and the muffler/manifold, but since the engine can move relative to it’s
mount you either have to accommodate significant movement, or support it to the
engine by some other means then the header pipes.
And then there is the matter of the exit pipe(s) and
secondary mufflers. Those have to be supported as well – an unsupported
length of pipe extending from the muffler is an ideal candidate for some
vibrational resonance which will fail the system somewhere. And the further
away from the engine centerline, the greater the loads.
My exhaust system is shown in the first attached photo.
This is in a pusher configuration. It is an inconel tangential
manifold/muffler supported to the engine by short inconel header pipes which
are welded to a heavy RB steel flange. It has a convex ‘head’ at
the front, and a conical outlet to the exit pipe. It has internal vanes
welded at an angle on the inside surface opposite the exit from the headers
(you can see the welds on the outside) to help break up the pulses and direct
the exhaust toward the exit. They also prevent possible “swirl-flow
choking” which could increase back pressure. There are
‘straightening’ vanes in the conical exit section.
The exit pipe is clamped (custom heavy SS clamp) to the
inlet pipe of the secondary muffler (I’ll call it a resonator). The
resonator is also of my design and is made of 321 SS. It is basically a
straight through 2 ¾” pipe that is drilled full of ¼” holes (about
100), contained within outer 5” dia. pipe. The inner pipe has an
orifice plate at the center which has a 1 5/8” opening. This
orifice produces some restriction to the flow through the resonator to force
some of it outward through the holes, and back through the holes to exit.
The purpose of the resonator is to knock down the pressure peaks a bit
more. Measurements on the dyno showed that resonator knocked another 8 db
off the sound level and had no noticeable effect on the HP.
The plug in the resonator closes a port originally
intended for the O2 sensor. But it didn’t work well in that location
because the temperature was too low (interesting, huh). I had to move it to the
inlet pipe.
Last but not least, there is a SS support at the end which
clamps solidly to the redrive. The clamp is designed to be rigid laterally, but
to also be an effective heat choke. This supports the resonator, and
reduces the likelihood of any resonance vibration in the system.
I originally thought that the resonator internals may not
last more than 50 hours, but at 95 hours they are still solid. Which brings up
another point. It is easily inspected. I can see those internals from the
exit end, and I can stick a screwdriver or ratchet handle or whatever; in there
and bang around to be sure things are sound. I inspect all the welds in
the exhaust system every time I remove the cowl, or at least every 10 hours or
so. Make your system inspectable, and keep an eye on it.
I wouldn’t call it “quiet”, but
I’ve had people say they like the way it sounds. Time will tell its
reliability.
Best,
Al Gietzen
Posted Dec,2009:
Here is a posting; with attachments,
that I sent to the list about 6 months ago.
The inconel is .045 or .050”;
I’m not certain which. The fabricator (Mark Sadickas (sp?);
referred by Dave Atkins) had the material. Materials and labor came to $1950.
The secondary muffler is .032”
321SS outside, with through-pipe and ends of .049”. The outside of
the secondary stays relatively cool – I learned that the O2 sensor would
not get hot enough there to give good readings, suggesting maybe about 900F.
Al
If you go through the archives, you'll find lots of examples of
failed
muffler designs. Many by your's truly. I think I've tried every
concoction
known to man and the Swiss. They all worked... for a while.
My best overall design (see attached) is a 2" tube, full of holes
inside
a 5" tube. All made of 16ga SS, all welded together. Needless
to
say,
the flange is more like 3/16" - 1/4" SS. The inside end of the
2" tube
is
welded to the end cap of the 5" tube. That blocks off the one end of
the
2"
tube and secures it from movement. The exhaust end of the 2" tube is
welded
through a 2" hole in the other 5" end cap. Rather than drilling
the
2"
tube full of round holes, we cut slots with a saw. Then take a big flat
blade
screwdriver, stick it in the slot and bend it over. This creates an
oblong
hole. (Much easier than drilling into SS. This is what will go on
the
Volmer.
The sound is quite acceptable, it fits inside the cowl and Jim M.'s
version
lasted the life of the aircraft... 600+ hours.
Neil
PS:
Are you considering Rough River?
-----Original
Message-----
From:
Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf
Of Al Wick
Sent:
Saturday, June 07, 2008 4:57 PM
To:
Rotary motors in aircraft
Subject:
[FlyRotary] Re: Mistral Crash Analysis
C'mon
guys. You do this every time there's a crash. Instantly go into
rationalization
mode. It's unhealthy. Greatly increases risk builders won't
take
action. Increases risk you won't research it thoroughly.
A
healthy response would be:" Here's another example of how our engines
produce
unusually destructive exhaust temperature and pulses. We have a rich
history
of broken exhaust components. We need to be very thorough when
designing
and building exhaust."
I
designed my own muffler. It had two inlets, two outlets. So if (when) my
muffler
failed, it could never block both pipes. I also put loose safety
wire
around my pipes, because on a pusher loosing pipe wipes out prop. So
basically,
I assume stuff will fail, then design it to control the way it
fails.
I've heard of rotary guys doing same type of thing. This is a good
time
to share those key items.
On
your car, they deliberately design products to fail a certain way. They
will
make a component weak, so it fails first. They do that with wheels and
hubs.
So when the muffler fails, little pieces come apart, not big sections?
You
guys do a great job of sharing successes, design and construction
details.
This is another opportunity.
-al
wick
<No
doubt you are on the money, Rusty. When folks are already predisposed
to
bad mouth the rotary - this will only be more ammunition. "See! even
with
umpteen million dollars you can't get one to fly" {:>).
But, I
serious
doubt it will effect many who have researched the rotary and come to
understand
its benefits - as for the rest, who cares {:>)
>
I'm certain it was a relief to Mistral that the culprit was not one of
their
engine components.
Whew! a close one for sure.
Hi Ed,
Unfortunately, I bet the majority of people will only hear "Mistral
rotary",
"lost power", and "crash" :-(
Rusty (RV-3 taking forever.)
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