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From: "Al Wick" <alwick@juno.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Exhaust and Muffler designs.
Date: Mon, 9 Jun 2008 17:53:47 -0700
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Well written Al, lot's of good info. I'm a little concerned for your rear 
bracket. If you kick exhaust on your car, you will see it is soft mounted. 
Typically the tail pipe can move up to 1/4" or more. Hard mounting forces 
stress concentration at the attach point. Basically, the pipe turns into a 
sine wave shape while running. If you fire up your engine at night, cowl 
off, near fluorescent lights, there is good chance you could actually see 
the pipe flex. Strobe light even better.

Anyway, I'd be tempted to convert your rear bracket to "soft" point, just by 
making it a clearance fit. Or you could say "bull", and just inspect the 
weld adjacent and the attach surfaces every X hours.

I had to resolve a multimillion $ warranty claim years ago. Engineer 
designed hard point attach to 1" coolant line. Lines failed at around 10k 
miles or so. Fatigue at attach point. Lot's of trucks on the side of the 
road leaking coolant.

-al wick


----- Original Message ----- 
From: "Al Gietzen" <ALVentures@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, June 08, 2008 1:14 PM
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 ž" pipe
that is drilled full of ź" 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


            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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-Al Wick
Cozy IV powered by Turbo Subaru 3.0R with variable valve lift and cam 
timing.
Artificial intelligence in cockpit, N9032U 240+ hours from Portland, Oregon
Glass panel design, Subaru install, Prop construct, Risk assessment info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html 

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