Exhaust Pipe Concern

[david010]

Greeting,

 

Nice forum Nick. It will take a while to get acquanted with all the options, but it is neat.

 

Yesterday I jerked the cowling after 25 hours, which I am doing regularly, to check things over. (I have constant pipe anxiety and need to do it if for no other reason than to make sure all is well.)

 

I noticed wear on the pipes where they exit the aft baffle. The current policy is to have a tight fit where the pipes exit the engine and it seems with this arrangement there is bound to be pipe wear at the contact point of the exit. Question is, how much pipe wear is tolerable? I would prefer none, but that is impossible with a "tight fit" to prevent loss of cooling air and the pipe if it should break forward of the baffle.

 

This issue needs to be addressed as more time is logged with this installation. I am at 370 hours and do not think the pipes will last another 370 hours at the rate they are wearing at present.

 

Dave Domeier

Cozy MKIV N10CZ

 

PS Does this message feature have a spell checker?

[unick3]

After 1500 hours on the longez I never had a problem with wear. My baffles are trimed about 1/4" from the pipe. There is some rubbing of the Al on the steel but I think it was just discoloration more than metal removal.

 

If you are concerned about breakage (given your experinces) I am sure you have installed a hose clamp, or bracing near the baffle to prevent the pipe from exiting the engine compartment?

 

 

ps: somewhere there is a spell checker... I will try and find it.

[david010]

Nick,

Re "There is some rubbing of the Al on the steel but I think it was just discoloration more than metal removal."

 

There is definitely metal missing from #2 and #4 pipes. It can be felt.

 

The pipes must rest on something coming aft, especially #4 being about 18 inches long with no support other than the slip joint, and that rest spot is the hole through the baffle. I have clamps forward of the baffle and one large one aft holding the 2 together. They will not leave the airplane even if they do break as the aft clamp is safety wired to the baffle, but the forces at work while the two pipes are in contact with each other are unknown at this time.

 

Your 1500 hours without an exhaust event would indicate this may be of no concern. But it happened with an approved welded heat muff system and my lack of confidence on the subject is slow in coming back and in fact stalled out. I still fly several times a week but not without thinking about a busted pipe. It behooves all of us to check this area often as the consequences of an event therein are rather serious.

 

dd

[unick3]

I had the same feeling about flying when blew up TWICE in flight. I guess I didnt learn from the first time so after the second one, I sent the engine off for overhaul and it was trashed....

 

It took 100+ hrs for my confidence to return and I am a lot more careful than before...

 

You know that old saying about old and bold pilots...???

[Nat Puffer]

David,

I wonder if you have your two pipes on each side fastened tightly together? My #2 and #4 pipes are held tightly together with the shroud around the heat muff, and on the other side, #1 and #3 are held tightly by a number of wraps of safety wire (I should replace that with a hose clamp), as well as being a tight fit where they go through the rear baffle. Since the exhaust pipes and the rear engine baffles are both attached to the engine, there shouldn't be a lot of relative motion between the pipes and the baffle. One of our first builders, Mike Pinnock, didn't install the shroud around the heat muff because he lived in Florida and didn't need cabin heat. The lack of support for the #4 pipe caused it to vibrate, fatigue, and break. Rather than exiting through the prop, it stayed inside the cowling, but the exhaust gas from #4 cylinder traveled along the centersection spar and out through the gap between the strake and the wing. The hot exhaust gas softened the layups in the centersection spar enough so that when he parked overnight, the wings sagged in incidence. Sooo, I think it is really important in a 4 pipe exhaust system to support the pipes really well by locking them to each other and having a tight fit where they go through the baffle. I have 660 hours on my Mark IV now, of which 560 is with the Lycoming, and I have no noticeable wear on my pipes. However, pipes do not last forever, and bear checking occasionally.

Regards,

Nat

[niceez]

David,

 

A nice tight fit is not as important as you might think. Baffling design has much more to do with it.

 

I have no baffling around my pipes and the cooling is still cooler then it should be. Then again, I have well refined baffling.

 

Melville did an article on it during the mid 1980's. He found that opening up the holes around the exhaust improved his cooling.

 

I will be switching to downdraft cooling to make the plane faster and cool the engine the way it was designed to be cooled. I have always thought it should be downdraft. Pipes will then exit the inside the main cheeks.

 

- Dale

[Nat Puffer]

I can understand David's concern, since he broke an exhaust pipe, it went through the prop, broke off a blade, gave his engine a pretty hard shaking, and forced a forced landing. The cause, I believe, was later determined to be that the pipes, particularly the long one, #4, which broke, were not adequately secured, so they were able to vibrate and fatigue. The situation was aggravated by the two pipes (#4 & #2) not being tied together, and too large a clearance around the rear engine baffle so the pipe could fall through the baffle into the prop. Soooo, how to prevent?

The forward end of the pipes are attached to the exhaust port flange via a slip joint, which holds the pipe in place with strong, stainless steel springs (two per pipe). Then where the pipes go through the aft engine baffle, there should be zero (like no) clearance. Since both the pipes and the baffle are attached to the engine, there should be little, if any, relative movement. The baffle should be reinforced with 1/8 inch aluminum where the pipes go through, to maximize the support and minimize the wear. Next, on the side of the engine where the longest pipes are (the passenger side) there should be a shroud around the heat muff, which clamps the two pipes together, to minimize relative movement, if any. And lastly, one should clamp both pipes together on both sides of the engine (in front of the aft engine baffle) so that if one pipe broke, the other pipe would hold it from departing the airplane via the prop.

All of these things, taken together, will greatly prolong the life of your exhaust pipes. However, these pipes, even though made of stainless, are repeatedly heated to 1500 deg F (red hot) and will not last forever. So it is good practice to inspect them whenever you remove the cowlings for oil changes or annuals. We do not know what the life expectancy is, but ours are still going strong after 500 hours.

[david010]

Nat,

The reason the #4 pipe broke was heat differention across a weld at the heat muff. The pipe cracked exactly where the weld was and should no longer be of concern since all have been replaced (or should have) with a clamped on heat muff.

 

I've had the pipes secured together aft of the baffle with a large hose clamp plus clamps forward of the baffle. The clamp aft of the baffle is a very poor fix as the #2 and #4 clamp broke recently and the only thing that kept it from the prop was safety wire through the screw head and secured to the baffle. Others are doing this as I first saw it at OSH or SNF. Any type of securing device aft of the baffle should be removed as it is a greater hazzard than the pipe breaking.

 

I remain somewhat concerned about securing the pipes to each other. It seems they would function best if independent and not touching each other. (free float) Time will tell.

 

dd

[Nat Puffer]

There are at least 2 occasions we are aware of where the #4 pipe has not been adequately supported where it goes through the rear baffle, and it has fatigued and broke. A number of years ago, it happened to Mike Pinnock, who lives in Florida. He was flying over the Gulf and heard the sound of the engine change. When he reached land he learned that #4 pipe had broken, fallen down into the cowling, and the #4 exhaust had exited into the cowling. By the next morning the wings had sagged about 1/4 inch, leading edge down. What had happened, was that the hot exhaust had traveled along the centersection spar and exited at the end of the strake. The centersection spar had softened, which caused the wings to sag overnight.

The second case, the builder had allowed about a 1/4 inch of space around the pipes where they exited the rear baffle, the pipes had vibrated, and #4, which is the longest, had fatigued, broken, went through the prop, broke a blade, and cause an emergency landing. Fortunately the pilot was able to reach a runway and the landing was executed safely.

The designer recommends that there be no clearance around the pipes where they exit the rear baffle for 2 reasons. 1) We need high presure underneath the cylinders to force cooling air through the cylinder fins. If there is clearance between the pipes and the baffle, it will allow the air to leak out, reduce the pressure, and adversely affect cylinder cooling. 2) If there is clearance around the pipes where they go through the baffle, the pipes are not adequately supported, will fatigue, and brake. #4 pipe is the most likely to break, because it is the longest.

If there is wear, either the baffle or the pipe, where the pipe goes through the baffle, the logical thing to do is to increase the surface contact. We recommend a 1/8 inch aluminum reinforcement of the baffle at that point. If that isn't enough, then 1/4 Inch. We also recommend that the two pipes on each side be strapped together with a hose clamp forward of the aft baffle, so that in the remote chance that one pipe might break, the other one will hold it from going through the prop.

Lastly, it should be remembered that even for stainless steel, being repeatedly heated to 1500 deg F and vibrated is very extreme duty, and the pipe will not last forever, so it is very likely that it may have to be replace sometime in the future. The plans model Mark IV now has 600 hours on it and the stainless steel pipes still look good, probably because we have followed the above advice.

[brendanw]

When we started the engine, we had the cowling off and our

tech advisor saw the plans way of running the pipes through the

baffling and gave us a down check on that method. He said that a metal

to metal connection would eventually lead to damage to the pipes or the

baffle or both.

 

Would using a 1/2 inch phenolic sheet fashioned to the the present

pattern on the M drawings do the job, or would the phenolic not stand

up to the pipe heat?

 

I note that Nat was quite emphatic about needing a tight seal between the pipes and the baffling. Others have stated that a tight seal doesnt affect the cooling performance. Any suggestions would be welcome.

[Kent Ashton]

I don't think phenolic will take the heat but I also don't think the exhaust-through-the-baffles is such a big problem. Exhausts and baffles are both attached to the engine and move together although the baffle gets a bit of twisting as the engine works against the cowl. I have 1/16" gaps around the exhausts and I seal them with aluminum colored silicone sealant from the home stores. It lasts a good while before it deteriorates. I get rub marks on the pipes but no alarming wear pattern. I used thicker aluminum (.040) around the holes. I have seen others rivet .063 or .125 reinforcements around the holes.

 

Have a good retainer and inspect closely each time you have the cowl off. For a pipe retainer, I welded loops of stainless welding rod on each pipe and looped several turns of .063 stainless wire around the pipes and through the loops. It lets the pipes flex but will one if it breaks--I hope.

[argoldman]

I have 1/16" gaps around the exhausts and I seal them with aluminum colored silicone sealant from the home stores. It lasts a good while before it deteriorates.

Why not use the high heat silicon-- blue or red from auto supply stores???

[Drew Swenson]

Or you could take the berkut13.com approach:

 

http://www.berkut13.com/exaust08.jpg

http://www.berkut13.com/exaust07.jpg

http://www.berkut13.com/exaust06.jpg

[brendanw]

Or you could take the berkut13.com approach:

Wow! Are these steel or aluminum? I suppose if there was a flange all the way around and not just on the inboard side, one could safety wire some rubber baffling material on very nicely. Great job though!

 

I think I agree that the phenolic material wouldn't stand up to the heat. Our current approach is along the lines of what's been suggested - install some aluminum 'collars' onto the aluminum baffling to make the edge thicker, and end up leaving about a 1/16" gap around the pipes. We'll likely be investing in a whole bunch of high-heat silicone as well.

 

Thanks for the info everybody!

[Drew Swenson]

You can read about his install here (they are stainless):

 

http://www.berkut13.com/berkut24.htm

[argoldman]

Or you could take the berkut13.com approach:

 

http://www.berkut13.com/exaust08.jpg

http://www.berkut13.com/exaust07.jpg

http://www.berkut13.com/exaust06.jpg

 

Beautiful welding of the stainless.

 

One of the main concerns that I have for this installation is when the pipes heat up and expand will they not jamb in the fixture tubes eliminating the advantage that the tube in the tube is supposed to create. An easy fix for that would be to mount the plate, on which the tubes are welded, to the, it looks like the baffling, by a piece of flexible but firm rubber. This would act as a suspension mechanism as well as an air stop.

 

just a thought