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JonSmith
Joined: 21 May 2010
Posts: 110
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 Posted: Fri Aug 24, 2012 12:54 am Post subject: XS Exhaust System Cracks |
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Hi, I'm suffering from exhaust cracks - specifically in both of the rear cylinder downpipes on the inside of the tight bend. The cracks form around the circumference of the bend and when first discovered were approx 75% around each rear pipe! They are very hard to spot normally as the location is right out of line of sight and well hidden. Each one has now cracked twice, the first occasion I found them both cracked and had them welded up, since then I've found them individually (just found the second one gone again).
History - Fitted secondhand Rotax 912ULS last year (replacing the existing Subaru) with a brand new Firewall Forward kit from Europa, including of course the brand new exhaust, this kit I had purchased 2 years prior ready for finding a Rotax engine. The first flight of this engine and new FWF set up was June 2011 and has done 25 hours of flying total since then. The cracks were first spotted after 15 hours of flying.
I am 100% confident that I have fitted the exhaust (each time!) carefully and correctly with no strain or stress on any of the components. The engine always extremely smoothly with no vibration.
I have just e-mailed the factory about it but was just wondering if anyone else has suffered exhaust problems like this or can maybe offer advice?
A photo of one of the cracked pipes is attached.
Cheers, Jon
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pete(at)lawless.info
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| Posted: Fri Aug 24, 2012 7:22 am Post subject: XS Exhaust System Cracks |
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Hi Jon
I had a similar problem on a Europa supplied front pipe on my Classic a few
years back. Ended up replacing the standard exhaust with a stainless unit
from CKT Aero & Automotive Engineering. Bought second hand from an owner who
was converting his Classic front end to XS.
Regards
Pete
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JonSmith
Joined: 21 May 2010
Posts: 110
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| Posted: Fri Aug 24, 2012 10:20 am Post subject: Re: XS Exhaust System Cracks |
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Hi Pete, thanks for your reply. I should have said that mine already is an XS version, stainless steel by CKT. On mine its both the rear pipes that keep cracking (twice for both of them). The front ones have so far been ok. Attached is a photo of one of the rear pipes - the other was similar.
Cheers, Jon
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pete(at)lawless.info
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| Posted: Fri Aug 24, 2012 10:59 am Post subject: XS Exhaust System Cracks |
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Hi Jon
My crack ran along the inside of the bend along the length of the pipe,
rather than round it. It is noticeable that the crack is just along the
bottom of the weld for the exhaust spring, though if that is anything to do
with the problem I could not say.
We had fun with the CKT exhaust pipes last year 3 of the 4 flanges(?) where
they bolt to the heads cracked off. The replacements are considerably
better welded.
I dealt with CKT direct for my replacements. Chris Piper is the man to talk
to. 01884 242211 if you need the number.
Pete
PS I seem to remember G-TERN parked outside the hanger at Kemble?
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JonSmith
Joined: 21 May 2010
Posts: 110
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| Posted: Sat Aug 25, 2012 2:54 am Post subject: Re: XS Exhaust System Cracks |
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Hi Pete, thanks again for your reply, interesting to hear about your problems too. Thank You also for the CKT contact details if I come to need it. I think I've been supplied with a duff one. Waiting to hear back from Europa. In the mean time I'm being ribbed about pictures of my crack appearing on the internet......
You're absolutely correct about G-TERN at Kemble. That's where I bought it from. You've got an excellent memory because that was nearly nine years ago!
Cheers, Jon
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budyerly(at)msn.com
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| Posted: Mon Aug 27, 2012 11:52 am Post subject: XS Exhaust System Cracks |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Jon and Pete,
I have the same problem on two 912S engines on #3 and #1 cylinder.
One is clearly a tune problem which causes excessive vibration of the muffler and clear engine shake...
The other was more interesting. The pipe and spring combination seems to resonate during runs about 5000 RPM. I may have a harmonic problem.
Chris Piper (CKT) and I have discussed the problem and we are going to stainless springs and then checking again, but the engine has a left to right shake that causes the muffler to want to swing back and forth under the engine, but of course the exhaust bolts and friction prevent that, and the pipe seems to fail.
The spring fails then the pipe I expected went before the spring, but don't know for sure. Tuning the engine was the first step. As we all know, the 912S is a bit rough on start and stop, but generally smooth once above 2500 RPM. However, with the fixed pitch props when entering the pattern at 130 knots then throttling back for downwind, the prop (a Warp Drive on an Airmaster manual not auto control) tends to vibrate a bit during de-acceleration and the engine vibration is quite noticeable in the feel, but not visually. We broke #1 spring and safety wire from the vibration. #1 has an adjustable leg of course and did not crack the pipe. But I am concerned. Something is vibrating at a harmonic down there.
I am at a loss at the reason for the vibration crack on only one cylinder. One would think it would affect more than one pipe. Our checks were:
Does the silencer hit anything on the engine or frame? No.
Is the engine in tune. Yes
Is the prop balanced. Yes
Is the prop blade tracking true. Yes
Is the prop blade angle true. Yes
Is the pipe overcooled by ram air. Not really
Are the springs tight with no twist or torque imparted. Yes
Is the pipe securely seated in the silencer. Yes
Is the pipe over-torqued. No.
Does the crack start at a weld. Possibly.
Are the engine mounts sound and firm. Yes
Are the engine attach bolts and frame secure. Yes
I'm running out of ideas myself.
Regards and still looking for the solution:
Bud Yerly
Custom Flight Creations, Inc.
www.customflightcreations.com
(813) 653-4989
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frans(at)privatepilots.nl
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| Posted: Mon Aug 27, 2012 12:21 pm Post subject: XS Exhaust System Cracks |
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On 08/27/2012 09:52 PM, Bud Yerly wrote:
| Quote: | We broke #1 spring and safety
wire from the vibration. #1 has an adjustable leg of course and did not
crack the pipe. But I am concerned. Something is vibrating at a
harmonic down there.
I'm running out of ideas myself.
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Try to support the #1, somewhere near the middle. I literally mean
"near", make sure you don't accidentially hit the middle. At least try
to avoid the 1/2 and 1/3 point. This will likely dampen out the close
harmonics.
This would be the most easy solution. Otherwise you have to change the
lenght of the pipe, or to chase the vibration which is likely a
byproduct of the engine/prop combination.
Frans
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JonSmith
Joined: 21 May 2010
Posts: 110
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| Posted: Mon Aug 27, 2012 2:51 pm Post subject: Re: XS Exhaust System Cracks |
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Hi Bud, Many thanks for your input on this. Interesting to hear about the problems you've had too. Just to give you some data from me, my problem has been both the rear pipes cracking, ie cyls 3 and 4. I first found them both cracked together after about 15 hours flying on my new engine installation. They were both cracked about 75% around the circumference inside the tightest part of the bend. They may have been like that for quite a while as they are almost impossible to spot without a mirror when in situ. It was taking no. 3 off to fit the new HD starter I quite by chance found it cracked. I had them both welded up. After about 2 more hours flying I found no. 3 cracked again, following almost exactly the same line as the first time (but not quite on the weld repair - it was fresh metal that was failed rather than the repair itself). I had it welded again. Since then I did just a quick 20 minute flight and have found no. 4 has cracked again, same pattern. So each rear pipe has cracked twice now. Both front pipes have been ok to date. The cracks do seem to run through where the spring attachment loops are welded on as my previous picture shows. As far as I am aware my springs are stainless - as supplied by Europa (they look stainless to me!). The engine runs very smoothly - as you say it can be a bit clunky on start/ stop but not excessively. My propeller is a BSW (Bog Standard Warp!) ground adjustable 64" non tapered blades warpdrive fitted to my 912 ULS. I always keep the carbs well sync'd. I do notice sometimes a slight vibration during slowing down and/ or descending - if flying fast at a high power setting when I reduce power at high speed, at a certain point during the power reduction - usually when the throttle has been brought a fair way back a slight vibration/ resonance can sometimes suddenly kick in but it's only ever very slight, a slight tremble than an actual vibration. More of a slight change in sound. I've never thought anything of it. I don't really know where to go with this now as it seems to be an ongoing problem I have and I can't really fault anything else in my setup.
Cheers, Jon
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budyerly(at)msn.com
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| Posted: Mon Aug 27, 2012 5:20 pm Post subject: XS Exhaust System Cracks |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> Hohn,
I doubt that the setup is wrong.
My concerns on vibration is two fold.
First is crank shaft alignment. Having a love hate relationship with all things airplane, I am not criticizing the Rotax, but it is possible since it is a 5 section crank shaft or so, the shaft may have slipped as it is only pressed together. This is highly unlikely unless the engine was bought used and had a prop strike that was undisclosed. In this case the engine vibrates is over a wide range of RPMs. I had a 914 with this problem once. It never balanced right.
Next is that the cracks indicate to me that the failure is due to lateral movement of the silencer, or attempted movement, which is restrained by the pipes. This side to side stress means the harmonic could be coming from the piston induced horizontal vibration of the boxer engine. This could be from a lack of proper harmonic balancing of the crank which is corrected by use of a rotating weight or other means such as crank shaft lobes to dampen the harmonics. Since many LSA aircraft fly with the 912S, I doubt that they have recorded exhaust problems we can draw from so we are in the blind right now. I know the Ran's aircraft 912S installations have no exhaust pipe problems to speak of, but they do get failures at the weld points occasionally. I have never studied the exhaust pipe springs and silencer movement closely on the ground engine run-ups in our aircraft, and probably should.
Frans correctly noted that the exhaust needs support or exhaust pipe dimensions changes, to affect changes to the harmonic. Problems arise with supporting the exhaust as it does move with heat, and can only be supported to the engine such as on the 914. Perhaps some stainless brackets supporting the silencer to the frame may be appropriate. An X type support comes to mind on the aft pipes to stabilize the silencer. Even a .041 stainless safety wire may work. The 912 has no problems with this cracking problem in my shop, the 914 is supported extensively, and only the 912S seems to have the issue. So must I assume it is an engine specific problem. Probably not an accurate assumption as usual...
My welder inspected the crack as did Chris Piper at CKT and the type of stainless may be the problem, but the jury is still out on that. There is not enough data on this problem. Chris at CKT, has solved the horrible exhaust silencer failures of the past and beefed up the previously anemic down pipes and I thought we were all right, then this problem started to creep up. We are considering the pipe bend and weld when under heat stress is the problem. Changing the material and heat treating may solve this if it is not an engine related problem. Extra support as Frans suggests may be necessary.
The Warp Drive and other props too, do vibrate under compression braking during a de-acceleration but it is slight as you noted. This vibration doesn't bother me as it is not constant and is only momentary and changes in its harmonic (as measured by the giggle of my calf muscles which are not registered with the bureau of standards). So if the run-up and full power dynamic balance of the prop is fine, I am not worried about the prop causing this problem.
More will come out. In the mean time, I will try a safety wire X brace and see what happens.
Regards,
Bud
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