Rotax 912 (100 Hp) Muffler Cracks

[skippydiesel]

Hi All,

 

This 912ULS has done 720hrs/16years.

 

Just found what looks like a fatigue/vibration crack in the ring weld that holds the "tail" pipe on my muffler/silencer. (small sign of exhaust leak drew my eye). On closer inspection I also found a crack radiating from the ring weld about 1/4 of the way across the slightly domed end of the muffler/silencer.

 

Vigorous shaking of the tail pipe did not result in any discernible movement of the cracks.

 

Q: Is this a reasonable operational life expectancy for a muffler?

 

Q: Do I replace muffler/silencer?

 

Q; Do I get muffler welded?

 

Your collective thoughts on the pros/cons will be much apptreciated.

 

 

[mnewbery]

A1: Yes, in comparison to a car that got the same amount of use (about 140,000 kms at 200 km/h)

 

A2: CO poisoning is no joke. I would replace it as it was never manufactured to be repaired

 

A3: There will be other cracks that can't be seen yet plus a bit of rust on the inside so more welding will be needed soon enough if this is attempted. This is not a rare part either so remanufacturing is not considered as a last resort

 

 

 

Plenty of forum viewers will provide horror stories about having an aero part repaired/overhauled only to find it was more expensive than buying a zero hour part.

 

 

[Ultralights]

mine had the same problem, at about 750 hours also. what i found was that the internal baffles broke away from the outer muffler skin, resulting in cracksin the skin where the baffles were, i had the baffles welded back in, and metal strips welded over the areas where the baffles are welded to the muffler. havnt had a problem since

 

 

[skippydiesel]

Thanks for the replies - Have been doing a bit more research:

 

• Could not find any evidence of an embossed/engraved part number on my original muffler, sooooo seems likely my muffler was supplied/built by the kit manufacture and may not be a Rotax original.
   
   
  
• It costs about as much to purchase the exhaust parts kit Part No 973676 (including pipes doughnuts, springs, etc) as it does to just purchase the muffler on its own.
   
   
  
• There is a fully fabricated Rotax exhaust system Part No 973387 - quite a bit more in cost and it may not fit my application. Interestingly the tail pipe on this system is spring attached (not welded) this may indicate that Rotax feel that a solid welded tail pipe may be subject to fatigue cracking.
   
   
  

 

 

 

 

[facthunter]

Think most joints are spherical (ball faced) and held in place by springs, and can move a bit. Stainless steel is far more prone to fatigue cracking than mild steel. There's a lot of high frequency vibration going on in those motors. Generally the whole system moves with the motor, and sections well behind it are hard to support. Regular inspection is part of the deal. They are made as light as possible, like all parts of aeroplanes. Nev

 

 

[mnewbery]

Did the aircraft with the welded muffler get a new weight and balance after the modified part was fitted?

[SSCBD]

Did the aircraft with the welded muffler get a new weight and balance after the modified part was fitted?

Maybe children (man baby) should be seen and not heard and given the dummy. (I see the stirring of the pot).

 

 

[Ultralights]

Did the aircraft with the welded muffler get a new weight and balance after the modified part was fitted?

doesn't need a re-weigh and balance, when you know the weight of the added material, and its moment. then you can amend the W&B paperwork

 

 

[mnewbery]

Agree. Then there is this...

 

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch04.pdf

 

Over a period of time, almost all aircraft have a tendency to gain weight. Examples of how this can happen include an airplane being repainted without the old paint being removed, and the accumulation of dirt, grease, and oil in parts of the aircraft that are not easily accessible for cleaning. When new equipment is installed, and its weight and location are mathemati- cally accounted for, some miscellaneous weight might be overlooked, such as wire and hardware. For this reason, even if the FAA does not require it, it is a good practice to periodically place an aircraft on scales and confirm its actual empty weight and empty weight center of gravity.

 

 

[storchy neil]

doesn't need a re-weigh and balance, when you know the weight of the added material, and its moment. then you can amend the W&B paperwork BULL

 

check out flight manual you will find that it is mandatory for a new weight and balance on the installation off a repair to an aircraft neil

 

 

[Ultralights]

doesn't need a re-weigh and balance, when you know the weight of the added material, and its moment. then you can amend the W&B paperwork BULLcheck out flight manual you will find that it is mandatory for a new weight and balance on the installation off a repair to an aircraft neil

Unless the engineer, or the repair manual states that repairs under a certain weight are deemed negligible effect on W&B.

 

 

[mnewbery]

Storchy and Daffyd Llewellyn covered this already. Specifically CAO100.7 and (no) exemptions from CAR235. Browse the threads!

 

 

[mnewbery]

For the nerds the tolerance before a new W&B is required if nothing else has changed:

 

(a) for aeroplanes:

 

(i) the empty weight has changed by more than 0.5% of the MTOW or 10 kg, whichever is the greater; or

 

(ii) the empty weight CG has changed by more than 2% of the maximum permissible centre of gravity range or 5 mm, whichever is the greater;

 

So for a 544Kg MTOW 0.5% is less than 3Kg. In that case you'd use 10Kg

 

 

[facthunter]

Recalculation of CofG based on component change to a new mass, (or new position) is pretty standard practice, in aviation. If the C of G of the component is near the aircraft's C of G it has little effect, whereas if it's located a long distance ( at the tail for instance,) it will have a large effect on balance and sometimes the handling as well. This is all common sense but if in doubt anytime a reweigh will remove all doubt. It's not something you will do in 5 minutes though and you must do it correctly.

 

The two things where a pass mark in of 100%, was required , was W&B and Fuel management. Nev

 

 

[Ultralights]

so, the effect of welding a 10cm x 1 cm strip of 0.025 thick stainless steel, onto an area thats less than 2 ft from the CoG datum, according to the engineer, is laughable. flying through rain you would pick up more weight fwd of that position.

 

 

[facthunter]

Seriously, I can't see the amount of welding rod used would change anything to a noticeable degree. I think the comment was very TIC. IF anyone is concerned about their plane's C of G accuracy and you have a few hours to spare do a reweigh, from the mains and "other" wheel (aircraft horizontal and laterally level) weight figures.

 

LOOK UP HOW TO. IF it's not somewhere near what your book shows as in the allowable Cof G range, empty and loaded, have it done properly and compensate for it, by moving or adding some fixed weight and change your load sheet (s) as to basic index . Where your elevator sits in cruise is a rough indication, of how well it's all working. but it also reacts to horiz stab angle of incidence. (fixed and adjustable) If you are out of balance the plane may unstable in pitch and suffer tailplane stall (deadly) or not have design elevator effectiveness. Nev

 

 

[skippydiesel]

The tangential debates arising from the most humble of topics are most fascinating.

 

I would like to assure all those concerned persons; I will watch my weight & its location (not sure that this will change my eating habits) and that I have signed away a goodly slice of my Super to purchase an original Rotax muffler/exhaust kit from a well known Australian Rotax agent. I hope the new unit will last at least another 720 hrs + and it should keep my W&B as perfect as ever.

 

 

[DrZoos]

I think the exact location of your partially digested breakfast is of more concern.

 

 

[skippydiesel]

Okay ! Got my new muffler kit. All the exhaust pipe holes/sockets & spring loops seem to line up well. So far so good.

 

The big problem now is the tail pipe.

 

To fit my application, I need a 60 & a 90 degree bend both of about 220 mm outside radius. The 90 comes off the muffler stub. The 60 comes of the 90 and exits under the cowling

 

The Rotax muffler stub has an OD of 40 mm (as does the supplied L shaped bit).

 

I can get automotive stainless exhaust elbows of 38 mm ID that will "do the job" but the wall thickness is considerably thicker than the Rotax stuff & consequently heavier.

 

Q Anyone know of an Australian supplier of stainless exhaust bends of the same specification as Rotax ?

 

Q What do other Rotax 912 operators do to "plumb" a tail pipe ?

 

 

[mnewbery]

Does this mean the old bends are junk?

 

 

[mnewbery]

You could ask Tranzac or Pipemasters. Pipemasters make their own bends I believe

 

 

[skippydiesel]

Existing (auto stainless) tailpipe welded onto muffler. Yes I could remove it, but then would have to excavate remains of muffler stub - all a bit messy. Probably end up with a loose fit, extra welding (weight & mess) to stabilise. Would like to do a neater/ more professional job than what was there before.

 

 

[skippydiesel]

Man Baby (MB?) not junk but I would like to make a more elegant system.

 

Update - Have completed new tail pipe fabrication & attachment to new muffler.

 

· I purchased a 60 degree, stainless steel, 38 mm automotive elbow (noticeably thicker walled & consequently heavier than Rotax pipe).

 

· A local muffler shop (thanks for your help, skill & patience Peter) stretched/flared the appropriate ends of the pipes so that they would "dry" fit together, but still rotate for alignment.

 

· I test fitted the Rotax 90 degree pipe to the muffler exit stub & cut to length

 

· Shortened both sides of the 60 degree bend to minimise weight

 

· Plugged remaining strait Rotax pipe into other end of 60 degree bend.

 

· Temporarily fitted the lower engine cowl to facilitate adjustments of tail pipe to the cowl exhaust hole.

 

· Used craft clay "blobs" to space the pipe from the inside of the cowl wall.

 

· Marked all joints for reference (I use a liquid paper "pen" to do this).

 

· Removed tail pipe.

 

· Using a TIG welder, put two small tack welds on each joint.

 

· Assembled tail pipe to muffler to recheck fit.

 

· Removed muffler & tail pipe to tack weld tail pipe to muffler stub.

 

· Reinstalled full exhaust assembly to check for final fit.

 

· All good.

 

· Removed assembly and my son, who is a much better TIG welder than I, permanently welded all joints in place.

 

· Final install with new system with new springs, anti seize lube, high temp silicon on springs & safety wire.

 

· As a precaution against heat damage to cowling, I have wrapped tail pipe in automotive heat bandage.

 

· Ground run of aircraft - system all good.

 

(Like to show a photo but not sure how to copy to this system)

 

Q: Tail pipe quite long, must make it and/or connection to muffler, particularly prone to vibration damage - any thoughts on the matter??

 

 

[mnewbery]

Vibration. Excellent question!

 

The length and the possibility of vibration damage are related but not 1:1. For this to be an issue the "part" needs to be excited into a state where the vibration becomes an issue.

 

Note to all: This is about lateral mechanical vibration of the exhaust after the muffler can. Not about vibration caused by exhaust gas pulses.

 

In the most general terms the exhaust end (the stack) would need to vibrate laterally like half a tuning fork. More like a clock pendulum probably. The longer it is the lower the natural frequency. A one metre long stack of 38mm auto exhaust isn't going to vibrate anywhere near the frequencies of a Rotax engine. Way lower. The maths is icky and requires things like the Young's modulus of the pipe and the density of the metal. Halving the length doubles the natural frequency. A thicker pipe makes no change because the extra mass cancels out the stiffening.

 

Again this is not like blowing across the top of a bottle because that is a different kind of oscillation.

 

The heat bandage will help lower the natural frequency.

 

If you really wanted to push it, find a loud speaker and a signal generator. Stick the item of interest on top of interest and have a play.

 

With kind regards, your friendly physicist.

 

 

[skippydiesel]

Thanks MB for the dissertation - but what to do? If anything?

 

I was half inclined to weld a bracket/strut between the muffler end and part way along the tail pipe but don't like the unknown effect such an innovation may have on the harmonics and expansion/contraction of the system.

 

I have an idea:

 

· I have some stainless half loops, left over from the Rotax muffler kit.

 

· On the muffler, (tail pipe end) I could weld a loop to one (a) or both (b)of the bracing plates for the exhaust sockets.

 

· I could weld another loop © close to where the 60 degree elbow starts.

 

· I could bridge the spaces a-c & b-c with unequal length coil springs (approximately 90mm & 130mm).

 

· This would give the tail pipe a slight upward force in two slightly differing directions while allowing expansion /contraction.

 

Would this tend to "dampen" any tendency for the tail pipe to vibrate?