Exhaust Fabrication

[skippydiesel]

Seeking recommendations for a service provider, to fabricate a replacement  exhaust header (engine to muffler). Current header has insufficient clearance.

 

Prefer Sydney Basin, however may be possible to deliver aircraft to shop location within NSW.

[facthunter]

Very specialised job to do it properly and most seamed steel tube is too thick and ends up too heavy.. Stainless is thinner. Your plane will take  up a lot of room and that limits where you can get it done. Most bends are made to a curve applied to some aluminium wire as a pattern.. Good luck. Maybe some aftermarket kit will fit .   Nev

Edited February 4 by facthunter

[skippydiesel]

34 minutes ago, facthunter said:

Very specialised job to do it properly and most seamed steel tube is too thick and ends up too heavy.. Stainless is thinner. Your plane will take  up a lot of room and that limits where you can get it done. Most bends are made to a curve applied to some aluminium wire as a pattern.. Good luck. Maybe some aftermarket kit will fit .   Nev

Thanks Nev - 

 

I have all the necessary materials (Rotax supplied) but none of the experience and not enough material to screw it up more than once.

 

The wire idea is okay for the bend radiuses but not so much for accurate determination of overall length -  this is an existing installation, one of four pipes, so must fit,  cylinder head to muffler, as per original .

 

There is no kit - this is pretty much a unique configuration.

 

Options for delivery to workshop: Fly in/out for repeated fittings OR trailer in (sans wings) to stay at shop OR fabricator comes to home airfield.

[facthunter]

You can purchase preformed bends and do a cut and paste for ONE pipe. Neatly worked lobsterback.  Nev

[Marty_d]

If it's the same stuff I got with the muffler, Rotax supply a stainless "donut" so you can cut whatever curves you want out of it, as well as straight tube.  I did the cutting myself based on a wooden mockup of the engine then got an experienced welder to join them.  Having said that I was slightly too long in one so had to re-cut, I need to either get him and his welding equipment here to do it in place, or take the lot up to him and hope that it fits after.  I think I'll try to get him here.

[skippydiesel]

That's great Marty_d but my engine is already mounted, in the airframe, with all four exhaust /header pipes in place. One of the exhaust/headers is about 12-15 mm too close to the engine frame. How do you suggest I proceed?

[facthunter]

You will need a jig to locate the ends of the pipe and show that you have the increased room at the spot where it's a problem.  Nev

[Methusala]

I fabricated 2 exhaust systems for vw's. Both were 4 into 1 with nearly equal lengths. They were built on the engine installed on my VP2. I used 40 mmm erw steel tube and purchased mandrel bends which I cut to length and tacked it together with oxy and leather gloves. I then welded it up. Worked well and was acceptably quiet. Didn't weigh all that much either. My welding skills are not exceptional but I was determined to have a crack. Don

[skippydiesel]

14 hours ago, facthunter said:

You will need a jig to locate the ends of the pipe and show that you have the increased room at the spot where it's a problem.  Nev

The jig is the engine to muffler, as is.

[facthunter]

Well IF you want to transport the whole plane and have it in a big shed with enough spare space without running up a great bill,  see how that turns out. I also wouldn't be grinding and welding near a good motor.  Nev

[turboplanner]

15 hours ago, skippydiesel said:

That's great Marty_d but my engine is already mounted, in the airframe, with all four exhaust /header pipes in place. One of the exhaust/headers is about 12-15 mm too close to the engine frame. How do you suggest I proceed?

Got a photo?

One method is to heat the tube with oxy, making a jig to bolt the flange to and clamp the pipes to (out of the aircraft). The issue with using donuts is if you change the pipe length you change the tune.

[facthunter]

I doubt the exhaust is finely tuned. The inlet set up is far from tuned. ALL the Rotax 2 strokes NEED the factory exhaust system or you risk a power  reduction and uneven mixtures.  Nev

[turboplanner]

1 minute ago, facthunter said:

I doubt the exhaust is finely tuned. The inlet set up is far from tuned. ALL the Rotax 2 strokes NEED the factory exhaust system or you risk a power  reduction and uneven mixtures.  Nev

That’s the tune I was talking about; easy to take power off with just a few mm shorter or longer.

[skippydiesel]

The exhaust/header in question is the left rear. Photo does not show it however the clearance to the upper diagonal engine frame strut (white pipe) is insufficient.

[facthunter]

People who work on racing motorcycle exhausts shouldn't have a lot of trouble with that. White is the best for engine mounts as you see any cracks more easily..  Nev

[turboplanner]

16 minutes ago, skippydiesel said:

The exhaust/header in question is the left rear. Photo does not show it however the clearance to the upper diagonal engine frame strut (white pipe) is insufficient.

If it's the pipe attached to the engine by the two red nuts and it ends down at the muffler and it and the white tube cross just to the left of the white bottle, then you can make up a jig from RHS with a clamp welded to it to clamp the pipe securely at the muffler end, then make up a flange to replicate the Engine flange, and you can modify it out of the aircraft and hangar by clamping it in its lower position and bolting the top flange to the jig flange then have a good oxy person get the pipe the required distance away in the 90 degree direction from the frame. It may even be possible to heat the tube in the offending area and ovalise the pipe.

[onetrack]

Make up a cardboard template for the curves you need, and go visit a good local exhaust fabrication business. Exhaust tubing must be mandrel bent, the exhaust tubing bending machines are dedicated machines for the job.

Stainless steel is best for the application, but joins must be welded carefully with the correct electrode or wire. The stainless exhaust pipe is pretty thin.

[facthunter]

Stainless is tricky to weld and fatigues quicker.(cracks). There's enough length there to have a gradual bend. Make a new piece that slips over the  pipe each end.. You could clamp one end and weld the other. Nev

[skippydiesel]

6 minutes ago, facthunter said:

Stainless is tricky to weld and fatigues quicker.(cracks). There's enough length there to have a gradual bend. Make a new piece that slips over the  pipe each end.. You could clamp one end and weld the other. Nev

For a moment I thought you have presented the ideal solution Nev.

 

Then I realised a single cut & fill will change the entry/connection angles of the engine & muffler ends. If I go with your suggestion It will almost certainly have to be two cuts and fills.

[skippydiesel]

20 minutes ago, onetrack said:

Make up a cardboard template for the curves you need, and go visit a good local exhaust fabrication business. Exhaust tubing must be mandrel bent, the exhaust tubing bending machines are dedicated machines for the job.

Stainless steel is best for the application, but joins must be welded carefully with the correct electrode or wire. The stainless exhaust pipe is pretty thin.

I stand to be corrected  - Rotax supply straight pipe and  "'donuts" that must be cut to length to achieve the correct exhaust length & curvature (no mandrel bending - not sure the supplied pipe would put up with bending).

 

Only the turbo & I think fuel injected engines variants, are supplied with factory exhaust systems.

 

912UL & ULS are fitted into so many diffrent aircraft, that it would not be cost efficient to try and deliver a custom exhaust system for all the builder must make their own from the supplied parts (or source a system from elsewhere).

[skippydiesel]

32 minutes ago, turboplanner said:

If it's the pipe attached to the engine by the two red nuts and it ends down at the muffler and it and the white tube cross just to the left of the white bottle, then you can make up a jig from RHS with a clamp welded to it to clamp the pipe securely at the muffler end, then make up a flange to replicate the Engine flange, and you can modify it out of the aircraft and hangar by clamping it in its lower position and bolting the top flange to the jig flange then have a good oxy person get the pipe the required distance away in the 90 degree direction from the frame. It may even be possible to heat the tube in the offending area and ovalise the pipe.

You have the correct offending exhaust pipe.

 

"RHS" - do you mean CHS? (Circular Hollow Section). I have the correct Rotax exhaust pipe

 

"...ovalise the pipe."  Sorry not too happy with this idea, although I can see your point.

 

[turboplanner]

1 minute ago, skippydiesel said:

You have the correct offending exhaust pipe.

 

"RHS" - do you mean CHS? (Circular Hollow Section). I have the correct Rotax exhaust pipe

 

"...ovalise the pipe."  Sorry not too happy with this idea, although I can see your point.

 

Rolled hollow section.

 

In an exhaust pipe it's the sonic pulses which determine engine power rather than gas flow, hence length being critical, cross-section less so.

By squeezing the pipe oval where it fouls the frame, you don't change your length and you dont reduce your cross section.

[skippydiesel]

Rolled hollow section ?

 

In my world RHS stands for Rectangular Hollow Section

SHS - Square

CHS - Circular

[skippydiesel]

"In an exhaust pipe it's the sonic pulses which determine engine power rather than gas flow, hence length being critical, cross-section less so.

By squeezing the pipe oval where it fouls the frame, you don't change your length and you dont reduce your cross section."

 

Good point - The Rotax exhaust is 25mm. I need to improve clearance by somewhere between 10-15 mm to be sure that engine movement has been accommodated - particularly over time, should there be any "softening" of the engine mounts .

 

Questions:

• If I introduce a 10mm deep "depression" in the exhaust pipe how will this impact on a 25 mm pipe?
• What shape should the depression take - cone? - elongated flat bottomed flattening of one side? - elongated shallow/ deep /shallow?
• How to create the above dent ? - I have oxyacetylene, with which I can heat one side of the (removed) pipe and push/tap it to forme a 10 mm depression?
• Will heat to plastic stage have a negative impact on the SS pipe?
• Cold working (ball peen hammering)- always thought this negatively impacted on SS  integrity?

[facthunter]

You are better to only flatten one side if you're going down that path. The set up there is not tuned length as the front's are much longer than the rear and that engine has rich and lean cylinders in the simpler models because of the way the Inlets Phase..   Nev