Aluminum vs steel exhaust, specifically radial collector rings

[petertha]

I've been around RC a long time, but never owned a radial myself until I built the O5. At some point I'd like to make a collector ring for my O5 & future engines. And reading the posts here, familiar with trials & tribulations of stainless exhaust systems: annealing, filler, maybe bit more specialized roller benders, joints etc.

I just so happen to be on Heilemann site lurking around. I'm not clear if they are the manufacturer of KALEO or a sales distributer or a custom modifier. But somehow I missed the detail that they are made of 'T-6' aluminum & painted black. From what I read, the RC crowd over the years seem very satisfied with the product from running standpoint on both methanol & gasoline suggesting higher thermal expansion can somehow be dealt with. Now I'm wondering out loud if bending aluminum would be less drama over thin wall stainless or just a different flavour of careful requirements. I don't own a TIG but its on the bucket list. The connector tubes & joints & such would have to be tacked & welded vs silver solder or TIG brazing steel tubing alloys. When I took a class years ago, I managed to TIG some inches of 1/16" 6061 (we wont talk about apprentice puddles on the floor). What do you think? Is aluminum it rabbit hole B vs SS rabbit hole A?

https://www.heilemann-sternmotoren.d...-fg-19-r3-info

 

[petertha]

Aluminum headers & tuned pipes have been around RC forever, but I have no idea what the manufacturing end looks like

 

[Jasonb]

I think the designs of engines we have are based on what most can do in their home workshop and silver soldering is one of those things so materials have tended to be those that can be soldered. Not so much choosing the material for it's looks, weight or how it performs but how it can be worked.

Even with aluminium you would want a softer grade to bend as common T6 will be harder to bend than T2 or annealed. The temper may well be done on those parts after fabrication as even local heating at the joints would likely alter the temper.

You also need to think about wall thickness if you are going to avoid a chunky looking exhaust. Steel, stainless, brass and copper could all easily be soldered at 0.5mm wall thickness but hot easy to TIG unless you are good at it so having to use thicker wall to avoid melting the tube may look a bit clunky.

 

[Bentwings]

I think the designs of engines we have are based on what most can do in their home workshop and silver soldering is one of those things so materials have tended to be those that can be soldered. Not so much choosing the material for it's looks, weight or how it performs but how it can be worked.

Even with aluminium you would want a softer grade to bend as common T6 will be harder to bend than T2 or annealed. The temper may well be done on those parts after fabrication as even local heating at the joints would likely alter the temper.

You also need to think about wall thickness if you are going to avoid a chunky looking exhaust. Steel, stainless, brass and copper could all easily be soldered at 0.5mm wall thickness but hot easy to TIG unless you are good at it so having to use thicker wall to avoid melting the tube may look a bit clunky.

I’ve been in Rc aircraft since it began warbird were my thing so scale functioning exhaust was considered the cats meow. TIG welding thin aluminum tube in tight quarters is very hard . I switched to copper tube and standard fittings by careful trimming you c 60% ofvthe weight off copper welds very nice but things et hot very fast. I found silver brazing with the TIG torch to be the easiest and best looking . Non o these failed in service aluminum starts to anneal just over 400 deg F gas engines exhaust is about 900 deg f so are in a losing situation . A guy built and sold kinda scale aluminum exhausts for a while but these did not last long and were chunky big and heavy brass is marginal as you have to make bends rather than purchase. Copper gets hot fast but also gives up heat just s fast. I had a giant scale Corsair that had copper scale functioning ex haust I flew it for 13 years nearly every flyable eeek end . I YHINK the exhaust is still in the “ some day “ parts box . I never repaired from day one . I made a stainless steel exhaust forvthe giant P 51 but sold it .015 stainless steel very challenging to TIG weld even with perfect fits . Have to back flow argon or weld in weld bag . Not fun at all

 

[Peter Twissell]

I have had success TIG welding thin wall stainless tube, by making a 'liner' at each joint.
The liner is a short section of the same tube, with a segment cut out so its diameter can be closed up to a tight fit in the tubes to be welded.
The use of such liners allows the exhaust sections to be fitted together in situ on the engine and adjusted as necessary before tacking in place.
The ends of the sections are left dead square and fusion welded with no filler. The liners mean that the weld does not penetrate through to the inside, so no gas purge is necessary. The liners also prevent the weld area from 'sinking'.
These are the induction pipes for my radial engine, as welded.

They cleaned up nicely. I am very far from an expert with the TIG. At the top right of the image below you can see some of my practise welds.

 

[Jasonb]

What sort of diameter were those? I think Petertha is looking at around 6mm OD and maybe 0.5mm wall

 

[Peter Twissell]

These were 32mm diameter with 1mm wall thickness.
I have welded tubes using the same method down to 8mm diameter with 0.6mm wall.
The 8mm tubes were no more difficult than the larger sizes, requiring only a few test welds to get the current right.
I am using an RTech 160 amp hobby rig, which has good control right down to very low current.

 

[petertha]

Very nice @Peter Twissell Sorry for so many questions at once.
- by the stamping on the one picture, it looks to be 316L stainless?
- were you able to find an ID matching OD liner tube from available stock or did you resort to turning a segment to suite from bar stock?
- what is an example or typical length of liner segment between 2 adjacent pipe segments?
- regarding bending, can you give me some idea of the bend radius relative to diameter you achieved on those examples? Assume you are using alloy filler & your own bender/roller? Any pictures to share on that front? Are you pre-annealing the tube before bending?
- can you elaborate on the tubing / flange joint? It almost looks like tube goes into a tight fitting hole in the flange & welded with no filler?
- on this picture, are the tubing ends turned down slightly for some other purpose, or is that a liner segment?

 

[Peter Twissell]

Hi Petertha,

The material is indeed 316L.

The liners were made from short sections of the same tube. I cut a section out of the liner ring, so it could be closed up to fit in the tube.
Before cutting the section out, I put each liner ring up in the lathe and chamfered the ID at each end to preserve a smooth flow through the assembled pipework.

For the 32mm tubes, the liners were about 10mm long. for the 8mm tube, they were about 3mm long. Just enough so that they would support the tubes together for assembly on the engine and tacking.

I purchased preformed bends, known as mandrel bends. These are available with centreline radius down to 1D, i.e. the same as the tube diameter. For complex assemblies, I cut the required angle of bend from these 90 degree mandrel bends.

The flange joint is exactly as you describe, welded on the other side and then machined to a flat face. For another application (motorcycle exhaust), the tubes protrude through the flanges and are welded on the "outer" side. The protruding section of tube fits into the port on the cylinder head.

The ends of my induction tubes are turned down. I needed to create a seal where they pass into the supercharger housing at an angle. I achieved this by turning the tubes down and making some tubes to a tight fit over the turned ends. The outer tubes were then cut at an angle and the two parts pressed into place with a gap between them, creating an O ring groove at the correct angle to seal into the supercharger housing.

I plan to make a small tube bender, with a mandrel to prevent thin wall tube from deforming around tight bends.
I'm not sure how the manufacturers of the mandrel bends achieve such tight radii with zero deformation - the bends are near perfectly round section right through.

 

[petertha]

Thanks so much for sharing your experience. I was aware of commercially available stainless/steel bends, but could never find them in smaller tube diameters., at least in the range of exhaust pipes & collector ring. For example this starts at 25mm. I'd love to see the machine/process they were made on.

https://www.everyexhaustpart.com/cart/tube-and-exhaust-bends-and-elbows

 

[Bentwings]

I have had success TIG welding thin wall stainless tube, by making a 'liner' at each joint.
The liner is a short section of the same tube, with a segment cut out so its diameter can be closed up to a tight fit in the tubes to be welded.
The use of such liners allows the exhaust sections to be fitted together in situ on the engine and adjusted as necessary before tacking in place.
The ends of the sections are left dead square and fusion welded with no filler. The liners mean that the weld does not penetrate through to the inside, so no gas purge is necessary. The liners also prevent the weld area from 'sinking'.
These are the induction pipes for my radial engine, as welded.
View attachment 154004

They cleaned up nicely. I am very far from an expert with the TIG. At the top right of the image below you can see some of my practise welds.

View attachment 154005

I do a number of top fuel race car headers in stainless if you use one level higher numerically than the base level material you usual don’t have trouble if you back fill . It’s expensive as you can use incredible amount of argon on 2 1/2” dia systems I did a lot of industrial finiky stainless stuff gladbitvwas not my expense a lot was very thin stuff you also have to match filler rod size with material thickness . I don’t remember exactly but .023” was about as small as I remember the company had a wire drawing mill so they made a lot of their own filler you hadcto have clean gloves all the time Forget bare hand stuff I didn’t do that any way you get sun burned hands not fun. I did tank truck repairs for a while you had to wear

 

[Bentwings]

Aluminum headers & tuned pipes have been around RC forever, but I have no idea what the manufacturing end looks like

I’ve used the tuned pipe manifolds and headers shown in aluminum with glow engines. These held up pretty well . But repair if needed even though I’ve been around precision TIG for a long time is nearly impossible once glow fuel gets in them . Cleaning is the issue . TIG requires perfectly clean surfaces and an incredibly steady hand and endless patience . I have made aluminum exhaust gas fuel gets too hot . Upwards of 900 deg F the aluminum gets pretty weak up there with melt temp only another couple hundred degrees . Silver solder is not good either . That’s why I switched to copper systems copper TIG welds very nice as well as accepts silicone bronze brazing . I’ve done some really thin stainless but it hard to work with if you don’t have the tools a very good mandrill bender is a must. Just getting straight thin wire filler is hard you can make your own wire straightener as I did but it’s another complication.

 

[Bentwings]

I've been around RC a long time, but never owned a radial myself until I built the O5. At some point I'd like to make a collector ring for my O5 & future engines. And reading the posts here, familiar with trials & tribulations of stainless exhaust systems: annealing, filler, maybe bit more specialized roller benders, joints etc.

I just so happen to be on Heilemann site lurking around. I'm not clear if they are the manufacturer of KALEO or a sales distributer or a custom modifier. But somehow I missed the detail that they are made of 'T-6' aluminum & painted black. From what I read, the RC crowd over the years seem very satisfied with the product from running standpoint on both methanol & gasoline suggesting higher thermal expansion can somehow be dealt with. Now I'm wondering out loud if bending aluminum would be less drama over thin wall stainless or just a different flavour of careful requirements. I don't own a TIG but its on the bucket list. The connector tubes & joints & such would have to be tacked & welded vs silver solder or TIG brazing steel tubing alloys. When I took a class years ago, I managed to TIG some inches of 1/16" 6061 (we wont talk about apprentice puddles on the floor). What do you think? Is aluminum it rabbit hole B vs SS rabbit hole A?

https://www.heilemann-sternmotoren.d...-fg-19-r3-info

TIG is not something you simply pick up the torch and piece of wire filler. It takes patience and practice then some knowledge of material and tungsten
More advanced machines have more features to learn to use I worked in shops without all the bells and whistles yet managed to pass all the qualification tests . I even taught the shop guys for a year so they could pass qual tests I like to say learn to weld with oxy acetylene on steel first then you will have much easier time with TIG. Also forget welding aluminum with gas torch there people that can do it well but it takes Iv don the aluminum beverage can thing many times but it takes skill too too a lot
Best to waste of skill skill . Even so called aluminum brazing is not mastered over night .

Watch some videos

 

[Steamchick]

I find aluminium "solders" very effective for most hobby applications.
I learned "zinc" application to aluminium as a preparation for lead soldering a cable joint, when I was an apprentice. I used zinc rods a lot until the more modern Aluminium (with a few % silver?) rods became available - that are easy to use and nearly as strong as annealed aluminium. Joint design is critical where any strength is required. Also, ALL aluminium is fully annealed by any hot joining method, so if you are using a hardened and tempered alloy (check you grade!) for strength you will lose up to 2/3rds of the material strength where heated/joined! S it is common for hot-fusion joined aluminium structure to use very thick sections just to cope with the material properties after heating. (Partly why aeroplanes were riveted, not welded! - And why Welded Aluminium boats are prone to fail from early life fatigue cracking! - unless properly design and manufactured.).
K2

 

[ninefinger]

It depends on you're design goals. From a fabrication simplicity point of view, copper is the winner. Easy formability, easy joining with silver soldered joints. And you could even nickel plate to conceal the copper colour.
Biggest downside on copper is it will remain relatively soft and therefore easily deformed if bumped, and for a flying application its heavy.

 

[Peter Twissell]

I have made copper manifolds by plating on to 3D printed formers. They are approximately 0.8mm thick and fitted to my flat twin sidevalve engine, which will be fitted to a flying model.
My design has the manifolds well protected from damage.