4 stroke flexible headers - lifespan?

[Nigel R]

Gents

 

As a general rule, how long do you expect these flexible headers to last?

 

I just burnt through (or did it fail from vibration, not sure) one after maybe 90 minutes running time. Failed right at the connection to solid header.

 

Seems like fairly awful value for money at £30 a pop...

 

[Jon H]

Its normally vibration that gets them. Where possible, tie the exhaust to the crankcase bolts on the engine mount. This means the whole engine and exhaust move together as a lump. If one end is tied to the airframe this can be an issue. 

 

Also they need to be well supported at multiple locations to prevent resonant vibration causing a fatigue failure. 

[Denis Watkins]

Jon has covered it Nigel

But to add

Be mindful these are generally made up from pressed, thin walled, stainless steel.

We couldn't have picked a more brittle material where vibration is concerned.

Stainless passes the best gas and fuel test, but is bottom of the league for resilience and cracks at will.

[Paul De Tourtoulon]

If I remember right you should bend it followed by anneal it in the oven to soften it up, and do it now and again, My Helicopter had an Enya 90c

in it with the 'pucker' Enya one a long time ago and it didn't last long.

 

 here's what's left of it, 25 years on ? I never throw anything away,,,

[Nigel R]

Thanks gents

 

Not sure it's worth me trying again then, I have (had?) a 5" run from solid header to silencer which is fixed to airframe... no real options for changing the route or bends, supporting on the mount is not feasible.

 

Maybe a less neat install is called for, using the solid header then straight into the can... functional beats fancy-but-fragile.

[Jon H]

46 minutes ago, Nigel R said:

 functional beats fancy-but-fragile.

 

I am contemplating this very thing with my yellow spitfire. I have a fancy solution, but i already know its going to be a pain in the behind

[Denis Watkins]

I used one of these Insulated P clamps on my remote silencer, lasted 2 years now.

 

[David Ovenden]

I have a Saito 115 in an 82" PT19 model.  It has a Saito 90 degree adapter then a Saito flexible extension tube and then the standard Saito muffler.  That combo has done over 100 flights with no breakage issues. 

However, the engine is mounted on a Dubro anti-vibration mount and the flex tube is supported at 2 points on rubber mounts and the end of the muffler is supported on a rubber mount too.

[Tim Flyer]

I think key to fixed exhaust systems lasting a long time is that the clamp holds them firmly but allows small high frequency movements produced by the engine.
High temperature silicone inside the clamps works well . The clamping shouldn’t be too near the header. If the exhaust is clamped rigidly it is much more likely to crack the header. I make my clamps from aluminium sheet and just run a bit of high temperature silicone inside they look similar to Dennis’s picture above. Ps it’s a good idea to let the silicone set before clamping. 

Edited October 14, 2021 by Tim Flyer

[Paul De Tourtoulon]

i only use Ptfe tubing on my petrol and Methanol planes, and the only way flexible stainless tubing will last is when it is straight,,

 

 "my experience',,,

 

 With one exception my 25+ year old Marutaka Enya 90c powered Spitfire, and yes the same enya that broke the flexible in my Helicopter;

[Frank Skilbeck]

Accelerated fatigue testing is performed by putting a fixed load on a component and then applying a variable load, so if you have a tight bend and then apply vibration you are basically doing an accelerated fatigue test. 

 

Also very common on high pressure flexible hoses subjected to motion, i.e. production/export hoses on offshore oil production facilities, to incorporate bend stiffeners at the ends to prevent premature failure where the hose is moulded onto the end connection. 

[Nigel R]

Hmm.

 

Mine was one of those with the thread & nut on both ends. I assume that imparts a large degree of stress right where that coupling holds the flex pipe. That was where the pipe sheared off. No degree of 'give' in the clamping.

 

I'm guessing heat actually had very little to do with my failure.

 

As there's no chance of getting a straight run in my original go, I'll just use the existing header and live with the can itself being off centre.

 

Thanks again all for your replies

Edited October 14, 2021 by Nigel R

[Andy Stephenson]

Does anyone know where to buy the raw flex tube as I was thinking of making my own flex header.

 

Thanks,

A.

[Denis Watkins]

Inwood Models

had some

[Robin Colbourne]

The silicone pipes beloved of the car tune-up fraternity are handmade from several layers of silicone with woven reinforcement inbetween.
There should be potential for making a reinforced silicone exhaust with all the necessary bends built in.
Something like this:  Pre-formed reinforced silicone tube

 

Edited October 14, 2021 by Robin Colbourne

[Robin Colbourne]

Here's a video on how the reinforced silicone tubes are made.  That mangle looks like an accident waiting to happen!

 

Manufacture of automotive reinforced silicone tubes

[Stuphedd]

Working in the aircraft industry for years I noticed that a lot on the sensor wires were protected from damage by being housed in st st convoluted "tube " and when  scrapped found new homes on some of my 4 strokes ! 

I also found that Saito exhaust threads are the same as brake pipe ends !!

cheers

[Martin Harris - Moderator]

5 hours ago, Andy Stephenson said:

Does anyone know where to buy the raw flex tube as I was thinking of making my own flex header.

 

Thanks,

A.

Mick Reeves Models. 

[Andy Stephenson]

Martin,

 

Just found it on Mick Reeves site.

 

Thanks,

A

[Tim Flyer]

If you don’t mind brazing on both ends with brass tubing, the raw flexy pipe is just called “stainless steel flexible conduit tubing”’and Is available on e-bay and other firms in lengths of a metre upwards . 

[Andy Stephenson]

I have a plan to braze a custom exhaust for a 4 stroke. It's difficult to tell if the ebay offerings are fully sealed as it's not always obvious from the description so I think will go with the Mick Reeves product unless anyone has a proven supply for the correct type..

[Paul De Tourtoulon]

For the professionals,,,,?

 

https://www.krumscheid-metallwaren.de/zubehoer/metall-flexschlauch.html

 

 they have it all,,,,,,,,,,,,,,?

[Andy Stephenson]

I have a suspicion that the spiral wound type isn't fully sealed as it's only for mechanical protection where as the concertina type is what we need for exhausts.

[Engine Doctor]

Stuart Mckay  of Moki engines used to supply all exhaust components including Krumschied silencers so maybe able to supply flexi pipe?

[Ron Gray]

Just Engines