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John Lancaster

Exhaust springs break way too often

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Hi all. I'm constantly finding broken exhaust springs during my preflights (2015 CTLSi). And by constantly, I mean one broken spring per two or three flights. Sometimes literally every other flight. Have owned the plane two years and the problem seems to have gotten worse in recent months. Switched to stainless steel springs but they seem no more durable than the old ones. I smear with RTV but that's no help either, so far as I can tell.

Is this indicative of a larger problem? Anything I can or should do? Huge pain to replace a spring every time--or almost every time--I go to fly...

 

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I'll chime in too. I have a CTLSi that I take care of, and it goes through springs. Even if you install the longer stainless spring it has to be over stretched. The gap between the coils is almost as wide as the diameter of the wire the coils are made of. Does anyone know of a longer spring that is available. The loops welded to the pipes are already bent down as much as the can be and still hook the spring in the loop.

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Tom/John... I have 155 hours on the Hobbs now... I haven't popped a spring yet.  That orange (silicone-ish stuff) goop is applied to the springs... it looks like it is used for vibration dampening.   I will be flying in the AM to Woodstock for some repair, I'll take a photo of what I am speaking of.  

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Hi John,

Give me a call so I can ask some question and hopefully point you in the right direction. I have 4 912iS's I take care of and two at my field. No spring issues.

I do agree that some exhaust have the spring loops too far apart and that stresses the springs. There is really only supposed to be a 1mm gap in the spring coils. That's about the thickness of your thumbnail.

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Two things. First remove the safety wire that is holding the springs against the hot exhaust. This won't keep the spring onboard if it breaks in the middle. Only if the hook breaks. The other item is there isn't even close to enough silicone on the springs. It should be wider, thicker and forced into the coil spaces. Plus you could help keep a little extra temp off the springs if you wrap the header. This isn't a big deal, but every little bit helps.

 

Exhaust spring1.jpg

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Bill, he is saying that my wiring scheme is inferior to his.  Both are wired, his wiring scheme will hold the broken spring on until  it is noticed.  My factory wiring scheme runs the risk of the broken spring ending up in someone’s back yard.  Next maintenance Occasion, I’m going to have it changed.   

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2 hours ago, AGLyme said:

Bill, he is saying that my wiring scheme is inferior to his.  Both are wired, his wiring scheme will hold the broken spring on until  it is noticed.  My factory wiring scheme runs the risk of the broken spring ending up in someone’s back yard.  Next maintenance Occasion, I’m going to have it changed.   

Plus you're getting direct heat from spring to pipe contact. There is no air gap.

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In 100+ hours of owning my CT I've not lost a spring, but do deal with the length issue (low compression / loose fitting).  I have a family connection in spring manufacturing, real high end medical device springs.  Looking at the price of what market charges for these, and issues with lengths / forces developed, has me wondering if developing a range of slightly longer / shorter offerings, and in a more robust design / material, would be enough of a market.  It's not like there is a lot to making a spring in todays world of NC equipment.  One would think this is common across other Rotax exhausts?  Do those in the know here see this on other LSA's too?  Would appreciate some feedback to consider next step here.

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Broken springs aren't from the material or design, but with wrong installation. Springs do break, but considering that there is over 55K Rotax engines and over 6 million run hours it isn't that common. When installed properly springs should hold up for many years.

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My plane is maintained by an A&P who is very experienced on CT's, and with many data points, I have found to be extremely good.

I go through springs...maybe two a year.  I think that in my case it's very unlikely that the issue is do to incorrect installation.

Andy

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Just a thought, as I have no experience with exhaust springs breaking.

This COULD be a case of sympathetic resonance on a specific motor where at a certain RPM, along with a small imbalance somewhere in the engine/prop combo. This puts the springs in a  sweet spot where they will vibrate at a high frequency. This will eventually lead to metal fatigue and cause a spring to fail.

Changing the spring rate, Tension, or amount of RTV ( more or less of all 3) may change the mass/ resonance frequency enough to prevent the high frequency vibration.

Seriously, Using a guitar tuning app or similar on you phone, you could do a test to see what natural frequencies your springs are at. and then doing a little math, using engine RPM, Prop RPM, etc could get an idea of where some engine produced frequency could be if any of these are close together, there maybe something to this.

Just a thought. 

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"Changing the spring rate, Tension, or amount of RTV ( more or less of all 3) may change the mass/ resonance frequency enough to prevent the high frequency vibration.".

You got it. Most don't apply enough RTV and and either have too much tension or not enough.

 

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In the case of Andy's airplane the hooks for the springs are to far apart from manufacture. The long springs are over stretched. I need to find a suitable replacement that is a little longer. I was eyeing the springs on a Savannah the other day.

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John Lancaster, I recently had Kari at Tally Ho Aviation in Danbury, CT balance my Husky’s prop. He does an amazing job and takes the time to really get it perfect. I was losing springs on my CT all the time and cracked two exhausts. I wish I had taken the CT to him the for a balance. 
 

He charges a flat $225 rate. 
 

kent

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On 9/4/2020 at 12:49 PM, Roger Lee said:

Hi Tom,

Can you take a hammer and tap the spring loops on the pipe +a little closer and flatter on the pipe?

Roger,  there is barely enough room to get the spring hook through the loop the way I put them on as it is. I could maybe do it the other way, but it is a bigger hassle.

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Hi Tom,

I use 12" needle nose pliers with some grooves cut into the jaws to install or remove springs. I found it was the easiest way. Then I can use all my upper body weight most of the time. You can slide then straight in and push straight down on the spring hook to install or remove it.

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Only one broken spring in 600 hours on mine.

For installing springs of any kind, I have a short length of ornamental chain - the kind used for hanging lamps and such. One end hooked to the spring, then a screwdriver or the like on the other end serving as a “T-handle” to pull the spring up and over the hook on the pipe. Works quite well, assuming you have room to work.

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37 minutes ago, FastEddieB said:

Only one broken spring in 600 hours on mine.

For installing springs of any kind, I have a short length of ornamental chain - the kind used for hanging lamps and such. One end hooked to the spring, then a screwdriver or the like on the other end serving as a “T-handle” to pull the spring up and over the hook on the pipe. Works quite well, assuming you have room to work.

I use a similar method with safety wire. 

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