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  #71  
Old 10-30-2011, 09:12 AM
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AlexPeterson AlexPeterson is offline
 
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Originally Posted by AlexPeterson View Post
A perfectly flared, properly torqued tube can crack if there is not good alignment. In other words, the B nut should thread on by hand. If it isn't easy to do so, the tube is probably not aligned. Now add a little cyclic loading, either bending or tension, and it will fatigue crack. I'm guessing the tube in the first picture was not cracked at the time of flare or tightening. I certainly could be wrong.
I thought this might stir some responses??
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  #72  
Old 10-30-2011, 09:17 AM
Captain Avgas Captain Avgas is offline
 
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Originally Posted by Walt View Post
So regardless of what material you elect to use the, the tools, methods and quality control is the determining factor of whether or not you will end up with a good part.
I would have thought that "quality control" specifically means using the correct material in the manufacture of any product. All industry documents that I have read would seem to indicate quite clearly that grade 3003 tubing is not recommended for brake lines on aircraft.

While I agree that correct flaring technique is thoroughly important (obviously), I don't think there is any proof available that all of the RV brake line failures can be attributed solely to poor flaring technique. Misalignment of the flare and/or overtorquing of the B-nut during repetitive reassembly may account for many of the failures. Consider that the flare at the top of the brake line on RVs never seems to fail....only the bottom flare. Could it be that the bottom flare is always the one that is poorly fabricated. That is not likely.

More likely is that the bottom flare is potentially subjected to more cycles of reassembly misalignment and/or overtorquing as it is the connection that is constantly being disconnected for maintenance. Each misalignment and/or overtorquing may deform the flare progressively until it eventually fails in service.

My argument is that 5052 is arguably less critical to flare in the first instance than 3003 (at least in my experience), less susceptible to damage from multiple retorquing (it's harder), and less likely to fail due to the cyclical stresses placed on it due to RV gear movement (has a higher fatigue strength).

In the hands of an amateur builder who may do no more than a couple of dozen flares in his life there is probably a lot more safety factor in opting for 5052 alloy than the 3003 alloy that no mainstream technical aviation document recommends.

In terms of the dangers of repetitive overtorquing of tubing flares I refer again to the document by Fred Moreno:
http://forums.matronics.com/download...43a40358ba848e

Having said all of this I must admit that I did not use any type of tubing at the brake caliper on my RV7A. I used high pressure hose. I think Vans design is flawed in that area and the fact that so many other builders use hose in that location as well means that many obviously agree with me.

When there is any doubt in my mind on crucial design issues I tend to have a good look at several certificated aircraft to ascertain what constitutes best practice before I proceed.
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Last edited by Captain Avgas : 10-30-2011 at 07:23 PM.
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  #73  
Old 10-30-2011, 09:51 AM
gereed75 gereed75 is offline
 
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Been following this closely - my conclusions:

3003 is a servicable choice done well. 5052 is a better one. In either case, your flares better be of high quality and your B-nuts not over tightened. 5052 would seem to provide a higher margin for less than professional craftmanship.

I am an engineer. I have a background in professional aviation and quality control. Even with that, I judge my personal level of workmanship to be only a 6.5 out of 10. My tube bending and flaring were done at that same level of "servicable but not great" workmanship. That tubing has served well in 200+ hours of service.

This thread has given me reason to look again at the quality of this critical system where I (like most builders) used a less than optimum material put together with less than optimal skills.

Now armed with a copy of SAEJ533, I will inspect my fuel tubing system (fuel injection with AFP pump and filter) for quality - with the expectation that I will probably be re-doing this critical system with 5052 and much improved quality and workmanship.

This will cost only a few $'s and a weekend's time.

I relate all of this because I suspect that I am about the "norm" for builders out there, and if I think this is a prudent course (knowing so much more now than I knew before), then it is probably a prudent course for lots of other builders.

That is the value of this forum. This is the type of cultural knowledge and standards that we can foster in our community that will create safer builders, airplanes and fewer accidents. Walt is setting a higher, more professional standard for us all to follow.

Current builders - proceed with 3003 but understand that quality craftsmanship and care is essential in this critical area to ensure safe results.
Hacks like me - redo better the second time (and while your at it, why not use the better material).

Thanks for starting the thread and thanks for all the thread contributions that led to improved knowledge.
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Last edited by gereed75 : 10-30-2011 at 09:55 AM.
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  #74  
Old 10-30-2011, 10:55 AM
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CharlieWaffles CharlieWaffles is offline
 
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Has anyone figured out the weight penalty for flex lines versus 3003 or 5052? From what I've read, the downside to flex lines is limited life and weight. But how much more does a full set of flex lines (aft of firewall) really cost in terms of weight?
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  #75  
Old 10-30-2011, 11:59 AM
aerhed aerhed is offline
 
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Default Most important thread in long time

Really glad to see this thread. I don't want to see it reduce to this vs that. The subject is crappy flares & tube fitting. Not material or flex vs tube. Crappy flares & fit. A good flare that is properly aligned can stand some torque abuse, etc. I've seen perfectly flared SS tubes fail from stress & vibration. That's engineering and trial by fire. Half the equation. The other half is......crappy flares.
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  #76  
Old 10-30-2011, 12:27 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by CharlieWaffles View Post
Has anyone figured out the weight penalty for flex lines versus 3003 or 5052? From what I've read, the downside to flex lines is limited life and weight. But how much more does a full set of flex lines (aft of firewall) really cost in terms of weight?
The flex lines used are usually Teflon which should not have any limited life issues.
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  #77  
Old 10-30-2011, 06:57 PM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by az_gila View Post
The flex lines used are usually Teflon which should not have any limited life issues.
Well Gil, I don't usually disagree with you, but at Oshkosh last year, i talked with a couple of different manufacturers of Teflon hoses, and both said that they need to be replaced at intervals "set by the airframe manufacturer". Neither one indicated that their lifetime is indefinite.

Paul
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  #78  
Old 10-30-2011, 08:30 PM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by CharlieWaffles View Post
Has anyone figured out the weight penalty for flex lines versus 3003 or 5052? From what I've read, the downside to flex lines is limited life and weight. But how much more does a full set of flex lines (aft of firewall) really cost in terms of weight?
Part by part, inch by inch, I don't have any numbers. However, I removed most of the flex hose FWF on the -8 and replaced it with stainless hard lines. Where I was forced to run flex hose due to component movement, I kept it as short as possible. I'm guessing this -8 was a "typical" example of a fuel injected RV.

Going to SS hardline I saved an incredible 7 pounds. The trash bag full of discarded firesleved hose was really something to behold.
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  #79  
Old 10-30-2011, 09:08 PM
N427EF N427EF is offline
 
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Default Weight v.cost

More than cost of the weight is the weight of the cost.

First off, Vans own airplanes have accumulated well over 20,000 hrs using the soft 3003 fuel line supplied in our kits. Learn how to properly flare and torque the B nuts and you have nothing to worry about.

Not only are hoses impractical in many tight locations due to a larger bend radius but hoses are also too fat to fit in many locations behind the firewall. Weight of those hoses is just one of the downsides but the cost of a complete set of hoses could easily be more than 10 times the cost of aluminum tubing.
Even with unavoidable trial pieces that most of us create when doing fuel lines it would be difficult to exceed a cost of $50.-in material. You can buy
2 hose fittings for that price and you can figure out the rest for yourself.

The picture in the original post shows an "acceptable flare" with sufficient wall thickness but sharp edges and a crack probably the result of missalignment or vibration.
I use a scotch brite pad to smooth out the edges of flares and I never miss an opportunity to install an adel clamp on a nut plate for support.
One thing that bothered me on the RV-10 fuel system behind the tunnel was the oil canning of the firewall. In essence that wobbly surface would transmit the engine vibrations throught the fuel hose and the bulkhead fitting to the back of the firewall fuel lines and thus cause potential for cracked aluminum
flares.
Here is what I did to alleviate the problem.

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  #80  
Old 10-30-2011, 09:12 PM
fehdxl fehdxl is offline
 
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If you could, please take a look here any provide positive/negative comments about how I made up my flares.

Thanks in advance for your time!

-Jim
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