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#4 Brake lines - Why?

Toobuilder

Well Known Member
#4 AN hose, tube and fittings are essentially ubiquitous in light aircraft brake systems. Considering that we are dealing primarily with pressure vs. flow in this system, why not run #3 or even #2 lines? Any thoughts on why #4 is the standard?
 
Physical Robustness?

#4 AN hose, tube and fittings are essentially ubiquitous in light aircraft brake systems. Considering that we are dealing primarily with pressure vs. flow in this system, why not run #3 or even #2 lines? Any thoughts on why #4 is the standard?

I would think mechanical robustness could be one? Larger fittings should be more resistant to kicks, tugs, etc. (thinking the portion of the system on gear legs, calipers, etc.)

That said, I used -3 hoses down the gear legs with -3 elbow fittings into the calipers on my -7. Works just fine.
 
I'm just guessing but #4 is most likely the best compromise for size with respect to performance, weight, robustness, availability, and component cost.
 
-3 works fine

-3 works just fine on my plane. For the flexhoses at the brakes, I used Russell-Edelbrock motorcycle brake hoses with SS AN3 fittings on the ends. They're Teflon w/ SS overbraid and a vinyl sheath. Very nice.
 
Truth be told, I'm planning the switch to #3 on the Rocket anyway.

But then I was thinking: If #3 is OK, how about #2?

So this thread is my "research".
 
Truth be told, I'm planning the switch to #3 on the Rocket anyway.

But then I was thinking: If #3 is OK, how about #2?

So this thread is my "research".

I also used all -3an fittings with premade Teflon braided hoses like Snopercod. I have an RV8 with the Grove gear legs, and it was nice to use the smaller lines out of the bottom of the gear legs to the brake calipers. I thought about using 2an, but I seem to remember the fittings and premade lines difficult, or impossible to find in the fittings I needed
 
Or you can do what i did. Use #4 AN fittings and #3 lines. Works great and the #4 fitttings are more robust. Also would allow you to keep your fittings if you had #4 fittings on already.
 
Or you can do what i did. Use #4 AN fittings and #3 lines. Works great and the #4 fitttings are more robust. Also would allow you to keep your fittings if you had #4 fittings on already.

Smaller diameter tubing in a larger fitting??? :eek:
 
Or you can do what i did. Use #4 AN fittings and #3 lines. Works great and the #4 fitttings are more robust. Also would allow you to keep your fittings if you had #4 fittings on already.

I did the same. Makes life easy.
 
My 61 year old C-180 uses the factory original solid 1/4" aluminum lines all the way from the fuselage to the brakes. No problems and no ADs on it.

And on that plane if there were a systemic brake problem, there would certainly be an AD.

I don't plan to use flex lines near the brakes on my RV-3B.

Dave
 
We build brake hoses from both -3 and -4. Vans, and several other manufacturers suggest -4 ( they even have -4 adapter fittings in the kits) for the brake hoses.
Finding -2 hose is tough, finding alot of -2 hose ends for teflon is tougher.
Tom
 
It used to be accepted practice in the homebuilt world to run plastic tube for with home depot compression fittings for brake hose - and I'm sure there are plenty of RV's still so equipped. This leads me to the conclusion that the actual operational requirements of the system are pretty modest. I'd think that long term durability and environmental factors are the primary consideration when selecting components (which is why plastic is now out of favor). That said, standard automotive inverted flare fittings and steel, stainless steel or aluminum tube are common and more than durable enough. Many automotive brake system lines are 3/16 and smaller, so the fittings are out there.

If #2 hose is a problem (from brake cylinders to firewall) then its pretty easy to run the #3 hose to #2 tubing and adapt at the firewall fitting.
 
I also would have little doubt the -3 would handle the pressure, however as you know, you will amplify the pressure in the tubing when you downsize. I would lay odds that you would blow seals before you would burst the tubing, but why not do the math? Grab a standards book, look up the specs of the tubing in the alloy and temper you are considering, and go at it. There are even handy calculators on the web that you can plug the figures into if you know them. You don't even need to come up with the actual PSI, just a factor between the -3 and -4. That will give you the difference in a percentage of safety factor.
 
Fluid Mechanics and Tube Stresses

I also would have little doubt the -3 would handle the pressure, however as you know, you will amplify the pressure in the tubing when you downsize. I would lay odds that you would blow seals before you would burst the tubing, but why not do the math? Grab a standards book, look up the specs of the tubing in the alloy and temper you are considering, and go at it. There are even handy calculators on the web that you can plug the figures into if you know them. You don't even need to come up with the actual PSI, just a factor between the -3 and -4. That will give you the difference in a percentage of safety factor.

2 things wrong here. 1. Reduction of tubing size in the system will not "amplify" pressure. For a relatively static system (0- to minimal fluid flow), the fluid pressure will be equal throughout the system regardless of tubing size. 2. Generally, smaller diameter tubing can handle higher pressure than larger diameter due to lower hoop stresses.

Skylor
 
I have no doubt that any reasonable material selection will handle the pressure of the system (see the above referenced plastic hose, or the fact that the brake lines are COPPER on my certified Tri Pacer). My only real question is why 1/4 tube became the standard. I'm perfectly willing to accept something simple like “economy” or “availability of materials”, but I was hoping to uncover something more technical – perhaps related to viscosity of 5606, damage tolerance, etc…

However, if our only reason is “…we’ve always done it that way…”, then I’m pretty comfortable breaking that paradigm.
 
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2 things wrong here. 1. Reduction of tubing size in the system will not "amplify" pressure. For a relatively static system (0- to minimal fluid flow), the fluid pressure will be equal throughout the system regardless of tubing size. 2. Generally, smaller diameter tubing can handle higher pressure than larger diameter due to lower hoop stresses.

Skylor
I stand corrected and neglected the no, or minimal flow. The specs in my handbook call out the same max. PSI rating for all sizes the same, but I get what you are saying. Perhaps that is why I flunked out of engineering school ;)
 
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Keep it small

I have no doubt that any reasonable material selection will handle the pressure of the system (see the above referenced plastic hose, or the fact that the brake lines are COPPER on my certified Tri Pacer). My only real question is why 1/4 tube became the standard. I'm perfectly willing to accept something simple like ?economy? or ?availability of materials?, but I was hoping to uncover something more technical ? perhaps related to viscosity of 5606, damage tolerance, etc?

However, if our only reason is ??we?ve always done it that way??, then I?m pretty comfortable breaking that paradigm.

I have been around aerospace forever. I have seen more -4 stuff than -3, so to me -4 is more common. On the other hand the -3 is smaller and may make bleeding the brakes easier, probably will get a more solid fill with -3 lines. If this were a race car i would worry more about a solid fill, but being used only for taxi and the last little bit of landing it probably doesnt matter much. I wouldnt go bigger than -4 thou, the air may never get out during bleeding.JMHO
 
Just as a data point, for my gliders I use 1/8" OD Nylaflow tubing for the entire brake circuit. In areas where the tubing gets flexed on gear actuation or is subject to abrasion, I armor it by running it through a section of 1/4" OD Nylaflow.

This is basically the system that has proved adequate for the early Rutan aircraft. I suspect that it might not really be good enough for something heavier or that taxis farther or faster, but at the lower end of the scale it works plenty well and saves a bunch of weight, complexity, and parts count.

--Bob K.
 
I looked at the burst strength for some of the 1/8 inch plastic tubing I procured for my pitot/static lines and IIRC it was 350 to 400 PSI - plenty for brakes.

However, as you point out, the heat generated on one good stop and the lack of long term environmental durability takes it out of consideration for the Rocket. If not for those two issues, I'd string that stuff in a heartbeat!
 
2 things wrong here. 1. Reduction of tubing size in the system will not "amplify" pressure. For a relatively static system (0- to minimal fluid flow), the fluid pressure will be equal throughout the system regardless of tubing size. 2. Generally, smaller diameter tubing can handle higher pressure than larger diameter due to lower hoop stresses.

Skylor

yep, you beat me to this.
 
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