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BreninTW
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Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 12:29 am

The thread on the G4 757 that had some tyre issues got me thinking ... I know that aircraft tyres are typically inflated using Nitrogen, but does atmospheric air get into the tyre as well?

Over the last 10 years or so, it's become more popular for high-performance cars to have tyres inflated with Nitrogen. However, I have read various articles and studies that show that Nitrogen-inflated tyres don't perform all that much better than atmospheric-air inflated tyres. One of the hypotheses that was raised is that because automotive tyres are installed in the open, they have a significant proportion of atmospheric air, which the Nitrogen does not displace.

So ... that got me wondering, are aircraft tyres fitted in a Nitrogen chamber? Or does the volume and inflation pressure (understanding that aircraft tyres are inflated way above the 2.5 bar my car's tyres are inflated to) introduce sufficient Nitrogen to the tyre that the atmospheric air becomes a negligible quantity?
 
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fr8mech
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 1:49 am

No, they are not aired up in a nitrogen chamber. Yes, they probably have some "air" in them. And, yes, the amount is probably insignificant to the purposes of using nitrogen:

-inert
-lower expansion coefficient than air
-dry

[Edited 2015-08-27 18:50:20]
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surfpunk
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 2:08 am

Well, nitrogen is not inert, but it is drier and has does have a lower expansion coefficient than air.

When I was in the Navy, I worked for a period of time in the tire shop on the USS Ranger. We had an enclosed cage in which all tires were inflated, as the pressure in them would be upwards of 300psi, depending on the aircraft, and whether it was a main gear or nose gear tire. We did use nitrogen to inflate them, although some air was certainly in place, since the wheels were two-piece, and bolted together, plus the ambient air inside the tire well itself would account for some, but it was a very small percentage when you're getting to pressure levels that high.
 
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fr8mech
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 2:32 am

Quoting surfpunk (Reply 2):
Well, nitrogen is not inert,

You I know that. What I meant to write was that it does not support combustion. When I think "does not support combustion", the word "inert" comes to mind. I realize it's not the chemical definition of "inert".
When seconds count, the police are minutes away, or may not come at all.
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kalvado
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 12:20 pm

Quoting fr8mech (Reply 1):
-inert
-lower expansion coefficient than air
-dry

expansion coefficient is pretty much the same. And drying air is not a big problem, actually even simple compression tends to remove some water. Then you can add a silica gel absorber to make air perfectly dry.
fire issues are probably the biggest thing.
 
Mender
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 1:09 pm

Don't forget that atmospheric air is mostly nitrogen to begin with. (78.09%)
 
boeingfixer
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 1:52 pm

Quoting surfpunk (Reply 2):
Well, nitrogen is not inert, but it is drier and has does have a lower expansion coefficient than air.

In the context of it's use, in this case tire inflation, nitrogen is considered an inert gas as it's non-reactive in the environment that it is used. It is used mainly to prevent combustion of the tire under high internal pressures and heat. There have been instances of tire explosions due to the use of compressed air to inflate them.

Cheers,

John
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nomadd22
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 2:39 pm

One of the main reasons for using nitrogen is that you don't make things worse when the tires burst in a fire.
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KC135Hydraulics
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 3:38 pm

That's interesting that you say nitrogen is inert. On the C-17, there is a system called OBIGGS (onboard inert gas generator system) which creates mostly pure nitrogen for the purposes of rendering the fuel tanks inert. Apparently someone has their definition of inert incorrect, but I can't even begin to imagine who, as I am not a chemist.
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Tristarsteve
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 4:42 pm

Quoting KC135Hydraulics (Reply 8):
On the C-17, there is a system called OBIGGS

Nitrogen generating systems are fitted to all? new airliners. Our latest A320 come fitted with them from Airbus. You would never know if a technician didn't tell you. There is no cockpit indicators or switches. A failure will produce an ECAM message, but it hasn't happened yet. There is an overboard oxygen outlet under the belly, and beside the vents in the wings as a tiny placard advising maintenance staff to take precautions when entering the tanks.

I know that single seat fighters have oxygen generating systems, can the oxygen from the NGS be used? Boeing and Airbus vent it overboard.
 
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Polot
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RE: Inflating Tyres ... Does Air Get In?

Fri Aug 28, 2015 4:45 pm

Quoting KC135Hydraulics (Reply 8):
Apparently someone has their definition of inert incorrect, but I can't even begin to imagine who, as I am not a chemist.

Inert just means not chemically reactive. N2 will technically chemically react so it is not truly inert, but not readily (generally under extreme pressure and/or temperatures) so it is typically considered inert for practical purposes.

Contrast that with O2 (found of course in air, which is why air is not inert) or H2 which we all know are pretty reactive as evidenced by their combustibility.

Source: I'm a chemist.
 
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DocLightning
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RE: Inflating Tyres ... Does Air Get In?

Sat Aug 29, 2015 4:47 am

Quoting KC135Hydraulics (Reply 8):
That's interesting that you say nitrogen is inert. On the C-17, there is a system called OBIGGS (onboard inert gas generator system) which creates mostly pure nitrogen for the purposes of rendering the fuel tanks inert. Apparently someone has their definition of inert incorrect, but I can't even begin to imagine who, as I am not a chemist.

I'm also not a chemist, but I studied enough in college for my Biology degree.

In the sense of "inert," there are two definitions. One is a gas with no chemical reactivity at all. Only the noble gases are called this (helium, neon, krypton, argon, radon) as they have complete outer valence shells and cannot participate in chemical reactions that form compounds (actually, that's almost true...noble gas compounds have been reported, but not for helium). Noble gases are sometimes called the "inert" gases.

The other definition of "inert" is a gas that does not have significant chemical reactivity in its typical environment and one that cannot sustain combustion. N2, CO2, and H2O in gaseous form are all inert in this sense. All can, under other conditions, undergo chemical reactions, but in the typical environments in which we encounter them, these gases are inert.
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IFixPlanes
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RE: Inflating Tyres ... Does Air Get In?

Sat Aug 29, 2015 9:45 am

The AD 87-08-09 gives some explanation about the inflation with dry nitrogen or other gases shown to be inert.
never tell an engineer he is wrong ;-)
 
Legs
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RE: Inflating Tyres ... Does Air Get In?

Sat Aug 29, 2015 10:18 am

Quoting Tristarsteve (Reply 9):
can the oxygen from the NGS be used

I use to work in a shop that overhauled OBOGS (On Board Oxygen Generating System) and I know a bit about OBIGGS, so I can field that question. The OBIGGS by-product doesn't meet the requirements for breathing air, let alone aviators dry breathing oxygen. It's atmospheric air minus most, but not all of the nitrogen, so moisture and contaminant counts are way up. It's also at a fairly low pressure, which makes it harder to integrate back into a breathing O2 system.
 
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Classa64
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RE: Inflating Tyres ... Does Air Get In?

Sun Aug 30, 2015 4:20 pm

I work in the automotive trade and Nitrogen filling of tires is the biggest scam around, since the air I put in a tire goes through a drier in our shop and is 80% nitrogen anyway there is no real benefit unless its a racing application or aircraft. The machines used by some dealerships fills and empties the tires several times to purge any real air out of the tire but I am sure there is always going to be a small amount left in the tire.

What do you do when your in an area that has no nitrogen and you have a plane with a low tire? Fill it with air and let it go, curious as to what the rules are. People drive away all the time with a low tire because we have no nitrogen, ends up costing them a tire and possibly an accident once it disintegrates, there loss.
"Freedom is the miles i'm rolling on"
 
yeelep
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RE: Inflating Tyres ... Does Air Get In?

Sun Aug 30, 2015 4:42 pm

Quoting Classa64 (Reply 14):
What do you do when your in an area that has no nitrogen and you have a plane with a low tire?

On the 737 we can top off the tire with dry air, if it does not exceed 5% of tire volume.
 
thegman
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RE: Inflating Tyres ... Does Air Get In?

Sun Aug 30, 2015 7:23 pm

Quoting Tristarsteve (Reply 9):
Nitrogen generating systems are fitted to all? new airliners. Our latest A320 come fitted with them from Airbus. You would never know if a technician didn't tell you. There is no cockpit indicators or switches. A failure will produce an ECAM message, but it hasn't happened yet. There is an overboard oxygen outlet under the belly, and beside the vents in the wings as a tiny placard advising maintenance staff to take precautions when entering the tanks.

I know that single seat fighters have oxygen generating systems, can the oxygen from the NGS be used? Boeing and Airbus vent it overboard.

That's interesting that the A32x have it with no controls at all. In the C-17 it can be turned off and not used at all, in fact during training it is not used at all. Not using it can create a nuisance "Tank not inert" message after every touch and go, but other than that it effects nothing in the aircraft.

On another note, OBOGS is only on 2 USAF aircraft that I know of, T-6 and F-22 and we all know the problems the F-22 has had with that.
 
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BreninTW
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RE: Inflating Tyres ... Does Air Get In?

Mon Aug 31, 2015 2:45 am

Quoting yeelep (Reply 15):
On the 737 we can top off the tire with dry air, if it does not exceed 5% of tire volume.

Does this require a notation to be made in the maintenance log? What happens if the tyre is topped off more than once, and thus the air volume exceeds the 5%?
 
Legs
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RE: Inflating Tyres ... Does Air Get In?

Mon Aug 31, 2015 9:06 am

Quoting thegman (Reply 16):
OBOGS is only on 2 USAF aircraft

Its also used on the F/A-18E/F/G as well, and has had some issues in that airframe too.
 
yeelep
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RE: Inflating Tyres ... Does Air Get In?

Mon Aug 31, 2015 4:38 pm

Quoting BreninTW (Reply 17):

Yes the amount of air put in the tire has to be documented. On our -400's a 15 flight hour deferral is created to deflate the tire and reinflate with nitrogen. On the NG's, if 5% will be exceeded it has to be deflated and reinflated with nitrogen, there is no deferral.
 
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flylku
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 01, 2015 1:09 am

Quoting legs (Reply 13):


I use to work in a shop that overhauled OBOGS (On Board Oxygen Generating System) and I know a bit about OBIGGS, so I can field that question. The OBIGGS by-product doesn't meet the requirements for breathing air, let alone aviators dry breathing oxygen. It's atmospheric air minus most, but not all of the nitrogen, so moisture and contaminant counts are way up. It's also at a fairly low pressure, which makes it harder to integrate back into a breathing O2 system.


I've worked with several clients who had developed molecular sieve technology that I believe is used to "filter" nitrogen out of the air for these types of systems. Amazing that we can manufacture such a device.
...are we there yet?
 
sccutler
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 01, 2015 3:21 am

Quoting Mender (Reply 5):
Don't forget that atmospheric air is mostly nitrogen to begin with. (78.09%)

Exactly.

Paying extra for "nitrogen fill" on car tires is such a total and utter scam.

Never pay f or this unless they throw in the enhanced muffler bearing.
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boeingfixer
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 01, 2015 9:44 pm

Quoting IFixPlanes (Reply 12):

The AD 87-08-09 gives some explanation about the inflation with dry nitrogen or other gases shown to be inert.

Pretty much confirms what I said in Reply 6.

Quoting boeingfixer (Reply 6):
In the context of it's use, in this case tire inflation, nitrogen is considered an inert gas as it's non-reactive in the environment that it is used. It is used mainly to prevent combustion of the tire under high internal pressures and heat. There have been instances of tire explosions due to the use of compressed air to inflate them.

Cheers,

John
Cheers, John YYC
 
KELPkid
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RE: Inflating Tyres ... Does Air Get In?

Wed Sep 02, 2015 12:35 am

Quoting Classa64 (Reply 14):
What do you do when your in an area that has no nitrogen and you have a plane with a low tire? Fill it with air and let it go, curious as to what the rules are. People drive away all the time with a low tire because we have no nitrogen, ends up costing them a tire and possibly an accident once it disintegrates, there loss.

Aeromexico lost a 727 that way...an MLG tire was filled with air. The aircraft developed a wheel well fire from that.

Transport category jets practically demand that the air inside the tires is inert. The problem? The gas inside the tires is under tremendous pressure, and oxygen at really high pressures does some interesting things (even if it is only 20% of the gas mix in the tire!). Rubber, and some of the other hydrocarbon compounds in the tire, are close to their flash point in a high pressure atmospheric environment (if you get the tire warm by say, excessive braking or doing your takeoff roll). The tires might just start spontaneously burning on the inside  
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JohnM
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 08, 2015 7:25 pm

Quoting KELPkid (Reply 23):
and oxygen at really high pressures does some interesting things

Not a chemist here either. However I have spent lots of time with 4,500 psi of breathing air on my back in fires (normal atmosphere air compressed). That would be about 20% oxygen content. No issues of course. Our trucks carried 6,000 psi around in our storage systems. The strict national standards wouldn't allow that if it was not 100% safe.
 
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Polot
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 08, 2015 7:50 pm

Quoting JohnM (Reply 24):
Not a chemist here either. However I have spent lots of time with 4,500 psi of breathing air on my back in fires (normal atmosphere air compressed). That would be about 20% oxygen content. No issues of course. Our trucks carried 6,000 psi around in our storage systems. The strict national standards wouldn't allow that if it was not 100% safe.

Oxygen under high pressure does present problems- that is why those compressed air tanks (and presumably the tanks located in the truck) are manufactured with thick steel or aluminum alloys, not primarily out of, admittedly heavily re-enforced, rubber...
 
KELPkid
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RE: Inflating Tyres ... Does Air Get In?

Wed Sep 09, 2015 5:20 pm

Quoting Polot (Reply 25):
Oxygen under high pressure does present problems- that is why those compressed air tanks (and presumably the tanks located in the truck) are manufactured with thick steel or aluminum alloys, not primarily out of, admittedly heavily re-enforced, rubber...

You'd think that, but my son just got to see the local fire dept. on back to school night at his school. One of the things that they got to do was to try on the firefighting gear. I noticed that the breather tank looked like it was carbon fiber or some sort of composite. I asked the fireman, who said it was "fiberglass." It still weighed 45 lbs. by itself (I'm guessing that most of the weight is the air charge inside-didn't realize it was so high!)  
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pygmalion
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RE: Inflating Tyres ... Does Air Get In?

Tue Sep 15, 2015 10:24 pm

Its the hot rubber give off gasses that react with the oxygen in the air... similar issues with race tires. Aircraft tires are limited to 5% oxygen maximum for this reason.... The tires last longer too. The oxygen reacts with the rubber in the tires.

Thats really the only benefit to putting N2 in your car tires... it can make then last longer. Many performance cars dont get that many miles on them and tires are replaced due to age not wear. It can make sense to use N2 in those cases.

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