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Nitrogen in tyres

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Nitrogen in tyres

Old 16th Jan 2016, 15:57
  #21 (permalink)  
 
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chu chu

I have my doubts about compressed air feeding an external fire. It isn't going to create any kind of lasting overpressure, so it won't cause a significantly higher concentration of oxygen molecules as a general matter.

Have you ever lit a fire using a wad of paper or kindling?
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Old 16th Jan 2016, 19:27
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Sure. A gentle, sustained flow of air works wonders. A sudden blast will either cause a brief flare-up or blow the flame out altogether.
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Old 16th Jan 2016, 22:35
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U.S. Navy Rationale

Back in the dark ages 40 years ago when I was flying off carriers, tire pressures were usually 400 psi for carrier operations and 300 psi for field operations. Those high pressures exerted a lot of stress on the tire especially during carrier landings. Navy pilots were taught that inflating tires with an inert gas precluded oxygen under pressure reacting with the rubber and weakening it.
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Old 17th Jan 2016, 20:08
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Think the accident that started the change was a DC8 in Saudi i think which had a wheel well fire and the tyres exploded leading to loss of the aircraft , sure someone else will have the relevant details
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Old 18th Jan 2016, 02:02
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FAR part 25.733(e):
"(e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels."

From Michelin Aircraft Tire Service Manual:
"NOTE: Michelin recommends that all tires, regardless of position or aircraft rating, be inflated with dry nitrogen of at least 97% purity. Some OEM (airframers) recommend 99.5% purity. When adding 97% pure nitrogen to an “empty” tire, the nitrogen concentration will end up about 95% when the pressure reaches about 125 psi. When the operating pressure is less than 125 psi and the desired concentration is ≥ 95% nitrogen by volume, use a double inflation process if the source nitrogen is 97% purity."

Consider the Space Shuttle tires:
"Like most aircraft tires, the Space Shuttle tires are filled with nitrogen because of its stability at different altitudes and temperatures. Due to the extremely heavy loads, these bias ply tires are inflated to 340 psi (main gear) and 300 psi (nose gear). The main landing gear shuttle tires are only used one time, and the nose landing gear tires are used for two landings."

Here what they looked like after one landing:



Last edited by riff_raff; 18th Jan 2016 at 02:15. Reason: More information
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Old 25th Jan 2016, 00:20
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Originally Posted by TURIN View Post
Left hand threads on the O2 hose unions help. I've still seen it done though.
Left hand threads are used on inflammable gases, i.e. propane, acetylene.
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Old 28th Jan 2016, 10:19
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Originally Posted by Ozlander1 View Post
Left hand threads are used on inflammable gases, i.e. propane, acetylene.
I didn't know that. Good job we don't have those hanging around on the ramp.
I don't think an oxygen/flammable gas combo would go down very well in the crew O2 system.
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Old 28th Jan 2016, 12:00
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The reason N2 is used in aircraft (and F1 and general cars) is the following:

1. Pure nitrogen is dry and does not absorb moisture. This helps with corrosion, but the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landing.

2. N2 molecules are larger than than O2, and so pressure loss is slower.

3. Its inert and wont contribute to a fire.
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Old 29th Jan 2016, 02:39
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the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landing
Not the case at all hoss. The real reasons are in this document.

Federal Aviation Administration
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Old 29th Jan 2016, 04:44
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I seriously recommend Nitrogen in Car tires, better fuel economy and less road noise

Here in NZ its cheap, and to get them checked and topped up when necessary is FREE!!!

Best Regards
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Old 29th Jan 2016, 05:56
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1. Pure nitrogen is dry and does not absorb moisture. This helps with corrosion, but the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landing.

You started so well, but......whilst we are never too old to learn, and despite my lifetime involvement as an engineer, I have never heard, or read, anything about wheels becoming inbalanced due to, erm, frozen water in the wheel hub.... 9/10 for creative thinking....1/10 for factual content. Do you, by any chance, have any documented evidence concerning aircraft wheel and tyre assemblies to support this please ?....because, having perused some excellent material from Mr Goodrich / Dunlop et al, I cannot find, or recall, any reference to this phenomenon.

2. N2 molecules are larger than than O2, and so pressure loss is slower.

Oh ?

3. Its inert and wont contribute to a fire

At last ! success !.....
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Old 29th Jan 2016, 07:32
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I was told by an automotive tyre guy that pressure loss with nitrogen is slightly less.

The molecule is larger than the oxygen molecule.

Rubber does a pretty good job at containing air/gas. At a molecular level, some gas very slowly leaks through the material. It might take several weeks for a pressure loss to become evident - maybe months - but it will gradually deflate.

The higher the inflation pressure, the greater/faster the loss.

Makes sense. However, at the end of the discussion, I opted for regular air. I don't mind recharging the tyres every few months when/if required.
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Old 30th Jan 2016, 18:41
  #34 (permalink)  
 
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ASN Aircraft accident Boeing 727-264 XA-MEM Las Mesas
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Old 1st Feb 2016, 11:20
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Lessons not always handed down...

Evidently there are a lot of young aviators contributing to this thread. And I wonder if the operative that serviced that B727's tyre with air in 1986 had ever read the following accident report. It was still fresh in aviation circles when I started flying:

https://en.wikipedia.org/wiki/Swissair_Flight_306

And it has figured here (on AH&N) in 2010:

http://www.pprune.org/aviation-histo...ml#post5710327
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Old 4th Feb 2016, 19:01
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In Alaska we use nitrogen for all the reasons given here plus it is not as affected by the cold. If the tyre is inflated with compressed air and then flown to the Slope for example, the tyre pressure drops as the temperature drops and we have had tyres roll off the rims when taxiing. With nitrogen the pressure is more stable and since the change we have had no problems with tyres in cold conditions.
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Old 4th Feb 2016, 19:12
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Originally Posted by boofhead View Post
In Alaska we use nitrogen for all the reasons given here plus it is not as affected by the cold. If the tyre is inflated with compressed air and then flown to the Slope for example, the tyre pressure drops as the temperature drops and we have had tyres roll off the rims when taxiing. With nitrogen the pressure is more stable and since the change we have had no problems with tyres in cold conditions.
Nitrogen and dry air will respond exactly the same to changes in temperature - it's called the ideal gas law. The potential problem with compressed air is if it isn't dry and there is significant water vapor present. The change in state of vapor to liquid (or visa-verse) is the potential issue.

Bottled compressed air is normally dry (running it through some sort of dryer is part of the process) - as is bottled nitrogen. It's the air out of the shop compressor that may be a problem.
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Old 4th Feb 2016, 19:22
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Indeed. I recall refilling a tyre at an OAT of -32C with shop compressed air. The tyre valve promptly froze with ice in the open position, releasing a large quantity of air, resulting in an even more under-inflated tyre.. Nitrogen has benefits for cold weather ops.
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Old 4th Feb 2016, 23:38
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Originally Posted by tdrace
"My gut feeling is that it's a myth (the molecular weight of N2 is lower than O2, so everything else being equal the N2 molecule would be slightly smaller than O2) but there could be some subtle way the molecule reacts with the rubber that makes it effectively larger. At any rate the difference would be in the mud"
As you'd expect, diffusion rate of O2 and N2 through various membranes has been studied for over a century, and is well understood.

Diffusion rate is defined by Fick's Law, and is dependent on the frictional coefficient of the molecule, not just its physical size. The frictional coefficient is based on (a) Stokes Radius (not physical diameter) of the molecule, and (b) The viscosity coefficient of the gas at the given pressure and temperature. I'm guessing that N2 has a greater frictional coefficient than O2, due to either greater viscosity and/or larger Stokes Radius than O2, and that probably explains the lower diffusion rate.

The equations to calculate these are complex, but it's not a myth. If any physics major is reading this, they could probably give more details.

http://en.wikipedia.org/wiki/Fick%27s_law_of_diffusion
http://en.wikipedia.org/wiki/Stokes_radius
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Old 5th Feb 2016, 01:59
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Last time there was a thread about this I found a U.S. Department of Transportation (I think) study on nitrogen on tires. My recollection is that it found that the nitrogen concentration in air-inflated tires increased slightly over time. But I don't think the study concluded whether this was due to oxygen diffusing from the tires more rapidly, or due to it's being "consumed" in oxidizing the tire walls.

In any event, the change in concentration was quite slight, meaning that if the oxygen was diffusing faster, it wasn't by much. And when you consider that air is only 20% oxygen to start with, a slight decrease in the diffusion rate for that 20% isn't going to make much difference in tire pressure.
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