TYRES AND PRESSURE

[Guest speight]

Just a question out of interest;When plane gets to high altitudes, why do the tyres not burst?Are they in a pressurised compartment?

[michal 44]

No, they are not in pressurized compartment.There are 2 factors you overlook:1. Pressure inside aircraft tires is many, many times greater than atmospheric pressure so there is no reason they would burst because of built in tolerance. Do you car tires burst when you drive at altitude of 13,000 feet? 2. If I am not mistaken tire pressure in big airliners can be regulated by pilot in flight or at least can be automatically maintained by automation.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[CF104 0]

The biggest worry with tires at high altitudes, if filled with regular air, is the possibility of spontanious combustion. This is due to the nature of the rubber used in tire manufacturing and how regular air, with oxygen present, poses an explosion hazard at high altitude. This is why tires on aircraft are filled with dry nitrogen gas which is inert.MJ is correct in that most large aircraft tires are filled with at least a 10:1 ratio compared to standard atmosphere at sea level.As for automatically filling/regulating tires in the air, this doesn't happen. Pressures and temperatures can be monitired but that's all. The only aircraft that I know of that can lower and raise tire pressures while in flight is the C-5A/B.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

[Guest speight]

Thanks

[Guest frankathl]

>MJ is correct in that most large aircraft tires are filled>with at least a 10:1 ratio compared to standard atmosphere at>sea level.Hi John,When an aircraft is at, say, 35,000 feet, isn't the ratio much greater, because of decreasing external air pressure with altitude? I thought that was the point of the original post?BR,Frank

[michal 44]

>When an aircraft is at, say, 35,000 feet, isn't the ratio much>greater, because of decreasing external air pressure with>altitude? Nope. What counts is not the ratio but pressure differential because this is what the actual rubber "feels". The same for example applies to the cabin pressurization. If the tire pressure differential at sea level is say 9 atm it is going to be 10 atm when very high.Thanks John for corecting me how aircraft tire pressure is controlled.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[CF104 0]

Hi Frank,One has to take into account that air pressure at S.L. is only 14.69 psi where as a normally inflated main on a heavy could be as high as 190 psi. That's a 12.93:1 pressure ratio. Air pressure at FL350 is only 3.46 psi giving a ratio difference of 4.24:1 over S.L. pressure. Summing the ratios gives the tire a ratio of 17.17:1 at FL350 and an equivelant pressure of 252 psi. Only a 32% increase in overall pressure. Tires and wheels are designed to withstand this change in pressure.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

[CF104 0]

I also forgot to add that the pressure change I noted above is only a perceived change due to the changes in outside pressure. The tire itself will still only have 190 PSI in it if checked with a gauge as it is a sealed vessel.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

[michal 44]

As a physicist by profession, I am going to dispute those calculations. Pressure ratios, or adding pressure ratios really have nothing to do when considering forces trying to rupture a tire. It is strictly the difference between the inside and outside pressure that counts. The situation is perfectly analogous with cabin pressurization and in that case there is even a gauge that shows a pilot whether the design limit of pressure differential (not a ratio) have been reached or not.Using the numbers above if the tire pressure is at 190 psi, then taking this tire to FL350 where outside pressure drops by about 11 psi is equivalent (from the standpoint of forces acting on the rubber) to having this tire at 190 + 11 = 201 psi at sea level.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest frankathl]

Hi John,I get what you're saying. Thanks very much!Frank

[Guest frankathl]

Hi Michael,You can blame me for focussing on pressure ratios(my excuse is that I am neither a physicist nor an aircraft mechanic! :-)). In any case, I think I'm clear on the general point, that the change in differential pressure between sea level and altitude is not significant compared to the high pressure maintained within the tire.BR,Frank

[michal 44]

>>is not significant compared to the high pressure maintained>within the tire.You got it Frank. In our case with tire pressure at 190 psi, the rubber will only have to work about 5% harder when at cruise altitude.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest speight]

The reason I asked about tyres in the first place, was that when I made enquiries about taking my push bike on a plane, I was told that before the bike can be loaded, the tyres must be deflated to prevent them exploding in the baggage hold.

[michal 44]

>was told that before the bike can be loaded, the tyres must be>deflated to prevent them exploding in the baggage hold.Some idiot or overzealous worker could come up with something like that. Cargo compartment is pressurized to 8,000 ft - no way for tires to explode (people bike regularly at this altitude). And even if they did explode the amount of expanded energy would be minimal.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Guest gremel]

I've been running nitrogen in my car tires for years. It has been used in a/c & racing cars for a number of years. The results could translate into a longer lasting wear since their are less fluctuations in pressure which would dramatically affect the wear.Regards,jack