Re: B747 technical questions

Date:         04 Aug 97 21:28:53 
From:         shumaker@eisner.decus.org
Organization: DECUServe
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In article <airliners.1997.1633@ohare.Chicago.COM>, "Michel Gammon" <jmgammon@sympatico.ca> writes:
> shumaker@eisner.decus.org wrote in article <airliners.1997.1560@ohare.Chicago.COM>...
>> The most common reason for using nitrogen for tire inflation where the
>> pressure must remain constant or at least predictable for wide
>> temperature variations (e.g.: racing car tires which routinely exceed
>> 180 F tread temperature) is that bottled nitrogen is _dry_.  It's the
>> water and water vapor that cause wild pressure increases with
>> temperature; any dry gas would work as well.  Nitrogen happens to
>> be widely available and cheap.
>
> As a chemist I dispute this.  Gas volume when uncontained or gas pressure
> when contained is directly related to temperature.  That is why  you are
> always admonished to check tire pressures on your car when they are cold,
> for reproducible accuracy.

The same instructions also admonish you to use sources of air which has
low moisture content.

> Water is a liquid and therefore will not be subjected to the same rules and
> also is incompressible.  Water vapour is microscopic liquid water droplets,
> and steam is water in the gaseous phase.

There is a significant volume increase when liquid-phase water changes
into gaseous-phase water, and the phase change begins to occur over the
range of temperatures which tires encounter.  The problem with mixtures
of gases and liquids is that while the gaseous components obey the gas
laws, the liquids -- if they undergo phase changes -- do not.

This effect is widely cited by tire manufacturers and application
engineers as the reason to use dry gases for tire inflation.

> My guess for the reason for nitrogen instead of air is that nitrogen is
> inert.  During an emergency stop before V1 when maximum braking is used, I
> suspect the internal tire temperatures can get very high (the external
> surface will at least be afforded some air cooling, while the gas in the
> tire will be insulated from external air and thus will not be cooled).  I
> suspect that if air is used to fill the tires, the internal temperature of
> the tire can theoretically exceed the auto-ignition temperature of the
> rubber, thus igniting the tire and causing a catastrophic failure (which
> often occur anyway probably as the pressure limit on the tire is exceeded).

And this temperature is not sufficient to cause all liquid-phase (including
fine droplets) water to turn into gaseous-phase water?

Mark Shumaker
shumaker@eisner.decus.org