Re: B747 technical questions

Date:         01 Aug 97 04:04:17 
From:         "matt weber" <mweber@cyberltd.com.au>
Organization: Customer of Access One Pty Ltd, Melbourne, Australia
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Gary Welch <garywelch@flex.net> wrote in article
<airliners.1997.1566@ohare.Chicago.COM>...
> Merlin Dorfman <dorfman@netcom.com> wrote in article
> <airliners.1997.1484@ohare.Chicago.COM>...
> > Jay Biederman (jbb@seanet.com) wrote:
> > : Actually the tires are inflated with nitrogen gas. This is because the
> > : tire pressure stays fairly constant over large variations in
> > : temperature...
> >
> >      It's been a long time since I took thermodynamics, but if I recall
> > correctly after all these years, the pressure of any gas will increase
> > linearly with the absolute temperature if confined to a fixed volume.
>
> Nitrogen is somewhat lighter than oxygen.  The difference may seem trivial
> but I've read that the air in the tires of a 747 weighs in the 100s of
> pounds (I've heard of Helium being used in some aircraft).  The main reason
> for using nitrogen is probably that it's less reactive than oxygen and thus
> less likely to affect the rubber at high temperatures.

The difference in weight between Nitrogen and air is trivial, air is 80%
nitrogen, most of the rest is oxygen, which is only about 10% heavier.

 As for the air in the tires weighing hundreds of pounds, not likely. At
normal temperatures and temperature, 22 liters of Nitrogen weighs about 28
grams. 1 pound =454 grams,  1 lb = 90 gallons of air.
100 pounds = 9,000 gallons of air. I have a hard time believing there is
100 pounds of air even in a full set of 747 tires. It certainly isn't going
to be hundreds of pounds.

You are however correct, tire casing on aircraft are retreaded many times,
so it is desirable to keep the casing in as good shape as possible. The use
of Nitrogen prevents the casing from oxidizing on the inside. Don't care
much about the outside because you re-tread it anyway.