On Mon, 15 Sep 2008 15:25:21 -0700 (PDT), a wrote:

On Sep 15, 6:20*pm, a wrote:
On Sep 15, 6:09*pm, "Viperdoc" wrote:

The thread on brakes raised a question from the past- why nitrogen in the
tires of big jets and heavies? Air is around 78% nitrogen anyway, and the
coefficient of expansion of the remaining 20% that's oxygen can't make that
much difference in volume.

Why not just use dry air? I couldn't imagine that dry air or dry nitrogen
could make that much difference in corrosion, either.

I seem to recall someone giving me the rationale for this a long time ago,
but also seem to remember thinking it didn't make that much sense at the
time.

The legend is that *the oxygen is reacting with the rubber. Even
though compressed air has the same fraction of oxygen as does the
atmosphere, the fact that it's compressed, it's been argued, increases
its reactivity. I can't validate the legend.

I got curious, did a quick check. The process also reduces water
vapor, and N2 does not migrate through the rubber as fast as does O2,
so pressure stays more predictable. So it's more consistent tire
pressure, less corrosion.

Howevwer even in air the % of O2 is considerably smaller than N2. I
practice I've never been able to see a difference. Possibly with the
old style "natural rubber" innertubes in aircrat tires it would make a
difference, but I doubt any of us would see any difference in todays
car and aircraft tires. I've not had to put air in the tires on
either car in over two years. It's been over a year on the Debonair
and the mains are still up to pressure. The nose gear which still has
the old Natural Rubber has to be filled about every two to three
months. OTOH I've nver tried N2 in it.


Technically O2 leaks about 3 times as fast in Natural Rubber as does
N2. However it that proved out in practice we'd actually be purifying
the N2 in the tires and after refilling them 3 or 4 times there would
be almost no O2 left.



Roger (K8RI) ARRL Life Member
N833R (World's oldest Debonair)
www.rogerhalstead.com