tyre pressures at altitude
 

When you take your bicycle on a wide-body, they ask you to take the air out of the tyres (even though the apparent altitude inside the aircraft is only about 8000ft - I think).

What about the tyres on the aircraft itself?

My understanding is that the big boys use nitrogen, which does not alter in volume to a great degree with altitude (and that any increase due to lower outside air pressure is offset by reduction within the tyre due to lower temperatures)?

For GA then, do we rely on this cancelling out (expansion due to altitude - compression due to temperature) alone? I don't put nitrogen in my tyres!

Is this pressure issue something you should consider if heading for a high altitude field? Perhaps take off 'on the low side'?

Sorry, still Sunday morning!

Sam.

Nitrogen expands more or less as much as any other gas.

But yes, the stress on the rubber increases with altitude. But not a lot. Let's say your bike tyre is 50psi. At FL400 the increase in tyre stress is equivalent to something like 62psi. Not significant.

A requirement to deflate tyres is based on ignorance.

The tires on my bike are inflated to 5 bar above ambient. Ambient of course being one bar. If I were to take my bike into outer space, that would mean that all of a sudden the pressure is now 6 bar over ambient, or a 20% increase. That would stiffen up the ride a bit but probably not burst them.

Aircraft tires, at least the ones on the PA-28, are inflated to 1.6 (nose) to 2.0 (main) bar over ambient. Taking them into outer space makes this 2.6 and 3.0 respectively, an increase of about 62% (nose) / 50% (main) max. That might be enough to burst them. But indeed, you have to take the PA-28 into outer space, and you have to disregard temperature effects. At more realistic altitude, say 10.000", the ambient pressure is still around 0.7 bar so you're talking about a 20% increase in the nosewheel max.

If the tyres on an airliner were in any danger of damage from being exposed to low ambient pressure at high altitudes they wouldn't have much of a chance coping with a landing where each one may have to bear the force of more than 25 tons per wheel slamming into the runway thus effectively making each tyre take nearly 40 tons of pressure at touchdown on a bad day.

If the temperature on the ground is 15C, at 10,000ft it will be –5C. I cannot be bothered to work it out, but I would have thought this would have had a significant impact on the pressure in the tyre.

Rod1

As Rod correctly points out, the drop in temperature pretty much wipes out the increase in pressure due to expansion caused by the drop in ambient pressure.

Pretty obvious really that it is not an issue as we have been using air filled tyres for a very long time.

Nitrogen makes no difference to the equation, air and nitrogen have the same density and behavior for the purposes of this discussion.

Replies to this thread of very high quality, obviously the PPRuNe intelligentia are up and about on a Sunday morning!

If someone could be bothered (I can't), you could construct an equation for the variation of tyre pressure above ambient with increasing altitude, noting (i) pressure times volume divided by temperature is constant for any gas (Boyle's law) and (ii) plugging in an expression for the reduction in temperature with altitude.

Overall, as others have said, the effect is not significant, and as others have noted certainly very far less than the increase in pressure when the tyre slams into the runway! This effect (I mean altitude, not landing!) is not considered in aircraft certification.

KP

KP, I do actually have such a chart....... :O:O

It is actually a chart we use for blending exotic breathing mixtures but the result is the same with a minor tweak!

Also when we inflate tyres, they are in contact with the ground and have the weight of the aircraft distributed on them as well. So once the tyres leave the ground, surely the pressure will decrease a bit as well.

Oh dear! :rolleyes:

Oh dear!
 

You are right, coodem. The natural shape of the tyre maximises its volume, and minimises its pressure. When bearing the weight of the aircraft the pressure will increase very slightly as a result of the distortion that gives the tyre its footprint.

They ask you to let your bike tyres down so the bike won't roll around as easily! Nothing to do with the drop in pressure at altitude. If it was then they also ask you to drop the air pressure in the forks and rear shock (assuming you are riding a full suspension mountain bike) and they don't.

Coodem and Rightbase:

A little experiment for you, go outside and set your car tyre to 30psi, then jack the car up and measure again and come back here and tell me you get a different reading to 30psi.

Here's an experiment for you to do McGoo:

Blow up a balloon, tie the end, then sit on it.

Then try and work out why it burst if the pressure hadn't changed when supporting your weight :E

Sorry, but Coodem and Rightbase are correct. If I inflate a tyre to, let's say, 50psi on the bench, then install the wheel onto an aircraft and let it off jacks, the pressure is slightly higher. This is not hypothetical - I've done it many times. The increase is around the 5% mark with weight on; I'll try to find something more definitive.

EDIT
Ah, this is it - bottom of page 302:
http://www.dunlopatl.co.uk/tyrecare/dm1172/DM1172.pdf

I think we are talking minute amounts, the balloon obviously we would be substantially decreasing volume so increasing pressure, but i'm not convinced about the tyre, I fitted just last week a new set of tyres to the car and the fitter set the pressures on the bench, I checked them again on the car and could see no differance, whether the fact they are low profile and so had stiffer sidewalls so negligible volume change?

Thanks for the link stevef, interesting, never noticed it ever on car tyres, unless the percentage makes it very marginal on the gauge.

Apologies Coodem, i've just read again and noticed you did say "a bit", missed that first time.

:ok:

Richard Branson has just announced that he is to commence PA28 rides into outer space....:rolleyes:

Richard Branson has style; he would not associate with a spam can. His machine is modern composite and I want a go.

Rod1

His machine burns tire rubber. I'm sure there must be a punchline in there wrt. this thread.