Why don't your tyres explode

[surface wind]

Sitting very bored in the tower the other afternoon a Sleazy Jet departed. I noticed that the tyres are not stored in a 'hold' on a B737. Why don't they explode? It must be like a submarine in reverse. Surely as the a/c climbs the air presure decreases, but the air pressure in the tyres must be the same. Am I just too bored or just a thick ATCO?

El Surface Wind-o

:eek: :eek: :eek:

[PPRuNe Dispatcher]

It's simple physics.

At sea level the pressure differential in a big jet tyre is very approximately 200 lb/sq-in. If the tyre was in a vacuum, the pressure differential would increase by only 15 lb/sq-in.

..Mik

[expedite_climb]

Okay I did A level physics and it cant be that simple as i dont understand that explanation !

Anyway - dont forget even if an aircaft has gear doors, the gear wells are not pressurized - so really it is no different.

I would like to know though why the 737 is designed like that. If the 757 has to depart with one missing there is a performance penalty due to extra drag...

[Tinstaafl]

Lots of a/c don't have wheel well doors, from lighties to large.

The undercarriage retraction mechanism & the wheel well is designed so that the wheel fits relatively snuggly in the available space.


Tyre pressures:

The true pressure inside the tyre is - correctly - the sum of the differential pressure read on the guage + ambient. The tyre pressure we all tend to think about is a relative pressure ie the gauge reads the difference to what is inside the tyre to what is outside.

Ambient pressure at sea level is only about 15 lbs/sq.in. so even if ambient was reduced to zero ie a vacuum, the differential pressure would rise by only 15lb/sq.in.

[surface wind]

I think I'll stick with controlling. Must find something more constructive to do with all that free time!!

:D

[OzExpat]

Tinny ... do ya wanna try again mate - that answer puzzled the bejayzuz outta me too... :eek: :D

[steamchicken]

The difference between the pressure inside the tyre and outside it is what matters. (as the outside (ambient) air pressure counteracts the pressure inside the tyre.) Therefore the force acting on the rubber from inside will be the tyre pressure minus the ambient air pressure multiplied by the area of the tyre. ie(TP-AP) * Area.

As the ambient air pressure at sea level is around 15lbs/sq in, the maximum possible increase in the force would be of 15lbs/sq in - not really very much. The tyre is sufficiently strong to withstand this.

Any clearer?

[Easy226]

Interesting! I'm currently doing A-Level maths and Physics and the explanations below confuse me! The ATPL exams might be harder than i thought!!

[Bellerophon]

BIK_116.80

Interesting post about the design of the B737 wheel well.

Just out of interest, for most practical purposes, the ambient atmospheric pressure at FL600 can be assumed to be 1 psi.

Regards

Bellerophon

[GoneWest]

You've all missed the simple answer...

Why don't your tyres explode ..."because they don't"

...or the flight instructor answer...."because they are designed not to";)

[tom775257]

One way I find to quantify the relatively small pressure change between low and high altitude is to compare it to when I scuba dive. If I descend 10 metres under water, total pressure change from the surface is the same as if I went from the surface into a vacuum. i.e. every 10 metres leads to about 1 bar pressure change (in the case of diving, an increase). I happily dive at 40 meters. :rolleyes:

[overstress]

Easy 226

Better hope for an easy physics paper on the day then - this is aircraft science, not the rocket variety (which tends to be simpler) ;)

[HiDrvr]

Further to Tom's SCUBA comment, there's a wreck of Swanage full of Land Rovers and a certain club decided to raise one. However, they reinflated the tyres FIRST. They then raised the Rover from 35m to the surface, thereby increasing the differential pressure by 3.5 atmospheres with a resultant 4 bangs.

[OzExpat]

My thanks to steamchicken and BIK_116.80 for an explanation that ties in with what Tinny said. Don't know how I could have missed what he was saying but at least now its all clear to me. Dunno that I'd liek to sit an exam on the topic but, thankfully, CASA had had their last opportunity back in 1992 and blew it coz I passed! :D

Really glad there wasn't any questions on tyre pressures in that exam!

HiDrvr ... things might've been worse if the said Landrover had been equipped with a spare tyre too... :D :D :D :D :D

[M.Mouse]

tom775257

So why don't you explode when you surface?

[malanda]

My slightly-related question : how much rubber is worn off in that cloud of smoke on landing? I was musing over whether it would worth fitting a device (motor/windmill) to spin the wheels up first.

[Lunar Landing]

Does the wind not do that enough?

[Willit Run]

The Rovers TYRES, as you folks spell it, were increased over 3 atmosperes, while aircraft tires, as we spell it, are only subjected to less than one atmosphere change.
It really is very simple. have a beer and think about it, in fact, have 2 beers and think about it!

[powerless]

Now explain what the air temperature does to this tyre pressure!

[TwinNDB]

I can remember when i first started flying i was surprised to see the wheels stop spinning shortly after takeoff. I had excected them to keep spinning all the way through the flight as surely the wind blowing on them would make them spin (assuming fixed gear here guys).

Then, later when thinking about it i guessed on a couple of reasons as to why they dont:

1) A tyre (and wheel) is circular in shape (we hope!), so any wind as a result of your speed that 'hits' the wheel will hit all the forward parts with equal pressure. ie; the pressure on the wheel on an angle 45 deg up and down from the centre (like the equator on earth) will be the same. With no differential in the pressure there is no offset force to start the wheel spinning.

2) This one is probably to a MUCH lesser extent. The mass of the wheel is too great for the given speed.

On a side note here, I know some flying schools teach their students to gently press the brake pedals after takeoff to stop the wheels spinning earlier and thereby stopping any vibration through the aircraft. Most of the guys I know used to do that until we had something happen a while ago that we (the CP, engineers and pilots) think happened because of the above procedure. On landing one of our guys had a tyre come off the rim of the wheel, no great harm done but it was a nasty shock to the guy say the least. What they found on inspection of the tyre was that the valve on tyre (where you put the air in :rolleyes: proper lingo escapes me) had been broken. The cause, they believe was that as you brake after takeoff to stop the wheel rotating you are doing just that, stopping the wheel rotating but because of momentum the tyre actually 'pulls' ahead of the wheel minutely (the amount of movement dependant on the pressure of braking) and it was this tyre 'slip' on the wheel that had caused the valve to be disloged. The result is obviously the air escaping and a flat tyre. :(

Any thoughts on the above guys...

Twin

[OzExpat]

Twin ... I'd have thought this was the same as tyre "creep", which was one of the things that I was told to check for during the daily inspection, when I first learned to fly. I'm pretty sure that "creep" happens for a lot of reasons other than touching the brakes after take-off.

When flying my first retractable u/c aircraft, I well remember the instructor telling me to touch the brakes after take-off, to ensure the wheels had stopped spinning before retraction. I'm not aware of any prangs that could be attributed to this practice, but have seen many instances of tyre creep.

I'm still inclined to stop the tyres rotating before retracting them these days. And if I'm in any doubt about the amount of creep that I detect, I'll always get an engineers opinion about replacement because I know that it could spoil me day if I ignored it.

Any other ideas out there?

[tom775257]

<<tom775257 So why don't you explode when you surface>>
Sorry to go off topic:
Simply breathe normally on a standard ascent, and allow the pressure to equalise from within your sinuses/ears. Sub-aqua does has the unfortunate side effect of upon ascent, the entire contents of your sinuses ends up out of your nose, all over your face due to the increasing volume of air in your head. This is especially bad if you have a cold..... Seriously though, on a fast ascent you run the risk of damaging your lungs. Regarding pressure and the like: When diving in Egypt, due to getting thirsty under water (water water everywhere, but.....), we used to take capris-sun down with us to drink (a drink in a foil pack), as it had no air in it, it didn't compress, hence no problem.
On a vaguely aeronautical note, our club tried many things to enhance communication under water. The best way by far we found was a pair of the old pneumatic headphones given to PAX. The person you want to talk to puts on the headphones; you blow out the water, then speak through it. Worked very well! I used to have a set attached to my stab jacket.
Cheers, Tom.

[Techman]

On all large aircraft the brakes are automatically applied when the gear is retracted.
So it should work just as weel on small ones, by manual application.

[mcdhu]

In the same vein, but only different, I have oft wondered why the brake temperatures don't cool from typical after t/o temps of 100 - 150C to much less than 15C after upward of 4 hours at -60C. Any ideas?
Cheers,
mcdhu
'Interesting - but only marginally so!'

[Trislander]

To get a simpler view on this tyre (tire, US) explosion business, I find its much easier to work with numbers as well, rather than word equations alone.

Example:
Tyre pressure gauge (gage, US) indicates tyre is inflated to 600 lbs/square inch.

Outside (or ambient) air pressure gauge indicates 15 lbs/square inch on the ground.

Outside air pressure gauge at a typical airliner cruising altitude indicates 2 lbs/square inch.

Actual tyre pressure=Indicated tyre pressure - outside air pressure

Tyre pressure on ground=600 - 15
which equals 585 lbs/square inch.

Tyre pressure at altitiude=600 - 2
which equals 598 lbs/square inch.

So the total difference between the pressure on the ground and at altitude equals 13 lbs/square inch (15 - 2).

The tyre is only therefore increased in pressure by 13 lbs/square inch, which is not a lot, so the tyre is not under enough strain to burst or explode.

I hope that has helped!

Cheers, Tri

[flapsforty]

M.Mouse, regarding your question about exploding; an other type of explanation.

Breath in at depth and the pressure in your lungs equals the water pressure on the outside.
Go up without exhaling and the decrease in pressure will allow the air to increase in volume.
If you breathe in at depth and hold your breath while ascending you will in fact "explode" your lungs.
Which is why many diving schools hammer it into the newbies' skulls to make a continuous sound during ascent; this makes exhaling madatory and avoids lung injury.

And what's so complicated about them tires huh? ;)

[M.Mouse]

Thanks to those taking the trouble to explain why tom775257 doesn't explode when surfacing from a dive.

I guess I left out the :D :D :D . It was tongue in cheek! Sorry!

[knobbygb]

I doubt tyre creep could be caused by braking after takeoff, unless the mass, and therefore momentum of the tyre was significantly higher than that of the wheel hub. Much more likely to happen during landing.

I was told by an instructor that landing in a crosswind without getting the aircraft lined up with the r/w puts was a major source of creep due to the sideways forces pulling the tyres away from the rim at the same time they start to rotate. Any truth to this?

On gear retraction... Would gyroscopic progression be a reason to brake the wheels before retraction? Obviously only for gear that retracts perpendicular to the rotation of the wheel, i.e. most main gear. Heard this mentioned on this board before but nobody knew for sure.

[Tinstaafl]

Stopping gyroscopic effects is an important reason for stopping the wheel rotation prior to retraction.

[flapsforty]

M. Mouse: http://www.stopstart.fsnet.co.uk/smilie/thumbs.gif

[Tinstaafl]

Trislander

I think you've made an error with your formula.

For the tyre gauge to indicate anything the pressure on one side of the gauge must be greater than the other. It's a differential instrument. In this case it's comparing pressure inside the tyre to ambient pressure outside the tyre.

To use your example, for the gauge to indicate 600 psi the pressure in the tyre has to be 600 psi greater than outside the tyre.

The pressure outside the tyre is 15 psi so the total tyre pressure must be 615 psi & not 585 psi in order for the guage to read 600 psi

Actual (total) tyre pressure = indicated plus ambient




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