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

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!!

OzExpat

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

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.

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 naked ladies posing with air powered machinery 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!

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...

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 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