Tyre pressure argument

[Guest Redair]

Greetings each, there are some who know me who say that I am argumentative, I say I'm not, they say I am etc.

 

So, having found myself at one of those points in the day where I have nothing better to do, I had an argument with myself, then invited a couple of others to join in, now I am extending the argument to all.

 

The problem is this: If my tyre pressure should be 14psi (approx. 1 atmosphere) and I have a flat tyre, my tyre pressure is correct. If I then get the compressor fired up and pump 14psi into my tyre to inflate it, I really have 28psi and have therefore over-inflated it. Allowing for all those barometric variations, I should be able to remove the valve from my tyre, wait a few minutes and then put it back in to get the correct pressure. Not quite the plane on the conveyor argument, but worthy of debate, maybe?

 

Redair.

 

 

[rankamateur]

Perhaps the pressure you are trying to achieve is one atmoshere plus 14 p.s.i., which would not result in over-inflation.

 

 

[M61A1]

Try reading it as 14PSID (differential).

 

 

[Tex]

Exactly M61A1

 

 

[dazza 38]

Greetings each, there are some who know me who say that I am argumentative, I say I'm not, they say I am etc.So, having found myself at one of those points in the day where I have nothing better to do, I had an argument with myself, then invited a couple of others to join in, now I am extending the argument to all.

The problem is this: If my tyre pressure should be 14psi (approx. 1 atmosphere) and I have a flat tyre, my tyre pressure is correct. If I then get the compressor fired up and pump 14psi into my tyre to inflate it, I really have 28psi and have therefore over-inflated it. Allowing for all those barometric variations, I should be able to remove the valve from my tyre, wait a few minutes and then put it back in to get the correct pressure. Not quite the plane on the conveyor argument, but worthy of debate, maybe?

 

Redair.

You have got to be kidding ? You are over thinking things. Just saying

 

 

[crashley]

yes you have 1 atmosphere inside your tube but you also have 1 atmosphere on the outside as well so you will have to take your plane into space for it to work !!!!!!!!!!!!

 

 

[Guest Nobody]

Someone needs to learn the terms "gauge" and "absolute". psig and psia.....

 

 

[M61A1]

Someone needs to learn the terms "gauge" and "absolute". psig and psia.....

Yep, that's a better way to look at it.

 

 

[Guest Maj Millard]

It's only flat on the bottom isn't it ???........so you've still got one atmosphere in the top right ?....

 

 

[facthunter]

To apply a pressure to the inside of the tyre of 1 atmosphere (14.7 psi) the gauge will read differential pressure which is what you are after. Your gauge doesn't read absolute pressure as there would have to be a vacuum for it to compare to. If you climb up to a higher altitude the gauge would read more as the outside pressure would decrease. Your radiator cap is similarly affected and if you open a hot thermos at height you will get pressure inside and steam coming out, but the boiling point it lower as the pressure drops. Nev

 

 

[Guest Andys@coffs]

So overthinking thing^2

 

Lets assume:-

 

1) Tyre is tubeless and naturally has the shape we expect

 

2) Aircraft is on a jack such that the wheel is off the ground and the valve is removed, allowed to equalise and then reinserted and sealed (1 atm present)

 

3) let off the jack

 

Then as soon as the aircraft weight acts on the tyre you will no longer have 1atm present, the deformation of the tyre ensures that the pressure must rise.

 

To prove it, unseal the valve and much air will hiss out and the tyre will get flatter if possible.

 

The point? if you pressurise a tyre without weight of aircraft to the pressure called for in the POH then as soon as its on the aircraft and aircraft weight is applied you will end up with pressure above that called for in the POH

 

Andy

 

 

[horsefeathers]

So overthinking thing^2Lets assume:-

 

1) Tyre is tubeless and naturally has the shape we expect

 

2) Aircraft is on a jack such that the wheel is off the ground and the valve is removed, allowed to equalise and then reinserted and sealed (1 atm present)

 

3) let off the jack

 

Then as soon as the aircraft weight acts on the tyre you will no longer have 1atm present, the deformation of the tyre ensures that the pressure must rise.

 

To prove it, unseal the valve and much air will hiss out and the tyre will get flatter if possible.

 

The point? if you pressurise a tyre without weight of aircraft to the pressure called for in the POH then as soon as its on the aircraft and aircraft weight is applied you will end up with pressure above that called for in the POH

 

Andy

Welllllll, yes what you say is true, to a point

 

However, if you do pressurize the tyre when jacked, then replace tyre on the ground, there is so much less compression of the tyre compared to the unpressurized case, that the pressure wont rise to the same extent. So yes you will go above the POH pressure, but not by very much :)

 

 

[facthunter]

The deformation will reduce the volume but not as much as one might think . The sides of the tyre bulge out, but there would always be a reduction, with the distortion.

 

A similar effect happens with temperature. Use the correct values for the universal gas equation. PV/T = Constant. Temperature is absolute (Kelvin.. minus 273 C is zero K). IF you wanted to go to the ultimate you would correct for ambient pressure too. Nev

 

 

[Guest Andys@coffs]

I never specified it would be huge...and I did start the post with "So overthinking thing^2"

 

The other factor is the pressure altitude at which you inflate the tyre.....inflate at a high pressure altitude to the specified POH pressure and then land at sea level and see what's in the tube.... (which is what Nev alluded to with his thermos analogy)

 

The OP was splitting hairs...I just split the already split hairs....because we can ! As was said universal gas law PV/T=K (or better recognised as PV=nRT) covers the only possibilities. Both the weight on wheel and the pressure altitude play with the P and V function

 

Andy

 

 

[facthunter]

I agree Andy. I read your post that way. Nev

 

 

[ahlocks]

Pump until the tyre looks roundish.... done!

 

 

[facthunter]

I've seen people kicking them. That must do something, like keep them in their place, but perhaps damage can ensue. Ive seen signs with "NO tyrekickers please" on adverts. Nev

 

 

[ahlocks]

Ah damn..... I forgot about the foot tyre interface differential equation.

 

 

[facthunter]

I was going to mention that. Nev

 

 

[Bruce Tuncks]

You were right first time Redair. Just put the valves in and fly to the moon . The tyres will be flat on takeoff but you will land with them inflated just right.

 

I'd come too but my Jab doesn't go that high.

 

cheers, Bruce

 

 

[Tomo]

Problem is, you have 14.7psi on the outside and 14.7psi on the inside. If only balloons pumped themselves up as soon as you open the packet...

 

 

[facthunter]

CEO's do that. Nev

 

 

[Marty_d]

Do they? I've never bought a packet of CEO's.

 

 

[ben87r]

I thought a pack of CEO's would do exactly the same as opening a packet of balloons, make allot of nose but not really do anything at all...

 

 

[facthunter]

There's only one in each packet. CEO (singular) Nev