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Atmospheric Pressure On Tires

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Ok gang I am in school taking aircraft maintenance when our teacher starts talking about atmospheric pressure, guage pressure and atmospheric pressure.


So the teacher says that if we take a car and check the pressure in the tires here in Thunder Bay which is 653' AMSL and drive it to Calgary which is 3557' AMSL that the tire pressure should read higher in Calgary than in Thunder Bay. Now in my way of thinking as we increase in altitude pressure decreases. Thus there would be less pressure on the outside surface of the tire and the pressure inside the tire would decrease as well. Anyway it seems that I am the only student who has yet to understand that the tire pressure should increase. Anyone out there willing to take a crack at an explanation in order for me to understand.




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Puddle Jumper



The air in the tire is comprised of . H20-Water, Co2,Nitrogen, and other atoms/molecules.(if you need to get a better understanding of chemistry learn about Avagadro an his number- the formula to calculate the amount of atoms/ molecules in 1 mole.)


Think of it this way. At Sea Level, the molecules are compacted by the atmospheric pressure and are held by this force. Assuming that the temperature is constant the molecules will try to break farther appart as the pressure decreases -increase in altitude. The pressure on the outside surface of the tire will increase as the volume of molecules spreads farther appart ,after all, there is less to restrict them from equalizing with the outside air pressure.


Eventualy the air in the tire will try to balance with the outside air pressure. It's all in the seal!


If you have any more questions I would be pleased to help.

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It also depends on the speed rating of that tire and how fast you are driving. Set your tire at 35 lbs when cold and then do it again after some high speed driving....expect anywhere up to 15 lbs more pressure on that reading. If you exceeded that speed rating on the tire, it won't take that extra heat from the high speeds and you got yourself an exploded tire or worse.


So let the altitude in Calgary be higher and let the pressure decrease with that altitude and it all equals out. Park the car overnite in Calgary after that trip and it might be a good idea to check the tires in the AM during your walk-around.

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As I'm sure your Instructor has already told you Puddle Jumper, the "Gauge Pressure" is the difference between the atmospheric, or ambient pressure and the "total pressure" of your system. If your tire is at 30 psi at sea level, the total pressure will be around 44.7 (30+ ISA Sea Level Pressure of 14.7).


As you climb in altitude the ambient pressure will decrease because there is "less" air pressing down on you from above. So at 4000 feet in Calgary, the ambient pressure will be approximately 13 psi. Since the total pressure remains the same, your tires will read approximately 32 psi to make up the difference.


Think about starting to fill your tire from zero at sea level. The gauge shows 0 psi because the pressure in the tire is the same as the pressure in your lungs... it is reading the "difference" between what's in the tire and what's outside the tire. So when you pump it up to 10 psi, the gauge is saying "hey... there's 10 psi more in that tire than there is outside that tire".


Now if we go to Calgary, or say Mexico City at 7800 feet, because the pressure is going to be less outside the tire (11.1 psi in Mexico City) your gauge (which is measuring the difference remember) will say "hey there's 13.6 psi more in that tire than there is outside that tire (10+ (14.7-11.1))".


Hopefully this helps (and hopefully I don't sound too pedantic), but if not there're tons of really good websites that explain stuff like this in everyday language with excellent illustrations and examples.


HV :hide:

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Well leadingedge,


I'm assuming that your question is hypothetical. I don't know of any aircraft (or any body period) that could perform a vertical loop at a constant velocity. The whole thing with a loop, much like an object being swung vertically with a piece of string, is the transfer from one form of energy to another during the whole process.


Conversely, if your dealing with a body rotating horizontally, it's easier to consider because you can eliminate the effects of gravity (that is; gravity is constant and can be dropped from the equation).


Back to your loop question; because you're converting kinetic energy to potential energy on the way up, your velocity decreases (the old conservation of energy law). So obviously your velocity must increase on the way down in order to keep the equation in balance.


If you had a jet aircraft that applied more thrust on the way up and could somehow "brake" on the way down you might be able to maintain a constant velocity of 140m/s, but it would really be "cheating".




P.S. That is a fast and tight loop... more like a carnival ride than an aircraft... or a rollercoaster maybe...

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