alphafloor
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737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 7:21 pm

I have a some questions regarding the pressure of the tires of a plane and gear bays. When the tires of a 737 retracts the gears bay do not close, the tires are still visible when they are retracted after take-off as seen below.

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I wonder in which proportions these tires increases volume at cruising altitude due to the lower pressure of the environment. I assume the pressure of the tires also increases when climbing and the tires are strong enough to deal with this increase of pressure. I also wanted to know if the gear bays of the 777, 747, MD-11, a320 are pressurized or not ? Why the 737 don't have a closed gears bay ? Isn’it more reliable to have closed gears bay ?

Al
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greasespot
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 7:36 pm

As far as I know no wheel wells are pressurized. I guess Boeing did not used enclosed bays because there is a lot less monkey motion required for gear up. IE: no doors to open or close. Therefore there is no doors to rig or actuators to change. So no doors is in a sense more reliable than having doors.

I do not understand what you mean by which proportions. The tires do not have any chambers so they increase and decrease equally.

Greasespot
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EMBQA
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 7:42 pm

I'm sure with the drop in atmospheric pressure as the aircraft climbs in altitude the tire pressure will go up slightly....but your talking ever so slight. The tire pressure will change more when they heat up after landing.

And No....No wheel wells that I can think of are pressurized.
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Klaus
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 7:44 pm

Landing gear bays are generally unpressurized.

The tire pressure is so high anyway that the additional difference between sea level and cruise altitude doesn´t make a big difference. They´d probably survive a total vacuum just as well. On touchdown and taxi they´ll have to deal with much worse.

And the standard answer regarding the 737: The rationale given was that the development cost, the added weight and complexity of full doors (and a "deeper" wheel well) would not have been justified by the relatively small reduction in drag.
 
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HAWK21M
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 9:40 pm

B737s do have LG Doors in fact three on each gear called the Inner,Mid & outer.But these are fixed & rotate with the gear & yes the Outbd side of the Outbd wheel is exposed.Why add to the weight.
regds
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320tech
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 10:18 pm

Think of it this way - tire pressure is around 200 psi. Sea level pressure is about 15 psi. Pressure at 35,000 feet is, well, I dunno, and I just got off midnight shift, so I'm not looking it up - but suppose it's 5 psi. Total relative increase - 10 psi. Big deal.
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alphafloor
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RE: 737 Gear Bays - Tire Pressure

Mon Aug 16, 2004 10:53 pm

Thanks everybody for the answers !!

Al
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videns
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RE: 737 Gear Bays - Tire Pressure

Tue Aug 17, 2004 3:53 am

And nobody has mentioned the change of temperature as the aircraft climbs. As the temperature in the wheel decreases, so does the pressure.
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prebennorholm
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RE: 737 Gear Bays - Tire Pressure

Tue Aug 17, 2004 5:25 am

The eventually slightly increased tire pressure at altitude is peanuts compared to the pressure increase when the bird is slammed down on the runway on one wheel only in crosswind.

The relative pressure increase is much higher on a car tire when crossing a high mountain pass. A car with a nominal tire pressure of 30 PSI may experience 34 PSI on a high mountain pass - well over 10% increase. Still we don't even notice that. A 737 with 200 PSI will never experience more than a 5% increase at altitude.

I would guess that the heat from the brakes - transferred to the wheels - after a landing produces a much higher pressure increase than any altitude the 737 can reach. But that's guesswork, I don't know.
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avioniker
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RE: 737 Gear Bays - Tire Pressure

Tue Aug 17, 2004 6:43 am

Just to throw something into the mix;
If the area of a tire expands due to a decrease in external pressure then the internal volume would also increase therefore the internal pressure will decrease (psi, remember?). Pressure (psi) is measured as a representative value of the force exerted on a square inch of internal surface area.
Aircraft tires are designed to resist that sort of expansion but I just thought I'd throw that in.
For you trivia buffs, the standard for aircraft sidewall flex on landing is 35%. An automobile tire is designed to regularly flex 80 to 90%.

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MaerskMech
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RE: 737 Gear Bays - Tire Pressure

Fri Oct 22, 2004 9:23 pm

10-15 minutes after landing the pressure is between 240 and 260 psi. thats the figures I have measured myself. If the pilot made a high speed landing the pressure might be even higher.
the normal pressure in our company is 210 psi. (737-700).
 
Goldenshield
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RE: 737 Gear Bays - Tire Pressure

Sat Oct 23, 2004 3:39 am

Also, the tempurature plays a lot into it as well. As the the air gets colder, the pressure decreases, and as it gets warmer, it increases.

There is also the gas law, or P/DT = R, or Pressure divided by density times tempurature equals a relative constant. So by using this, you can reduce pressure by decreasing the tempurature, by reducing mass, and by increasing the volume. Since a tire is an enclosed object, we cannot reduce the mass, so tempurature and volume play a part here; therefore, as the aircraft rises, the volume in the tire may increase due in part to the nature of rubber, but it is offset by the cooling of the air outside the tire, and over time, inside as well. On landing, the heat created by friction would be enough to raise the pressure inside the tire, while the volume would still remain the same, or maybe a percent or two different due to the rubber heating.

This would explain the tire pressures that Maersk has noted here.

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MaerskMech
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RE: 737 Gear Bays - Tire Pressure

Sat Oct 23, 2004 4:44 am

It's the brakes that heat up the tire. Don't think that the aircraft rise. cause you really can't see any difference if there are 145 psi or 240 psi. If the tire rises it is millimeters we are talking about. The tires are very rigid.
 
EMBQA
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RE: 737 Gear Bays - Tire Pressure

Sat Oct 23, 2004 6:54 am

As was found out today, there is very little room up inside a 737 wheel well.

[Edited 2004-10-22 23:57:08]
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stealthpilot
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RE: 737 Gear Bays - Tire Pressure

Sat Oct 23, 2004 9:53 am

The topic of pressure was well covered so i wont get into it  Smile
The question about why there are no bay doors, well simply because they dont need to be there. There is a negligable difference in drag with LG doors so it's the same thing aerodynamicaly to not have them. saves wieght, moving parts, space, complexity, money..... lots to think of.
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studentflyer
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RE: 737 Gear Bays - Tire Pressure

Tue Oct 26, 2004 11:13 am

In relation to aircraft tyres, is it true that instead of using regular air, they use nitrogen? If so, what advantages does nitrogen content in a tyre have compared with normal air? I have heard that this is the technology used these days, so I'm wondering if it's true or not.

Thanks in advance,
AK
 
LeanOfPeak
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RE: 737 Gear Bays - Tire Pressure

Tue Oct 26, 2004 2:12 pm

Just to go into nauseating detail...

Conceptual design of a new aircraft is basically a series of trade studies. You start with a payload, a range, a cruise speed, and a required runway performance, etc. That eventually gives you, based largely on historical trends, such things as weights, fuel requirements, wing areas, etc.

Once you have an estimated empty weight for the aircraft and some modicum of aerodynamic data for the various proposed configurations, you can start to trade off.

Gear doors add weight. If you just take off, fly around the pattern, and land again, it costs you more fuel to haul that weight around than the fuel it saves aerodynamically. At some set distance you can assume for the length of a typical trip for the proposed aircraft, the (small) aerodynamic cleanup of the doors will save more fuel than it costs to haul the weight of the doors.

Beyond that trip length, eventually you might save enough fuel (Assuming a given fuel price) to overcome not just the weight of the inner gear doors, but also the up-front purchase-price difference of developing and building the doors, as well as the extra maintenance costs they incur. If the aircraft is planned to fly farther than that distance more often than not, you put on gear doors. If not, you don't.

That same type of trade is applied to most design decisions on the aircraft, sometimes returning to decisions you've already made to see if a design change impacts the trade study for another design decision.

This is the reason you see winglet retrofits on 737's. With an increase in fuel cost, aerodynamic cleanup becomes proportionally less expensive. Eventually, the aerodynamic cleanup (Particularly on longer trips) pays for itself and turns a profit. At the fuel prices expected when the aircraft was designed, it wasn't thought it would pay off, but it will now.

Of course, sometimes a very small performance or cost penalty will be accepted for marketing reasons (OEM winglets sometimes being one of these cases). However, if you told the board of a major airline that they should buy your airplane because it has inner main gear doors, you'd probably be laughed out of the office, so they tend to succumb to the black-and-white, rational trade-study analysis more often than not.

Finally, if you can't justify the time and expense of a trade study or if a good estimate of weight or aerodynamics can not be made, the decision might be made based on what the engineers like, what management likes, what looks like it might work best, or what's easiest to do.  Smile

As for the Nitrogen-filled tires, I understand some of the reason to be that Nitrogen is not flammable.
 
MaerskMech
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RE: 737 Gear Bays - Tire Pressure

Wed Oct 27, 2004 2:41 am

Nitrogen is free of water and moist. that prevent that ice would build up inside the tire.
 
Lucky727
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RE: 737 Gear Bays - Tire Pressure

Wed Oct 27, 2004 3:19 am

When I posted a similar question a while ago I got some pretty good answers - which complement the ones above...check out reply #7

http://www.airliners.net/discussions/tech_ops/read.main/97822
··· [·] oooooooo [·] oooo oo ooooo [·] ooooooooooooooooooo [·]
 
studentflyer
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RE: 737 Gear Bays - Tire Pressure

Wed Oct 27, 2004 12:09 pm

Thanks for the answers guys, esp Lucky727 who linked his post to this, and MaerskMech who answered it a bit further  Smile.

Thanks guys.

Regards,
AK
 
air2gxs
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RE: 737 Gear Bays - Tire Pressure

Wed Oct 27, 2004 11:24 pm

Nitrogen is inert. It will not sustain combustion. And it does not re-act with the rubber or the surrounding atmosphere in the wheel well should the tire vent in flight.
 
flyer737sw
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RE: 737 Gear Bays - Tire Pressure

Thu Oct 28, 2004 1:11 pm

WN uses -0 +5 at 200psi on their mains...

Kevin

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