Why do airplane windows have tiny holes?
slate.com
slate.com
So the hole is there so that air gets through and the pressure difference is at the outermost pane, OK. What's wrong with it being on the middle pane? What would happen if there was no hole?
You could get around that by having the panes airtight and produced in a moisture free environment, but that would be more complicated. Think about failure rates in double paned windows which don't have to undergo the stresses on an aircraft. I'd assume even the outer panes aren't completely airtight to keep things simple:
Right at the beginning they should have said that the hole is for "releasing the moisture from the air gap and stop (most) fog or frost from forming on the window."
The hole actually dilutes the safety wee bit: "In the extraordinarily unlikely event that the outer pane fails, the middle pane takes over. And yes, in that case, there would be a small leak of air through the breather hole—but nothing the aircraft’s pressurization system couldn’t easily cope with."
TL;DR: Aircraft window hole compromises safety to improve the view :-)
Sure, because they are sharing the normal load, it is less likely to break in the first place. But, you'd have essentially doubled the strength, but removed the backup. That's not a good trade-off.
First for the panes:
The article says the two panes are for safety reasons and I buy that. Equally important in my opinion is another reason. It's the same reason you probably have double panes in your home: thermal and noise insulation. Air has terrible thermal and noise conduction. So you need two panes and air between them.
Why the hole:
Remove moisture, I buy that. Allow pressure to equilibrate between the passenger cabin and the air gap between the panes. I buy that too but the article does not really explain why this is necessary.
If the two panes would be sealed absolutely tightly the pressure between them would stay at normal level in any case. As an aircraft climbs, the air pressure drops in both the cabin and the outside air — but it drops much more outside, as the aircraft’s pressurization system keeps the cabin pressure at a comfortable and safe level.
Without the hole and with perfect seal if the pressurization fails for some reason you'd have maximum pressure gradient on both panes. The redundancy of the second pane would be lost.
> If the two panes would be sealed absolutely tightly the pressure between them would stay at normal level in any case.
I think higher pressure inside the plane would push out on the inner pane, pushing it closer to the outer window, so the pressure between would go up compared to the outside.
That is to say, cabin pressure would be highest, pressure between the panes would be in the middle, and pressure outside the plane the lowest. But ALL THREE pressures would be lower than air pressure on the ground.
> Without the hole and with perfect seal if the pressurization fails for some reason you'd have maximum pressure gradient on both panes. The redundancy of the second pane would be lost.
The air in between the panes would provide some "padding" so the outer pane would receive less pressure than the inner pane, but there's not much air between the panes so the padding, and difference in pressure, would likely be small, so yes, redundancy would be lost.
If sea level pressure was trapped between the two windows you'd have pressure in the opposite direction on the inside window whenever the cabin altitude increased, which would happen every flight and induce cyclic stress on the sill that it wasn't designed for.
If the inner cavity between the two windows did not have a hole, the full sea level pressure would be pressing outward against the outer window. But with the hole, only about 75% of that pressure is present.
Additionally, if there were no hole, the inner cavity between the windows would be positively pressurized with respect to the cabin. This means the force on the inner window would be backwards from what it would have to bear if the outer window failed.
wait, what? this isn't how pressure works... am i missing something here?
| |
[cabin] | [gap] | [outside]
0.75atm | X atm | 0.23 atm
| |
I think it's the lack of air in double-paned windows that helps. Usually double-paned windows have a vacuum so that air molecules can't move from the outer pane to the inner pane. Surely air is a fine conductor of heat if we consider our weather system. And surely it's also a fine conductor of sound, if we consider our hearing system. In soundproofing applications, open air channels are the top priority, and the simplest way to dampen the sound is by adding mass. Sound doesn't travel at all through a perfect vacuum though (no sound in space), so typical double-paned glass should have an effect there.
I think the purpose of the hole is to ensure that stress does not build up on the redundant pane so that it is only used in the case where the main pane fails. When you build redundancy into a system, it's important to ensure that failure of both pieces doesn't happen at the same time. Either can fail on its own, but they can't both fail.
So you want the middle window to be as close to the ideal condition as possible when the outer window fails. By adding the hole, you accomplish this. Moreover, once the outer window fails, the hole ensures that the middle doesn't need to take the full pressure differential and allows the pressure to equalized in a somewhat controlled manner. Yes, the pressure in the cabin will drop, but there are oxygen masks that will drop if it gets too low and the pilots can always descend to a lower altitude. The people onboard will have the time necessary to take both necessary actions.
Design evolution at work.
Another issue the Comet had were the engines buried in the wings, which makes them less accessible for servicing and more dangerous to the aircraft in the event of failure/explosion.
http://flighttraining.aopa.org/magazine/1998/November/199811...
The fact is that the percentage of lift that is attributable to angle of attack is far greater than that due to reduction in air pressure across the curved part of the wing.
As the angle of attack is increases, the point of minimum pressure moves forward and the size of the adverse pressure gradient increases.
btw. In most most airfoils zero angle of attack and zero lift axis differ. The wing generates lift even without the angle of attack. Chambered wings are exception to this.
Commercial jetliner fuselage wall thicknesses are typically around 1-2mm. They don't call 'em flying tincans for nothin! Think about that next time you fly!
I was just coming back to edit my reply to say that the difference appear to be 0.5:2, or the soda can holds roughly 4x the difference.
Ah, but I'm tired, maybe I read/thought wrong.
The difference of pressure between an soda-can and air (at ground level) would appear to be between to and three atmospheres worth (or like sea-level and a depth of 20-30m/~60-90', if the rule-of-thumb I've learned is about correct; see also: Why is parachuting into water OK, while diving and then flying a bad idea?).
It would appear the interior and exterior and an air-plane typically differ at about half an atmosphere (5m/15' water).
So that pretty much answers what I was hesitant to suggest out loud - that even though the fuselage of an airplane is proportionately thinner, the soda can has to withstand a greater pressure difference. Thanks!
If we consider a 3 m wide airplane, with a 1.6 mm thick fuselage at cruise altitude (around 40k feet), you get around 56.5 kPa of pressure difference, and the above equation gives you approximately 52 MPa. (data from a real aircraft that I cannot mention).
For a coca-cola can, internets say 380 kPa of pressure, for a 6.6 cm diameter can, and 0.15 mm aluminum sheet. That results in approx. 85 MPa.
So as you can see, even though you have roughly 7x more pressure in the soda can, its much smaller diameter severely reduces the stress on the walls, resulting in about 1.5x the circumferential stress.
Hope that answers your question.
edit: several errors in my back-of-the-envelope calculations. sorry.
stress * 2 * thickness * dx = p * 2 * r * dx.
[ stress * walls cross-sectional area] = [ internal pressure * projected internal area ]
Solve for stress, you get:
stress = p * r / t
Regarding your question, yes, as the radius goes to infinity, the stress goes to infinity. The area where the pressure is applied grows with r, but the cross-sectional area where the stress is applied is still (w * thickness * dx.) This equations work well for thin-walled cylindrical pressure vessels (r > 5t is general rule of thumb). For a cube, you would have to develop the equations, but keep in mind that you will have a singularity/discontinuity on the walls because of the right angle.
edit: good to have a reference just in case: http://ocw.mit.edu/courses/materials-science-and-engineering... [PDF ALERT]
So a planar face of a cube cannot satisfy the equilibrium equations? Interesting ... so then a cube will necessarily bulge so the radius is enough to satisfy the equations, right?
My take.. The difference between inside and outside of soda can is approx 175 (kPa). The difference between inside and outside of aircraft cabin at the cruising altitude is 56.6 (kPa). So Soda can bears differential of approx 3 times than the pressure differential that aircraft cabin structure supports, with material 4 times thinner.
Aircraft is 12 times more safer than a soda can (!)
Interesting.. aircraft safety engineers at work.
Reference:
http://en.wikipedia.org/wiki/Cabin_pressurization
http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...
Both are engineered in a way to make pressure not a problem. The can is subject to stress if you refill it, the airplane is a bit overengineered so it's not subject to stress. Increasing the width of any wouldn't lead to an increase on their safety.
An aircraft fuselage needs to withstand THOUSANDS of cycles of pressurization / depressurization during its service life, which is usually much more critical then the static pressure load, due to metal fatigue. This is why aircraft service life is given in flights (which corresponds to one pressurization cycle) and not in flight hours or miles or whatever. This is also why aircraft used on longer routes tend to last for more years (longer flights = fewer pressurization cycles).
edit: If anyone wants a reference, here goes one http://www.airspacemag.com/need-to-know/what-determines-an-a...
edit2: Additionally, consider checking my response to another comment. The diameter of the fuselage is so much bigger than the soda can that the actual stress on the walls are roughly only 1.5x bigger on the soda cans.
http://www.vfrmagazine.net/wp-content/uploads/2012/09/fretti...
I didn't get around to researching it, but here it is. It fell into my lap.
The load on each pane is lower so I would think they'd be less likely to fail.
A few hypothesis:
- If the main failure mode is a defect that doesn't depend too much on pressure, then we have increased likelihood of failure;
- The manufacuring cost of pressurizing the midpane is high;
- Having a varying load direction on the external pane (inward on ground and outward in sky) is not good.
Any opinions?
The other reason cited by OP, fog and frost, would still be valid if it is not desirable to package the panes in a humidity-free environment.
Without the hole the system would probably be better modeled as a single pane with an airgap between its inner and outer edges. It's likely that both panes would fail at the same time.
I know it's routine to manufacture even household glass panels with different gases sealed in their interior (I think they put Argon inside? some low conductivity gas), but afaik it's at atmospheric pressure.