Am I blind or is the question not really answered?
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?
My main guess (I'm not an expert) is that it's to ensure that the double-window system is fully redundant. Without the hole, in the event of an outer-window failure the inner window would already have gone through a variable and probably large number of hours of stress from differing air pressure. With the hole, the middle window is (hopefully) effectively factory-fresh with zero service hours at the moment it takes over. That would be especially good if the failure of the outer window had anything to do with a quality defect in that window (or mishandling of it) instead of being completely adventitious: in that case, maybe it's not the only window pane in the aircraft with a dodgy service-life rating? If the outer windows have to be replaced every n flying hours, then not stressing the middle windows would also make sure that they don't have to be replaced as often. (This seems a bit clearer from reading the io9 version http://io9.com/why-is-there-a-hole-in-airplane-windows-17036... , though I don't think it's stated explicitly there either.)
The purpose of the smudge pane should be obvious.
There are two panes and a hole left to explain.
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.
Just to put a number on it, the cabin pressure at high altitudes is given by Wikipedia as around 0.75 atm. (It varies by aircraft model.)
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.
I think it's saying if the two panes were sealed without a hole at ambient pressure when the plane is manufactured then the outer pane at elevation would have ~1atm of pressure pushing out on it and the middle pane would have ~1atm pushing in on it. With the hole the outer pane only has cabin pressure pushing out on it, which is ~75% of 1atm.
If "what to make X" is your design choice, you would not want to have it always pegged at 1 atm, as it would be if it was/could be completely sealed at sea level. It's simplest to have it equalize with the cabin pressure.
When looking at designs of these types, I find it's instructive to think about what catastrophic event the designers are trying to mitigate. In this case, my guess is some sort of decompression where the pressure inside and outside of a window is rapidly equalized and the people onboard don't have time to react in an intelligent fashion.
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.
This explanation makes a lot of sense--I agree that not having the hole removed redundancy--but I'm not sure some of your details are exactly right.
> 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.
"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."
I think this is incorrect. Assuming the windows are sealed at ground level, the pressure between the panes would be slightly below ground level at altitude, due to flexion of both inner and outer panes; the pressure in the cabin is substantially lower than ground level; and outside pressure is the lowest of all. Thus the pressure between the two layers is the highest of the three compartments at altitude, subjecting both layers to considerable stresses with every pressurization/depressurization cycle. Now if you sealed a near-vacuum inbetween, that could help with thermal insulation and prevent condensation, but would cause more stress on the ground.
It seems likely to me that the sill is designed to resist pressure principally from inside the plane to outside the plane.
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.
> 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.
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 didn't know that there are indeed double-paned windows that have most of the air removed from the space between the panes. That was interesting news to me.
> a separate but related function of the hole: to release moisture from the air gap and stop (most) fog or frost from forming on the window.
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:
I'm just guessing, but I think the breaking of the middle pane would damage the outer pane anyway, since the pressure is coming from inside. That would leave you with two broken panes.
Agree, the answer to the question was not articulated properly.
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 :-)
Um... that's not how I read it. It's about controlling the failure modes. The pane with the hole is the "backup"--without the hole, it would be stressed roughly the same as the outer pane during normal operation. This has a major consequence: when one breaks, the backup will probably be in the same sorry state as the primary before it broke, so it will break soon too! Not a very good backup!
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.
Its also interesting that the corners of the windows are rounded - it's to avoid points of concentration for metal fatigue. We learned that lesson the hard way. When the De Haviland Comet was designed with square ones this was still poorly understood and there were several severe failures. The quest to find the cause of these failures at the time could be seen as the birth of modern air crash investigations.
The windows weren't quite square panes, they were slightly rounded, but not enough.
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.
Engines buried in the wings was first and foremost aerodynamic problem. Today we know that intakes must be kept from messing with the airflow above the wing where most of the lift is generated.
Roughly 80% of the lift is generated above the wing in typical aircraft. Angle of attack is part of it and not a counterargument.
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.
That's a weird text. At the beginning the author lists reasons why it's clearly Newton's third law, and then just basically says: "But then an 'authority' on aerodynamics waved hands around telling me it's not. The end." Maybe I misread something, but nowhere in the examples that are apparently inconsistent with Bernoulli's law, and for which the professor claims they actually support it, does he actually provide an explanation.
Sure. A follow-up question I would have asked: how do propellers work? By accelerating large volumes of air, force is generated according to Newton's Second Law. So do wings accelerate large volumes of air? Of course, as one can see at any airport, large vortices trail behind and below the wings of any airplane or glider. So how is a wing different than a propeller?
I didn't say it wasn't complicated. What I said was most lift is due to angle of attack.
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.
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!
> If you shrank a 777 to the size of a soda can, its skin would be about four times thinner than the average can's.
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 (!)
You are neglecting the fact that a soda can is pressurized once, depressurized when opened for consumption, and then wasted.
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).
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.
Well, except that aircraft pressure is applied outside-in while soda can pressure is inside-out. Put some outside-in pressure on a can, and see how well it handles it.
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.
Sorry I'm confused by your statement. The air pressure inside an aircraft during flight is much higher than the air pressure outside. So the pressure is also inside -> out, same as a soda can.
Scaling objects doesn't preserve their ability to withstand forces: for example cross-section areas scale with square of scaling factor and masses/weights scale with the cube of scaling factor; so objects are close to collapsing due to their own weight if you scale them up.
This is potentially a pretty stupid question - but are the differences in pressure roughly the same between a full, pressurised soda can and a cruising-altitude pressurised airline cabin?
I'm not sure why you'd think it's a stupid question (what is a stupid question?) -- but according to the Internetz[1,2], I think the answer is no.
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).
I think the internet has just made me a little over-cautious, especially when being around so many smart people as there are on HN :)
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!
From a 2-minute google search, it looks like both are ~10 psi pressure difference (psig). The numbers I found were 7-10 psig for a plane and an estimate of 17 psig for the soda can. So they're quite similar and it's not a stupid question.
More important than pressure differences, is the circumferential stress on the fuselage/can, which is what the walls are actually withstanding. For a pressurized circular container, it is given by p*r/t, where p is the pressure, r the radius and t the wall thickness.
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.
That's an odd model for me. So as the radius goes to infinity for a fixed thickness and differential the stress goes to infinity too? But take a pressurized cube with reinforced edges. You can say it's faces have infinite radius (or as small as you'd like) -- yet the stress should be finite? How do you reconcile this? Or does that work only for circles? (which would be odd since stress is a local concept to me)
I'm not sure of your background, but it is actually a pretty simple concept. To satisfy static equilibrium on a given transverse "dx" segment of a cylinder, the walls must balance the internal pressure (in this case taken as the pressure difference), so that:
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.
Ah makes more sense now, thanks for the explanation. I'm an undergrad in EE so mechanics is not my strong point.
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?
Holy crap. I actually put this in my notebook to look into. I was flying home from Atlanta a few weeks back and I was staring at the hole, wondering why it was there.
I didn't get around to researching it, but here it is. It fell into my lap.
In fact, one could package the three panes in a normal (or humidity-free) air without the hole. Assuming the cabin pressure is close to ground air pressure, the pressure difference will still be on the outermost pane as in the OP, and if the outermost pane fails, it will come automatically on the middle pane as in the OP. In other words, this reason cited by the OP does not explain at all why a hole is there. Am I missing something here? :-)
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 space between the two panes will be pressurized. I'm not sure what the exact figures are (I'm not a phycisist) but I would guess that without the hole the outer pane will be subjected to some force F due to the presure, and the inner pane will be subjected to some force F/2, or even equal.
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.
You don't need to be a physicist: if the panel is sealed there will be a constant pressure inside. The pressure on each face is then P_ext-P_int. It's just as likely that both panels will fail at the same time in that case as in the case with a hole; but the bonus in this case is that you can set up the internal pressure so both panels have lower nominal pressure (but the internal at least should be able to still handle the full pressure difference).
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.
Many people assume that panes must be fixed solidly within a solid frame. ISTM the frame itself could be somewhat flexible, to accommodate Boyle's Law and the regular sea-level-to-8k-ft pressure cycle. There may well be other factors involved, especially the beautiful simplicity of drilling a hole. However, if for any particular use case the hole proves problematic, we might expect to see the backup pane in such a flexible arrangement.
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?