Why airplane windows have round corners (2016)
jasonlefkowitz.net
jasonlefkowitz.net
"The accident report's use of the word "window" when referring to the Automatic Direction Finding (ADF) aerial cutout panel[121] has led to a common belief that the Comet 1's accidents were the result of its having square passenger windows."
> Design and construction flaws, including improper riveting and dangerous concentrations of stress around some of the square windows, were ultimately identified. As a result, the Comet was extensively redesigned, with oval windows, structural reinforcements and other changes.
There's an ongoing debate on this on the article's talk page.
Here's a diagram of what the ADF "windows" are: https://images.app.goo.gl/3AUW6JsPV6tj39fq9
It's kinda heavy, which is another reason why cargo versions don't have windows.
Also, while I have read the story about the Comet many times before, I vaguely recall one telling of it that noted the problem wasn't simply due to square windows, but some other contributing factor, an adjustment to the design that caused extra stress on top of the corners.
It kind of seems like common sense that engineers wouldn't have been as oblivious as the standard telling implies.
Most disasters don't happen from anyone being dumb per se, but from the interaction of different people or groups who lack the full context of what the other is doing, and I believe that was the case here.
An opening on the top for a radio receiver was improperly riveted, instead of being glued. The rivet holes caused stress concentration, which resulted in metal fatigue and eventual failure and explosive decompression. When this root cause was discovered, it was also realized that the window corners also caused stress concentration.
The actual claimed explanation:
> when an aircraft’s interior is pressurized and de-pressurized repeatedly, over and over again for many months, the strength of that aircraft’s metal body slowly weakens — a phenomenon that became known as metal fatigue. And when the holes you cut into that body to hold windows have sharp corners like squares do, thanks to a process called stress concentration the weakness builds up much faster in those sharp corners than it does elsewhere
You can read the rest of the article if you want to read about the history of the de Havilland Comet, the world's first commercial jet airliner.
He went through bit more details said topic in another video "Why Are The Dreamliner's Windows So Big": https://www.youtube.com/watch?v=7-I20Ru9BwM
What if we could lead all this air to an external compensating device in a more ordered and less violent way?.
I'm thinking on some kind of "external airbag", elastic membrane, rubber balloon, etc, so we would reduce the time of event duration and soften or control the disordered exit of air (and maybe to save also some valuable oxygen from being totally lost). If unfortunately the accident happens, any corpse, object or piece would be trapped in the balloon instead to be sucked out and dissapear (this is better for the family than not having nothing to bury at least).
I suppose that to implement such device, like a rubber ring window in a handful of windows and doors, wouldn't be technically defiant and could save lifes (or alleviate the harm done) until emergency landing. Would be also a inmediately visible flag for airport rescue team to go directly to the area with more possible victims.
https://en.wikipedia.org/wiki/Southwest_Airlines_Flight_1380
We know how to build solid airplanes made 100% with rigid materials except in a single point (We know that rubber tires are the right way to cope with repeated landing impacts). Maybe we should explore a different way and include more deformable materials in strategic points to absorb the damage made by a possible impact. Maybe the concept of an "boxer's eyelid airbag" for windows in airplanes, would deserve some exploratory research.
You'd need a venting system which itself would detect sudden pressure change and blow first -- effectively a pressure circuit breaker. That itself would all but certainly introduce multiple new catastrophic failure modes.
Your suggestion generalises to flying an airplane's pressure hull within another pressure hull. This scales poorly in a regime where weight and payload space are highly constrained (though it has similarities to nonpressurised oil tanker double-hull design). Yo dawg, I hear you like airplanes, here's an airplane you can put your airplane in so you can fly your airplane in an airplane....
Better to design-proof against metal fatigue-induced ruptures in the first place.
I've never been described as that before.
That is not the idea either
It's better to think through the problems created by sudden rupture:
1. Pressure loss and insufficient oxygen. Reducing flight levels to ~13,000 feet addresses this, emergency oxygen provides for several minutes' supply for passengers, hours for flight crew.
2. Suction through void; crew, passengers and other internal contents may be sucked into / out of the void. This can and has compounded initial structural failures through pressure-hammer effects (especially Aloha Air), and is hard on crew, passengers, and contents subjected.
3. Internal windflows. Cabin contents themselves become missiles. Securing these (seatbelts, latches, straps, etc.) minimises risks.
4. Progressive structural compromise due to venting airflow, jetstream, or air hammer. This may occur over miliseconds to minutes, and is the primary result of total aircraft loss in most cases. Designing to minimise rupture propogation is the key countermeasure.
Controlled aircraft depressurisation is not itself immediately fatal if countered quickly (supplemental oxygen, reduced altitude, emergency landing).
At very high pressure differentials and sudden decompression, traumatic injury is unavoidable. The Byford Dolphin incident (warning: NSFL) would be a prime example. Though the structure itself was not totally compromised.
Pressurised aircraft fuselages are already designed to minimise the risk you're describing to the limits of materials and cost considerations. Double-bagging isn't practicable.
Aircraft in very high-risk environments (typically military aircraft) have been unpressurised or minimally pressurised (the F-16 maintains a 5psi differential to ambient above 23k ft). Explosive decompression by enemy (or friendly) fire is a key risk.
Irony.
Good on him for leading the innovation, but it seems obvious to me today that "first" rarely means "most profitable" or "longest surviving".
Sometimes dealing with it can be a pain, for example if you need two rectangular things to sit together along two faces, since you have to radius both corners with a larger radius on the smaller object. One way to combat this is to drill a hole in the corner, removing additional material but creating a little 270ish-degree-C-shaped radius in place of the sharp corner. Take what would've originally been a sharp corner, but then drill it out (example: [1]). What you lose on less material, you more than gain back with eliminating the stress concentration. This has the advantage that then a rectangular object can fit snug into the corner, because the smaller's sharp corner just sits inside the larger's C-shape cutout.
The best visualisation tool of this phenomenon that I learned is force lines [2]. Where they bunch up closely together is where high stress occurs. It follows naturally that a sudden sharp corner will concentrate all those lines close to it, whereas a gentler radius encourages a more gradual transition. Think of the lines as similar to Isobars (probably the most well-known contour line) [3], in that the lines represent points of constant pressure/force, and so will sort-of repel adjacent lines (since there needs to be a smooth gradient between those lines). The more gradually they're forced to change, the more gradual that gradient between lines will be (represented by them being farther apart).
Anyhow, all of this is also why you see countless concrete footpaths, driveways, etc. with cracks growing out from sharp corners in the concrete. Look at any internal corners where the concrete runs in an L shape. You'll not uncommonly find a crack growing from it. However, no one really cares (except for aesthetic properties) because it doesn't cause a functional problem, and forming concrete into straight sides is orders of magnitude easier than forming smooth curves.
Bonus points if you notice that it's generally not parallel to one of the edges that form the corner, and can figure out why :)
(I'd be guessing if I gave my answer, but I'm somewhat confident it'd be right: thermal expansion/contraction affects both sides simultaneously, meaning that the shear stress runs generally at 45 degrees to the corner rather than perpendicular to one, which would be the case if only one side expanded/contracted.)
[1]: https://img3.gmdu.net/35780.0.jpg [2]: https://en.wikipedia.org/wiki/Force_lines [3]: https://en.wikipedia.org/wiki/Contour_line#Barometric_pressu...
Also, the only Nazi aircraft of any type with root-integrated jet engines is the obscure Horton 229 prototype - and apart from that single cosmetic similarity, it has absolutely nothing in common with the de Havilland Comet.
"The Junkers Ju 49 was a German aircraft designed to investigate high-altitude flight and the techniques of cabin pressurization. It was the world's second working pressurized aircraft, following the Engineering Division USD-9A which first flew in the United States in 1921.[1] By 1935, it was flying regularly to around 12,500 m (41,000 ft)."