
Why airplane windows have round corners (2016) - smacktoward
https://jasonlefkowitz.net/2016/09/why-airplane-windows-have-round-corners/
======
gmueckl
Stress concentration, especially at sharp corners, is a well known phenomenon.
During the second world war, the US manufactured transport ships in an
assembly line fashion. The were launching these ships at rates of more than
one per week. A lot of them had the same design flaw: the cargo hatch corners
were not properly rounded off. This made them starting points for cracks in
the hull. Amd once a crack is started, it continues to extend relentlessly
under stress. As the story goes, a few of these ships were lost because the
cracks grew so long that the hulls broke and sank.

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tyingq
Wikipedia says the crashes weren't caused by the windows:

 _" 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."_

[https://en.wikipedia.org/wiki/De_Havilland_Comet](https://en.wikipedia.org/wiki/De_Havilland_Comet)

~~~
starpilot
This seems to contradict other parts of the Wikipedia article saying the
windows were the problem:

> 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.

~~~
tyingq
I read that as solving a potential future issue, versus solving the issue that
caused the crashes.

Here's a diagram of what the ADF "windows" are:
[https://images.app.goo.gl/3AUW6JsPV6tj39fq9](https://images.app.goo.gl/3AUW6JsPV6tj39fq9)

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perl4ever
Interestingly, the fuselage fragment picture looks like the windows _were_
rounded.

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.

~~~
stcredzero
_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._

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.

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JulianMorrison
So, why aren't they circular like portholes?

~~~
taneq
You can probably get 80% of the stress reduction from 20% circularity.

~~~
Gibbon1
Definitely that. Also for a cylinder under pressure the hoop stress is twice
the longitudinal stress. Which says to me that vertically oriented rectangular
windows are optimal. Probably even more so since the windows are aligned.

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jessriedel
The only thing worse than not having a tl;dr is having a clickbait tl;dr: "The
answer is simple: to keep that window from killing you". (Translation: "To
find out more, read the article!")

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.

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jjakque
Real Engineering made a video on this exact topic, explaining the essence in
less than 2 minutes: [https://www.youtube.com/watch?v=7rXGRPMD-
GQ](https://www.youtube.com/watch?v=7rXGRPMD-GQ)

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](https://www.youtube.com/watch?v=7-I20Ru9BwM)

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pvaldes
Ok, is interestiong, so lets eval the explossive decompression issue.

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.

~~~
dredmorbius
Venting from multiple, engineered, locations rather than a single explanding
rupture would spare the airframe from catastropic disintegration. It's still
hell on the cargo and pilots.

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.

~~~
pvaldes
> Your suggestion generalises to flying an airplane's pressure hull within
> another pressure hull

That is not the idea either

~~~
dredmorbius
Practical remedies generally resemble ripstop designs, preventing propogation
of structural failure, rather than attempting to contain the pressure.

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.

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_bxg1
> Too late for the British aerospace industry, which saw competitors like
> Boeing and Lockheed, slower to market with their own jetliners but perceived
> to be more safe, seize and hold the crown of king of the Jet Age for
> America.

Irony.

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ken
> "A jet airliner, in other words, was an airliner that could revolutionize
> aviation — and that meant the first company to bring one to market stood to
> make an absolutely fantastic amount of money."

Good on him for leading the innovation, but it seems obvious to me today that
"first" rarely means "most profitable" or "longest surviving".

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NamTaf
Stress concentration factors like this are the bane of all structural
engineering. Anything with too sharp an internal corner will concentrate
stress and lead to material fatigue failure if there's any stress there. Given
I've spent all morning staring at it in some finite element analysis work, I'm
going to ramble about it to give myself a break.

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](https://img3.gmdu.net/35780.0.jpg)
[2]:
[https://en.wikipedia.org/wiki/Force_lines](https://en.wikipedia.org/wiki/Force_lines)
[3]:
[https://en.wikipedia.org/wiki/Contour_line#Barometric_pressu...](https://en.wikipedia.org/wiki/Contour_line#Barometric_pressure)

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ganesh7
You could also say the great innovators did not understand the technology they
looted from the Germans. Or the similarities in design, e.g. integration of
turbines in the wings, of the Comet and Nazi jet planes are merely
coincidental I assume.

~~~
dTal
The placement of the engines has nothing to do with metal fatigue from
thousands of repeated 40,000 feet cruises with a pressurized cabin anyway; no
German jet fighter was pressurized, so it's doubtful that they understood the
technology either.

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.

~~~
ganesh7
Uhuh:

"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)."

~~~
dTal
What on Earth is your argument? Everyone was experimenting with cabin
pressurization - that wasn't even the first one. Are you seriously suggesting
that that specific aircraft - a pre-war, single-engine piston craft - in any
way influenced the design of the Comet?

[https://en.wikipedia.org/wiki/Cabin_pressurization#History](https://en.wikipedia.org/wiki/Cabin_pressurization#History)

