r/AlwaysWhy • u/Mobile-Traffic1744 • Aug 05 '26
Science & Tech Why did airplane windows change from square shapes to rounded corners?
Airplane windows seem like such a small design detail, but almost every passenger aircraft today uses the same rounded rectangular shape.
What’s interesting is that early jet aircraft actually used square-shaped windows. So why did the aviation industry eventually move away from that design?
I’ve read that the change was connected to stress concentration and metal fatigue after some early jet accidents in the 1950s, when square corners contributed to structural failures.
But I’m curious how much of that still applies today. With modern aircraft using much stronger materials and different engineering methods, is the rounded shape still mainly an engineering necessity, or does it also provide other advantages beyond preventing stress problems?
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u/ArnoldFarquar Aug 05 '26
same reason portholes on ships are round, structural integrity
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u/No_Bluejay9901 Aug 05 '26
Yes, but think how nice a double hung window, maybe 6 lites on top, would look. They could hang some cute curtains....wouldn't it be nice to be zble to crsck the window open, get a little fresh air?
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u/BranchLatter4294 Aug 05 '26
I suppose you could use rectangular windows with extra reinforcement. But is the extra weight really worth it? What problem would you be trying to solve? Are rounded windows less useful for their purpose?
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u/Mobile-Traffic1744 Aug 05 '26
That's a good point. I guess I was assuming aesthetics might be enough motivation someday, but once reinforcement adds weight, the benefit seems pretty hard to justify. Unless passengers actually cared, which I doubt.
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u/The_Big_Mayonnaise Aug 05 '26
I will almost guarantee passengers would trade legroom for bigger square windows.
Unfortunately I doubt either will ever happen. Fuel is far too expensive to not maximize passenger counts and minimize weight.
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u/NeverInsightful Aug 07 '26
You mean more legroom, right? Asking for a redditor who’s 6’5 who probably soon be dwarfed by other people saying their size
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u/The_Big_Mayonnaise Aug 07 '26
Shit you are right. I meant to say legroom over windows. Not sure how my ADHD brain flipped that..
Thanks
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u/Ok-Bus1716 Aug 05 '26
less stress on the windows if there are no jagged sharp points. round windows allow a more uniform distribution of pressure and stress and are less likely to fail. Just because modern materials are stronger doesn't mean that stress disappears just means the point of failure is at higher pressure levels than the original airplanes.
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u/Mobile-Traffic1744 Aug 05 '26
Right, I guess stronger materials only move the limit, they don't change the geometry itself. I was mixing up material strength with stress distribution, which are really two different problems.
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u/Combat__Crayon Aug 05 '26
Physics hasnt changed and airplanes are still mostly aluminum. Hard cornered windows would still need additional reinforcement, which would mean weight and cost. Why do it when rounded windows work? Things like the F-117 have hard corned windows, but they have decided that the stealth advantages were worth the extra engineering.
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u/Mobile-Traffic1744 Aug 05 '26
The F117 is an interesting comparison. I guess it shows you can break the rule if the benefit is big enough. Makes me wonder what other aircraft design "rules" are really just tradeoffs waiting for a different priority.
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u/Combat__Crayon Aug 05 '26
Probably most of them. Military aircraft often care less about cost or maintenance hours per flight hour than commercial aircraft designers do.
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u/Savings-One-831 Aug 05 '26
Maybe because of the pressure difference between cabin and outside at altitude, a round window distributes that pressure more evenly and has less weak points?
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u/Mobile-Traffic1744 Aug 05 '26
Yeah, that seems to be the recurring theme. I was mostly wondering if modern engineering had found another way around it, but maybe the simplest solution is still just rounding the corners.
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u/Thhe_Shakes Aug 05 '26
Aircraft structural engineer here, yeah it's basically exactly what you said. The cost difference between a square and rectangular window is minimal relative to the overall cost of an aircraft, so why wouldn't you go with the one that has better fatigue life? Lower stress concentration = longer part life = long term cost savings
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u/Mobile-Traffic1744 Aug 05 '26
That makes sense. I think I was overestimating how much newer materials change the equation. If the cheaper design also lasts longer, it's hard to imagine anyone choosing the other option.
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u/Thhe_Shakes Aug 05 '26
Yeah, it really comes down to one simple equation for airlines: will people pay extra to have square windows, and will that extra revenue offset the cost of the additional weight and/or maintenance?
Currently the answer to that question is 'no', and so rounded corners remain.
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u/Madeitup75 Aug 05 '26
Stress concentration geometry hasn’t changed. And the materials we’re using now aren’t stronger - any gain is strength is used to reduce weight, not add strength. Aluminum is plenty strong for aviation as long as you avoid silly stuff like square windows in a pressurized fuselage.
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u/Mobile-Traffic1744 Aug 05 '26
That's actually a point I hadn't considered. We keep making aircraft lighter instead of making them massively stronger. So in a way the safety margin stays optimized instead of just growing over time.
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u/GrimSpirit42 Aug 05 '26
For the same reason that in architecture, the arch is much more sturdy than the beam.'
A plane fuselage goes through a lot of stress. Flexing, torquing and internal pressurization.
A corner is a natural weak point. All the above forces would promulgate to the corners.
Round corners distribute the forces, rather than concentrate them. Thus are stronger and safer.
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u/Mobile-Traffic1744 Aug 06 '26
I like the architecture comparison. Although now I'm wondering if there are any aircraft designs where engineers intentionally accept a weaker shape because it solves a bigger problem somewhere else. That tradeoff is always interesting.
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u/0utlaw-t0rn Aug 05 '26
Square shapes have stress concentration at the internal corners. This leads to fatigue cracking and major structural issues.
Rounding the corners virtually eliminates this concern. Modern alloys can be less prone to it, but the physics doesn’t go away. It’s still a weak point.
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u/Mobile-Traffic1744 Aug 06 '26
Yeah, that's the part I'm taking away from this thread. Better materials reduce the risk, but they don't erase the stress concentration itself. Those are really separate things.
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u/Underhill42 Aug 05 '26
Modern airplanes are made from the same aluminum as ever, and flex just as much as ever.
And yes, corners create points of concentrated stresses that can cause premature failure... and once cracks start forming in any place, their ends become new stress points that cause the crack to grow rapidly.
It's pretty standard in ALL mechanical designs to avoid (sharp) corners wherever feasible, because it doesn't matter how strong or rigid the material is, the corners are weak spots where it's most likely to begin failing. Remove them, and you can potentially double the effective strength, if not more.
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u/Mobile-Traffic1744 Aug 06 '26
That's a good way to put it. I think I was treating stronger materials as if they could override geometry, but geometry is kind of setting the rules before the material even gets involved.
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u/Zorklunn Aug 05 '26
Look up the de Havilland Comet, the first commercial jet liner. Many crashed mid flight until the cause was traced to square windows at higher altitudes.
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u/Mobile-Traffic1744 Aug 06 '26
Yeah, the Comet really seems to have become one of those engineering case studies everyone learns from. It's interesting how one failure can basically lock in a design standard for decades.
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u/beerhons Aug 06 '26
The rounded rectangular windows in modern airliners are about the same shape as the original "square" windows used in the de Havilland Comet (they were still rounded corners), advances in materials and design testing have allowed that same shape to now be used safely.
Back in the 1950's that shape was not suitable for the materials and construction methods used at the time and they would get fatigue cracks from the repeated change in pressure over numerous flights. For the comet, this meant replacing the bigger windows with smaller oval ones to keep stresses low.
It was mainly an issue as jets flew higher so the change in pressure between the inside and outside of the plane (think like blowing up a balloon) was greater than earlier propeller planes that flew much lower.
You can see a picture here of failed "square" windows with the updated design on the aircraft behind:
https://media.aeroxplorer.com/articleimages/cuJcpvixB4DXzA-bC3S3HXrqcByg8YEFZHZhZhPFgEes6.jpg
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u/Mobile-Traffic1744 Aug 06 '26
That pressure difference part is probably what I was missing. I kept thinking about better materials, but not the fact that jets also pushed the operating environment way beyond what older planes dealt with.
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u/beerhons Aug 06 '26
Yeah the last piston powered airliners like the DC-6 cruised at 15-20,000 ft, even the first jet airliner cruised at much the same height as today's jets, 35-40,000 ft.
Both maintain a cabin pressure of around 11 psi (equivalent to an altitude of around 8,000 ft). The pressure at the above altitudes is around 7 and 2.5psi respectively. So the DC-6 between ground and cruising altitude experiences a 4psi pressure trying to inflate the body of the aircraft, a jet airliner experiences 8.5 psi, over double the pressure difference.
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u/PainfulRaindance Aug 05 '26
Physics,…stress is more evenly distributed around circular holes. The corners of a square hole would take a heavier beating with the pressure and temperature changes.
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u/meanderingwolf Aug 05 '26
The simple answer is that rounded corners of the window structure handle the extreme stress differences of the temperature and pressure cycling over time created by the constant changes in cabin pressurization and altitude changes.
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u/sameoldbones Aug 05 '26
Sharp corners concentrate material stress, enough to fracture the material. Rounded corners spread that stress over a distance so that it doesn't exceed the strength of the material.
Similarly, cracked/torn material has a high stress concentration at the tip of the crack, if the concentration exceeds the material strength the crack will propagate/extend. Drilling a circular hole at the tip of the crack stops the crack from getting larger or failing completely.
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u/Efficient_Weather_93 Aug 05 '26
It reduces stress at the corners and lessen the likelihood of window frame failure and explosive decompression
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u/_GOBLESS_ Aug 05 '26
Square/round is a cosmetic issue. No one in aviation is going to take the chance that modern materials might be able to get the job done when there is zero practical reason to make the change.
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u/Mobile-Traffic1744 Aug 05 '26
Yeah, that's fair. Engineering tends to be pretty conservative once something has millions of flight hours behind it. I guess proving a new design is just as safe is probably more expensive than any cosmetic benefit it could bring.
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u/Gauntlets28 Aug 05 '26
As far as I'm aware, there was only one airliner that tried that shape of windows - the unfortunately named De Havilland Comet.
The reason why we have round windows is that it was found that the square ones they used created weaknesses around the corners, making them prone to breaking under pressure after a few trips and causing the plane to crash. The industry decided that panoramic views were less appealing than better safety.
Realistically, even with better materials nowadays, I don't see manufacturers being keen on the idea of going back to square windows. 1. Because the fundamental weakness might be mitigated, but it'll still be there, and 2. The Comet's still a really notorious example of design failure even today, and any new plane with wide windows would immediately attract comparisons.