Why are plane windows round?

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We often learn the most from our failures, this is particularly true for advancements in the field of engineering. Unfortunately for the engineers in the aviation industry, the prices to pay for failure are high. This will be a reoccurring theme in my videos, trust me. The flip-side of this unforgiving industry, is that it consistently provides learning opportunities for engineers, because failure is not an option when peoples lives are at risk.

One of the greatest examples of this occurred during the development of cabin pressurisation. The problems caused by cabin pressurisation didn’t develop until the introduction of the first commercial jet powered aircraft, The De Havilland Comet. It entered service in 1952 and initially proved to be a massive success, but just one year into service catastrophe struck. Three Comets suffered fatal mid-flight disintegrations and the entire fleet was grounded until the cause was identified.

The root of the problem was double-edged. The introduction of jet engines required planes to fly even higher in order to make the fuel hungry engines economically viable (less drag in the upper atmosphere means less fuel is needed). As a plane increases in altitude the external atmospheric pressure lowers to a greater extent than the internal cabin pressure. This creates a pressure differential that causes the fuselage to expand ever so slightly. Engineers accounted for this, but the effects of repeated pressure cycles over time were not well known at the time. Over thousands of cycles and metal begins to fatigue and cracks can form at high stress locations.

The effects of stress concentration were also not well understood at the time. Stress concentration occurs when the flow of stress is interrupted. Square windows, in contrast to modern oval windows, provide a significant barrier to the smooth flow of stress. Because of this stress peaks at the sharp corner of the window, and this is exactly where investigators determined the origin of failure to be.

These combined phenomenon proved to be fatal. Today all airliners feature oval windows to avoid this stress concentration and comprehensive fatigue testing is required before a plane can be approved by the FAA. We often learn the most from our failures, this is particularly true for advancements in the field of engineering. These are now two basic concepts that every materials engineer is taught, these events allowed us to further our understanding of materials and prevent further failures.

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So odd looking back on this. I spent a long weekend over Chinese New Year to learn animation to make this video. Feb 2015. About 3 months after my best friend died and I was really struggling mentally. Looking for meaning. Spent a few months iterating on it until I landed on this version. Then I quit my job in September 2015 to pursue YouTube, not out of bravery, but just complete desperation. 3 months before I uploaded this. What the fuck was I thinking?! I'm insanely lucky this all worked out. Thank you all for giving my life meaning.

RealEngineering

now this, this is how things should be explained. straight to the point, and not over explaining, or using words that are unfamiliar to the un-educated.

Great video, Real Engineering.

DeadlyTigers

I have never, ever, been interested in engineering. Im more of a biochem whiz. But then I stumbled on your videos, and you make this really interesting! Keep doing what your doing!

suki

Thug life... Been a pilot ten years and had no idea...

ragnarox

Visually appealing and very informative :) Keep it up! ;)

ChristyLau

liked and subbed for your non dragging outro ;)

Ropetable

That was quite interesting, thanks for the upload !

engineeringworld.

It's fun to see the first few videos of such big channels. It really emphasizes that many people start small.

mysteriousshadow

Taught my son's this last week while shopping at Costco! Your explanation was much better than my cell phone drawings. Right angles are avoided in any high stress steel structure for this reason. Thanks Real Engineering!

Native_love

I always wondered why do plane windows have a clear plastic window on the inside? I assume it's because the outer glass gets very cold and could be painful to touch, as well as attracting condensation.

psammiad

I've been studying Industrial engineering for a year and I already knew all this information!
Hope I'll be an engineer some day.

PeWaRaWNintendoFan

Thanks! That was, as they say, short & sweet. I *_LOVE_* the fact that I can go on YouTube, Google, or Wikipedia & basically look up anything I want (or need!) to know. I was born in '65, so I saw firsthand the revolution that allowed both video games & home computers (I owned everything from a Vic20 to a TRS80 to a Commodore 64C computer, & spent countless hours at the bowling alley & the mall arcade.) The evolution has been amazing. But the single biggest impact (on me, certainly) has been the dependence on - & trust in - the internet as a source. Not that I believe everything I read, of course! But I was just thinking the other day, how much it's changed the very way I think. If I'm talking to someone, or watching some show or movie or video, or listening to music - & it brings up a question I can't immediately answer - I know I can find out in a minute or two (& often do!) That's a *_huge_* change from having to wonder, or go ask somebody, or call a friend or your Mom, etc. I don't know if people my age consider it that way (anymore), & of course, younger people haven't experienced anything else. Anyway, thanks again! ℝ𝕚𝕜𝕜𝕚 𝕋𝕚𝕜𝕜𝕚.

richarddeese

I know this video has been out for 2 years, but I just wanted to let you know that it inspired me to delve deeper into material science learning. After seeing how the square windows could cause a plane to crash while circular ones wouldn't, I was curious about what kind of effects other shapes had on material. I was able to run an experiment using a tensile tester where I tested out different shaped holes and different patterns of holes. I learned a lot, including some things that surprised me, and it definitely broadened my worldview by changing the way I see materials around me. Thanks for making awesome videos like this that inspire deeper levels of learning. :)

KellinKingdom

I knew the de Havilland Comet would be mentioned the moment I saw this video's title.

I'm sure modern planes are much more efficient and better engineered, but from a purely aesthetic point of view, you can't deny there is something beautiful and almost magical about having the engine nacelles built into the wings.

YEdwardP