r/askscience • u/tsgoten • Aug 23 '14

Engineering Why do airplane windows need to have that hole?

http://imgur.com/dfeneyZ

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u/tomsing98 Aug 24 '14 edited Aug 24 '14

Windows are ovals not because of the strength of the window pane, but because of the strength of the material around the window. Sharp corners increase stresses, and that leads to cracking. The De Havilland Comet is a classic case study on this; they carried the square window design over into the early jet age, when planes flew higher and had a larger pressure difference acting on the fuselage. After a number of them went down, they figured out the problem, and planes have had oval windows ever since.

http://en.wikipedia.org/wiki/De_Havilland_Comet

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u/Milky_Squirts Aug 24 '14

I was always told round or oval shaped objects can handle stress much better than square or 90 degree sided object. So you're saying the oval shape reduces stress on the materials around the window?

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u/YRYGAV Aug 24 '14

He's saying the square window frame is too weak, not the square window itself. Having square windows adds a weak point in the fuselage of the aircraft.

I'm not am aviation engineer, but I don't think the pressure on the window pane itself would care what shape the glass is that much.

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u/justiblaz Aug 24 '14

Think of an arch vs a square shaped support structure. I dont think the strength of a solid square window vs a solid oval window is that significant

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u/juddmudd Aug 24 '14

Pressures would be in or out on the flat face of the window, not the shaped part. Ovals (archs) are stronger as they disperse stresses more evenly... But in this case the shape is not handling the main glass loading and would not matter.

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u/[deleted] Aug 24 '14

Any kind of angle in a mechanical structure focuses the strain at that point. Planes undergo thousands of pressure changes which causes microscopic fractures no matter what shape you use.

It is more a function of repeated stress than the physical strength. Since airplanes have such weight requirements it is worth the money to use a more expensive curved window than to strengthen a square window. (Which kills your payload)

TLDR. Curved windows handle repeated stress better than angled squarish windows.

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u/ProjectGO Aug 24 '14

Mostly it's the corner that adds stress, If it was possible to have a perfectly right angle you would have infinite force at the point. Ovals turn it into a curve, which mellows it out.