Why Airplane Windows Are Round

  • Wednesday, May 31, 2023

In her response to the opinion letter 'Congratulations, Democrats,' Mrs. Brenda Washington includes this seemingly unrelated thought-provoker:  "Here's a trivia for you two old goats. Up until the 1950s airplane windows were square. Do you know why airplane windows would later be made round, and why?" Mrs. Washington and I are loosely acquainted and, I believe, casually friendly.  I'll soon be 80 years old, so I surely qualify as an 'old goat.'  And I'm an experienced mechanical engineer, a genuine honest Purdue Boilermaker -- so it seems fair for me to try to answer her question.  Publicly.  But I suspect I'm setting myself up to be labeled dumber than her preschool great-nephew out West.

My immediate thought was, and remains, airplane windows are round for a legitimate reason, and it's not merely fashion or style.  The reason airplane windows are round is to minimize the structural stresses associated with them.  The basic fuselage of a large passenger plane is a tube, which endures a variety of forces and stresses.  There is the obvious stress of holding the wing, engines, and tail together; there are lots of forces involved there.  Add the forces due to flying, and the support of internal weights of fuel, passengers, cargo, etc.  And for high-flying passenger planes, the interior of that big tube-in-the-sky is pressurized so the passengers can breathe easily.  The flying plane's fuselage is continually being bent and twisted in every direction, besides its tendency to explode due to internal pressure.

Cutting holes anywhere in that tube, for doors and windows, sounds kind of dumb, huh?  It removes material that is essential to its strength, to its survival; holes obviously weaken the whole affair.  Engineers eventually learned (generations ago) that, while round holes are bad enough, square (or rectangular) holes are way more bad.  A round (circular or elliptical) hole-in-the-wall allows structural forces to 'flow' around it smoothly; yes, the round hole weakens the tube, but minimally.  A hole with straight edges and sharp corners, though, concentrates forces and stresses near those corners; such sharp corners in mechanical parts are called 'stress risers' or 'stress raisers' for a good reason. 

I suspect really up-close inspection of early airliners' 'square' windows will reveal that they actually had rounded corners -- rounding off the corners of a hole is one way to reduce the concentration of stresses, while maintaining the traditional 'square' appearance of windows in our houses and buildings.  Cosmetic trim around the windows may have perfect square corners but, trust me, the underlying metal parts have radiused inside corners.

By the late 1950s high-flying jet airliners were taking over, and old photos reveal they had straight-edged windows with large radius corners -- sort of semi-square, semi-round.  And as altitudes, speeds, loads, and sizes of the planes increased, passenger windows became more and more nearly circular -- elliptical, at least, everywhere curved with no sharp corners anywhere.  Purely round windows are the simplest way to minimize the concentration of stresses in the airplane fuselage.  It's truly pure and simple.  I doubt it's a matter of style or economics, certainly not done to save on window material, but it is the best way to maintain the structural integrity that everyone in any airplane wants as they gaze out those round windows.

That's my honest engineer's answer, but Mrs. Washington's original question still has me scratching my head.  She asked 'why,' 'and why?'  Two whys; why twice.  So I keep waiting for the other shoe to drop.  First, I'm afraid I missed something, because her usual opinions introduce a racial element even where none was originally present.  Or maybe, in this case, it's just a little kid's trick question that has nothing to do with engineering.  If it's an honest question, my answer stands; if it's anything else, all bets are off. 

If Brenda hollers 'Gotcha!' and makes me look dumber than a 5-year-old, so be it; she won't be the first to do that, because I've always been way too willing and cooperative in those experiments.  And it looks like I've gone and done it again, huh?

Larry Cloud

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