Why airplane windows are round?

Did you ever think why airplane windows are round? Why not they are square or some other shape?

As airplanes became more popular in mid 20th century. So, they started flying in higher altitude to save fuel. Flying high means less drag, because air density is lower in upper atmosphere. So, less fuel is wasted. Flying high leads to comfortable flight, as less turbulence is there.

But, to make passengers survive at higher altitude, cabin must be pressurized. So, to make this possible, the shape of cabin is changed to cylindrical shape. So that it can handle the internal pressure. But, one design flaw is not corrected when the cabin pressurization is first introduced. It is square windows.

When the airplane goes into higher altitude, the external atmospheric pressure is lower as compared to internal cabin pressure. This creates the pressure difference inside and outside the airplane. This makes the airplane to expand slightly.

When any material changes its shape like this, stress is developed in that material. As the material is stretched more and more, stress increases. Eventually, when stress rises so high, then material breaks.

There are lots of factors which elevates this stress. One of them is shape of the window. In airplanes, shape of the window has huge effect on the level of stress. If no window is present, then stress passes smoothly. But when any obstacle like window is present, then stress have to change its direction and hence increases. This is called Stress Concentration.

In square shaped windows, the sharp corner of the window develops higher stress, so they cause cracks in airplane. In modern day round window, window provide smooth path to stress and hence doesn’t cause cracks.

This elevated stress created by the square window leads to cracks which is the reason of crashes of de Havilland Comet (1st commercial jet airline) in early 1950s. Futher investigation found the real reason of crash, so in modern airplanes we have round oval shaped window.

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