Science Puzzle
Why Are Aeroplane Windows Round?
Look at the windows on any airliner today and you'll see they have rounded corners. The world's first jet airliner, the de Havilland Comet, entered service in 1952 with windows that were nearly square.
Why did aircraft designers abandon square windows?
The Answer
Sharp corners are where cracks begin. Rounded corners spread the load out and stop them starting.
At cruising height the air outside is far too thin to breathe, so the cabin is pumped up with air. That pushes the fuselage outwards and stretches its metal skin a little on every flight, and it relaxes again on landing. Wherever there's a hole in the skin, such as a window, the stretching force has to flow round it. At a smooth curve it flows round gently. At a sharp corner it crowds into a tiny area, and the stress there can be several times higher than anywhere else.
One stretch does no harm. Thousands of them can. Each cycle can grow a microscopic crack at the most stressed spot a little further, a process called metal fatigue. In 1954 two Comets broke apart in flight within three months of each other. Investigators sealed a whole Comet fuselage in a water tank and pumped it up and let it down again and again. After the equivalent of about 3,000 flights it split open, with the failure starting at the corner of an opening in the cabin.
Every airliner since has had rounded windows and carefully shaped openings. You can see the same idea at home: a sheet of paper with a small notch in its edge tears easily from the notch.
The principle: Stress concentration. Load flowing round a hole crowds together at sharp corners, so cracks start there, while rounded corners spread the stress and resist fatigue.