Why are train windows and airplane portholes made with rounded corners

Everyone knows that in modern passenger cars, the windows have rounded corners. Portholes in airplanes tend to be round or even completely round. Why do designers choose this form? Is it only related to the design or are there more serious reasons?

High demands are placed on passenger cars. They must be strong, have a rigid structure and a long service life. It is not so easy to achieve this — during operation, the wagons are subjected to high loads of various types.

The most dangerous are dynamic loads that can destroy the car right during the trip. A one-piece carriage with no openings would be an ideal solution. But it is impossible to imagine passenger cars without windows. It is they who become the weakest points of the design.

For example, the windows of old houses can be cited. If you look closely, you can see that cracks originate in the corners of many of them, sometimes very long and wide. This is due to the fact that serious mechanical stresses occur in the corners of the windows of buildings. They are the ones that eventually cause the destruction of bricks, metal and even concrete.

But the structures of buildings do not experience dynamic loads and therefore the share of wagons accounts for more severe tests. To reduce the impact of stresses, the corners of window openings are rounded. This solution significantly extends the service life of the wagons, and in addition gives them a modern look.

In the case of airplanes, the stresses are even more dangerous and unpredictable. The first Comet jet passenger airliners in the UK were produced with square windows. This led to at least two plane crashes with human casualties. Fatigue of the material in the corners of the windows led to cracks and destruction of the fuselage. The planes simply fell apart in the air, leaving people with no chance of escape.

Therefore, the designers decided to use rounded porthole windows that do not have weak points in the form of corners. This fuselage design is considered classic today and is used by all aircraft and helicopter manufacturers.