Why are airplane windows rounded instead of square?

Nowadays, airplanes have become an indispensable means of transportation in our daily travel. Some friends may find that the windows of airplanes are round or oval when they are on the plane.

Why are airplane windows not the square windows we often see? Why are airplane windows designed this way? Is it just for aesthetic considerations? In fact, there is a lot of knowledge here. At the same time, it is also the experience gained through painful lessons.

In fact, the first airplane windows were not rounded, but square. In 1952, the Comet, developed by the British company De Havilland, was born. It was also the first civilian airliner powered by a jet engine. This aircraft was not only fast, but also had a sealed cockpit, which allowed it to fly higher, and its stability and comfort were unprecedented. People thought that the appearance of this airliner would be a historic breakthrough for airliners, but the reality was not satisfactory.

In the short period of 18 months from October 1952 to April 1954, 6 of the 17 Comets in flight had accidents, killing 99 passengers and crew members.

On January 10, 1954, a Comet took off from Rome's Ciampino Airport and disintegrated and crashed into the Mediterranean Sea 20 minutes after takeoff, killing all 35 people on board. No one witnessed the disaster, only some uncertain and incomplete radio signals were left, and there was no obvious reason to explain the crash of the plane. British Prime Minister Churchill at the time said that "no effort should be spared to uncover the mystery of the crash of the 'Comet'."

After some effort, investigators determined that metal fatigue was the likely cause of the accident, so they conducted tension tests on the fuselage surface. The test results showed that the pressure near the window corners was much higher than expected, and the pressure on the fuselage was also higher than previously tested and expected, simply because the window shape was square.

Why are square windows the culprit?

It turns out that because the Comet jetliner flies at a high speed and altitude, the air pressure in the closed cabin needs to be increased or decreased from time to time during takeoff and landing to ensure the comfort of passengers, which also causes the body to bend and expand invisibly. Over time, this will lead to metal fatigue and instability.

According to the knowledge of material mechanics, when a rectangular window is exposed to high pressure, stress is easily concentrated at the four corners, which are also the most vulnerable places, causing cracks inside. Under the combined effect of load and internal cracks, the fuselage material will deform or even break. Designing the window with rounded corners can reduce the problem of uneven stress, so rounded corner windows are now widely used in aircraft.

This design detail was so easily overlooked at the time that representatives from competing companies Boeing and Douglas said that their aircraft design engineers had not considered it, and if the Comet was not the first aircraft to disintegrate due to this, their aircraft might have been the first. So after this accident, the windows of the aircraft were designed with rounded corners.

What other "secrets" are there about airplane windows?

Careful friends may have also noticed that there are usually holes on the windows of airplanes. It sounds quite dangerous for an airplane flying at an altitude of 10,000 meters to have a hole in it for no reason. However, the tiny holes on airplane windows are actually specially designed. Not only are they not dangerous, but they also play a key role in protecting the safety of the people inside the airplane. They also have their own name, called " vents ."

In fact, every window on the plane is made of three layers of plexiglass, the main material of which is acrylic fiber. The outer layer of window glass is in direct contact with the outside world, the inner layer of window glass is located on the passenger side, and the ventilation holes are located on the middle layer of window.

Due to the difference in pressure inside and outside the cabin, with this small hole, the higher air pressure inside the cabin can directly act on the outermost layer of glass. When the regulating effect of the small hole reaches its limit, the first glass to break will also be the outermost layer. This ensures that the cabin is still sealed and intact. Moreover, it can also play a role in anti-fog and moisture removal, allowing us to enjoy the beautiful scenery above the clouds on the plane.

References

[1]https://mp.weixin.qq.com/s/jVRspidxP616jSQeY_qMQA

[2]https://mp.weixin.qq.com/s/t-1LuHrvS7aFiYMC3K3kDg

[3]https://mp.weixin.qq.com/s/4wdhPqiVae7TDnVDTHkUtg

[4]https://mp.weixin.qq.com/s/ouiXlma4gMmyWrOMFC7Wgw

Planning and production

Source: Science Popularization

Editor: Yang Yaping

Proofread by Xu Lai and Lin Lin