Why are plane windows round? This seems like a simple question, but let's take a look.
As passenger planes became more popular, planes needed to fly higher. Flying high means less drag because the air density is lower in the upper atmosphere, so less fuel is wasted. And flying high also means a more comfortable flight because the plane can now fly above the turbulent lower atmosphere. But some design changes need to be made to allow this.
First, the cabin has to be pressurized to create a survivable environment for the passengers. Second, the cabin must be cylindrical in shape because a cylinder can resist the new internal pressure. But one design flaw was not corrected when cabin pressurization was first introduced. The windows remained square. So why is this a problem?
As the plane increases in altitude, the external atmospheric pressure lowers more than the internal cabin pressure. This creates a pressure difference between the inside and outside of the plane, causing the plane to expand ever so slightly. When a material changes shape like this, stress is created in the material. As the material is stretched more and more, this stress begins to rise. Eventually this stress can rise so high that the material breaks.
And there are a lot of factors that can elevate this stress. One of them is the shape of the material. In planes, the shape of the windows has a huge effect on the level of stress. Stress will flow smoothly through material, but if an obstacle is placed in its path, like a window, it needs to change direction, and this causes the pressure to build in certain locations. This is called stress concentration.
Comparing modern-day oval windows and square windows, you can see that the square windows provide a larger barrier through the smooth flow of stress. And this means that the stress builds in the sharp corners of the window. This elevated stress created by the square windows actually caused cracks to form in the de Havilland Comet, causing three tragic crashes. The investigations that followed led to our greater understanding of stress concentration, and that is why all plane windows are now oval in shape.
- 「非常、極其」- Ever So
This creates a pressure difference between the inside and outside of the plane, causing the plane to expand ever so slightly.
- 「導致、通往」- Lead To
The investigations that followed led to our greater understanding of stress concentration, and that is why all plane windows are now oval in shape.