January 30, 2016 Leave a comment
Ever wondered why airplane windows are always round or slightly curved? Well actually they were not always round. In fact, till 1950’s they were rectangular in shape.
As the planes became more popular, planes needed to fly high, flying high meant less drag because the air density is lower in the upper atmosphere, and lesser drag meant less fuel is wasted. Flying high also meant lesser turbulence because the plane could fly above the turbulent lower atmosphere.
Certain design changes had to be incorporated to allow planes to fly higher. The cabin has to be pressurized to create a survivable environment for the passengers. Second, the cabin must be cylindrical in shape because cylinder can resist the new internal pressure. But one design flaw which was not corrected when the cabin pressurization was first introduced – the windows remained square. So, why is this a problem?
As a plane increases in altitude, the external atmospheric pressure lowers more than the internal cabin pressure. This causes a difference in the pressure in 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, stress begins to rise. Eventually the stress can rise so high so that the material can break. And there are a lot of factors which can elevate this stress. One of them is the shape of material. In planes, the shape of windows has a huge effect on the level of stress. Let’s see how.
Stress will flow smoothly through material, but if an obstacle is placed in its path, it needs to change direction and this causes pressure to build in certain directions. This is called stress concentration.
Comparing square windows to oval windows, we can see that square windows provide a larger barrier to the smooth flow of stress which means that the stress builds in the sharp corners of windows.
This elevated square stress formed due to square windows actually caused two planes to fall apart in the air in 1953, killing 56 people. It took engineers some time to realize that it was due to the square shape of windows. Since then, airplane windows have always been oval or round in shape.