An Engineer, Polarized sunglasses and round airplane windows

When you induce stress on an object and see it through your Polaroid sunglass, then you witness these amazing rainbow patterns.


This property of a material where the changes in optical properties of a material is used to determine its stress distribution is known as Photoelasticity.

The simplest way to understand stress distribution in a material is by using a sponge and some wooden planks.

Observe how the lines on the sponge change when one applies the load:

Uniformly Distributed Load


                                  PC:  University of Manchester

Concentrated Load


When a concentrated load is applied, the lines closer to the loading point become radially distorted but the effect of this distortion dies out as moves away. *

If those lines made sense to you, then the lines that you see through your polarized sunglasses are no different.


      Photoelastic visualization of contact stresses on a marble in a C-clamp


In addition, the patterns that you observe are directly proportional to load that you apply. You vary the load, you vary the pattern observed.



Why are airplane windows round?

How does knowing the stress concentration help you at all ? When you are an Engineer, knowing the stress concentration tells you the critical stress points in a structure ( or points of probable easiest failure )


                     Stress concentration in Square v/s Oval windows

As this Real Engineering video goes on to explain when square windows are used in an aircraft, there is a greater accumulation of stress in the edges than the oval windows.

This increased stress, lead to cracks forming near the sharp edges of the window and causing major havoc, which is why all modern aircraft windows are round.

That being said, it is ironical to note that pilots on aircrafts are not supposed to wear Polaroid sunglasses while flying!

( Check out the previous post to know more)

* Saint- Venant’s principle


On Taj Mahal and Lift in airplanes

This is an interesting story that is probably popular among those in the aerospace community on how flaps help provide lift.


During the World War II, a C-87 cargo plane (pic above) was all set to take off from Agra airport, India. The pilot had specifically asked for a small load of fuel for takeoff.

(because the C-87 did not climb well when heavily loaded  )

But the ground crew accidentally filled it to its full capacity and  forgot to tell the pilot about it.


The pilot realized this only halfway through the runway and was already committed for take off.

With a three ton overload on the plane, the plane was heading for a fatal crash with one of the towers of the Taj Mahal which was being repaired at that time and was swarming with workmen.


The pilot gave full throttle but it still refused to rise up.

And in a desperate attempt, he lowered the flaps fully and instantaneously the plane ballooned upwards.


Surely, it lost some of its forward speed due to the increased drag. But it comfortably cleared the famous tomb, averting an impending disaster. So yeah, flaps on an airplane are no joke.

Have a great one!

* Why does lowering flaps increase lift?

** Physics of stall

*** Stories are great at linking words with experience. And this aids a lot in the learning process.