# 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:

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)

# Earth’s rotation and Space shuttle launches

Last week we were talking about wind patterns and how they affect flight time. But it is also worth mentioning that Space shuttles are launched almost at all times from West to East to take advantage of the earth’s rotation

## How does earth’s rotation affect shuttles ?

Earth is a spherical body rotating with some angular velocity. And as a result of this, the equator is rotating at a higher velocity than the poles. By launching a space shuttle from the equator you are getting a ‘speed boost’.

This means that if a shuttle is launched from the pole, it has to accelerate from 0 to 17000mph to reach orbital velocity.

But if a shuttle is launched from the equator, it only needs to accelerate from 1025 to 17000mph. (that 1025mph initial velocity is given by the earth free of charge)

This saves valuable amount of fuel required for propulsion

## Polar Orbits

Not all rockets are launched from the west to east and the direction is determined by the purpose of its payload.

The satellites that are used for mapping for instance follow a Polar Orbit i.e they move from north to south or vice versa and therefore during launch they cannot take advantage of the earth’s rotation.

## Florida or California

Another characteristic of launching satellites is that the launching
stations are generally located near the coast just in
case of failure of the launch, the satellite falls in an uninhabited area.

NASA primarily uses Kennedy Space Center, Florida for east-west launches and Vandenberg Base California for polar orbits for the very same reason. ***

Rocket science is just truly breathtaking.

*** This statement holds true for most launches.

## The Physics of the ‘Stall’

The proposition that more the angle of attack, more the lift does not hold at all angles.

At about 14 degrees, something weird happens and the aircraft instead of soaring the skies starts to plummet to the ground.

When this happens it is known as a stall.

## What causes Lift ?

The main thing to know is that a difference in pressure across the
wing–low pressure over the top and higher pressure below–creates the net
upward force we call lift.

Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .

## The Concept of a Boundary Layer (BL)

There is a high chance that you might have heard this word even in a casual conversation about wings and that’s because its an important concept in the context of aerodynamics and associated fields.

To understand the physics of a stall, lets consider the interaction of a moving air on a flat plate.

The nature of airflow over a wing/plate is the result of stickiness or viscosity of air.

The first layer sticks to the wing/plate not moving at all.

The second layer in frictional contact with the first moves slowly over it.

And the third layer moves somewhat faster than the second

Thus layer by later the flow builds up to the free stream velocity or airspeed. These layers of flow are known as boundary layers.

What happens to the BL during a stall?

During a stall, these successive tiers of air that form the boundary layer lose their gripping on the surface and break away into turbulence.

( what i mean by turbulence is the chaotic wiggling of the test leads attached to the wing in the animation )

It takes a pressure difference between the top and bottom parts of the wing in order to produce lift. But when the flow of air becomes turbulent ( i.e during a stall ), this pressure difference is no longer established.

As a result of which, the lift drastically decreases and the aircraft starts dropping to the ground.

## How to get out of a stall ?

Stalls can cause problems only when the pilot is not aware that the aircraft is stalling. ( Unlikely but has caused accidents in yester times )

As the airplane loses altitude, its nose dips down and airspeed picks up quickly. This restores the lift and the pilot would be able to regain control and bring the aero-plane into level flight.

## How are stalls detected ?

On light aircraft there is a reed (much like used on a musical wind
instrument) mounted on one wing root, which is angled such that at the
Angle of Attack which would cause a stall, the reed “plays” which can be
heard in the cockpit.

Here is a view of where this system is mounted on a Cessna

On some aircrafts, it is a similar principal, however instead of a
reed, it uses a fin which at critical AoA pushes a micro-switch which
activates a buzzer/horn inside the cockpit.

Here is the assembly on a Beech 18

Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes  to supply input to flight computers for the purpose of calculating AoA.

Source

Review:

A lot of important stuff regarding aerodynamics in this post. Here’s a summary of the post:

Boundary Layer concept  — >  Why do aircrafts stall ? — >  How to get out of one  — > How are stalls detected ?

That’s all folks!

Hope you enjoyed today’s post and learnt something new.

Have a good one !

This post covers the fundamental principles from which the subsequent posts queued up for this weekend are derived from. Stay tuned.. It is gonna be wild ride.

# 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!

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

In mathematics there is a concept known as ‘Conformal Mapping’ which allows you convert a given shape to a completely different one by making a transformation.

In the joukowski transform you take all the points on a circle and apply the following transform:

And the resulting transformed points resemble an aerofoil shape. Pretty cool huh ?

** Conformal mappings are a really cool topic in complex analysis but also equally extensive. If you want to know more about them click here

## Hangout session with a Rocket Scientist!

We put forth all your questions that you had asked to Marielle during yesterday’s hangout session. It was truly a transcending experience, we hope you enjoyed it as well.

There were a couple of questions that went unanswered due to time constraints. They will be answered by Marielle on her tumblr – missaerospace.

Thank you so much, Marielle for taking the time to answer all the questions and sharing other valuable information and experiences that would aid rocket science aspirants. 😀

EDIT : Marielle has cordially answered all the other questions asked during the hangout session. you can check them out here

# Landing on Aircraft Carriers.

Landing on an aircraft carrier is an extremely challenging task. A
shortened moving runway surrounded by the mighty oceans makes it only
harder.

But pilots( especially navy ) are trained to land on aircraft carriers and a couple of simple engineering designs aid in this enterprise;

## The arresting gear

Arresting gear, or arrestor gear, describes mechanical systems used to rapidly decelerate an aircraft as it lands.

There are 4 cables in separated lines that the pilots aim for whilst landing.

When the tailhook of the jet engages with the wire, the aircraft’s kinetic energy is transferred to hydraulic damping systems, this slows down the aircraft tremendously.

## What if they miss?

It does happen! Pilots do miss the line while attempting to land.

They keep full speed until they are 100% sure that they hook up  ( in case
they miss the cables ). Which means they are still at full speed for
about 2 seconds at the end with the cable extended to max.﻿

If they don’t hook up to the line, they simply go around.

## Vertical Landing

Some jets also have the ability to vertically land on the flight deck.

They are known as VTOL’s ( Vertical take off and landing ) aircrafts.They can hover, take off, and land vertically.

## Catching aircrafts with a net

The barricade/barrier system/crash net is quite literally a net that is used to slow down an aircraft.

It is employed only under emergency situations or for aircrafts that operate without a tailhook.

A successful landing without a nosewheel

The barricade webbing engages the wings of the landing aircraft, wherein
energy is transmitted from the barricade webbing through the purchase
cable to the arresting engine.

That’s all folks!

Hope you guys enjoyed this post. Have a good one!

Aircrafts.. Glazing through the skies with their huge wings, transporting people and goods across frontiers. We see/listen to these beasts soaring through the skies on a regular basis.

But almost every time what we are referring to an aircraft, they are fixed-wing aircrafts. i.e their wing configuration does not change flight..

## Behold the Variable Sweep Wing

But there are also wing configurations that can be changed during flight and these are known as Variable-sweep wings.

A variable-sweep wing, colloquially known as a “swing wing”, is an airplane wing,
or set of wings, that may be swept back and then returned to its
original position during flight.

Dassault Mirage G with swept wing (top)

Typically, a swept wing
is more suitable for high speeds, while an un-swept wing is suitable for
lower speeds, allowing the aircraft to carry more fuel and/or payload,
as well as improving field performance.

A variable-sweep wing allows a
pilot to select the correct wing configuration for the plane’s intended
speed.

F-111

## Where is it used ?

There is a good chance that you haven’t heard about this, and thats because in this day and age they have been superseded by other advanced versions.

One of the major problems with this concept is that swept wings add weight, radar cross section, and additional mechanical maintenance.

These aircrafts were built for speed. By altering the swept angle, they were able to achieve higher speeds, which was the need of the hour back in those days.

But with
today’s engine technology, stealth tech, and radar systems, our pilots
can maneuver or avoid threats altogether through surveillance aircraft,
rather than depend strictly on speed.

Pretty Cool eh?

## The Physics of the ‘Stall’

The proposition that more the angle of attack, more the lift does not hold at all angles.

At about 14 degrees, something weird happens and the aircraft instead of soaring the skies starts to plummet to the ground.

When this happens it is known as a stall.

## What causes Lift ?

The main thing to know is that a difference in pressure across the
wing–low pressure over the top and higher pressure below–creates the net
upward force we call lift.

Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .

## The Concept of a Boundary Layer (BL)

There is a high chance that you might have heard this word even in a casual conversation about wings and that’s because its an important concept in the context of aerodynamics and associated fields.

To understand the physics of a stall, lets consider the interaction of a moving air on a flat plate.

The nature of airflow over a wing/plate is the result of stickiness or viscosity of air.

The first layer sticks to the wing/plate not moving at all.

The second layer in frictional contact with the first moves slowly over it.

And the third layer moves somewhat faster than the second

Thus layer by later the flow builds up to the free stream velocity or airspeed. These layers of flow are known as boundary layers.

What happens to the BL during a stall?

During a stall, these successive tiers of air that form the boundary layer lose their gripping on the surface and break away into turbulence.

( what i mean by turbulence is the chaotic wiggling of the test leads attached to the wing in the animation )

It takes a pressure difference between the top and bottom parts of the wing in order to produce lift. But when the flow of air becomes turbulent ( i.e during a stall ), this pressure difference is no longer established.

As a result of which, the lift drastically decreases and the aircraft starts dropping to the ground.

## How to get out of a stall ?

Stalls can cause problems only when the pilot is not aware that the aircraft is stalling. ( Unlikely but has caused accidents in yester times )

As the airplane loses altitude, its nose dips down and airspeed picks up quickly. This restores the lift and the pilot would be able to regain control and bring the aero-plane into level flight.

## How are stalls detected ?

On light aircraft there is a reed (much like used on a musical wind
instrument) mounted on one wing root, which is angled such that at the
Angle of Attack which would cause a stall, the reed “plays” which can be
heard in the cockpit.

Here is a view of where this system is mounted on a Cessna

On some aircrafts, it is a similar principal, however instead of a
reed, it uses a fin which at critical AoA pushes a micro-switch which
activates a buzzer/horn inside the cockpit.

Here is the assembly on a Beech 18

Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes  to supply input to flight computers for the purpose of calculating AoA.

Source

Review:

A lot of important stuff regarding aerodynamics in this post. Here’s a summary of the post:

Boundary Layer concept  — >  Why do aircrafts stall ? — >  How to get out of one  — > How are stalls detected ?

That’s all folks!

Hope you enjoyed today’s post and learnt something new.

Have a good one !

I am almost every time put in a trance whilst spectating an aircraft/jet takeoff . There is always something interesting in the occurrence that makes me go nuts!

And this time around, there are these series of rings that one can see in the exhaust plume of a jet engine when it takes off ( usually when the afterburner is on).

I had no clue about the phenomenon nor did I know how to express it in ‘search engine’ terms to find a match.

But upon discussion with some of friends, I was shown this video of a space shuttle launch that seemed to produce a similar pattern.

Hmm.. Interesting

## Shock Diamonds

These set of rings/disks that are formed in the exhaust plume are known as Shock Diamonds or Mach discs ( and by many more names ).

And usually occurs at low altitudes when the pressure of the exhaust plume is lower than the atmospheric pressure.

## How does it form ?

Since the atmospheric pressure is higher than the exhaust, it will squeeze it inward. This compresses the exhaust increasing its pressure.

The increased pressure also instills a increased temperature.

As a result, ignites any excess fuel present in the exhaust making it burn. It is this burning that makes the shock diamond glow.

Source

The pressure is now more than the atmospheric pressure, and the exhaust gases start to expand out.

Over time,
the process of compression and expansion repeats itself until the exhaust pressure becomes the same as the ambient
atmospheric pressure.

In other words, the flow will repeatedly contract and expand while gradually equalizing the
pressure difference between the exhaust and the atmosphere.

The same occurs in rocket engines as well.

## What if?

What if the atmospheric pressure is less than the exhaust plume ( like at higher altitudes ), would we still see shock diamonds ?

Yup, we would! And here’s a picture of it too ( The Bell X-1 at speeds close to Mach 1 )

The same phenomenon as discussed above occurs except that the cycle starts with the exhaust gases expanding to atmospheric pressure first.

Did you enjoy this post?

There is an extensive explanation of shock diamonds given by shock waves which this post does not cover.

And this beckons the start of Supersonic Fluid Dynamics – a marvelous field of its own. If this captivated you, it is definitely worth a google.

Cheers!