How to photograph shock waves ?

fyfd:

This week NASA released the first-ever image of shock waves interacting between two supersonic aircraft. It’s a stunning effort, requiring a cutting-edge version of a century-old photographic technique and perfect coordination between three airplanes – the two supersonic Air Force T-38s and the NASA B-200 King Air that captured the image. The T-38s are flying in formation, roughly 30 ft apart, and the interaction of their shock waves is distinctly visible. The otherwise straight lines curve sharply near their intersections. 

Fully capturing this kind of behavior in ground-based tests or in computer simulation is incredibly difficult, and engineers will no doubt be studying and comparing every one of these images with those smaller-scale counterparts. NASA developed this system as part of their ongoing project for commercial supersonic technologies. (Image credit: NASA Armstrong; submitted by multiple readers)

How do these images get captured?

It may not obvious as to how this image was generated because if you have heard about Schlieren imaging what you have in your head is a setup that looks something like:

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But how does Schelerin photography scale up to capturing moving objects in the sky?

Heat Haze

When viewing objects through the exhaust gases emanating from the nozzle of aircrafts, one can observe the image to be distorted.

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Hot air is less dense than cold air.

And this creates a gradient in the refractive index of the air

Light gets bent/distorted

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Method-01 : BOSCO ( Background-Oriented Schlieren using Celestial Objects )

You make the aircraft whose shock-wave that you would like to analyze pass across the sun in the sky.

You place a hydrogen alpha filter on your ground based telescope and observe this:

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                  Notice the ripples that pass through the sunspots

The different air density caused by the aircraft bends the specific wavelength of light from the sun. This allows us to see the density gradient like the case of our heat wave above.

We can now calculate how far each “speckle” on the sun moved, and that gives us the following Schlieren image.

Method-02: Airborne Background Oriented Schlieren Technique

In the previous technique how far each speckle of the sun moved was used for imaging. BUT you can also use any textured background pattern in general.

An aircraft with camera flies above the flight like so:

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The patterned ground now plays the role of the sun. Some versions of textures that are commonly are:

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The difficulty in this method is the Image processing that follows after the images have been taken. 

And one of the main reasons why the image that NASA has released is spectacular because NASA seems to have nailed the underlying processing involved.

Have a great day!

* More on Heat hazes

** More on BOSCO

*** Images from the following paper : Airborne Application of the Background Oriented Schlieren Technique to a Helicopter in Forward Flight

**** This post obviously oversimplifies the technique. A lot of research goes into the processing of these images. But the motive of the post was to give you an idea of the method used to capture the image, the underlying science goes much deeper than this post.

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.

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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

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                                  PC:  University of Manchester


Concentrated Load

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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.

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

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

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              Source

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 )

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                     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

Why do pilots use non polarized sunglasses ?

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Polarized lenses are not recommended for use in the aviation
environment.

While useful for blocking reflected light from horizontal
surfaces such as water or snow,

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polarization can reduce or eliminate the
visibility of instruments that incorporate anti-glare filters.

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Polarized lenses may also interfere with visibility through an aircraft
windscreen by enhancing striations in laminated materials (known as photoelasticity)

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

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and mask the
sparkle of light that reflects off shiny surfaces such as another
aircraft’s wing or windscreen, which can reduce the time a pilot has to
react in a “see-and-avoid” traffic situation.

– FAA

*Source:  Polarized v/s non polarized cockpit images

In 2015, a 777-200 made the Newyork-London route in 5 hours,16 minutes where the usual journey time is ~7 hours.

The flight reached ground speeds of up to 1200 km/h (745 mph),riding a powerful jet stream of up to 322 km/h (200 mph) tailwinds and breaking the sonic barrier ( 1224 km/h (761 mph)).

Tail and headwinds

The principle is analogous to those high school problems in relative velocity:

“A man rows a boat in a river. The velocity of the
boat is … Find the stream velocity”

If you are headed downstream i.e in the same direction as the river stream you will reach your destination faster than if you were rowing upstream.

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Similarly a tailwind is one that blows along the same direction of the aircraft increasing the net speed of the aircraft ,and headwind is one that blows in the opposite direction and slows the craft down.

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So, does this mean that if you are moving at v kmph and there is a headwind of -v kmph, you would just hover? Hell yeah!

Take a look at this video:

Wind shear

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A phenomenon known as ‘wind shear’ occurs when the wind speed changes abruptly, which can cause turbulence and rapid increase/decrease in velocity of flight.

This can be really challenging during landing since if the headwind turns tailwind, there is a possibility of the aircraft overshooting the runway due to the increased velocity.

What causes this ?

The aviation industry takes advantage of trade winds and jet streams in order to cut time off the flight and save fuel.

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Tradewinds are caused by the unequal heating of the atmosphere
at different latitudes and altitudes and by the effects of the Earth’s
rotation (Coriolis effect).

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                           Trade wind pattern. Credit: Earth Wind Map

Jet streams on the other hand are this narrow current of fast moving
winds in the upper troposphere flowing west to east. And riding one can
definitely make your travel time shorter.

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                               Jet streams in the northern hemisphere

As a result of jet streams, within North America  the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by more than that amount if it must fly west against it.

How do pilots know about this ?

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Pilots receive a weather briefing actively during flight. Included in the briefing is the best combination of jetstreams and other wind patterns that the pilot can take advantage of saving time and fuel.

Many airports have runways facing in different directions in order to allow the pilots to use the runway that faces the wind during take off/landing.

Have a great day!

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.

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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.

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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.

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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.

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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.

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The Smoke Angels.

Wingtip vortices shown in flare smoke left behind a C-17 Globemaster III.

How does an aircraft fly?
Think of it like this, due to the design of the wing, larger number of
air molecules are hitting the bottom portion of the aircraft than the
top.

As a result, a upward force acts on the wing, hence the wing lifts!

This works fine till we get to the wing tips.

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In
the wingtip, the air from a higher pressure wants to move to the region
of lower pressure. And as a result, this forms vortices ( fancy name
for the swirling motion of air ) known as Wingtip Vortex. ( because its
formed in the wing tips!!! )

And it is due to the ramification of this, that we obtain those gorgeous smoke angels. Pretty cool huh ?

Have a great day !

fuckyeahphysica:

What Is That Hole in the Tail of an Airplane?

When it comes to aviation, you just cannot take anything for granted. Everything serves a purpose, even the hole in the tail of the airplane.

APU ( Auxillary Power Unit )

The primary purpose of an APU on an aircraft is to provide power to start the main engines.

You gotta get them started and provide sufficient air compression for self sustaining operation, right ?

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For smaller jet engines, one can accomplish this by electric motors.

But when the engine is a colossal giant, you need something much more than a quotidian electric motor to get it to start running.

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What is it ?

The APU is essentially an air turbine motor, i.e a turbine that is used to produce power by using the air as a fluid

It is attached to the rear end of the aircraft. The hole in the rear end is used to direct the exhaust out of the aircraft i.e APU exhaust.

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The APU is started by a battery or other means. But once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft’s main engines.

What other purpose does the APU serve?

APUs are also used to run accessories while the engines are shut
down. This allows the cabin to be comfortable while the passengers are
boarding before the aircraft’s engines are started.

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                                             Source

Electrical power is
used to run systems for preflight checks. Some APUs are also connected
to a hydraulic pump, allowing crews to operate hydraulic equipment (such
as flight controls or flaps)
prior to engine start.

image

This function can also be used, on some
aircraft, as a backup in flight in case of engine or hydraulic failure.[

GPU ( Ground Power Unit )

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Many a times you can also find the aircraft connected by some wires. This is the Ground power unit
and supplies the aircraft with electricity while the generators or the auxiliary power unit (APU) are not running.

Some
airports reduce the use of APUs due to noise and pollution, and ground
power is used when possible.


What we behold as merely a small hole on the tail has so much depth to it .Pretty cool eh?

Have a good day!

image

What Is That Hole in the Tail of an Airplane?

When it comes to aviation, you just cannot take anything for granted. Everything serves a purpose, even the hole in the tail of the airplane.

APU ( Auxillary Power Unit )

The primary purpose of an APU on an aircraft is to provide power to start the main engines.

You gotta get them started and provide sufficient air compression for self sustaining operation, right ?

image

For smaller jet engines, one can accomplish this by electric motors.

But when the engine is a colossal giant, you need something much more than a quotidian electric motor to get it to start running.

image

What is it ?

The APU is essentially an air turbine motor, i.e a turbine that is used to produce power by using the air as a fluid

It is attached to the rear end of the aircraft. The hole in the rear end is used to direct the exhaust out of the aircraft i.e APU exhaust.

image

The APU is started by a battery or other means. But once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft’s main engines.

What other purpose does the APU serve?

APUs are also used to run accessories while the engines are shut
down. This allows the cabin to be comfortable while the passengers are
boarding before the aircraft’s engines are started.

image

                                             Source

Electrical power is
used to run systems for preflight checks. Some APUs are also connected
to a hydraulic pump, allowing crews to operate hydraulic equipment (such
as flight controls or flaps)
prior to engine start.

image

This function can also be used, on some
aircraft, as a backup in flight in case of engine or hydraulic failure.[

GPU ( Ground Power Unit )

image

Many a times you can also find the aircraft connected by some wires. This is the Ground power unit
and supplies the aircraft with electricity while the generators or the auxiliary power unit (APU) are not running.

Some
airports reduce the use of APUs due to noise and pollution, and ground
power is used when possible.


What we behold as merely a small hole on the tail has so much depth to it .Pretty cool eh?

Have a good day!

image

Landing on Aircraft Carriers.

fuckyeahphysica:

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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.

image

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.

image

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.

image

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.

image

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.

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                     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.

image


That’s all folks!

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

Landing on Aircraft Carriers.

image

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.

image

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.

image

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.

image

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.

image

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.

image

                     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.

image


That’s all folks!

image

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