What is aircraft thrust?

What is aircraft thrust? 

What is thrust

An average aircraft weighs approximately 450 tons (Here I’m referring to the takeoff weight) which is approximately the weight of 900 whale testicles.
Now how is a body that heavy going to be able to propel itself to achieve lift?
A little background check on the little devil that decide how fast this man-made birdie can move...

What is Thrust?


Thrust: The holy matrimony of the Pressure formula and Force equilibrium gave birth to this simplified formula for thrust.
Image result for turbojet thrust equation nasa
Here the mass flow exiting the engine is the sum of the mass flow rate of air and the mass flow rate of fuel(added at the burner). But since this mass flow rate of fuel is relatively small, we will ignore the term in our simplified equation.

T = Ma(Ve-Vi)
From this, the science humans found that thrust is directly proportional to :
 1.mass flow rate of air (mass of air flowing per unit time...we like bringing time into the picture when we work with a flow in motion aka fluid dynamics, in another article we’ll tell you why)
 2. the velocity difference between the entrance to the engine and the exit of the engine. (The thrust equation is derived considering the engine as the isolated system)

It doesn’t take a rocket scientist to know that if we increase one of the two or both of them were going to get what we want. WHAT DO WE WANT? THRUST! WHEN DO WE WANT IT? NOW!
How do we achieve in increasing one or more of these two quantities?
Suck. Squeeze. Bang. Blow.

Image result for heinkel he 178
Heinkel 178, Pleasure to meet you.

Imagine a Heinkel 178 cruising in the air on a late August morning over the greenest fields of 1939 Germany. The test pilot Erich warsitz sits in his cockpit not knowing that he would go down in history as the first man to fly an aircraft powered by a turbojet engine. The gas turbine engine was a man-made marvel and took on the world war by a storm, putting its piston predecessor to shame.
Sounds radical doesn’t it, but what really happens on the inside?
There’s nothing left to do but to march through the engine, one component at a time.
Image result for turbojet engine
Just your average jet engine.

Suck: The diffuser is a welcome mat to the engine. It’s job? To reduce the velocity of the incoming air. Why? Because slower air is easily tamed by the components following it. How does it do this? It’s a diverging section by geometry. The continuity equation would tell you that area and velocity are inversely proportional. So when the area increases in the section, velocity will decrease creating a slower flow.
This doesn’t really sound like a sucking mechanism does it? Well, the sucking happens because of a pressure difference. You see a flow is established when there is a difference in pressure. High pressure to low pressure: that’s all the navigation that fluid particles comprehend. (Why? In the microscale all atoms look to settle in a place of a lower energy, lower pressure means it has lower internal energy associated with it)So in order to suck in the air, the diffuser must be at a lower pressure level than the atmospheric pressure of free stream outside. How do you make this happen? By geometry. The more the wall of the diffuser diverges, the more the air that sticks to its boundary begins to feel like it’s not wanted anymore (nobody likes a clingy neighbour...get the hint bob)
So the wall is put at an angle such that, at a distance from the diffuser inlet, the flow separates from the boundary swirling inwards and making a low-pressure region there. This low pressure pulls the higher pressure free stream towards it and they make sweet sweet love. Thus generating a flow.

Squeeze: the second is the compressor. It’s job? To increase the pressure of the air (compress) how does it achieve this? It’s composed of a series of rotating and static blades called rotor and stator. The energy equation (Bernoulli equation) tells us that the sum of pressure head, kinetic head, and a gravitational head is a constant.
Or energy is conserved. The rotor creates velocity increase (by virtue of its rotation) and this translates to a pressure increase at the stator, satisfying the conservation of energy. Moral: to properly satisfy, one must properly squeeze.

Bang: Thirdly comes the combustor. The slow squeezed air comes into the combustor hoping to get hot. Combustor’s job? To create kinetic energy increase. When fuel and air are mixed and set on fire by a spark plug, this results in a massive increase in kinetic energy. (And temperature duh) how does this kinetic energy increase? Fuel and air mixing and being set on fire is a chemical reaction. So this chemical energy is going to be converted into kinetic energy. This hot faster-moving gas goes on to the next component: Turbine. The Siamese twin of the compressor with regards to mechanical parts does the exact opposite of what the compressor does, decreasing the pressure.
Hot fact: The turbine and the compressor are Siamese twins in another aspect. They’re joined at the hip. A shaft connects the turbine to the compressor driving it.
Wait, what?
This would mean that once a sustained flow is established in the engine, the turbine and the compressor work in synergy: driving each other. That’s exactly what happens. The kinetic energy produced in the combustor is what really produces the propulsive thrust. Without the combustor, it would just be a sustained operation of a compressor turbine system. And without the compressor turbine system, the engine would only be an uncontrolled explosion courtesy of the combustor. Hot damn.

Blow: the exhaust is a nozzle which implies that it is a converging geometry. It increases the velocity by the same principle employed at the diffuser. The faster air is expelled through the nozzle thus increasing the difference in velocity portion of the thrust formula. How do we increase the mass flow rate? In a turbofan configuration, there is a portion of air which is allowed to pass along the side of the engine, not through the various components: bypassing it. Kinda like a get out of jail free card, you don’t need to be sucked squeezed banged and blew, to be a reliable contributor to the system. Or so my mother tells me.
Now addressing the question that was posted. How does this help with a lift?

Image result for lift equation
Lift formula

The formula for lift shows that lift and the forward velocity are like two peas in a pod, one increased, the other does too.  When thrust increases, the forward velocity increases...which leads to the increase in lift. (It should be noted that you must increase the other parameters of area and angle of attack to attain enough lift to take off ). That's exactly why you see aircrafts taking a running start before they lift off from the runway.

And now you know how to get 900 whale testicles to take flight.

Thanks for reading!

How are Military aircrafts named?

Ambiguous Alphabets Explained

How are Military Aircraft named?

How are Military Aircrafts named?

The Northrop Grumman B-2 Spirit.
The Lockheed Martin F-22 Raptor
The North American P-51 Mustang.

What do these have in common? Besides being some SICK aircrafts in military aviation, they all have mysterious alphabets in their name which really don’t make sense. OR DO THEY?

Let’s try and decode what their names indicate.

So the letters stand for the primary function of the aircraft.

A10 Thunderbolt

A10 Thunderbolt

 The A in A10 thunderbolt stands for attack. These tactical metal birds can mess up a place with more precision than a bomber. How they do that is by being able to fly close to the enemy grounds, launch airstrikes and flee the scene like it’s nobody’s business. They ARE equipped to engage in air to air combat but it’s not what they were MADE for.

B2 spirit

B2 Spirit
B2 Spirit

The B in B2 spirit stands for bomber, designed to drop bombs on the ground bases. They’re on the heavier side (not weight shaming or anything) and thus not as manoeuvrable as an attack aircraft.

EF111 Raven

EF111 Raven
EF111 Raven
The E in EF111 Raven stands for electronic installation. These modern marvels have the exceptional ability to engage in electronic attack (using electromagnetic energy to jam enemy systems) and electronic protection(harnessing EM power to protect themselves from being pulled into an electronic attack) among other awesome things to do with EM energy.

F-22 Raptor

f22 Raptor
f22 Raptor

The F in F-22 Raptor stands for Fighter. Made for dogfights, let’s admit these belligerent devils look the coolest in the skies.

KC-135 Stratotanker

KC-135 Stratotanker

The K in KC-135 Stratotanker stands for Kerosene tanker. These are like war nurses in the skies. When a fighter who has just gotten out of a dogfight is running out of fuel in the air, Fuelrence tankingale comes bearing fuel. They can perform air to air refuelling. Where a rod pops out mid air and attaches itself to the aircraft in need of fuel.

P-52 mustang
P52 mustang
P52 mustang

The P in P-52 mustang stands for Patrol or Pursuit. They are fighters  They later came to be referred to as the F series.

SR-71 Blackbird

SR-71 Blackbird
SR-71 Blackbird

The R in SR-71 stands for Strategic Reconnaissance. Spies in the skies more like it. They collect information over enemy regions and return to ground base. In a recon mission they are often accompanied by fighters, as they are not equipped to defend themselves against enemy fighters.

T-67 firefly

T-67 firefly
T-67 firefly

The T in T-67 firefly stands for Trainer. The teachers. These aircrafts give pilots in training the experience they need to get up and flying.

U-2

U-2
U-2
The U in U-2 stands for utility. They are used for transporting people or freight

YF-23

YF-23
YF-23

The Y in YF-23 stands for prototype. These prototype aircrafts may or may not make it to the country’s fleet.

X-15

X-15
X-15

The X in X-15 stands for Research. These aircrafts are still in the rudimentary research phase where they will be made to undergo rigorous testing before they get the chance to make it to the coveted hangars.


Check our previous articles on
How are Boeing planes named?
How are Airbus planes named?


Thanks for reading!

WE Expedition: Circumnavigation of the earth

Around the world in 90 days!

WE Expedition: Circumnavigation of the earth

Do you know about the Jules Verne's 1872 adventure novel 'Around the world in 80 days'? If yes, then you probably know how cool circumnavigation is. Oxford dictionary defines circumnavigation as The action or process of sailing or otherwise traveling all the way around something, especially the world. Travelling all around the earth isn't that exciting? (Unless you are the moon, then that's usual thing, Lame joke!) In the Jules Verne's novel, Englishman Phileas Fogg with his servant Passepartout uses various transportations to accomplish the circumnavigation. In 2004, Movie of the same name as the novel was made, directed by Frank Coraci.
This was complete fictional though! 
But there are few successful circumnavigations in the real world. 

World's first Circumnavigation of the Earth

Magellan–Elcano circumnavigation

The Magellan–Elcano circumnavigation was the first circumnavigation. It was a Spanish expedition under the command of Ferdinand Magellan that sailed from Seville in 1519. They left Spain on 20th September 1519 with 5 Ships and 270 men. They traveled from Spain to East Asia through the Americas and across the Pacific Ocean. This trip was concluded by  Juan Sebastián Elcano after the death of Ferdinand Magellan at Philippine. At the end of the circumnavigation, Juan Sebastián Elcano returned Spain with only 1 ship and 18 men on 6th September 1522. 

As Aircraft Nerds, we are more interested in aerial circumnavigation, aren't we?

World's first Aerial circumnavigation of the Earth

First aerial circumnavigation
Douglas World Cruiser Aircraft Chicago
In 1924, The team of aviators of the United States Army Air Service (United States Air Force) conducted the first aerial circumnavigation of the world. In this trip, they covered 27,553 miles (44,342 km) in 175 days. 4 Douglas World Cruiser aircraft named Seattle, Chicago, Boston, and New Orleans left Santa Monica, California for circumnavigation on 4 April 1924. Douglas World Cruiser was modified version of DT Torpedo Bomber. 

Lucky Lady II
Lucky Lady II - Non-Stop circumnavigation
Not amazed yet? So, here is the exciting info. Lucky Lady II USAF Boeing B-50 Superfortress is the first aircraft to circumnavigate around the earth Non-Stop. In 1949, it was supported by in-flight refueling. The flight lasted for 94 hours and 1 minute. 

Enough talking about the past. Now, let's talk about Indian mother-daughter duo who is going to create history in the circumnavigation of the earth. They have named their this journey around the world as WE Expedition.

WE Expedition!

WE Expedition

What is 'WE'? 'WE' stands for Women Empower. WE is a movement which will inspire women & girls all over the world and help them to grow. WE expedition is the world’s first circumnavigation by an Indian woman pilot in a motorglider. 

On 10th March'18, Indian Capt. Audrey Deepika Maben with her daughter Amy Mehta is all set to go around the globe with microlight motor glider Mahi (which means Great Planet Earth in Sanskrit)! Actually, Mahi is Sinus 912, two-seat, ultra-light, high-wing, cantilever monoplane developed by Pipistrel with the wingspan of 14.97 m (49 ft 1 in) and max. take-off weight of 544 kg (1,200 lb). It is powered by Rotax 912UL, 4-cylinder liquid cooled engine. 

Sinus 912
Sinus 912
WE Expedition is a 90 days long journey. Mother-daughter duo will cover 40,000kms and visit 21 countries with 54 stops.

According to the official website of 'WEfly', World records set by this WE Expedition are as follow:
  • A World Record for the first motorglider circumnavigation by a woman
  • A World First for a mother-daughter team expedition
  • The First motorglider circumnavigation by an Indian pilot, man or woman
The special thing about this journey is it will not only create multiple World Records but also it will motivate woman across the globe. Fund collected by this journey (WE Udaan Scholarship) will help deserving girls across various cities and towns in India to enroll in aviation training.

More Details!

To get detailed information about WE Expedition you can visit their official website

This article is small attempt to support WE Expedition. We wish WE Expedition to be a great success. You can support WE Expedition by contributing to WE Udaan Scholarship. Click on the link below to contribute to WE Udaan Scholarship

Thanks for reading!

The Immelmann turn

The Immelmann turn

f22 raptor

There it was...The british BE 2 was swooping into world war 1 skies after having successfully evaded the German Fokker EI. Oh but the poor brit in the cockpit had little knowledge of the aerobatic tricks the german pilot had in store for him. You see the German, was Max Immelmann. And Max Immelmann was the Mann....wait for it....
Who gave birth to THE Immelmann turn.

Image result for Fokker EI
Never seen before photo of Immelmann thinking about how he's going to do some twisted turns

You're kidding right? You don't know?
What is the Immelmann turn? Ugh noob. It's alright I'll tell you.

Take a look at  Immelmann turn first,




The half loop, half roll beast of a manoeuvre was executed usually after an attack to reposition the aircraft for another attack . Fighter pilots just couldn't get enough of the chase. 
How do you execute an immelmann turn? Either you get yourself a fighter capable of withstanding such engine power OR you have a really vivid imagination.
Let's go with the latter. But in order for this imagination ride to begin you need some intro into a couple of concepts.
Kinetic energy: The energy associated with a body in motion. Anything which moves has kinetic energy.
Potential energy: The energy associated with a body displaced to a height from a reference position.

Rollercoster
With the analogy of a rollercoaster, the ride has maximum potential energy and zero kinetic energy at the top as it relatively at a standstill and at a considerable height from the ground. This potential energy is traded for kinetic energy when the ride goes down the slope gaining momentum, and losing height.

Angle of attack: The angle between the chord line and free stream velocity.
Stall angle of attack: the angle of attack beyond which the aircraft begins to stall i.e. Lift is lost. WHAT? How does this happen? As the angle of attack increases we know that the lift increases (because angle of attack and lift coefficient CL are directly proportional) but too much of something is good for nothing. When you keep on increasing this angle there’s going to be a point (refer graph) where there will be separation of flow, and once flow separates there’s going to be no possibility of lift. (There are stall reversing manoeuvres’ though)

Related image
the first image shows attached flow over airfoil and lift (Coefficient of lift) linearly increases up until 16 deg (second image). This is when the flow starts separating and after this angle, flow becomes detached and lift becomes negative. NOT GOOD.
Rudder: The control surface on the vertical stabilizer which yaws the aircraft (learn about yaw here
 : LEAVE )
Related image
This swooshes the aircraft left or right
Ailerons: The control surfaces on the wing to roll the aircraft. (go learn : we know you want to.. )
Related image

 “Energy is neither created nor destroyed, it can only be changed from one form to another” – someone important.
This energy conservation principle drives many things, our little manoeuvre included. 
Let me paint your imagination with physics.

Image result for immelmann turn
The Immelmann turn
You first accelerate to gain kinetic energy (the energy associated with a body in motion), then he pulls the aircraft to a climb trading this kinetic energy for potential energy (the energy associated with a body which is at a height from a reference point). This climb happens when the angle of attack of the wings are constantly increased up until just before stall . MAD RIGHT? So then you apply full rudder to yaw the aircraft and the corresponding aileron to roll the aircraft to bring it back to level flight. This brings you back in position to get another shot at your enemy. 
And now you know.

Thanks for reading!

What is the dutch roll?

SKIES HAVE EYES

What is Dutch Roll?

It’s the 7th of June 1967, the six-day war is unfolding on the ground. But our eyes stay fixed to the skies where the swept wing greats, the Mirage 3 battles it out with the MiG 21 in a dogfight which is sure to make it to the books. Diving into the battlefield from the cloud ceiling, Giora Romm in the cockpit of the Dassault Mirage 3 engages in a fight for air superiority against the MiGs. A few minutes in, he is seen with an enemy MiG on his tail shooting for the kill. He then sets the Mirage to rock about its axis, to dodge the gunfire and escapes the danger successfully. PAUSE.


Dutch roll- Mirage 3
Hey, wanna check out my cockpit?

What he did there,  is a brilliant maneuver in military aviation, called The Dutch Roll.
This might sound like a mouth-watering puff pastry which tests the prowess of some cute dutch bakers, but all it tests is the roll and yaw stability of an aircraft....which is way cooler.
Before we go into how a dutch roll happens, let’s look into a couple of crazy circus acts our aircraft can do:
Dutch Roll - Aircraft Moments
The rotational motions of an aircraft
Rolling (banking) – Aircraft rotation about the longitudinal axis. This can be brought about by one wing having a higher lift than the other wing. (ailerons jutting in opposite directions).  For example, if the left wing has more lift, right-wing tilt down and the aircraft rolls right 

Yawing- Aircraft rotation about the perpendicular axis. Rolling and yawing come as a package deal. So your aircraft rolls left. The lift vector which was initially perpendicular to the wing jutting upwards is tilted to the left. Thanks to vector algebra, this tilted lift vector has a horizontal component associated with it called Sideslip (somebody makes this a dance move already). This skids/ yaws the aircraft in the direction of the roll in our case yaws left. 

Pitching- Aircraft rotation about the lateral axis. Can happen when the elevators on both the wings are operated in tandem. We don't need pitching motion in the dutch roll so it isn't explained here. But feel free to check out how pitching works. Knowledge is boundless. 

SO WHAT REALLY HAPPENS IN A DUTCH ROLL?

Put on your imagination caps so I can paint you this three-dimensional moving picture.



Dutch Roll - Swept angle
Rolling right creating the horizontal sideslip
Assume you roll your Mirage right, and the lift vector tilts right. The horizontal component of the tilted lift, sideslip yaws the aircraft to the right. This yawing causes the free stream velocity or the velocity of the wind to hit the wing at an angle. Now, pay close attention to the sweep (Wing swept angle) of that beauty. (learn about swept back wings here: yay information) This angle at which the wind hits the wing causes more chordwise component of wind on the right wing. (lift which matters is only generated when the flow is parallel to the chord. Go learn about lift here: click diz ). So more air is flowing just how we want it (parallel to the chord) on the right wing and not as much on the left. This translates to more lift on the right wing than the left. Still with me? This unbalanced lift, higher lift on the right side and lower on the left creates another roll, except now it's to the left. The right wing with more lift brings with it drag. This is the lift-induced drag explained in our article on the types of drag. : Drag your butt over here 

Dutch Roll - Swept Wing
Yawing right, the relative wind hits the leading edge like this
This right wing drag pulls the plane right (yaws right). This is when the vertical stabilizer (fixed vertical wing on the tail end of aircraft) does its thing by reversing this yaw, bringing it lined up with the nose. Remember our Mirage is still rolling left? This induces a left sideslip. This gives a yawing to the left. And this goes on as a repetitive rocking and rolling motion until of course the rudders are employed to bring it back to stable flight. You can think of the Dutch roll as you when you're 5 drinks in, and the rudder is your sober best friend. 

Although military aircrafts are thrown into dutch roll intentionally, their commercial cousins go into a dutch roll due to turbulence or external disturbance. They are not built for such instability: because that's what this is,  an instability condition. Which is why we say fighter jets are built for maneuverability (aka instability) while commercial aircrafts are built for stable level flight. (if they weren't our flights would be one hell of a ride). In this constant tradeoff between stability and maneuverability, as a design engineer you have to ask yourself this question: 

Is your aircraft equipped to rock and roll? 

Thanks for reading!


What is the Magnus effect?

What is the Magnus effect?
Magnus Effect
They see me spinnin', they hatin'. 

Spinning Cylinders

In 1930, Plymouth A A 2004 flettner airplane used a bunch of spinning cylinders (flettner rotors) to achieve flight. This puts forth the question: CAN SPINNING CYLINDERS FLY?

By the Magnus effect, that is a possibility.

Magnus Effect

Almost a hundred years before this flettner aircraft took flight, in 1852, Heinrich Gustav Magnus was working on projectiles from firearms. The bullets seemed to take a deflected path rather than the destined path. Pissed off, he grabbed the bullet by the collar and asked it what the hell it was doing. “It’s the pressure difference man! Look I’ve got a family to feed ” said the bullet. Thus he proposed the Magnus effect. True story.

Fast forward to the 20th century, there came a revolutionary theorem barging into the abode of aerodynamics and that was the Kutta Jowkowski theorem. Kutta and Jowkowski sat together and whipped up some dank mathematics to prove our man Magnus’s effect.
As opposed to going into the mathematics of the Magnus effect, and analytically proving its existence, we’ll look into the physics of it and see what sort of sorcery it is.  
Magnus effect
Spinning cylinder
Suppose a cylinder is spinning in the clockwise direction. There’s going to be attached flow right above and below the cylinder creating a boundary layer. (layer where friction effects are felt). After this attached flow, there’s going to be flow separation in the aft of the cylinder (One could say after but Aircraft Nerds like cool words). This flow separation creates low pressure vortices. (The physics behind this attachment and separation has been elaborated in the article on Coanda effect linked below) Let’s step outside the boundary layer for a bit and examine the velocity above the cylinder. It’s going to be higher than the velocity below the cylinder. Why? Because it spins. Any spinning object rotates about it's axis by an angular velocity. This angular velocity is induced by the velocity acting tangent to the circle at every point. Over the top of the cylinder, this clockwise spin or rotation creates a tangential component of velocity which is added to the free stream velocity (velocity of the flow V which is a constant at a distance from the cylinder) creating a velocity Vtop. Why is it added? because the flow velocity and the spin velocity are in the same direction. And in the bottom of the cylinder the velocity is reduced from the free stream velocity (V) creating a lower velocity Vbottom. Why is it subtracted? Because the flow velocity and the spin velocity are acting in opposite directions. So the velocity over the top is greater than the velocity below. 

Magnus effect Angular velocity
Tangential velocity over top > Tangential velocity below

Bernoulli: “That makes the pressure over the top lesser than the pressure below the cylinder!”

What he said. So as mentioned in the linked article on Coanda effect, flow moves from high pressure to low pressure. This difference in pressure creates a force which pushes the flow in that direction. In our spinning cylinder, there’s high pressure below the cylinder and so the force acts upwards. This upward force is, drumroll, please…
Lift. 
Or Magnus force.
 Now look at the aircraft again, see those spinning cylinders separated by discs? They're flettner rotors. These flettner rotors take advantage of the Magnus effect to generate lift.
Random cricket fanboy: Curiouser and Curiouser..
what?
Related image
Topspin on a cricket ball
Random cricket fanboy: This sounds an awful lot like topspin and backspin in cricket.
Right you are random fanboy, right you are.The Magnus effect explains the deviations of spinning balls in ball sports like golf, tennis, baseball and ofcourse cricket. In top spin the ball goes down because the Magnus force acts downwards. In backspin the ball rises.

Now you know how to throw a killer serve with physics.

Click here for more enlightenment: What is the coanda effect?

What is coanda effect?

Why are fluids clingy? Coanda Effect? 



Lift on an airfoil is a by-product of the attached flow over the airfoil. (this is explained in the article on lift linked below). But how do the air molecules know that they have to climb up the curvature and follow the contour of the airfoil. Is there a rule in nature which says that being clingy is socially acceptable behavior?
Yes, in fact, there is, and it's called the Coanda effect.
Image result for streamline flow airfoil
fluids- where an attachment is not a disorder

What is the Coanda effect?

It is the tendency of a jet of fluid (gas/liquid) to curve towards a surface. In other words, it's the attachment complex of a fluid. 

Why are fluids clingy? Let's delve into physics behind the Coanda effect.

Setting: a stream of water flows over a convex object (works on concave objects as well)

Cast: 
Fluid: anything that has molecules not too closely packed to be considered a solid.

Bernouilli equation: energy equation that says if pressure increases (pressure energy), velocity decreases (kinetic energy) and vice versa. So as to maintain constant energy.

Image result for pressure in a container
Pressure on the walls of a container
Hydrostatic pressure: The pressure that comes out of particles in the fluid bombarding against one another. The pressure at a point in a non-moving fluid is directly proportional to the height of the fluid above it. It can be thought of as the pressure exerted by the fluid on the walls that restrain it.
 Continuity equation- the equation that says an area is inversely proportional to velocity. (If you wanna remember this: picture a fat man run.
Image result for boundary layer
Viscosity prevails only in the boundary layer
Viscosity: the friction that exists between the layers of the liquid. When the fluid flows over boundary the boundary acts like it’s deadbeat husband, holding it back, not giving it the permission to go as fast as it can. So the layer above it has significant velocity decrease(for subsonic flow). The layer over that is a little faster, and so on, until it’s out of the boundary layer, and the fluid layer moves as fast as it can. Viscosity tells you all that you need to know about a dead weight, the further you move away from them, the better your life can be.

Entrainment: The movement of one fluid by another. (can exist between two layers of the same fluid at different velocities). You can visualize this concept as how an audience sways to a musician’s tunes. standing awkwardly not knowing what to do one minute, to rhythmically shaking what your mother gave you the next. It’s not magic, it’s just emergent behaviour….but yeah it’s pretty close to magic.

Movie:  Tiny little fluid particles are on their way over the curved surface. So at time t=0 they listen to inertia and they don’t curve downwards around the curve, they move on down the same line (they don’t get far but wait for it). This moving layer entrains the layers right below it (past the curve). This entrainment causes the layers to leave that region leading to a reduced pressure there. Why reduced pressure? where there is a lack of fluid particles (owing to the fluid layer eloping with it’s second cousin), there is a lack of pressure being exerted on the walls of the surface. (Like when you’re finally done with high school ,but you feel a pang of loss for some reason). At this point in time, there’s a higher pressure on top and lower pressure below, this pressure difference creates a force which acts downwards. Force gives you displacement so this facilitates the movement of the fluid to move from high P to low P curving downwards.

Crazed fan’s plot interpretation on the pretentious blog: All of this happens at t=0.0000000000001, and so you don’t really see them trying to move down the same line. They get curved by the fluid military (force arising from pressure difference), leaving the law of inertia in a blubbering mess. Valiant attempt, my little-wet friend.

Click over here, go on, I dare you. How do wings generate lift?