Since well before the first  Wright brothers flight in 1903,

and all the way to the present day, mankind  has been fascinated by taking to the skies.

Once thought to be impossible, heavier-than-air  flight is only a reality because of the lift

generated by aircraft wings. But lift is a complicated topic, 

and even to this day engineers have  lengthy debates about how it's created.

So what exactly is lift? When fluid flows past an object, or an object

like this plane wing moves through a stationary  fluid, the fluid exerts a force on the object,

which can be split into a component acting in the  same direction as the fluid flow, called drag,

and a component acting perpendicular  to the flow direction, called lift.

When talking about lift we're mostly interested  in streamlined bodies like this airfoil,

which are carefully designed to produce  a lot of lift, but to minimise drag.

Lift-producing airfoils can  obviously be found in airplane wings,

but also in many other applications, like  wind turbine blades, or propeller blades.

They're also used in the wings of Formula 1 cars,

which are designed to generate downforce so that corners can be taken at higher speeds.

Airfoils come in a huge range of shapes and sizes. One designed for an aircraft wing won't

be optimised for a propellor blade, for example. And a wing designed to fly at supersonic speeds

will have a very different profile compared to one designed to fly slower than the speed of sound.

Airfoil profiles can be defined using a few different parameters. The forward-most edge

of the airfoil is called the leading edge, and the trailing edge is at the back of the airfoil.

Drawing a straight line between the leading and trailing edges gives us the chord line.

The angle between the chord line and the flow direction is called the angle of attack.

Drawing a line which is midway between the upper and lower surfaces gives us the mean camber line.

Camber describes how curved an airfoil is. We can have positive camber or negative camber,

and a symmetrical airfoil has zero camber. Camber and the angle of attack are important

parameters that will have a large influence on how much lift an airfoil can generate.

So how does a humble teardrop shape generate enough force to

lift heavy aircraft off the ground? As the fluid flows around the airfoil

it creates two different types of stress which act on its surface.

First we have the wall shear stresses. These stresses act tangential to the object's surface,

and are caused by the frictional forces that act on the airfoil

because of the fluid's viscosity. Then we have the pressure stresses.

They act perpendicular to the object's surface,

and are caused by how pressure is distributed around it.

Lift is the resultant of these two stresses in the direction perpendicular to the flow.

The only way a fluid can impart a force onto an object is through these stresses. Integrating the

stresses in the lift direction over the surface of the airfoil gives us the lift force.

For streamlined bodies like airfoils, the shear stresses will mostly be acting

in the same direction as the flow. They will make a large contribution to the drag force,

but won't contribute a significant amount to the lift force. And so we can neglect them and

say that the lift acting on an airfoil is caused by the way pressure is distributed around it.

A typical pressure distribution looks something like this. The pressure is low above the airfoil

and high below it, which creates a net force with a large component in the lift direction.

If we plot the pressure profile along the top and bottom surfaces, we can see that the low pressure

on the top surface is larger in magnitude than the high pressure on the bottom surface.

So the suction pressure on the top surface is what contributes most to the total lift force.

We can also see that the majority of the pressure difference is coming

from the forward-most part of the airfoil.

In truth there's nothing particularly special  about the shape of an airfoil that allows it to

generate lift. Any object that creates an uneven  pressure distribution will generate a force in

the lift direction, like a flat plate at an angle  relative to the flow, for example. Airfoils are

just optimised shapes that have been carefully  designed to have high lift-to-drag ratios.

Without a difference in pressure above  and below an object there can be no

lift. A symmetrical body like this bullet  doesn't generate any lift force because

there's no pressure difference around it. So we know that lift is caused by the pressure  

distribution around the airfoil. But where  does the pressure distribution come from?

The answer to this question is complex, and  there's much debate about the best way to

explain it in a concise way. We can broadly  split the different explanations into two

groups - those based on Bernoulli's Principle  and those based on Newton's third law.

Bernoulli's Principle explanations  focus on the velocity of the fluid. 

If we look at how fluid flows around the airfoil,  we can see that close to the leading edge there's

a point where the fluid velocity is reduced  to zero - this is called the stagnation point.

Outside of the thin boundary layer surrounding the  airfoil, the fluid flowing above the stagnation

point, over the top surface of the airfoil, travels faster

than the fluid travelling over the bottom  surface, as we can see from these particles. 

Bernoulli's Principle tells us that  when the velocity of a fluid increases, 

it's pressure must be reduced, which is just  a statement of the conservation of energy.

This means that the increase in velocity above  the airfoil creates an area of lower pressure,

and the reduction in velocity below  it creates an area of higher pressure,

and this pressure difference  creates the lift force.

But then we need to explain what  causes the difference in velocity.

One explanation is that the geometry of an airfoil  causes the flow to be pinched together above the  

airfoil, but not below it. Because of the conservation of mass,  

this results in increased  velocity above the airfoil.

A more complete but less intuitive  explanation for the difference in velocity

is based on the concept of circulation. The flow around an airfoil can be thought

of as the superposition of idealised uniform  irrotational flow, and circulatory flow. 

Without circulation, the flow around  the airfoil would look like this.

This is clearly non-physical, since the  fluid can't turn such a sharp corner at

the trailing edge, and so the airfoil  must be generating some circulation. 

If we impose a condition that says that  the flow above and below the airfoil must  

be parallel when leaving the trailing edge, we  can calculate the exact amount of circulation  

that must be generated by the airfoil to do  this. This is called the Kutta condition. 

Circulation has the effect of accelerating the  flow above the airfoil and delaying the flow

below it, which gives us the explanation we need  so that we can apply Bernoulli's Principle.

What about the explanations of lift  that are based on Newton's third law?

These don't consider the velocity  above and below the airfoil

but instead look more generally  at the behaviour of the fluid.

If we look at a wider area we can observe  that the effect of an airfoil can be felt far

beyond its immediate vicinity. Upstream of  the airfoil the flow is being swept upwards,

which is called upwash. And downstream the  flow is deflected downwards, which is called

downwash. A very large volume of air  is being displaced by the airfoil.

Newton's third law tells us that for every  action there is an equal and opposite

reaction. The airfoil must be imparting a  force on the air to create the downwash,

and so based on Newton's third law, there must  be a corresponding reaction force acting on the

airfoil. In other words an airfoil generates  lift by turning the incoming air downwards.

We can use the concept of circulation  again, this time to explain how the

upwash and downwash are created. In summary, a lift force acts on an airfoil

because of the pressure distribution around it.  The exact cause of this pressure distribution

is complex, and can be explained in several  different ways, which approach the problem

from different angles. Explanations based  on Bernoulli's Principle and on Newton's

Third Law provide valuable insight into how  lift is generated, although both approaches

have limitations, partly because they're  based on cause-and-effect relationships.

The problem is that there isn't always  a clear cause-and-effect relationship

between the different phenomena which  are involved in generating lift,

whether we're talking about the fluid velocity,  the pressure distribution around the airfoil,

or the down-turning of the fluid. In reality  all of these things are happening simultaneously

and are mutually interacting. Nevertheless, these explanations

are useful and can lead to a more  intuitive understanding of lift.

We can easily imagine for example that  increasing the camber of an airfoil

will allow it to deflect a larger amount of  fluid, and so will increase the lift force.  

The same is true for the angle of  attack. Increasing the angle of attack

deflects more fluid and increases lift. However there are limits to this logic.

Once the angle of attack reaches a  certain critical value, we can observe

a sudden decrease in the lift force. For this  airfoil it occurs at around 16 degrees. 

At this angle of attack the boundary layer  is no longer able to remain attached to the

airfoil and it detaches from the surface,  creating a wake behind it which affects

the pressure distribution around the airfoil,  significantly reducing lift and increasing drag.

I covered flow separation in detail  in my video on aerodynamic drag.

The sudden reduction in lift is called stalling,  and it can be very dangerous for aircraft.

Different airfoil shapes can have drastically  different lift characteristics.

This airfoil is cambered. If  an airfoil is symmetrical,

and so has zero camber, the lift force  will be zero for zero angle of attack.

Aerobatic aircraft usually use symmetrical  airfoils since they allow planes to fly upside

down more easily. Lift is generated by lifting the  nose of the plane to create an angle of attack.

Modern aircraft wings are equipped with  flaps and slats which allow the shape of

the airfoil to be adjusted and optimised  for the different phases of flight.

During take-off for example you want high lift.  Extending the flaps increases the camber of the

wing, which increases lift, and so flaps are  extended during take-off. But the extra lift

comes at the expense of increased drag, and so  the flaps are retracted when cruising, since

high lift is no longer needed and drag should  be minimised to improve fuel consumption. 

This video has really only scratched  the surface when it comes to developing

a complete understanding of lift. If  you'd like to dive a little deeper,

you can start by checking out the extended  version of this video over on Nebula,

where I've covered some more advanced  aspects, like how circulation is induced,

and how the Kutta-Joukowski theorem can  be used to calculate the lift force. 

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And that's it for this introduction to  aerodynamic lift. Thanks for watching!