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  • Thanks to CuriosityStream  for sponsoring this video.

  • 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 airfoilwe 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 impartingforce 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 itThe 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 velocitythe 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 reachescertain 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 surfacecreating a wake behind it which affects

  • the pressure distribution around the airfoilsignificantly reducing lift and increasing drag.

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

  • The sudden reduction in lift is called stallingand 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 liftExtending 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

  • Nebula is a streaming platform built by  independent educational creators. It's a  

  • place where we can upload our usual content, but  also experiment with longer videos or new formats,

  • and it's completely ad free. All of my content is on Nebula,

  • including extended versions of some of my  videos. There's loads of great completely

  • original content on there too, like Mustard's  fascinating look at the F-117 Nighthawk.

  • To make Nebula even better we've  teamed up with CuriosityStream.

  • CuriosityStream is the best place to go for high  quality documentaries. It has thousands of titles,

  • like Pioneers in Aviation, a three part series  that tells the story of the key characters and

  • developments in the history of aviation, from the  earliest Wright brother flights to the Cold War.

  • Or Engineering the Future, that looks  at how new technologies like electric

  • aircraft are likely to shape our future. If you sign up to CuriosityStream using this link,

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  • great way to support this channel, and all  of the other creators on Nebula as well!

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

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