Understanding Aerodynamic Lift

Thanks to CuriosityStream for sponsoring this video. Since long before the Wright brothers' first flight in 1903, and to this day, humanity has been fascinated by taking to the skies. Although previously thought to be impossible, flying with heavier-than-air objects is only a reality thanks to the

lift

generated by airplane wings. But elevation is a complicated topic, and even today engineers have long debates about how it is created. So what exactly is elevation? When a fluid passes over an object, or an object such as an airplane wing moves through a stationary fluid, the fluid exerts a force on the object, which can be divided into a component that acts in the same direction as the flow. of the fluid, called drag, and a component that acts perpendicular to the direction of flow, called

lift

.

When we talk about lift, what we are most interested in are

aerodynamic

bodies like this airfoil, which are carefully designed to produce a lot of lift, but minimizing drag. Aerofoils that produce lift can obviously be found on airplane wings, but also in many other applications, such as wind turbine blades or propeller blades. They are also used on the wings of Formula 1 cars, which are designed to generate downforce so they can take corners at higher speeds. Airfoils come in a wide range of shapes and sizes. One designed for an airplane wing will not be optimized for a propeller blade, for example.

And a wing designed to fly at supersonic speeds will have a very different profile compared to one designed to fly at speeds slower than the speed of sound. Airfoils can be defined using a few different parameters. The leading edge of the airfoil is called the leading edge and the trailing edge is at the rear 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 direction of flow is called the angle of attack. Drawing a line that is halfway between the top and bottom surfaces gives us the line of mean curvature.

Camber describes how curved an airfoil is. We can have positive camber or negative camber, and a symmetrical airfoil has zero camber. Curvature and angle of attack are important parameters that will have a great influence on the lift that an airfoil can generate. So how can a humble teardrop shape generate enough force to lift a heavy aircraft off the ground? As fluid flows around the airfoil, it creates two different types of stress acting on its surface. First we have the shear stresses of the wall. These stresses act tangentially to the surface of the object and are caused by frictional forces acting on the airfoil due to the viscosity of the fluid.

Then we have pressure stresses. They act perpendicular to the surface of the object and are caused by how pressure is distributed around it. Lift is the result of these two tensions in the direction perpendicular to the flow. The only way a fluid can impart a force on an object is through these tensions. The integration of the stresses in the lift direction on the surface of the airfoil gives us the lift force. For

aerodynamic

bodies such as airfoils, the shear stresses will act primarily in the same direction as the flow. They will make a large contribution to the drag force, but will not contribute significantly to the lift force.

We can then ignore 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 like this. The pressure is low above the airfoil and high below it, creating 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 greater in magnitude than the high pressure on the bottom surface. So, the suction pressure on the top surface is what contributes the most to the total lifting force.

We can also see that most of the pressure difference comes from the most forward part of the airfoil. In truth, there is nothing particularly special about the shape of an airfoil that allows it to generate lift. Any object that creates an uneven distribution of pressure will generate a force in the direction of lift, such as a flat plate at an angle to the flow, for example. Airfoils are simply optimized shapes that have been carefully designed to have high lift and drag ratios. Without a pressure difference above and below an object there can be no lift. A symmetrical body like this bullet does not generate any lift force because there is no pressure difference around it.

So we know that lift is caused by the distribution of pressure around the airfoil. But where does the pressure distribution come from? The answer to this question is complex, and there is much debate about how best to explain it concisely. In general terms, we can divide the different explanations into two groups: those based on Bernoulli's principle and those based on Newton's third law. Explanations of Bernoulli's principle focus on the velocity of the fluid.  If we look at how the fluid flows around the airfoil, we can see that near the leading edge there is a point where the velocity of the fluid reduces to zero: this is called the stagnation point.

Outside the thin boundary layer surrounding the airfoil, the fluid flowing above the stagnation point, on the upper surface of the airfoil, travels faster than the fluid traveling on the lower surface, as we can see in these particles .  Bernoulli's Principle tells us that when the velocity of a fluid increases, its pressure must decrease, which is just a statement of the conservation of energy. This means that increasing speed above the airfoil creates an area of ​​lower pressure, and reducing speed below creates an area of ​​higher pressure, and this pressure difference creates the lift force. But then we need to explain what causes the speed difference.

One explanation is that the geometry of an airfoil causes the flow to compress above the airfoil, but not below it. Due to conservation of mass, this results in an increase in speed above the airfoil. A more complete but less intuitive explanation of the speed difference is based on the concept of circulation. The flow around an airfoil can be considered as the superposition of idealized uniform irrotational flow and circulatory flow.  Without circulation, the flow around the airfoil would look like this. This is clearly not physical, as the fluid cannot turn such a sharp corner on the trailing edge, 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 as it leaves the trailing edge, we can calculate the exact amount of circulation the airfoil must generate to achieve this.

This is called Kutta condition.  The circulation has the effect of accelerating the flow above the airfoil and retarding the flow below it, which gives us the explanation we need so we can apply Bernoulli's principle. What about explanations of lift that are based on Newton's third law? These do not consider the speed above and below the airfoil, but rather analyze the behavior of the fluid more generally. If we look at a wider area, we can see that the effect of an airfoil can be felt far beyond its immediate vicinity. Upstream of the airfoil, the flow is dragged upward, which is called upwash.

And downstream the flow is diverted downward, which is called downwash. The aerodynamic profile displaces a very large volume of air. Newton's third law tells us that for every action there is an equal and opposite reaction. The airfoil must impart a force on the air to create the downdraft and therefore, according to Newton's third law, there must be a corresponding reaction force acting on the airfoil. In other words, an airfoil generates lift by turning incoming air downward. We can use the concept of circulation again, this time to explain how updrafts and downdrafts are created. In summary, a lift force acts on an airfoil due to 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 Newton's third law provide valuable information about how lift is generated, although both approaches have limitations, in part because they are based on cause-and-effect relationships. The problem is that there is not always a clear cause and effect relationship between the different phenomena involved in the generation of lift, whether we are talking about the speed of the fluid, the distribution of pressure around the airfoil or the spin towards below. of the fluid. In reality, all of these things happen simultaneously and interact with each other. However, these explanations are useful and can lead to a more intuitive

understanding

of the survey.

We can easily imagine, for example, that increasing the curvature of an airfoil will allow it to deflect a greater amount of fluid and therefore increase the lift force.   The same goes for the angle of attack. Increasing the angle of attack deflects more fluid and increases lift. However, this logic has limits. Once the angle of attack reaches a certain critical value, we can observe a sudden decrease in lift force. In this aerodynamic profile it occurs around 16 degrees.  At this angle of attack, the boundary layer can no longer remain attached to the airfoil and detaches from the surface, creating a wake behind it that 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 a stall and can be very dangerous for airplanes. Different airfoil shapes can have drastically different lift characteristics. This airfoil is curved. If an airfoil is symmetrical and therefore has zero camber, the lift force will be zero for zero angle of attack. Aerobatic aircraft typically use symmetrical airfoils, as they allow aircraft to fly upside down more easily. Lift is generated by lifting the nose of the aircraft to create an angle of attack. The wings of modern aircraft are equipped with flaps and slats that allow the shape of the aerodynamic profile to be adjusted and optimized for different phases of flight.

During takeoff, for example, you want a lot of lift.  Extending the flaps increases the camber of the wing, which increases lift, so the flaps extend during takeoff. But the additional lift comes at the expense of greater drag, so the flaps are retracted when cruising, as much lift is no longer needed and drag must be minimized to improve fuel consumption.  In reality, this video has only scratched the surface when it comes to developing a complete

understanding

of elevation. If you want to dig a little deeper, you can start by watching the extended version of this video on Nebula, where I covered some more advanced aspects, such as how circulation is induced and how the Kutta-Joukowski theorem applies. can be used to calculate lifting force.  Nebula is a streaming platform created 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 free of advertising. All my content is on Nebula, including expanded versions of some of my videos. There's also plenty of great, completely original content, like Mustard's fascinating look at the F-117 Nighthawk. To make Nebula even better, we've partnered with CuriosityStream. CuriosityStream is the best place to find high-quality documentaries. It has thousands of titles, such as Pioneers in Aviation, a three-part series that tells the story of key figures and developments in aviation history, from the Wright brothers' first flights to the Cold War.

Or Engineering the Future, which looks at how new technologies such as airplanes are likely toelectric, shape our future. If you sign up for CuriosityStream using this link, you'll get 26% off the annual plan and get Nebula for free. That's CuriosityStream AND Nebula for less than $15 a year. So, to get the package, head to curiositystream.com/ficientengineer or click the link in the description. Signing up is a great way to support this channel and all the other Nebula creators too! And that's it for this introduction to aerodynamic lift. Thanks for watching!