How airplanes fly is one of the most fundamental questions in aerospace engineering. Given its importance to flight, it is surprising how many different and oftentimes wrong explanations are being perpetuated online and in textbooks. Just throughout my time in school and university, I have been confronted with several different explanations of how wings create lift.
Most importantly, the equal transit time theory, explained further below, is taught in many school textbooks and therefore instils faulty intuitions about lift very early on. This is not necessarily because more advanced theories are harder to understand or require a lot maths. In fact, the theory that requires the simplest assumptions and least abstraction is typically considered to be the most useful.
In science, the simplicity of a theory is a hallmark of its elegance.According to Einstein (or Louis Zukofsky or Roger Sessions or William of Ockham…I give up, who knows), “everything should be made as simple as possible, but not simpler.” Hence, thestrength of a theory is related to:
- The simplicity of its assumptions, ideally as few as possible.
- The diversity of phenomena the theory can explain, including phenomena that other theories could not explain.
Keeping this definition in mind, let’s investigate some populartheories about how aircraft create lift.
The first explanation of lift that I came across as a middle school student was the theory of “Equal Transit Times”. This theory assumes that the individual packets of air flowing across the top and bottom surfaces must reach the trailing edge of the airfoil at the same time. For this to occur, the airflow over the longer top surface must be travelling faster than the air flowing over the bottom surface. Bernoulli’s principle, i.e. along a streamline an increasing pressure gradient causes the flow speed to decrease and vice versa, is then invoked to deduce that the speed differential creates a pressure differential between the top and bottom surfaces, which invariably pushes the wing up. This explanation has a number of fallacies:
- There is no physical law that requires equal transit times, i.e. the underlying assumptions are certainly not as simple as possible.
- It fails to explain why aircraft can fly upside down, i.e. does not explain all phenomena.
As this video shows, the air over the top surface does indeed flow faster than on the bottom surface, but the flows certainly do not reach the trailing edge at the same time. Hence, this theory of equal transit times is often referred to as the “Equal Transit Time Fallacy”.
In order to generalise the above theory, while maintaining the mathematical relationship between speed and pressure given by Bernoulli’s principle, we can relax the initial assumption of equal transit time. If we start from a phenomenological observation of streamlines around an airfoil, as depicted schematically below, we see can see that the streamlines are bunched together towards the top surface of the leading edge, and spread apart towards the bottom surface of the leading edge. The flow between two adjacent streamlines is often called a streamtube, and the upper and lower streamtubes are highlighted in shades of blue in the figure below. The definition of a streamline is the line a fluid particle would traverse as it flows through space, and thus, by definition, fluid can never cross a streamline. As two adjacent streamlines form the boundaries of the streamtubes, the mass flow rate through each streamtube must be conserved, i.e. no fluid enters from the outside, and no fluid particles are created or destroyed. To conserve the mass flow rate in the upper streamline as it becomes narrower, the fluid must flow faster. Similarly, to conserve the mass flow rate in the lower streamtube as it widens, the fluid must slow down. Hence, in accordance with the speed-pressure relationship of Bernoulli’s principle, this constriction of the streamtubes means that we have a net pressure differential that generates a lift force.
Flow lines around a NACA 0012 airfoil at 11° angle of attack, with upper and lower streamtubes identified.
Of course, thistheory does not explainwhy the upper streamtube contracts and the lower streamtube expands in the first place. An intuitive explanation for this involves the argument that the angle of attackobstructs the flow more towards the bottom of the airfoil than towards the top. However, this does not explain how asymmetric airfoils with pronounced positive camber at zero angle of attack, as shown in the figure below, create lift. In fact, such profiles were successfully used on early aircraft due to their resemblance to bird wings. Again, this theory does not explain all the physical phenomena we would like it to explain, and is therefore not the rigorous theory we are looking for.
Asymmetric airfoil with pronounced camber 
Another explanation that is often cited for explaining lift is that the airfoil pushes air downwards, i.e. there is a net change of momentum in the vertical plane between the leading and trailing edges of the airfoil, and by necessity of Newton’s third law, this creates a lift force. Any object that experiences lift must certainly conform to the reality of Newton’s third law, but referring only to the difference in start and end conditions ignores the potential complexity of flow that occurs between these two stations. Furthermore, the question remains through what net angle the flow is deflected? One straightforward answer is the angle of incidence of the airfoil, but this ignores the upwash ahead of the wing or anything that happens behind the wing. Hence, the simple explanation of “pushing air downwards”, however elegant and correct, is an integral approach that summates the fluid mechanics between leading and trailing edges and leaves little to say of what happens in between. Indeed, as will be shown below, upwash and flow circulation play an equally important role in creating lift.
Indeed, we can imagine a flow around a 2D cylinder shown in the figure below. The flow is symmetric from left-to-right and top-to-bottom and experiences no lift. If we now start the cylinder spinning at the rate in the clockwise direction shown, the velocity of air increases on the upper surface (reduced pressure) and reduces on the lower surface (higher pressure). This asymmetric flow top-to-bottom therefore creates lift. Note that the rotation of the cylinder has moved the stagnation point towards the rear end of the cylinder (where the bottom and top flows converge) downwards and therefore broken the symmetry of the flow. Hence, in this example, lift is created by a combination of a free-stream velocity and flow circulation, i.e. air is “spun up” and not necessarily just deflected downwards (in this example upwash ahead of the cylinder matches the downwash aft).
Flow around a cylinder
Flow around a rotating cylinder that induces lift
In the example above, lift was induced by creating an asymmetry in the curvature of the streamlines. In the stationary cylinder we had streamlines curving in one direction on the top surface, and by the same amount in the opposite direction on the bottom surface. Rotating the cylinder created an asymmetry in streamline curvature between the top and bottom surfaces (more curvature upwards then curvature downwards). We can create a similar asymmetry in the flow with a stationary cylinder by placing a small sharp-edged flap at the rear edge and positioned slightly downwards. Real viscous flow might not necessarily flow as smoothly around the little flap as shown in the diagram below, but this mental model is a neat tool to imagine how we can morphologically transition from a rotating cylinder that produces lift to an airfoil. This is shown via the series of diagrams below. This series of pictures shows that an airfoil creates a smoother variation in velocity than the cylinder, which leads to a smaller chance of boundary layer separation(a source of drag and in the worst-case scenario aerodynamic stall). A similar streamline profile could also be created with a symmetric airfoil that introduces asymmetry into the flow by being positioned at a positive angle of attack.
The reason why differences in streamline curvature induce lift is addressed ina journal paper by Dr Holger Babinsky, which is free to download. If we consider purely stead-state flow and neglect the effects of gravity, surface tension and friction we can derive some very basic, yet insightful, equations that explain the induced pressure difference. Quite intuitively this argument shows that a force acting parallel to a streamline causes the flow to accelerate or decelerate along its tangential path, whereas a force acting perpendicular to the flow direction causes the streamline to curve.
The first case is described mathematically by Bernoulli’s principle and depicted in the figure below. If we imagine a small fluid particle of finite lengthlsituated in a field of varying pressure, then the front and back surfaces of the particle will experience different pressures. Say the pressure increasesalong the streamline, then the force acting on the front face pointing in the direction of motion is greater than the force acting on the rear surface. Hence, according to Newton’s second law, this increasing pressure field along the streamline causes the flow speed to decrease and vice versa. However, this approach is valid only along a single streamline. Bernoulli’s principle can not be used to relate the speedand pressures of adjacent streamlines. Thus, we can not use Bernoulli’s principle to compare the flows on the bottom and top surfaces of an airfoil, and therefore can say little about their relative pressures and speeds.
Flow along a straight streamline 
However, consider thecurved streamlines shown in the figure below. If we assume that the speed of the particle travelling along the curved streamline is constant, then Bernoulli’s principle states that the pressure along the streamline can not change either. However, the velocity vectorvis changing, asthe direction of travel is changing along the streamline. According to Newton’s second law, this change in velocity, i.e. acceleration, must be caused by a netcentripetal force acting perpendicular to the direction of the flow. This net centripetal force must be caused by a pressure differential on either side of the particle as we have ignored the influence of gravity and friction. Hence, a curved streamline implies a pressure differential across it, with the pressure decreasing towards the centre of curvature.
Flow along a curved streamline 
Mathematically, the pressure difference across a streamline in the directionn pointing outwards from the centre of curvature is
whereR is the radius of curvature of the flow and is the density of the fluid.
One positive characteristic of this theory is that it explains other phenomena outside our interest in airfoils. Vortices, such as tornados, consist of concentric circles of streamlines, which suggests that the pressure decreases as we move from the outside to the core of the vortex. This observation agrees with our intuitive understanding of tornados sucking objects into the sky.
With this understanding we can now return to the study of airfoils. Consider the simple flow path along a curved plate shown in the figure below. At point A the flow field is unperturbed by the presence of the airflow and the local pressure is equal to the atmospheric pressure . As we move down along the dashed curve we see that the flow starts to curve around the curved plate. Hence, the pressure is decreasing as we move closer to the airfoil surface and . On the bottom half the situation is reversed. Point C is again undisturbed by the airflow but the flow is increasingly curved as me closer to D. However, when moving from C to D, the pressure is increasing because pressure increases moving away from the centre of curvature, which on the bottom of the airfoil is towards point C. Thus, and by the transitive property such that the airfoil experiences a net upward lift force.
Flow around a curved airfoil 
From this exposition we learn that any shape that creates asymmetric curvature in the flow field can generatelift. Even though friction has been neglected in this analysis, it is crucial in forcing the fluid to adhere to the surfaces of the airfoil via a viscous boundary layer. Therefore, the inclusion of friction does not change the theory of lift due to streamline curvature, but provides an explanation for why the streamlines are curved in the first place.
A couple of interesting observations follow from the above discussion. Nature typically uses thin wings with high camber, whereas man-made flying machines typically have thicker airfoils due to their improved structural performance, i.e. stiffness. In the figure below, the deep camber thinner wing shows highly curved flow in the same direction on both the top and bottom surfaces.
Deep camber thin wing with high lift 
Shallow camber thick wing with less lift 
The more shallow camber thicker wing has flow curved in two different directions on the bottom surface and will therefore result in less pressure difference between the top and bottom surfaces. Thus, for maximum lift, the thin, deeply cambered airfoils used by birds are the optimum configuration.
In conclusion, we have investigated a number of different theories explaining how lift is created around airfoils. Each theory was investigated in terms of the simplicity and validity of its underlying assumptions, and the diversity of phenomena it can describe. The theories based on Bernoulli’s principle, such as the equal transit time theory and the contraction of streamtubes theory, were either based on faulty initial assumptions, i.e. equal time, or failed to explain why streamtubes should contract or expand in the first place. The theory based on airfoils deflecting airflow downwards is theoretically accurate and correct (Newton’s third law: changes in fluid momentum over a control volume including the airfoil lead to a reactive lift force), but by being an integral approach it is not helpful in explaining what occurs between the leading and trailing edges of the airfoil (e.g. upwash is also a contributing factor to lift).
A more intricate theory is that curved bodies induce curved streamlines,as the inherent viscosity of the fluidforces the fluid to adhere to the surface of the body via a boundary layer.The centripetal forces that arise in the curved flow lead to a drop in pressure across the streamlines towards the centre of curvature. This means that if a body leads to asymmetric curved streamlines across it, then the induced pressure differential arising from the asymmetry induces a net lift force.
Edits and Acknowledgments
A previous version of this article referenced a misleading and incorrect example of a highly cambered airfoil as a counterexample to the theory of airfoils deflecting airflow downwards and the theoretical explanation using control volumes. Dr Thomas Albrecht of Monash University pointed this error out to me (see the discussion in the comments) and his contribution in improving the article is gratefully acknowledged.
DThanhvp. Photobucket. http://s37.photobucket.com/user/DThanhvp/media/American.jpg.html
 Babinsky, H. (2003). How do wings work?. Physics Education 38(6) pp. 497-503. URL:http://iopscience.iop.org/article/10.1088/0031-9120/38/6/001/pdf;jsessionid=64686DBCB81FEB401CFFB87E18DFE6DA.c1
What creates lift on a wing? ›
When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air.What are the 2 ways a wing creates lift? ›
Both the upper and lower surfaces of the wing act to deflect the air. The amount of lift depends on the speed of the air around the wing and the density of the air. To produce more lift, the object must speed up and/or increase the angle of attack of the wing (by pushing the aircraft's tail downwards).What creates lift? ›
Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion.How do wings generate lift force relate your answer to Bernoulli's principle? ›
Air moving over the curved upper surface of the wing will travel faster and thus produce less pressure than the slower air moving across the flatter underside of the wing. This difference in pressure creates lift which is a force of flight that is caused by the imbalance of high and low pressures.Where is the lift force on a wing? ›
Lift is the vertical (up and down) force acting on a wing. The focal point of this lift force is called the center of pressure. In flight, as the wing changes its angle to the oncoming flow of air, the center of pressure moves back and forth along the surface of the wing.How do you find the lift on the wings? ›
The lift coefficient is defined as: CL = L/qS , where L is the lift force, S the area of the wing and q = (rU2/2) is the dynamic pressure with r the air density and U the airspeed.How do birds form their wings and create lift? ›
Birds use their strong breast muscles to flap their wings and give them the thrust to move through the air and fly. In a way, birds use a swimming motion to get the lift needed to fly.How do wings produce lift upside down? ›
When a wing is tilted with the leading edge up relative to the incoming wind, the air tends to pile up under the wing, causing high pressure that pushes the wing up. The wing is riding on top of a bubble of dense air. This is the same reason kites fly.Which wing produces more lift? ›
Wings that can cause a bigger difference in air pressure from the top to the bottom of the wing will create more lift. For example, a wing that has relatively little curve to it will not create much lift. However, a wing with a large curve on the top will create more lift.What pressure creates lift? ›
Air pressure is the reason airplanes are able to produce lift. Due to the shape of an airplane wing, air on top of the wings moves faster than air on the bottom of the wings. Bernoulli's Principle states that faster moving air has lower air pressure and slower moving air has higher air pressure.
What are 4 causes of lift? ›
What Factors Affect Lift? The size and shape of the wing, the angle at which it meets the oncoming air, the speed at which it moves through the air, even the density of the air, all affect the amount of lift a wing creates.How do bird wings work? ›
From side on, you can see that a bird's wing is flat underneath and curved on top. This means that the air passes faster above it than underneath it. The difference in air speed creates air pressure underneath the wing, which lifts it up. Aeroplane wings use exactly the same shape to help give them lift.What is the force of lift and how does it affect a plane in flight? ›
The plane goes up if the forces of lift and thrust are more than gravity and drag. If gravity and drag are bigger than lift and thrust, the plane goes down. Just as drag holds something back as a response to wind flow, lift pushes something up.How does the wing generate lift and how is it affected by high lift devices? ›
The wing of an aircraft generates lift by accelerating air over it. This acceleration can be increased by tilting the wing up which is normally achieved by pitching the nose of the aircraft up which increases the angle of attack (the angle between the airfoil chord and relative airflow) on the wing.How are the wings of a plane aerodynamically shaped to create lift? ›
An airfoil (or aerofoil in British English) is any structure designed to manipulate the flow of a fluid to produce a reaction, which in an aircraft's case, is aerodynamic lift. The wings of fixed-wing aircraft feature airfoil-shaped cross-sections.How much lift on a wing? ›
A: An airliner wing may produce a pound of lift per square inch in level flight. That doesn't seem like much, but over the entire surface of the wings these pounds-per-square-inch add up.How does a lift work? ›
A motor at the top of the shaft turns a sheave—essentially a pulley—that raises and lowers cables attached to the cab and a counterweight. Gears connect the motor and sheave in slower systems. Faster elevators are gearless; the sheave is coupled directly.Where does lift act on a wing quizlet? ›
Wing area increases, camber increases. Where does lift act on a wing? Centre of Pressure.Why do birds lift their wings in the rain? ›
In light showers, you will see birds fluff up their feathers to keep warm, while in heavy rain, they will flatten down their feathers to make them even more water-resistant. Many birds are also able to preen their feathers with a layer of water-resistant oil to give them further protection.How do birds make their wings strong? ›
The power behind a wing beat comes mainly from the pectoral, or breast muscles. Other muscles adjust the wing's shape in flight, or fold it up. In strong-flying birds, the powerful wing muscles can make up a third of their body weight.
Why do birds lift? ›
25 in Nature Communications, show that birds actually draw on drag to support up to half their body weight during takeoff, and that lift helps them brake during landing.How do flat wings create lift? ›
Air has to flow a longer distance over that slight curve as compared to the bottom of the wing, just like an airplane. The faster air causes a pressure drop relative to the bottom of the wing, thus lift.Do longer wings mean more lift? ›
Yes, the length of an aircraft's wings do make a difference in the aircraft. The wings of an airplane help create lift, an essential force of flight. The longer the wings an airplane has, the more lift that can be created. This is important in sustaining the weight of the airplane and maximizing fuel efficiency.Which part of a plane creates a lift? ›
A plane uses its wings for lift and its engines for thrust. Drag is reduced by a plane's smooth shape and its weight is controlled by the materials it is constructed of.What type of force is lift? ›
Lift is an aerodynamic force ("aero" stands for the air, and "dynamic" denotes motion). Lift is directed perpendicular (at right angle) to the flight direction. As with weight, each part of the aircraft contributes to a single aircraft lift force. But most aircraft lift is generated by the wings.What does lift depend on? ›
Lift is a result of pressure differences and depends on angle of attack, airfoil shape, air density, and airspeed.Is lift caused by air resistance? ›
Four Forces Affect Things That Fly:
Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure. Thrust is the force that propels a flying machine in the direction of motion.
Lift—is a force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through the center of lift (CL) and perpendicular to the lateral axis. In level flight, lift opposes the downward force of weight.What is the greatest factor causing lift? ›
Suction above the wing is the greatest factor causing lift. Lift is generated when a certain mass of air is accelerated downwards. Lift is generated when the flow direction of a certain mass of air is changed.What are the 4 types of lift? ›
There are four main types of elevators: hydraulic, traction, machine-room-less, and vacuum.
How can a bird move forward to create lift? ›
In order to create thrust, birds flap their wings. As they push on the air, the air pushes them forward and up, generating both lift and thrust. When they lift their wings back up, they bend them slightly so that the wings don't push on the air as much on the way up.Do wings work without wind? ›
However, you need to know that even without wind, bird flies nonetheless. This is because flying has more to do with the “lift” than the presence of wind. As long as there is air, birds can fly. In terms of aerodynamics, there are four forces that greatly influence bird flight.What allows birds to fly? ›
Their arms have transformed into wings to power them along. Instead of heavy jaws and teeth, they have lightweight beaks. And instead of fur, they have feathers. These are light, streamlined and cleverly adjustable for flight control.How does lift help a plane fly? ›
A: Lift is one of the four forces of flight that help an airplane stay in the air. Lift helps an airplane counteract its weight in order to rise into the air and maintain altitude.How does the angle of the wing affect lift? ›
The angle at which the wing meets the oncoming air is called the angle of attack, and by changing this angle, you can affect how much lift a wing creates. If you tilt a wing upward, it creates more lift to a certain point. Tilting a wing up too much actually decreases lift because this can cause the plane to stall.Does a larger wing create more lift? ›
Increasing the wing area will increase the lift. Doubling the area will double the lift.How is lift generated in Delta Wing? ›
High-speed supersonic waveriding
Here, a shock body beneath the wing creates an attached shockwave and the high pressure associated with the wave provides significant lift without increasing drag.
Each wing was tested 20 times. It was concluded that Airfoil Three generated the most lift, with an average 72 grams of lift. Airfoil One generated the second most lift with an average of 35 grams. Airfoil Two was third with an average of 29 grams of lift.What wing shape produces the most lift? ›
An elliptical planform is the most efficient aerodynamic shape for an untwisted wing, leading to the lowest amount of induced drag.How much of the lift is generated by wings? ›
An airliner wing may produce a pound of lift per square inch in level flight. That doesn't seem like much, but over the entire surface of the wings these pounds-per-square-inch add up.
What is the best explanation for why bird wings lift? ›
Wings. The shape of a bird's wing is important for producing lift. The increased speed over a curved, larger wing area creates a longer path of air. This means the air is moving more quickly over the top surface of the wing, reducing air pressure on the top of the wing and creating lift.