Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
To get to the bottom of it, it might help to look at lift at a molecular level:
Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects:
- Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.
- Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they exert on their surroundings.
- Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.
Flow over the upper side of the wing
Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This contraction is needed to make space for that wing; in supersonic flow it will decelerate for the same purpose. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly, the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules change their direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.
Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.
Flow over the lower side of the wing
A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.
Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.
Lift can be explained in several, equivalent ways
Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.
Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.
If you look at the wing as a black box and only pay attention to the impulse of the inflowing and outflowing air, the wing will change the impulse by adding a downward component. The reaction force of this impulse change is lift.
Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!
Lift is a matter of definition
Lift and induced drag are both part of the pressures acting on the wing. If you add up all the pressure forces acting on a wing, their resulting vector will point slightly backwards. The streamwise component is drag, and the component orthogonal to the direction of movement is lift. This is just a defininion, made for simplicity.
Short answer: by exerting a downward force on the air around them.
Long answer: Some outreach people at NASA's Glenn Research Center have written up a very good multi-page explanation, dealing individually with each contributing effect, as well as some discussion of why explanations you might have heard at school don't work. Since the navigation there is a bit funky, I'll link each page individually with a short summary.
When a fluid moves over an object (or vice-versa), the pressure is different at different points. Because of this pressure difference, there's an overall force. You can use the Bernoulli equation to work out this force, but you need to know the speed of the fluid (at each point on the wing) to start. You can't just explain it with "the Bernoulli effect", because the Bernoulli effect applies just as much to anything moving through the air.
Both surfaces of the wing turn the flow of air. The bottom surface deflects it (the air bounces off the wing), while the curved top surface bends it around (the air sticks to the wing). The turning of the flow is what gives you lift rather than just drag. You can look at the turning as the source of the pressure difference in the Bernoulli effect, or you can think of it simply in terms of equal and opposite forces.
There's another way of modelling the flow turning, which isn't discussed on the NASA site. If you've heard of the Kutta-Joukowski theorem, this is what it relates to. When the air bends around the wing (or any object), there are two special points. At the front of the wing, some of the air goes over the top, and some goes under the bottom, but there's a point in between the two. The opposite situation happens at the back of the wing, where the air from the top surface meets the air that came the bottom way (but not the 'same' air: see wrong theory #1 below). These two points are called stagnation points. In a normal object, they're at the same level vertically each other, but because the back of a wing is sharp, the rear stagnation point will form behind it when the wing is moving quickly enough. That's lower than the front stagnation point, which implies that the net movement of air is downwards. That's where the flow turning comes from, and the theorem lets you calculate how much lift you get.
As I said, to invoke the Bernoulli effect, you have to explain why the air on the upper surface is moving faster. Teachers often claim that it's because the air on the top surface has to meet the air on the bottom surface. That's simply wrong, and there's a nice simulator to demonstrate it.
This page discusses when people realise the air "bounces off" the bottom surface of the wing, but neglect the top surface.
Some people imagine the top surface of the wing as a half of a Venturi nozzle (a nozzle which speeds up fluid flow by constricting it). This speed difference would give rise to a pressure difference (Bernoulli effect again), but it turns out the wing doesn't work like a nozzle at all.
This last page just sums up that the wrong theories start with well-known physics (Newton's laws or the Bernoulli effect), but then try to oversimplify everything to make them fit the situation, so they end up with explanations which make wrong predictions.
HOW AN AIRPLANE GENERATES LIFT
There are usually two popular fields of thought (excluding the debunked equal time theory) behind why an airplane flies; some think it is caused by an application of Newton's 3rd law, and others think it is caused by a pressure difference on the top and bottom of the wing. Basically both the "Newtonian" explanation and the "High/Low Pressure" explanation are right to a certain extent. NASA acknowledges this (see second reference below) in their article however their ultimate explanation is much more focused on mathematical application and less on a physical explanation.
Newton's 3rd Law
On the Newton's 3rd law side the net aerodynamic force is caused by a redirection of the relative wind downwards (known as "downwash"). If you look at the vector diagram describing the forces by the wing on the air its is shown that this redirection is caused by a force on the wind by the wing which points downwards and more or less perpendicular to the chord line of the wing (the line directly between the leading edge and the trailing edge). Because of Newton's 3rd law, this results in a force by the wind on the wing in the opposite direction (upwards and more or less perpendicular to the chord line); this upwards net aerodynamic force accounts for lift and induced drag (drag caused by the lifting processes of the airfoil, not to be confused with parasitic drag which is drag caused by the surfaces of the plane; a parachute trailing behind the plane would contribute to parasitic drag, and all airfoils produce some amount of induced drag when they generate lift).
On the bottom of the wing this redirection of air can be explained simply. The relative wind hits the bottom and is forced away from the airfoil by the airfoil's normal force.
On the top of the wing the air is redirected by a phenomenon known as the Coanda effect, resulting in a laminar flow (the relative wind follows the wing and is directed downwards by it). I will describe why the wind follows this laminar flow in greater detail when I explain the second major lift generating phenomenon that has to do with pressures (as you will need the information from that section to understand the Coanda effect)
There is a higher air pressure on the bottom of the wing relative to Patm (atmospheric pressure). This is because airstreams are concentrated when their paths are blocked and redirected by the airfoil. Higher concentration of air leads to higher pressure.
Likewise on the top of the airfoil airstreams are prevented from directly reaching the top surface of the wing, creating a void where there is a lower concentration of air particles and thus lower pressure. Because fluids naturally flow from high to low pressure the air at Patm well above the wing is "sucked" downwards and hugs the surface of the wing. However even with this laminar flow (as we discussed above) there still exists a low pressure zone on the top of the wing; the air from the laminar flow still isn't enough to restore that region to Patm. This can be found by looking at a pressure map of an airfoil -- you will see that there is a low pressure region on top of the wing even if laminar flow exists. This section should have also answered why laminar flow exists (see the last part of the newton's 3rd law part above).
Finally, because you have a higher pressure (force per unit of area) on the bottom of the wing than you do on the top of the wing, the forces on the airfoil are unbalanced and point upwards, in a similar direction to the net aerodynamic force caused by newton's third law (detailed above). This contributes to the net aerodynamic force.
Because of the lower pressure on the top of the wing relative to the bottom, the airflow on the top of the wing moves faster than on the bottom, according to Bernoulli's equation (basically in an airstream a decrease in pressure results in an increase in speed and vice versa) -- See the flow diagram at the top of this post. This may be why the "equal time" theory (that the airflow on the top of the wing has more distance to travel so it has to travel faster) is so widely accepted. The airflow on the top does travel faster but not because it's a longer distance.
This also accounts for "wingtip vortices" -- those swirling vortices of air that can be seen (under certain conditions) trailing behind the wings of a plane. This is because the high pressure air from the bottom of the wing swirls over the ends of the wing to try and neutralize the low pressure area on top (because fluids tend to travel from high to low pressure). They do increase the pressure on top of the wing (and as a result decrease the pressure on the bottom) somewhat, reducing the pressure difference, however since the airplane is moving not all the air traveling from bottom to top reaches its destination as the airfoil moves out of the way, leaving that air to swirl in a circular vortex. This stream of high pressure air reduces lift (because it decreases the pressure difference). This is why winglets were invented (The vertical wing extensions on the end of wings) -- to block some of this flow and increase lift (and therefore fuel efficiency). "Ground effect", or the phenomenon which increases lift when a plane is close to the ground is due to the ground getting in the way of the air trying to swirl up and neutralize the low pressure on top of the wing.
One more aerodynamic phenomenon that I will relate to this explanation is a "stall". When an airfoil stalls it looses a large amount of lift and can no longer counteract gravity, causing the plane to plummet to the ground. As a pilot I have practiced stalls many times and there are two noticeable things that happen leading up to a stall. One is that the airplane looses airspeed considerably as you start to increase the angle of attack. In this case what is happening is the total force on the wing is being angled backwards so it is mostly induced drag rather than lift (to a certain point increasing the angle of attack increases lift because it increases the total force on the airfoil however as the angle gets extreme lift starts to decrease and drag continues to increase). Finally when the airplane stalls you feel a sudden jerk downwards by the airplane as if a cord holding it up were just cut. In this case the wing has reached its critical angle of attack and the laminar flow on the top of the wing (as detailed above) has separated (because the lower pressure on the top of the wing can no longer pull the wind down to conform with its surface as the necessary force to change the wind's velocity vector by that large angle cannot be exerted by that pressure difference. Once the airplane stalls you must reattach the laminar flow to the airflow to "recover" from the stall -- in a plane you do this by pitching down with the yoke.
In the future I would love to expand this post with more mathematical explanations on how to calculate the lift of a given airfoil as well as exploring other related stuff like coefficient of lift, Reynolds number, how to calculate critical angle of attack, and related subjects. This field is generally dominated by empirical data and to break into some of it with some complicated math is hard but fun to do (not to mention the way of the future, especially as computers can now process these mathematical models for us and are much faster at doing so than experiments can be).
The simplest answer that I know that is that is still accurate is that for any object to move through the air, some force must push the air in front of it out of the way (gravity, engines, momentum etc doesn't matter). If more of the air is pushed downwards then upwards (by for example, wings) then the difference is called lift.
Wings generate lift pushing air downwards. As a kid I used to stick my hand out of the open car window and tilt it - there is an upward force. A flat plate does this.
So aircraft wings could be flat plates, but unfortunately flat plates create a lot of drag as soon as they create lift since the flow at the upper end detaches immediately (curly spiral in picture above). This effect could be reduced by using a cambered plate instead of a flat plate, reducing vortex on the upper surface:
But the issue remains that as soon as the cambered plate is tilted further, it creates a lot of drag, in the same way as the straight flat plate. A water drop shape is more drag efficient than a flat plate, by keeping the flow attached. And what is a wing cross section other than a cambered plate with a water drop cross section?
It gets a bit confusing and all when we look at accelerating air at the top and lower pressure etc, especially if we want to explain the creation of lift from that. Ultimately the lift is created by accelerating the air downwards, and continuity of mass implies that the air on the top side must accelerate. It is an effect rather than a cause.
Here is a link to John S. Denker's web book on airfoils. This is probably the definitive explanation of how wings work. John Denker has a bunch of websites worth checking out.
Bottom line: for a 150,000 lb. aircraft to stay in the air, it must impart 150,000 lbft of momentum to the air through which it passes. You can talk about air pressure differences (etc.) but that's only the beginning of the explanation. If you think equal transit time, or wing curvature is what makes wings work, this is a must-read.
A simple way to understand it is that the wing acts as a blade in a fan. Moving through the air at the correct angle causes a vacuum to form on top. The front tip must be round to allow the air to move smoothly and expand to create the vacuum.
Flat bottoms and other shapes are simply maximizing this effect but are not necessary. This is why it is possible to fly upside-down as long as the wing is hitting the air at the right angle. (Not at a right-angle.)
Update: See Own Experiments on Flow turning at the bottom of this post
I'm an independent science journalist, I did a lot of research about myths and false explanations around lift and this explanation is the outcome:
The Problem. As we know, the principle of the generation of lift in general and the Magnus effect is wrongly understood and explained false in many sources. The high flow speed around an airfoil bulge (or a spinning sphere /cylinder in the case of the Magnus effect) and the related low pressure (Bernoulli effect) is not the cause of the lift as often stated but is just assisting lift generation because it is an acceleration of the air. However, it is still an important factor in the mechanism of lift because it is part of the lift force (Force = Mass x Acceleration). This extra acceleration due to increased flow speed can be added to the normal acceleration that is involved with the force that causes a flow to turn.
The Real Cause. Also generally accepted is that the real cause of the lift is the air that is turned downwards by the angle or shape of the airfoil and this force causes a force in the opposite direction, as explained by, among others, NASA. Yet, the mechanism is still unclear for many people. I try to give a little more insight with some very easy self-developed experiments and examples that are easy to understand. (see also this video demonstration). We know that in order to turn a flow, a force is required, so the bigger the deflection, the bigger the force. A turning is actually an acceleration. During the turning there must be an equal force in the opposite direction (Newton’s third law). This is the actual lift on the airfoil. It is clear that a certain radius of flow turning (action) results in an equal radius of the opposite force (reaction). It is important to understand that the reaction of the airfoil on the accelerated airflow is caused by the interaction of the airfoil surface with the boundary layer.
Center of Pressure. The key to create action=reaction on the airfoil is the viscosity of the air as without the air sticking less or more to the airfoil, the necessary interaction would not happen.These forces act everywhere on the airfoil but the center of pressure (CP) occurs there were the average deflection is the biggest, so there is also the biggest action=reaction point. This is the point were the lift force acts on the airfoil. We can check this easily with deployed flaps. The flaps cause a bigger deflection of the air at the trailing edge, thus the center of pressure moves more to the trailing edge then without flaps.
The Real Lift Force. As the air is deflected downwards, the air exerts a force in the opposite direction which means that it adds up to the pressure on the underside of the wing with the result a bigger vector in the upward direction. But on the upper side of the wing now we have a smaller vector as the pressure is lowered because here is a deduction of the pressure caused by the force in the upward direction. The result is a net force upwards. This vertical pressure lowering is the real lift force.
Summarizing: We have a relatively low tangential pressure reduction (acting in the flow direction) which is the Bernoulli part and is the accelerating part of the lift force. And we have a huge vertical pressure reduction which is the Newtonian part of the lift force which actually causes the airfoil to move up and which determines where on the airfoil the center of pressure is located and where the resulting lift force acts. Most of the pressure we see on an isobars figure of an airfoil is vertical and only little is tangential. This corresponds to earlier measurements by aerodynamicists that the pressure reduction in the flow direction (Bernoulli) does not correspond to the actual generated lift. To understand the relation between the pressure reduction in the flow direction and the pressure reduction in the vertical direction, realize that the deflection of a flow in order to create lift is always accompanied by a pressure gradient, so if the flow speeds up over the top of the airfoil and lowers in pressure (Bernoulli’s principle) and then is turned downwards to create an upforce, the flow is decelerated and the pressure increases. This increase of pressure on the upper side of the airfoil is negligible compared to the decrease in pressure on the upper side caused by the air that is accelerated downwards, hence the airfoil moves up and we have lift.
One more Example. Imagine a flat plate wing flying at zero angle of attack with at the trailing edge a flap that is pointed downwards. Imagine only the airflow on the upper side of this wing. There is no acceleration and related pressure lowering of the flow as the flow doesn't pass any obstacle. It just encounters an adverse pressure gradient when it moves over the flap down because there is a decrease in flow speed thus an increase in flow pressure (Bernoulli). But as the flow is deflected down, a force in opposite direction acts at the same time and therefore on the upper side there is a much, much more important pressure decrease (because the force in the upward direction works against the ambient pressure coming from above). This decrease in pressure caused by the 'vertical' action is the real lift force.
Update: Own Experiments on Flow Turning. On september 26, 2018, during personal flow turning test experiments with self developed cardboard flying wing devices, I strongly found evidence for a theory that I had long suspected. This involves the importance of the distance of flow turning in relation to the steepness of the turning. Explained briefly: The distance of turning seems more important then the angle of turning. When throwing the wing, and when estimating the location of the center of pressure, the side with the longest turning always won it from the side with the steepest turning, no matter what the orientation of the wing was.
The test results:
--Short steep curve pointing downwards in the front, long less steep curve in the back pointing upwards.> Result: positive momentum, nose moves up.This is the effect of the curve in the back as a predominant down pointing curve in the front would generate a nose-down moment as this would be a negative angle of attack.
--Long less steep curve pointing upwards in the front, short steep curve in the back pointing downwards.> Result: positive momentum, nose moves up. This is the effect of the long less steep curve in the front as this is a positive angle of attack.
The results of my findings correspond to the fact that the flow turning at the leading edge of an airfoil is acually the biggest while it is not creating the biggest momentum. The turning to the trailing edge after the point of maximum camber however is longer, it wins, so it creates the CP momentum. It seems logical however that in a battle between two curves of the same length, the curve with the steepest angle wins.
One of my self-developed devices to do experiments with lift, flow turning and center of pressure: The FWSCLm Demonstrator (Flying Wing Stability & CL movement). The pen in the front can be moved in and out in order to regulate the center of gravity. The flaps in the back are used to increase or decrease the curvature of the wing profile in order to regulate the center of lift. side-view
How does a small ball generate centripetal force when it moves on a curved surface? The reason is gravity. When the small ball has a velocity along the red arrow, the small ball has a tendency to leave along the normal direction of the surface, so the force of the small ball on the curved surface will be reduced, thus the centripetal force of the small ball moving along the surface will be obtained.
We change the small balls on the surface into air. When the air does not move, assume that the force of air on the curved surface is F, and when the air has a velocity along the direction of the red arrow, the force of air on the curved surface is f, because the air has a tendency to leave along the normal direction of the curved surface, so F > f. So air has a centripetal force moving along a curved surface, which makes air move along a curved surface.
The force exerted by air on the curved surface is air pressure. A decrease in air pressure is a decrease in the force exerted by air on a curved surface.
The curved surface here is similar to the wing.
Lift is a force generated across a wing because of Pressure Difference. So,basically If you are able to achieve different pressure above and below a wing, you'd have lift. Now, from basic Newton's law, this force would be directed from the region of high pressure to the region of low pressure (Because the region of high pressure is going to push the surface by exerting more force onto it as compared to the region of low pressure which would push the surface with a relatively lesser force).
Now, the important thing is to create this pressure difference. This is achieved by exploiting an interesting property of fluid : A fast flowing fluid has lower pressure as compared to a slow moving fluid. This property can be proved by various mathematical means and is beautifully incorporated in the Bernoulli's Principle. Hence Bernoulli's Principle is a mathematical expression of an inherent property of a fluid.
Now, to get lift, the required pressure difference can be created by having a flow around the airfoil in such a way that the velocities of fluid below and above the airfoil are different. This is achieved by changing the shape of the wing (Camber) in such a way that it becomes asymmetrical. The asymmetry causes different velocities on the top and bottom portion of the airfoil because of the following reason:
When a fluid reaches the Leading Edge of the airfoil, some part of the fluid is displaced upwards, while some of it is displaced downwards. Due to the asymmetry of the Airfoil, the fluid which has moved upwards has less cross sectional area to move through as compared to the fluid which went under the airfoil. This difference in area available to the fluid for movement creates the difference in the speeds of the fluid in different regions. This property of fluid to move faster in areas of lesser cross section, and move slowly in areas of larger cross section can be derived in mathematical form by application of conservation of mass, and is called Principle of Continuity.
Hence, changed fluid velocities create a pressure gradient which in turn causes a force on the wing, which is called lift. Now, this lift can be in any direction (which could be found out by integrating very small forces on very small areas on the wing surface). The component of this force perpendicular to the direction of the velocity of the aircraft is called lift force, where as the other component parallel to the velocity of the airplane is then included in the drag force.
For very accurate representation of the equations governing fluid behavior, it can be argued that Bernoulli's Principle is incorrect. In this case Navier Stoke's equation is valid, but for understanding purposes, any time invariant (steady), in compressible, inviscid flow can be considered to obey Bernoulli's Equation.
Further, for a real fluid, it would not obey Bernoulli's Equation most of the times, but the general behavior of the pressure reduction with flow speed increase is still observed, though exact pressure drop can not be calculated through Bernoulli's Equation. In such cases Navier Stoke's equation is used to correctly calculate the pressure drop created due to increased velocity of the flow.
For symmetrical wings, the wing wont generate any lift if the flow see's the wing symmetrically, so that inherently means that a symmetrical wing with 0 angle of attact would not produce any lift. To get lift from a symmetrical wing, it is placed at some angle to the flow, so that the flow see's it "asymmetrically" and hence, the above explanation can be used to explain the life generated in this case.
Explanation for Planes flying upside down: For a normal plane to fly, a positive angle of attack is needed. Give this plane a velocity axis roll of 180 degrees, you get a plane with -ve angle of attack, and hence a negative lift. But a plane cannot sustain flight with negative lift, so what the upside down flying planes need to do is to increase the -ve angle of attack to positive, by pulling the nose up (that would be pushing the nose towards the sky in an upside down plane). This causes the angle of attack to change and become +ve. The +ve angle of attack means that the wing will now experience a life such that a upside down plane has lift in upwards direction (This is equivalent to a normal plane with - ve angle of attack and hence negative lift).
A plane flies by several mechanisms. The first is the Bernoulli effect caused by wing camber that generates a pressure differential pushing the wing upwards as it moves forwards through the air. Note that birds have cambered wings. However, it is possible to have a plane with completely flat wings and no camber at all, so it is a mistake to think this is the only source of lift (as some of the answers above have done).
The angle at the wing root is also important. If you stick your hand at an angle out of car window, you will feel it forced upwards. This same effect is accomplished in an aircraft by angling the wings slightly upward relative to the plane of the fuselage.
Finally, you should be aware that the reason a plane stays aloft has nothing to do with lift, but with the surface area it presents to the ground. The primary force holding a plane up is air resistance which is a function of this surface area. The force of this air resistance is much greater than the force generated by the previous two effects. For example, a major design criteria for a plane is whether it has a square fuselage or a round/oval fuselage. A square fuselage will present more surface area to the ground, thus having greater efficiency in staying aloft. For this reason, nearly all early aircraft had square fuselages. However, a round fuselage will be more efficient moving forwards than a square one, so in a plane built for speed, round is better. An aircraft with a round fuselage goes faster, but is less fuel efficient than one with a square fuselage.
The same argument holds true for wing area. The larger the wing, the more air resistance. For this reason, gliders have relatively large wings compared to powered aircraft. The drawback of a large wing is the same as that of a square fuselage: the plane goes slower.
So, to recap, there are three factors that keep an aircraft aloft: vertical air resistance due to downward facing surface area, the angle of the wings at the wing root, and the Bernoulli effect associated with the camber in the wings.