How is lift really created

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. 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.

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. 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.

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 , lb. You can talk about air pressure differences etc. 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. 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. 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. But the fact that the wing is moving through the air, with each parcel affecting all of the others, brings these co-dependent elements into existence and sustains them throughout the flight.

Soon after the publication of Understanding Aerodynamics , McLean realized that he had not fully accounted for all the elements of aerodynamic lift, because he did not explain convincingly what causes the pressures on the wing to change from ambient.

In particular, his new argument introduces a mutual interaction at the flow field level so that the nonuniform pressure field is a result of an applied force, the downward force exerted on the air by the airfoil. There are reasons that it is difficult to produce a clear, simple and satisfactory account of aerodynamic lift. Some of the disputes regarding lift involve not the facts themselves but rather how those facts are to be interpreted, which may involve issues that are impossible to decide by experiment.

Nevertheless, there are at this point only a few outstanding matters that require explanation. Lift, as you will recall, is the result of the pressure differences between the top and bottom parts of an airfoil. We already have an acceptable explanation for what happens at the bottom part of an airfoil: the oncoming air pushes on the wing both vertically producing lift and horizontally producing drag.

The upward push exists in the form of higher pressure below the wing, and this higher pressure is a result of simple Newtonian action and reaction. Things are quite different at the top of the wing, however. A region of lower pressure exists there that is also part of the aerodynamic lifting force. We know from streamlines that the air above the wing adheres closely to the downward curvature of the airfoil.

This is the physical mechanism which forces the parcels to move along the airfoil shape. A slight partial vacuum remains to maintain the parcels in a curved path.

This drawing away or pulling down of those air parcels from their neighboring parcels above is what creates the area of lower pressure atop the wing. But another effect also accompanies this action: the higher airflow speed atop the wing.

But as always, when it comes to explaining lift on a nontechnical level, another expert will have another answer. But he is correct in everything else. The problem is that there is no quick and easy explanation. Drela himself concedes that his explanation is unsatisfactory in some ways. So where does that leave us? In effect, right where we started: with John D.

This article was originally published with the title "The Enigma of Aerodynamic Lift" in Scientific American , 2, February How Do Wings Work? Holger Babinsky in Physics Education , Vol.

David Bloor. University of Chicago Press, Understanding Aerodynamics: Arguing from the Real Physics. Doug McLean. The advent of this model, and the complicated mathematical manipulations associated with it, leads to the direct understanding of forces on a wing.

But, the mathematics required typically takes students in aerodynamics some time to master. One observation that can be made from figure 7 is that the top surface of the wing does much more to move the air than the bottom. So the top is the more critical surface. Thus, airplanes can carry external stores, such as drop tanks, under the wings but not on top where they would interfere with lift.

That is also why wing struts under the wing are common but struts on the top of the wing have been historically rare. A strut, or any obstruction, on the top of the wing would interfere with the lift. The natural question is "how does the wing divert the air down? To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass.

Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 8. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton's first law we know that for the fluid to bend there must be a force acting on it. From Newton's third law we know that the fluid must put an equal and opposite force on the object that caused the fluid to bend.

Fig 8 Coanda effect. Why should a fluid follow a curved surface? The answer is viscosity: the resistance to flow which also gives the air a kind of "stickiness. The relative velocity between the surface and the nearest air molecules is exactly zero. That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.

Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached note that this occurs in less than an inch. Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface.

Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer". There are many types of wing: conventional, symmetric, conventional in inverted flight, the early biplane wings that looked like warped boards, and even the proverbial "barn door.

What all of these wings have in common is an angle of attack with respect to the oncoming air. It is this angle of attack that is the primary parameter in determining lift. The lift of the inverted wing can be explained by its angle of attack, despite the apparent contradiction with the popular explanation involving the Bernoulli principle. A pilot adjusts the angle of attack to adjust the lift for the speed and load. The popular explanation of lift which focuses on the shape of the wing gives the pilot only the speed to adjust.

To better understand the role of the angle of attack it is useful to introduce an "effective" angle of attack, defined such that the angle of the wing to the oncoming air that gives zero lift is defined to be zero degrees. If one then changes the angle of attack both up and down one finds that the lift is proportional to the angle.

Figure 9 shows the coefficient of lift lift normalized for the size of the wing for a typical wing as a function of the effective angle of attack. A similar lift versus angle of attack relationship is found for all wings, independent of their design. This is true for the wing of a or a barn door.

The role of the angle of attack is more important than the details of the airfoil's shape in understanding lift. Fig 9 Coefficient of lift versus the effective angle of attack. Typically, the lift begins to decrease at an angle of attack of about 15 degrees.

The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing.

This separation of the airflow from the top of the wing is a stall. We now would like to introduce a new mental image of a wing. One is used to thinking of a wing as a thin blade that slices through the air and develops lift somewhat by magic.

The new image that we would like you to adopt is that of the wing as a scoop diverting a certain amount of air from the horizontal to roughly the angle of attack, as depicted in figure The scoop can be pictured as an invisible structure put on the wing at the factory.

The length of the scoop is equal to the length of the wing and the height is somewhat related to the chord length distance from the leading edge of the wing to the trailing edge. The amount of air intercepted by this scoop is proportional to the speed of the plane and the density of the air, and nothing else.

Fig 10 The wing as a scoop. As stated before, the lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air. As a plane increases speed, the scoop diverts more air. Since the load on the wing, which is the weight of the plane, does not increase the vertical speed of the diverted air must be decreased proportionately.

Thus, the angle of attack is reduced to maintain a constant lift. When the plane goes higher, the air becomes less dense so the scoop diverts less air for the same speed. Thus, to compensate the angle of attack must be increased. The concepts of this section will be used to understand lift in a way not possible with the popular explanation. When a plane passes overhead the formerly still air ends up with a downward velocity.

Thus, the air is left in motion after the plane leaves. The air has been given energy. Power is energy, or work, per time. So, lift must require power. This power is supplied by the airplane's engine or by gravity and thermals for a sailplane. How much power will we need to fly? The power needed for lift is the work energy per unit time and so is proportional to the amount of air diverted down times the velocity squared of that diverted air.

We have already stated that the lift of a wing is proportional to the amount of air diverted down times the downward velocity of that air. Thus, the power needed to lift the airplane is proportional to the load or weight times the vertical velocity of the air. If the speed of the plane is doubled the amount of air diverted down doubles. Thus the angle of attack must be reduced to give a vertical velocity that is half the original to give the same lift.

The power required for lift has been cut in half. This shows that the power required for lift becomes less as the airplane's speed increases. In fact, we have shown that this power to create lift is proportional to one over the speed of the plane.

But, we all know that to go faster in cruise we must apply more power. So there must be more to power than the power required for lift. The power associated with lift, described above, is often called the "induced" power. Power is also needed to overcome what is called "parasitic" drag, which is the drag associated with moving the wheels, struts, antenna, etc. The energy the airplane imparts to an air molecule on impact is proportional to the speed squared.

The number of molecules struck per time is proportional to the speed. Thus the parasitic power required to overcome parasitic drag increases as the speed cubed. Figure 11 shows the power curves for induced power, parasitic power, and total power which is the sum of induced power and parasitic power. Again, the induced power goes as one over the speed and the parasitic power goes as the speed cubed. At low speed the power requirements of flight are dominated by the induced power.

The slower one flies the less air is diverted and thus the angle of attack must be increased to maintain lift. Pilots practice flying on the "backside of the power curve" so that they recognize that the angle of attack and the power required to stay in the air at very low speeds are considerable. Fig 11 Power requirements versus speed.

At cruise, the power requirement is dominated by parasitic power. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from Bernoulli's equation, the pressure is lower. The difference in pressure across the airfoil produces the lift. Before considering what is wrong with this theory, let's investigate the actual flow around an airfoil by doing a couple of experiments using a Java simulator which is solving the correct flow equations.

Below the simulator is a text box with instructions. Be sure that the slider on the right of the text box is pulled to the top to begin the experiments.

The applets are slowly being updated, but it is a lengthy process. This interactive Java applet shows flow going past a symmetric airfoil.

The flow is shown by a series of moving particles.