# Is the low air pressure on top of the wing the major contributor to lift?

I am kind of old time flyer. Started to fly with those primitive hang-glides with titanium frame and the yoke control, around 1966 and then after a few close calls and getting older I switched to small airplanes like Cessna 172 and Cherokee Warrior.

But from my decade of flying those hang-gliders I remember times when my wing fabric had collapsed because I must have been hit by a swift downdraft. I had to push it back up and hold it there a few seconds to stop stall. I needed to apply considerable upward force to push the fabric back up to airfoil shape. So I know how powerful the down-flow stream is.

If we consider the total pressure on top of the wing
$P_{wing} = P_{atm} - \frac{1}{2}\cdot\frac{dm}{dt}\cdot v_z^2$
and vertical speed $v_z = sin(\alpha)$, so by multiplying $v_z$ by the Wing surface we get the vertical down-flow and plugging it into the above equation we get a rough estimate of lift caused by low pressure on top.
I have checked many references on how lift is created and have not found any that refer to this major component of lift which I have personally experienced. Am I missing something?

I am adding a nice sketch of me rowing a boat. Using this setting as a poor man's wind-tunnel.
If you delay lifting your row at the end of its stroke it will glide back by the force of momentum of your boat. If you keep the row steady as shown on sketch and force it to drag behind you,what happens?
It creates a small swell, with water washing the outside face of the row a bit higher, shown on the sketch on red, and on the inside face of the row it creates a vortex at front and water washes the row a bit lower than lake level, shown in green.

My question is this: after reviewing many researches by labs and NASA and many other interested parties even prestigious universities, one finds less weight given to this significant part of the dynamics of lift: low pressure on top of the wing causing air to spill down to fill in by converting part of atmospheric pressure to kinetic energy, while relieving the wing top of full force of ambient atmospheric pressure?
I did a very rough estimate assuming the 176 square feet wing of Cessna 172 made of flat balsa and came up with approximately 850 lbs at 55 kn which is the rotation speed.

I am familiar with accepted theories like Newton's change of momentum of stream of air and NACA airfoils.

• Possible duplicate of How complete is our understanding of lift? – mins Jul 28 '16 at 0:48
• @mins how did you guess that I am referring to venturi effect? even though many planes do use it in the pitutt tubes. Neither do I mean stone through. I could google myself but I though here I could find somebody with deep understanding of flight which extends beyond what pops out of google search. I have been flying in variety of machines for 50 years. I am looking for just a bit deeper insight please! – kamran Jul 28 '16 at 2:59
• I'm having trouble reconciling your description with your drawings, especially the top one. Why is the arrow on the bottom right showing air moving up and forward? – TomMcW Jul 28 '16 at 18:59
• @TomMcW Sorry for the confusion the arrow shows the reaction of the flow seen by bottom of the wing not the flow itself. The wing is bending down the flow at an angle = approx two times angel of attack, therefor the force upward marked: resultant pressure. My emphasis in the two sketches is on the zone of low pressure on top face constantly sucking in the air from the top. – kamran Jul 28 '16 at 19:26
• I'm trying to figure out how your explanation differs from others. Every explanation I've ever read indicate that the reduced pressure on the top side of the wing does contribute a greater portion of the aerodynamic force than the underside. I don't see a difference. – TomMcW Jul 28 '16 at 19:54

You must have read the wrong sources. Low pressure on the upper surface of the wing is really the major source of lift. The surrounding air sucks the wing up as much as it pushes it up from below.

On a flat plate, the contribution of suction and pressure is about equal. On a wing with a thick airfoil, some additional suction is added on both sides due to the displacement effect of the wing, so the resulting negative pressure change on the upper surface becomes bigger than the positive pressure change on the bottom. At low angles of attack, you even get suction on both sides.

Your pressure calculation is maybe helpful for the mean pressure difference between both sides of the wing, but it will not give a correct value for absolute pressure. It is better to calculate lift from the momentum change imparted on the air by the wing, as shown in this answer.

Your example with the row is well chosen: The local height difference of the water to the undisturbed sea level is equivalent to the local pressure difference to the static pressure (a higher water level signifies higher pressure) and shows the approximate conditions on a section of the wing. The row is like an inclined flat plate. You can even change the angle of the row and see the additional trailing vortex as it floats away from the row.

Now for the hang gliders: They are peculiar in that the airfoil shape depends on local pressure, and early models did not use lengthwise stiffeners so the pressure over the fabric could invert when the wing caught a negative angle of attack for a moment. You are lucky that you could press the fabric up again: Several early hang glider pilots could not recover and fell to their death. This happened when designers tried to increase the aspect ratio of the initial Rogallo shape of their gliders. I only know a German word for this phenomenon: Flattersturz.

• I think the English equivalent to flattersturz is "uh oh!" – TomMcW Jul 28 '16 at 20:04
• Do you have a reference for the "equal contribution" of the top and bottom of a flat plate? I'm looking at things like the figure on the second page of s6.aeromech.usyd.edu.au/flat_plate_lift.pdf, where the suction effect looks larger to me, but I realize I can't be sure which integral is greater just by looking at the figure. – David K Jul 28 '16 at 20:14
• A couple of points regarding my glider: It was a delta shaped titanium tubing covered by a light sheet and kept tight in shape by thin cables. when there was not enough wind I had a friend tow me with a 200 ft clothe-lines attached to my car seat-belt permanently attached to CG of my kite with a rubber band release. My friend was responsible for safe launch by going fast so that I'd lift without being dragged on ground. Lost many courageous friends! - Collapse of canopy happens for variety of reasons like inversion of air current near the shear mountain cliffs which we didn't know then!! – kamran Jul 28 '16 at 21:03
• @PeterKämpf I'm like David K. In fact, I don't have time to search for it now, but I think it was you that indicated somewhere that the suction force was greater. – TomMcW Jul 28 '16 at 21:12
• @DavidK: The pressure on the lower side cannot be more than the stagnation pressure, while the suction peak on the upper surface can go all the way to vacuum. Therefore, near the leading edge suction is higher than pressure, but for most of the chord both are much the same. – Peter Kämpf Jul 28 '16 at 21:22

The question is: "Is the low pressure on top of the wing the major contributor of lift."

It depends how you define "the major" contributor. But the answer must be yes because low pressure on top of the wing is certainly one of the ingredients in the recipe for lift and airplane flight. So let's take a look at what's going on, and you can decide how major is major.

First - wings have no lift when they're standing still, and as the wing moves through the air - faster and faster - the recipe starts to come together. So speed is a major factor too. Speed as it flows across the top of the wing creates a cohesive flow of air which is very similar to water flowing in a river. Imagine throwing a stick into fast moving water and the stick is carried away. As the fast flowing cohesive air flows across the top of the wing, the molecules on top of the wing are caught up in the flow and they leave the spot where they were, leaving a vacuum behind. The faster the speed, the greater the flow, the greater the vacuum. So - cohesive air flow is also a major contributor.

Second we have no "lift" without a reaction from below. An upward force which is actually the force that holds the plane up into the air. Where does this lift come from? It comes from the dynamic struggle of the electron inside the atom. You see - electrons are so small that the solid skin of the wing is like a chain link fence to the electron and so the electrons inside the atoms below the wing can see (sense) the vacuum void, and they move 'en mass to fill the void - but there's a problem! The atom that hold the electron inside its belly is too big to enter the chain-link (solid) skin of the wing, so as the electron strives to fill the vacuum on top, it drags the atom into the underside skin of the wing. When the atom hits the surface of the wing, that is the moment when potential energy is converted into kinetic energy, and lift occurs. The greater the pressure differential, the harder the electrons tug on the too big atoms. So the mandate of the Universe that electron matter fills the void of nothing, is another "major contributor" to the airplane lift.

So yes - the vacuum is important, but its actually just a bait. The most important part of lift is when the energy is converted from potential to kinetic, and that is found in the internal struggle of the dynamic of the atom and electron relationship. But then again... the reaction wouldn't happen if the bait vacuum didn't exist, si I think I'll give the vacuum equal importance. It's just like the "chicken or the egg" conundrum. So, which is the most important ingredient in the recipe of airplane flight. It turns out that all the ingredients are important and work together as one. Vacuum is just one of the ingredients.

• Your idea that vacuum is just one of the ingredients is great. However, electrons move to positive charge areas, not to vacuum. Air flows from high to low pressure, analogous in concept, but not related, to electrical charge flow. You may wish to think of bottom lift as mass action/reaction. Think of water skiing. Not much lift from top there. Now make your skis 800 times bigger and try it in air. This is a significant portion of lift. But if you aerodynamicly reduce drag, you can ski with a 35hp motor instead of 100hp, and yes, with 2 skis or 1 ! – Robert DiGiovanni Jan 1 '19 at 20:55

The real cause of lift and the logical reason that an object moves up is indeed a pressure difference. But how this pressure difference is created is one of the most misunderstood things in physics.

The short answer is that a wing produces a force in upward direction (or downward in case of negative lift). This force works against the air pressure coming from above the wing, with the result a pressure decrease on top. But on the underside this force works now with the air pressure coming from below the wing, with the result a pressure increase from below. The net force is upward. I made this video demonstration to show how pressure difference due to flow turning is the cause of lift instead of pressure difference due to Bernoulli principle.

A longer answer is hidden in this physical explanation about the center of lift I wrote in another publication:

Much is written about the center of pressure or center of lift - which is actually the same - but few people know what it actually is in a physical sense. The center of pressure, shortened as CP or CL is an important factor in aerodynamics as its position relative to the center of gravity (CG) estimates in an important way the stability of a flying system.

To really understand what the center of pressure is, it’s important to first understand the true principle behind the generation of lift which is all about acceleration of air caused by the turning of that air. Whether a flow of air is turned down along the upper side of a wing or along the lower side, in order to turn the flow, it is required that a force is acting on it. The force that is responsible to turn air along both the upper and lower side is the viscosity of air. Compare this to a viscous fluid that sticks more to your hand then a less viscous fluid. As the air sticks to the wing surface, it is able to excert a force on the wing. This is to counteract the force that the wing acts on the airflow by turning it down. Or in another words: the well-known action=reaction principle, discribed by Newtons third law.

Center of Pressure is Action=Reaction Point

This action=reaction point is the center of lift or the center of pressure. It is called the center of lift because it is the point where the lift force acts on a lifting surface (wing) or lifting configuration (aircraft). It is called the center of pressure because this is the average point of all pressure acting on the lifting surface or lifting configuration. Realize that as the air is deflected downwards by the wing, the air exerts a force on the wing 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 airfoil now we have a smaller vector as the pressure is lowered because here is a deduction of the pressure on the wing caused by the force in the upward direction. The result is a net force upward. This vertical pressure lowering is the real lift force.

Wrong Explanation

A very common wrong explanation of the center of pressure is that it is caused by the pressure lowering due to the Bernoulli principle on top of the wing. The faster airflow over the top of the wing is indeed causing a pressure drop on this location but this is by far not comparable in magnitude to the pressure lowering caused by the flow turning as discribed above. Therefore, the faster airflow over the top of the wing is not the cause of the lift generation but the flow turning is the real cause.