When a plane moves forward, the air around it does not. Some of the space around the wing is filled with “new” air, but the area behind it is blocked from being filled up. This area is outlined in red in the image below.
This causes a vacuum or low pressure area to form, which sucks on the air above it. The result is that the air flowing over the wing is pulled into a curved path. Since the air isn’t really moving, when it is pulled down, its relative horizontal speed does not decrease. The total speed relative to the wing is now higher, and the air must spread out because the distance it has to cover in x units of time is now bigger. This means that the curved air above the red line is also at a lower pressure. This cycle would keep repeating, spreading the pressure difference. At the end of the wing, the air continues moving downwards because of its inertia.
The Bottom of the Wing
Assuming that the AOA is positive and/or the bottom of the wing curves downwards, the air flowing past it will run into the downwards slope of the wing. A “traffic jam” of air starts to form, which increases pressure. At the same time, the air is deflected downwards. Then, the high pressure region spreads out in the same manner as the low pressure region over the wing.
The air exiting the bottom of the wing will be at a lower downwards angle than the air over the top of the wing, so the flows will push each other into one direction— somewhere between the two.
Putting It All Together
The difference in pressure of the wings means that more air molecules are pushing on the bottom of the wing than on the top, which gives rise to an upward net force. The downwards deflected air adds to the lift.
A Question…
How does this all work in a wind tunnel if the vacuum is caused by movement? I’m sure there’s a dozen technical explanations, but what would physically cause the air to follow the curvature of the wing in this case?
I couldn’t possibly hope to explain it succinctly, but it’s to do with the Coanda principle; see below:
Not sure I fully understand this part of the question, what makes you conclude this? Flying into a headwind, even if stationary, does not reduce the magnitude of the low-pressure area (the red area) above the wing. If you’re flying at x knots and suddenly get a headwind of 10 knots, ceteris paribus, the magnitude of the low-pressure area will increase, causing an increase in lift.
In a wind tunnel, the air moves relative to the stationary wing, simulating the outside environment where it is usually the other way around. The relative speed at which the air passes over the wing is what generates lift - and determines its vector magnitude. In other words, the effect is the same if you move the wing really fast in an environment where the air is stationary versus moving the air when the wing is stationary. The effect is exactly the same.
It is, for this reason, you don’t see small Cessnas fall out of the sky when the headwind equals the aircraft’s ground speed. The aircraft in absolute terms relative to the earth will remain in the same place, but the lift generated does not change regardless if the wing is stationary with a constant flow of air, or the other way round.
I see - in that case, I’ll pass the baton to someone who can explain the effect better than I possibly could; I’m simply not educated enough on this matter to provide a nuanced response. I know @adit likes to dabble in these topics, maybe he can provide some further insights.
In a wind tunnel, the air is drawn through the tunnel by a large fan, creating a steady stream of air flowing over the wing or other object being tested. The shape of the tunnel and the objects inside it are designed to create a controlled and consistent airflow, allowing researchers to measure the performance of the object under different conditions.
As the air flows over the wing, it experiences changes in pressure and velocity due to the shape of the wing. The curved upper surface of the wing causes the air to move faster than the flat lower surface, which creates a region of low pressure above the wing and a region of high pressure below it. This difference in pressure creates lift, which allows the wing (and an airplane attached to it) to stay aloft.
The movement of the air in the wind tunnel is not caused by a vacuum, but rather by the fan that draws the air through the tunnel. The air follows the curvature of the wing because of the principles of fluid dynamics, which govern the behavior of fluids (like air) in motion. In simple terms, the shape of the wing causes the air to move in a specific way, creating lift and allowing the airplane to fly.
Air follows the curvature of a wing due to the Bernoulli’s principle and the Coanda effect. When an airfoil (such as a wing) moves through the air, it creates a difference in air pressure between the upper and lower surfaces of the wing. The shape of the wing, with its curved upper surface and relatively flat lower surface, causes the air moving over the top of the wing to travel faster than the air moving beneath the wing. According to Bernoulli’s principle, as the speed of a fluid (such as air) increases, its pressure decreases. Therefore, the air pressure above the wing is lower than the air pressure below the wing, which generates an upward force, known as lift.
The Coanda effect further explains why air follows the curvature of a wing. As the air flows over the curved upper surface of the wing, it wants to continue flowing in a straight line. However, due to the curved shape of the wing, the air is redirected downward, following the contours of the wing’s surface. This creates a downward momentum that results in an equal and opposite upward force on the wing, again generating lift.
Overall, the combination of the Bernoulli’s principle and the Coanda effect causes air to follow the curvature of a wing, generating lift and allowing an airplane to fly.
It is a very digestible explanation of lift, including the basic theories of circulation.
Air follows the curvature of the wing due to surface tension and viscosity. It is the reason the turbulent boundary layer and drag exists.
I’ll let @adit give his two cents as well. There are many good books regarding two dimensional aero-fluid dynamics that can be found online if you really would like to dive deep.
Edit: I’ll drop this here too, it is the core foundation of lifting principles and circulation.
An asymmetrical object moving relative to the air, turns the air.
Lift is simply that, turning of the air.
This is indisputable: the change in momentum of the mass of air in making the turn in direction is exactly equal to the amount of lift created. And this can be written by a straightforward and precise equation.
Lift =The change in momentum of air perpendicular to the wing (the mass moved and amount of curvature of that movement)
The above however only explains how the wing produces the amount of lift achieved, not how lift gets applied to the wing, how it pushes on the wing. The change in momentum force pushes up on the wing by means of the pressure drop:
Lift = force from the pressure drop above the wing relative to that below the wing
So the pressure drop is the effect opposing the creation of the downward momentum of air:
The above two separate principals being both fully equal to the total amount of lift, means they are numerically equal to each other:
Change in vertical momentum of the air (curving of the air) = The pressure drop force (top relative to bottom of wing)
To @Ksisky , I would suggest that all the calculus of the Kutta-Joulkowski theorem can be simply summarized in the above statement of equality, or again: changing the path momentum of the air (curving it) fully accounts for the cause of the pressure drop, or the force of lift.
After all, the extraction of air from above the wing to mobilize it downwards (for the path change, the curve) causing a pressure drop, does not seem anything but simply reasonable?
As for:
That’s an excellent presentation, and it contains important advanced considerations.
But I’ve been fascinated by the lingering disagreements among the various presentations of lift from all sources, whether pilots, academics, aeronautical engineers, government agencies etc. in producing a uniform basic explanation of lift. Why is there even any mention of disagreement? Can it, or should it, really be that mysterious?
I think all of the following ties into a good and true fundamental explanation of lift:
In the above you have curvature, which is both due to camber and AoA, and why the geometry of such curvature leads to bending of the relative wind.
For first order effects I don’t think
really needs to be considered. For “second order” more comprehensive physical effects, of course, some such concepts have relevance, but not surface tension which doesn’t apply to gases such as air.
My first inclination is to also ignore
for a “first order explanation of lift.” There are too many common explanations that mix “second order effects” in so thoroughly that a fundamental comprehensive explanation of lift gets abstracted into mystery.
As far as
I think it’s a historical source of confusion to try to explain the pressure difference isolated from the momentum change caused by the turning air.
Those two principals are joined at the hip in physical reality and so not joining them in a description of lift leads to a truncated understanding.
So the problem with the above is that, again, lift is fully equal to both downward movement of air and the pressure difference force considered separately. And those two equal but separate notions have to be integrated as being different aspects of the single phenomena.
I would personally avoid using the Coanda effect at all in any fundamental explanation of lift, simply because it’s very difficult to come across a first principles physical explanation of the Coanda effect. In other words, I suspect it adds to the mystery rather than help clarify what is missing in the explanation. Bluntly, you don’t need the Coanda effect for a basic, yet sound explanation of lift.
Not if the wing causes enough curvature of enough mass of air for that generation of lift to equal weight, AND the path of that curvature is gradual enough so as not to cause turbulence in the flow of that air (a stall).
It’s not camber that provides the curvature to sustain inverted flight. AoA does it all for stunt planes and fighter aircraft.
This is why I don’t like the equation of lift as usually taught, with AoA buried in the lift coefficient.
Angle of attack, in essence, is lift - curving the air (not to mention camber contribution, for wings that aren’t symmetrical)
Come to think of it, aren’t these all likely connected?:
I mean isn’t the Coanda principle really driven by a boundary causing supply deprivation of the fluid/gas medium.
Although you could maybe identify some nuance of difference in the technical definition, the overriding principle is really about changing the flow direction due to the solid object’s contoured shape causing a deprivation in the supply of the medium which needs to be filled in.
So the air follows the upper surface downwards.
And that’s how the air’s momentum direction is changed, which equals lift.
there’s air molecules pushing against the air, but the side of the air with the wing is not exerting any force on it because there is no air, so it is pushed into it by itself.
As in your original post diagram, as the wing moves forward in space, the volume it occupies becomes exposed. That exposed space would be a vacuum if air did not flow down to occupy that continuously emerging empty space.
The Coanda effect, I believe, is similarly caused by a solid object acting as on obstacle to the ready supply of air from moving along with the flowing fluid/gas. So on the opposite side of the flow from the solid object, which has open access to air supply, the greater air pressure forces the flow against the solid surface.
So if what I said is true, the boundary of the wing, blocking the air causing a low pressure area needing to be filled, is the common feature, as in your diagram.
Yes, except for that your explanation is a little harder to understand, but accounts for the wind tunnel scenario. Well— harder to grasp would be a better way of putting it.
