An aircraft wing experience a lift force when moving through the air due to high pressure on the lower surface and lower pressure over the upper surface, or part of it. Various theories have been proposed as to why this occurs.

Is there a theory of lift that proposes that the blocking of airflow creates a low pressure area on the upper surface of the wing?

See this diagram:

https://en.wikipedia.org/wiki/Lift_(...separation.jpg

It is obvious that the angled airfoil is preventing air molecules from travelling past the leading edge of the wing, thereby creating a low pressure area on the top of the wing. This is a real effect, demonstrated through experiment.

Originally Posted by knlever

It is obvious that the angled airfoil is preventing air molecules from travelling past the leading edge of the wing.

Is it?

Streaklines Animation (1 min 27 sec)

Of course I should have been more exact. But first, a link to a similar discussion with several useful links:

https://www.rcgroups.com/forums/show...-lift-theories

Wikipedia states :

Basic attributes of lift
Lift is a result of pressure differences and depends on angle of attack, airfoil shape, air density, and airspeed.
Pressure differences
Pressure is the normal force per unit area exerted by the air on itself and on surfaces that it touches. The lift force is transmitted through the pressure, which acts perpendicular to the surface of the airfoil. The air maintains physical contact at all points. Thus, the net force manifests itself as pressure differences. The direction of the net force implies that the average pressure on the upper surface of the airfoil is lower than the average pressure on the underside.

These pressure differences arise in conjunction with the curved air flow. Whenever a fluid follows a curved path, there is a pressure gradient perpendicular to the flow direction with higher pressure on the outside of the curve and lower pressure on the inside.[38] This direct relationship between curved streamlines and pressure differences was derived from Newton's second law by Leonhard Euler in 1754:

I should have said that some air molecules are prevented from moving past the LE. This causes the lower pressure downstream of the leading edge. With Euler's formula, however the radius of the curve top of the wing keeps changing, so this needs to be taken into account.

From the first few milliseconds of movement of the airfoil in the air, or the first cloud of air molecules reaching the wing of a wing in a wind tunnel, the air downstream of the leading edge finds itself at a lower pressure. This is because the movement of the wing has left an empty space that has to be filled with air molecules. If the wing is moving, the space will be filled with the air molecules ahead of the wing.

Will have a look at the following answers, maybe something there. Basically I am looking for a mechanical explanation of lift by referring to experimental results, without resorting to any other principles.

https://aviation.stackexchange.com/q...-generate-lift

Originally Posted by knlever

An aircraft wing experience a lift force when moving through the air due to high pressure on the lower surface and lower pressure over the upper surface, or part of it. Various theories have been proposed as to why this occurs.

Is there a theory of lift that proposes that the blocking of airflow creates a low pressure area on the upper surface of the wing?

See this diagram:

https://en.wikipedia.org/wiki/Lift_(...separation.jpg

It is obvious that the angled airfoil is preventing air molecules from travelling past the leading edge of the wing, thereby creating a low pressure area on the top of the wing. This is a real effect, demonstrated through experiment.

You got some of it right (the important part, actually) ... If you tilt any flat plate, into an oncoming airstream - it will create a vacuum behind the plate. In wings, it's this vacuum that causes the air moving over the top of the wing, to move faster than the air moving under the bottom of the wing. The vacuum helps accelerate the air. And as NASA states, it's the wings action of accelerating air downward that creates lift (the wing reacts with an opposing and equal upward force - to its action of accelerating air downward).

From NASA ...

https://www.grc.nasa.gov/www/k-12/airplane/lift1.html

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

Not to further kick a very wounded horse, but in reference to many prior go-arounds in several previous threads please note that NASA wrote "Lift occurs" and not *ALL lift occurs" or "Lift ONLY comes from".

A fire occurs when flammable material is ignited by a match

... or by a cigarette lighter

... or by lightning

... or by spontaneous combustion


Of course just as there are people who still think the Earth is flat or held up by a large man standing on a huge turtle, those refusing to understand or being unwilling to accept the fact that lift can come from many sources or phenomena .... please keep working on it (at home)

The streaklines animation, which I just viewed for the first time, illustrates the point I want to make. The lower two particles get deflected downwards by the wing. One of the upper molecules the light blue one, I think, travels along the surface of the wing, but the other two take a long arc over the top surface of the wing, leaving a large gap between them and the wing surface, illustrating the area of lower pressure.

I can accept the deflection of air by the lower surface, and the force of the impact of the air molecules on the bottor surface, however there is no way the molecules of air moving over the top surface can exert any force on the top surface of the wing, on the contrary it is the lack of force on the top of the wing by the random movment of gas molecules that results in low pressure on the top surface. Whether the air is turned or sucked up by a large vacuum cleaner makes no difference, its work is done.

The question is which comes first, the low pressure on the top of the wing or the speeded up airflow over the top? I would think the former.

Perhaps a study of why cars change contact pressure with the road would clarify or confuse the ongoing queries about lift.
The "flying Ferrari" accidents demonstrate that lift can come from many sources

The diagram illustrates my point. The molecules of air are blocked from the top surface of the wing, in reality, viscous forces cause the air to stick to the wing. If there was no air on the top of the wing it would be even better and the pressure differential would have been even greater.

It's all very clear to me now. So maybe a little step on the top of the wing will increase lift, a la Kline–Fogleman airfoil?

edit: the pressure distribution shown here

http://www.mh-aerotools.de/airfoils/...tributions.htm

Seems to indicate that the air is able to affect the flow of air ahead of the leading edge, in subsonic flow, anyway.

Edit: this link shows the pressure ahead of the airfoil, the high pressure area must be deflecting air away from the leading edge.

http://hyperphysics.phy-astr.gsu.edu...s/airfoil.html

Now what does it all mean to us wing-makers.

There's a potential issue with this description of lift. Your drawing (with the air separated from the upper surface) best represents the flow around a stalled airfoil. A stalled airfoil typically generates less lift than one with the flow attached to the upper surface.

seems to be a bit of confusion about attached FLOW and reduced pressure on upper surfaces.
There can be pressure difference with attached flow and basically FLAT NOT parallel surfaces.
The ice cream cone airfoil used by WExtra on many designs shows this to be true.
You don't need an airfoil which is shaped differently, top to bottom.
just a bit of change from zero AOA

Originally Posted by knlever

The diagram illustrates my point. The molecules of air are blocked from the top surface of the wing, in reality, viscous forces cause the air to stick to the wing. If there was no air on the top of the wing it would be even better and the pressure differential would have been even greater.

It's all very clear to me now. So maybe a little step on the top of the wing will increase lift, a la Kline–Fogleman airfoil?

edit: the pressure distribution shown here

http://www.mh-aerotools.de/airfoils/...tributions.htm

Seems to indicate that the air is able to affect the flow of air ahead of the leading edge, in subsonic flow, anyway.

Edit: this link shows the pressure ahead of the airfoil, the high pressure area must be deflecting air away from the leading edge.

http://hyperphysics.phy-astr.gsu.edu...s/airfoil.html

Now what does it all mean to us wing-makers.

Your explanation for lift is actually what Newton used when he tried to explain lift, he modelled a fluid as a stream of bullet like particles which don't interact with each other and so lift is caused by the particles hitting the front (lower surface) and not hitting the back (upper surface).

This isn't the correct physical explanation however. A fluid is accurately modelled as a continuous material which deforms around its surroundings. This is an accurate assumption to make as fluid particles interact with their neighbours something like 100,000,000,000 times a second.


I think the best explanation for lift which gives a physical understanding of it comes primarily from conservation of mass. An airfoil doesn't impose any hard constraints on the flow but it does still constrict the flow travelling past it. The same mass of air has to leave the section the airfoil is in as entered into it (imagine two vertical lines at the leading and trailing edges). That means the air has to speed up when going past the airfoil for mass to be conserved. The larger the obstruction, the more the air will speed up. This is why a more cambered wing (or a higher angle of attack) will produce more lift, the air is forced to speed up more to get around it and by bernoulli's principle, lower pressure will accompany this. If the wing has undercamber (I don't know the correct term but imagine an umbrella) then the air is able to expand into the lower surface so it doesn't have to speed up as much to get past it thus increasing the pressure there.

With all that said, no lift would be produced without viscosity. If there was no viscosity the flow would be able to turn around the trailing edge from the lower surface, and high speeds (and low pressure on the lower surface) would accompany this resulting in no lift on the airfoil. Viscosity acts to resist the deformation of the fluid so the viscous force forces the air to leave the airfoil parallel to the trailing edge. Thus preventing the lower pressure from occuring on the lower surface.

The airfoil is the shape it is so that the largest obstruction is toward the front where the pressure gradient is most favourable, which helps prevent separation.

I think bernoulli's principle can be understood physically as well on the level of the molecules. At a given temperature, all the molecules in a fluid parcel will have the same momentum, when the fluid parcel is stationary, the momentum is spread out in all directions and this is pressure. When the fluid parcel is moving, the momentum of the molecules is the same as this is set by the temperature, but some of this momentum is now "given" to moving with the fluid parcel reducing the total momentum of the molecules available for pressure. The takeaway from this is that higher speed doesn't cause lower pressure or vice-versa, they accompany each other

Just my 2 cents after reading about this for the last few months

Originally Posted by knlever

From the first few milliseconds of movement of the airfoil in the air, or the first cloud of air molecules reaching the wing of a wing in a wind tunnel, the air downstream of the leading edge finds itself at a lower pressure. This is because the movement of the wing has left an empty space that has to be filled with air molecules. If the wing is moving, the space will be filled with the air molecules ahead of the wing.


]

The air always surrounds the wing. At sea level it's about 14.7 PSI.
If you were to put a dam against the airflow near the forward edge of the upper surface, I would expect it to to a very high pressure differential at the top of the dam (low pressure spike as the air turns the corner)and the higher pressure area of the wing aft of the dam. Which would lead to a stalled condition. Sounds like a vintage roll control spoiler.
Because the low pressure is derived from a very small area, I would say that any lift achieved would be very low.
Be mindful that a normal wing typically generates maximum lift just before a stall, with a very high pressure differential over a fairly reasonable area (apparently), but the area aft of that low pressure area will be slightly turbulent. When the wing nears the stall, then stalls, the turbulent area walks forward up the wing, and the wing looses lift.
So, an even (low) pressure distribution, down the chord of the wing, is what you might shoot for.

Originally Posted by ShoeDLG

There's a potential issue with this description of lift. Your drawing (with the air separated from the upper surface) best represents the flow around a stalled airfoil. A stalled airfoil typically generates lest lift than one with the flow attached to the upper surface.

It may at that, however I was trying to illustrate the Newtonian concept of lift, that is, imagine, in a vacuum, a steady stream of particles hitting the flat plate, or better still, a flat plate travelling through space impacting a cloud of tiny meteorites. You will have a totally kinematic effect, and you will have momentum imparted to the flat plate: "lift".

I was trying to point out that this blocking of moving particles is responsible for the lack of particles on the upper side of the flat plate in the case of the plate in space (or saucer?) and in the case of a flat plate immersed in an ocean of fair, some rarefication of the fluid occurs at the top surface. Actually, this sort of thing happens in the ocean for propellers, it is called cavitation.

Cavitation is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities in the liquid where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave.

https://en.wikipedia.org/wiki/Cavitation

Your explanation for lift is actually what Newton used when he tried to explain lift, he modelled a fluid as a stream of bullet like particles which don't interact with each other and so lift is caused by the particles hitting the front (lower surface) and not hitting the back (upper surface).

It works for what I call 'kinematic lift', of course, but Newton's model does not explain pressure differences. I was using this model to explain the pressure differences and suggest that pressure differences precede speeded up airflow, when the airfoil first starts moving.

This isn't the correct physical explanation however. A fluid is accurately modelled as a continuous material which deforms around its surroundings. This is an accurate assumption to make as fluid particles interact with their neighbors something like 100,000,000,000 times a second.

And the speed of interaction is the speed of sound?

I think the best explanation for lift which gives a physical understanding of it comes primarily from conservation of mass. An airfoil doesn't impose any hard constraints on the flow but it does still constrict the flow travelling past it. The same mass of air has to leave the section the airfoil is in as entered into it (imagine two vertical lines at the leading and trailing edges). That means the air has to speed up when going past the airfoil for mass to be conserved. The larger the obstruction, the more the air will speed up. This is why a more cambered wing (or a higher angle of attack) will produce more lift, the air is forced to speed up more to get around it and by bernoulli's principle, lower pressure will accompany this. If the wing has undercamber (I don't know the correct term but imagine an umbrella) then the air is able to expand into the lower surface so it doesn't have to speed up as much to get past it thus increasing the pressure there.

I think that the best explanation of lift is the pressure differences caused by the movement of a body asymmetric to the flow. This is bound to happen- see cavitation.

With all that said, no lift would be produced without viscosity. If there was no viscosity the flow would be able to turn around the trailing edge from the lower surface, and high speeds (and low pressure on the lower surface) would accompany this resulting in no lift on the airfoil. Viscosity acts to resist the deformation of the fluid so the viscous force forces the air to leave the airfoil parallel to the trailing edge. Thus preventing the lower pressure from occuring on the lower surface.

Kinematic lift or lift due to momentum transfer from air particles does not need viscosity. I recall that flight at high altitudes is somewhat like this, but I am unable to find a reference.

I think bernoulli's principle can be understood physically as well on the level of the molecules. At a given temperature, all the molecules in a fluid parcel will have the same momentum, when the fluid parcel is stationary, the momentum is spread out in all directions and this is pressure. When the fluid parcel is moving, the momentum of the molecules is the same as this is set by the temperature, but some of this momentum is now "given" to moving with the fluid parcel reducing the total momentum of the molecules available for pressure. The takeaway from this is that higher speed doesn't cause lower pressure or vice-versa, they accompany each other

Is it possible to say if pressure differences cause high speed airflow? I believe it is, think of a cube of moving air approaching an airfoil in a wind tunnel. It would be interesting to see if the lift force appears after the moving cube of air passes over the wing or at some point before. The air will speed up only after passing the wing.

It all gets easier to understand if you accept that any difference in air pressure will normalize to ambient pressure as rapidly as it can
low pressure area absorbs higher pressures and high pressure flows to lower pressures
no mysteries - just rational behavior.
anything that gets in the way of this attempt at equalization of pressure , gets shoved around.
in this case - the wing goes with the flow.
from high to low.
who is this Newton guy?
Wayne?