Shoe

Fair enough, but several here have invoked Newton's Second Law to suggest a required balance between the rate of change of the air's linear vertical momentum and the lift force acting on the wing. The point I am trying to make is that balance doesn't exist (it's definitely not required by Newton's Laws). Newton would certainly make a distinction between linear and angular momentum.

Tim Green

Since air is being shoved down by the wing, then the wing is going to react with an opposing force - a force we refer to as lift.

There's no getting by this point - when something changes the momentum of the air, that something will react with a force equal to that momentum change and in the opposite direction. And wings do change the momentum of the air - and they do indeed react to this - by lifting.

rmh

Forget about Newton - his laws describe only a portion of lift.
Besides that , his singing is not all that great

rmh

You get an A for perserverance -
for objective thought - - les than an A
carry on

Shoe

Agreed (as long as by "momentum change" you mean "rate of momentum change"). My issue is you have been arguing the converse of this... that when something (e.g. air) exerts a force on another object it will experience a rate of change of its momentum. This is true ONLY if that something experiences an UNBALANCED force.

You illustrated a few posts back that a RC helicopter hovering over a scale results in the air experiencing no unbalanced force (upward force exerted by the scale on the air is equal and opposite the downward force exerted by the helicopter on the air). If you don't conclude from this that the air is experiencing no net rate of vertical momentum change, then you truly do not understand Newton's Second Law.

You have stated over and over again that a lifting wing imparts net vertical momentum to the air, yet you have provided nothing to show that a wing imparts more downward than upward momentum (other than a flawed argument that Newton's Laws require this to be the case).

LouW

I’ve been away from the forum for a couple of years and much to my surprise when I return, I find the same discussion going on about how a wing produces lift and arguing whether lift is a result of pressure distribution or air deflection. It’s interesting that the same worn arguments are still much in use.

The problem is kinda like arguing over, “which blade of a pair of scissors cuts the cloth�. The truth is both are necessary for the scissors to cut. One blade can’t function without the other. Much of the confusion concerning lift arises because of the concept that air is flowing over a wing. In the real world of flight the air doesn’t flow at all. It just sits there at rest waiting for something to happen.

By and by a wing comes along being pushed or pulled by something. As it moves through the air the air has to get out of the way since the wing is a solid body. It begins to pile up slightly and the pressure goes up a little pushing the air either upward or downward. (The line where this division occurs is called the “stagnation point�, and the slight increase is called “dynamic pressure�). What happens next depends if the wing is moving at some positive angle of attack relative to its zero lift angle. If so, this is what happens.

As the air going upward moves away from the stagnation point it continues to be pushed away from its original position by the passage of the wing until the pressure is back down to the static pressure of the air at rest. At this point, since air has mass and has been accelerated to some vertical velocity, it continues to move upward creating a reduced pressure above the wing. Meanwhile the air that moved downward from the stagnation point continues to be shoved downward by the passing of the wing and the pressure remains at static or somewhat higher. The difference between the reduced pressure above the wing and the slightly higher pressure on the lower surface operating over some area equals the lift. (First round goes to the pressure guys.)

The air that was set in motion upward soon stops when it encounters the air above that is still at the prevailing static pressure. The air above begins to move down into the lower pressure area over the wing. It also has mass and being accelerated downward continues to move in that direction uniting with the air that had moved downward at the stagnation point and continues to move downward after the wing passes. (Veola! deflection.)

Here’s what it looks like from the beginning:

The air is just sitting there waiting for something to happen. By and by a wing comes along being pushed or pulled by something. It is dragging above it a bubble of low pressure. The air below the wing can’t move into it so it just presses up on the wing holding it up (lift). The air above the bubble is exposed to the same force as is acting on the wing (same area, same reduced pressure) but being fluid (with mass) it moves downward with an acceleration proportional to that force (f=ma). After the wing has passed, the downward moving mass of air slows and dissipates in turbulence as the pressures equalize. Finally the air is still again and sits there quietly waiting for something else to happen.

Lift is ultimately the result of inertia forces that are the only way that a fluid can effect a body moving freely within it. The pressure guys are correct because,

Force = Pressure x Area

And the deflection guys are correct because,

Force = Mass x acceleration

When designing wings, the pressure paradigm is most useful because it gives numbers.
When flying wings, the deflection paradigm is most useful because it is easier to visualize. One can’t exist without the other. They are equally true, just different ways of looking at the same phenomenon.

(Of course air flows over the wing in a wind tunnel, but that’s just a simulator so we can measure stuff as if the wing was moving through the air.)

(If you could paint individual particles of air some bright color so you could see them, as the wing passes you would see them being pushed and pulled around, bouncing up or down or twisting as the pressures move them. Finally after the wing has passed you would see them calm down until they were at rest again never far from where they were when the wing stirred them up. The energy for all this turmoil comes from whatever is pushing/pulling the wing through the air.)

chuckk2

I see that the alligators are getting in the way again.
Some of this can be thought of initially as a flat plate moving in the air.
Next, think of a triangle, with the top side more or less parallel to the movement in the air, and
with the bottom side having a positive angle of attack.  The rear side creates mostly drag.
Then, start adding complexity to the top by moving one of the angles to about 1/3d the length of the top, and closing the rear.
Refinements are made to reduce drag and increase lift under specific conditions.
  

rmh

Yes - the long winded stuff about Bernoulli and Newton and millions of carefully plotted curves etc., describe refinements to the basics-which are simply shapes which cause a pressure unbalance as the solid shape passes through the air
add to this the "rules" about needing to bank to turn- on and on- and the swirling prop stream (that one is a winner) and the mysteries deepen - like where did the Easter Bunny come from.
I have had casual observers ask " How can that thing fly ?" because my little park flyer has flat plates for wings .
This from engineers with degrees !
It is a never ending mystery -
The real mystery being - how did something so simple get so twisted.

Iron Bottom

I agree, Mr Hanson. It is , however, more than a little disturbing to me that you don't like to hear Mr. Newton sing "Donkey Shame".

Propworn

My Feedback: (3)

I read through the whole thread no one mentions liftonium why?

rmh

Probably because it does not push down on the earth

ltdive

I was very interested in reading this whole thread and all this very confusing formulas, physics, and bad science
I fly full size aerobatics (a real plane ) in real world environments as well as GA aircraft and have many buddies in air force
Bernoulli effect is wrong no amount of low pressure above a wing will provide anywhere near enough lift to get a plane off the ground Sorry but true
the main thing the low pressure air does is to help keep the vital BOUNDARY LAYER attached to the wings upper surface thereby reducing drag and accelerating the air molecules
The high pressure under the wing is correct because the wing moving through the air compresses it
When the two meet you get thrust

Lift is just the wing riding on the air (like a speed boat on water) and the air being accelerated off the trailing edge provides thrust and high lift is ie take off and landing with high angles of attack increased surface area(flaps)forces more air in a downward direction creating more thrust. ergo lift but also more drag which is why you need more power for take off than straight and level flight

If pressure differential is such a big player then how can it keep a 20 tonne jet fighter stay in the air ? their wings are the thinnest of all Oh and thin wings produce less drag at shallow angles of attack as they have laminerflow wings ie same top and bottom profiles
Sure some of you will say pressure differential works for them at the high speeds they travel . So explain how that works at takeoff and landing . It don't

High camber wings produce higher lift at lower velocities because they produce higher thrust but at the expense of drag at higher speeds caused by loss of boundary layer
because the air cannot stay attached to such a curved plane at these higher speeds
Power will help here to keep the plane flying overcoming the drag to a point
The loss of the boundary layer also kills off almost all low pressure air above the wing but you still have the compresses air below the wing to keep you going for a while until all thrust from the accelerated air above wing stops = stall

A wing stall can happen at any speed any attitude all you need to do is change angle of attack .trust me we do this all the time in aerobatics

lift is like other have said NOT ROCKET SCIENCE think of a wing like a skipping stone over a pond

The REAL science comes from reducing DRAG
Like others have said a flat plate will produce lift but it also produces loads of drag

Some of you will huff and puff at me and that's fine all i will do is fly my plane upside down while you scratch your head as to how i am able to stay in the air

BMatthews

You might want to look at the pressure distribution diagram for various airfoils at various sizes and angles of attack. All the ones I've seen that are generating any amount of lift show that the pressure drop over the top of the wing is greater than the pressure rise on the lower side. I'm not huffing and puffing or making this up. Look at the test results from wind tunnel testing and the like and see for yourself.

ltdive

True the pressure drop is greater but still not enough to generate the lift required to lift a 20 tonne jet off the ground would require close to 40 tonnes of lift as 20 tonnes of lift is only equal to the planes weight so is in equilibrium there is absolutely no way the low pressure can generate that amount of power let alone lift an A380 at over 540 metric tonnes
Yes low pressure is there and you do need it for efficient flight but it is used for the boundary layer to reduce drag
remember drag is the enemy for all aircraft that is where the science is

BMatthews

A 20 ton plane only requires very slightly more than that to climb unless it's a jet fighter standing on it's tail and going up like a missle. At which point the wings are only along for the ride and the "lift" is coming from the engines. So no, the wings don't need to generate double the weight of the plane. They only need to develop enough added lift to provide the upward acceleration.

You're still trying to separate the upper and lower side in your comments. But you simply can't do that and be accurate. It's the pressure DIFFERENCE between the upper and lower side. Both work together to give the wing its lift. It's the addition of the pressure rise on the lower side and the pressure drop that combine to give us the added difference that equals the lift.

You use the deflection idea. But when working fully within a fluid it's not about deflection but about pressure variation. The only lift due to pure deflection is with a planing boat that is going fast enough that it has risen up and is skipping off the surface of the water. That can only happen when there's a hard interface of the sort between water and air.

Run the math and see what I mean. It's been some years since I did it but as I recall a 747 at max takeoff weight only has to generate a 1.5'ish psi difference between the upper and lower surface to fly. There's so much wing area that the massive weight is spread out that much. And to put this in perspective the average human can generate a little over 2 psi when blowing up a party balloon. With that in mind is it so hard to accept that the big wing and massive engines can generate the same sort of pressure difference as a set of human lungs?

Your original post also claims that air cannot stay attached to the wing surfaces. Yet there's literally scads of wind tunnel work that has been photographed showing that it CAN and DOES stick to such surfaces. And that's a good thing since air that isn't attached to the wing indicates a stall condition.

ltdive

You still keep talking about windtunnel testing why????
It is not deflection that give lift it is a combination of faster air on the upper surface meeting the air from the lower surface that gives downward thrust

If air pressure is the reason a plane has lift then give tell me how a jet engine witch uses hundreds of small airfoils can possibly work ???
according to you it is the lift caused by the pressure difference on each airfoil and this includes the large fan at the front that provides 7o%of the thrust
And how pressure difference can then compress the air if it was pressure difference then the air would not move enter the engine
and as for you saying a boat on water is not the same think again both are fluids acting in the same way take it below the surface and hey presto thrust is the result
and dont look at props they use low pressure tests to remove the low pressure from the back side of props the reduce drag and cavitation

in wind tunnel testing look at the direction the air travels after it passes the airfoil. it moves down . at a higher speed than when when it arrived
Stop continuing this ancient Bernoulli myth go to flight school

bjr_93tz

Max take off weigh of a Boeing 777 is about 660,000lbs and has a wing area of about 666,500 square inches. 1psi of low pressure across the entire top surface of the wing will do the job which is a bit lower than what's available (at sea level at least). The top and bottom surface both perform work, in just what proportions depends on the conditions.

Of course something can only be "pushed" by the higher pressure as a house roof isn't sucked off by the moving wind but pushed off by the stationary air in the ceiling below, but in that case it's the top surface creating the pressure differential, not the bottom.

As for your final sentence? Assuming that posters here can't comprehend an airfoil being flown at a negative angle of attack is fairly lame.

ltdive

It is not negative angle of attack when a plane is inverted so yes you don;'t get inverted flight
Angle of attack is the relationship between the wing and the air the air being static whether a wing is upside down or knife edge or any attitude
As to your Boeing 777 remember the fly at 30 to 40 thousand feet where air pressure is very low i doubt they get anywhere near 1psi

ltdive

how does a propeller work ?

ltdive

explain a laminar flow wing

ltdive

if a jet fighter only needs slightly more to fly then how can it do the same job fully load with ordnance it has to get the extra lift from somewhere

ltdive

if a jet fighter only needs slightly more to fly then how can it do the same job fully load with ordnance it has to get the extra lift from somewhere? it can't fly any faster.

flycatch

My Feedback: (26)

For those that argue the point made by the professor, don't. A book entitled "Radio Control Scale Aircraft" written by Gordon Whitehead supports his findings.

flycatch

My Feedback: (26)

A jet engine works because it creates a low pressure in-front of the intake. The thrust of the engine pushes the body of the airframe into this low pressure.

bjr_93tz

Are you serious? If the airstream is at a negative angle to the airfoils centreline then the airfoils angle of attack is negative. The airplanes orientation to the Earth (inverted or upright) isn't considered in this convention. Sure, "positive" is arbitary but once you decide which way is positive the other way is negative.

I'm not sure what pressure drop they'ed get but even blind Freddy can see the little condensation clouds a wing can generate on their upper suface and you need a significant pressure drop in a short space of time to get the required air temperature reduction for those to form.