Lift Generation

[Die Hard]

Your answer is far too simplistic to be meaningful in any way. It would be more correct to say that lift is created by deflecting air downwards.

And yes, changing the angle of attack on a laminar flow airfoil does indeed change the effective shape of said airfoil.



You can clearly see how the effective upper surface of the airfoil is considerably larger/longer than the lower surface measured by airflow.

Over-simplifying something as complex as aerodynamics really doesn't help at all.

[Golfer]

Pressure.

Bernoulli.

[Stoney]

...by deflecting air downwards...Over-simplifying something as complex as aerodynamics really doesn't help at all.

 

I just don't understand that combination of statements.

[SgtPappy]

Stoney.. about the AoA theory .. That's what I found out by reading all your posts and answering questions that those answers spawned. Thanks again! 

I told my physics teacher about how his theory was wrong. I elaborated and he then asked 'then how is it possible that symmetrical supersonics airfoils have lift?' At first I was puzzled. I looked at my diagram on the board and found.. well. The Spitfire and P-38 books I have show cross sections of the wing. Their wings are close to symmetrical around the chord line. The only thing giving them lift is the fact that the chord line is tilted up.

Rooting a symmetrical airfoil to the aircraft while giving that airfoil a positive angle of attack will produce lift. The faster you are, the less AoA needs to be applied to have the same amount of lift at lower speeds. And there we have it.

Thus my final theory is that air is deflected upwards when in contact with the airfoil (plane is at zero AoA, but wings are ALMOST always at a positive given AoA). The deflected air is pushed downwards by the surrounding fluid/air streams and is accelerated down and along the top of the airfoil, since the plane is moving forward (downwash). The air near at the bottom of the wing is pulled upwards by the viscosity of the air itself. As the air is deflected upwards, it pulls more air with it. Once the stagnation point is apparent, the air has 'split' and the air traveling under the wing is spread out since it was 'pulled' on. Thus, high pressure at the bottom. 

[bozon]

SgtPappy & Charge,

The air above does not "catch up" with the flow from below, period. It has little to do with the "longer path" either. That is one of the urban myths that are very hard to root out since it has been quoted so many times (wrongly). It is almost as bad physics as the "CO2-caused global warming" that itself became truth, just by being quoted so many times (wrongly).

What you have to realize is that there is a distribution of velocities above the wing, and more importantly - toward the trailing edge. What is said above about air being pushed downward is half true. Air is being pushed downward, but continuity requires that somewhere air will be pushed upward as well - air does not accumulate below the plane. If you really are interested in this approach, the more accurate thing to say is that the air acquires net downward momentum.

I will try to take a different approach and try to explain the basic idea of lift with just a "drop" of physics:

It is the trailing edge that is responsible for the creation of lift. The flow around a corner creates a vortex, or a whirlpool if you like. Move a flat surface, such as a teaspoon in your tea and you will see it easily - a vortex behind each edge of the spoon. Leaving out the detailed physics, the direction of circulation is normally (if you are not too wild) "into" the corner. Translating this to the airfoil - the trailing edge, "lowered" into the airflow, creates this corner and the direction of circulation is "upwards" around the corner. This is the trailing-edge vortex. The front of the wing is thicker and more curved in an attempt to prevent a vortex from begin formed there, the way it does around both edges of the teaspoon.

Now consider a physical principle: "In a flow around obstacles, the total vorticity of an inviscid fluid is conserved".

Vorticity is a measure of rotation in the flow. This is related to conservation of angular momentum. A fluid is a strange thing. You disturb it in one place and it affects the flow everywhere (when not supersonic), because it pushes against itself. A vortex created in one place can affect circular motion in another place.

Inviscid means that it has no viscosity and a vortex will not decay. "But the air is not inviscid", you may claim. That is true, but the thing that is called the Reynolds number, if it is high (in our case it is), means that the time it takes to a vortex to decay due to viscosity, is much longer than the time it takes to travel the size of the "system" (i.e. the wing). So the total rotation of the air (of the wing-size scale flow) does not change as it flows over the wing.

What does this mean? It means that there has to be another rotational flow that counters the rotation created by the trailing edge vortex. This is the global flow around the wing. The trailing edge vortex creates a flow with its upper part directed against the direction of the wind. If the flow is from left to right (as in most pictures), the direction of rotation will be counter-clockwise. The global flow around the wing will have a component of clock-wise rotation to counter it. This global "rotation" is superimposed on the bulk flow and so the effect is faster air above the wing (rotation speed added to the flow) and slower air below (rotation speed subtracted from the bulk flow). From this point Bernoulli's principle kicks in and you get upward force.

If you increase the angle between the wing and the flow (before it is affected by the wing) too much, the curvature of the leading edge is not enough to prevent a vortex being formed there as well. The direction of this vortex will be opposite to the one at the trailing edge ("inward" direction around a corner). This subtracts from the large scale rotation around the wing and reduces the lift. Your wing is becoming a teaspoon. If you push it even further, the edge vortices will breaks into several smaller scale vortices of opposing directions (cancel each other out in terms of total vorticity) and a large scale rotation will not be produced. Keep going, and the rotation will break into vortices of all scales - turbulence.

This is the simple explanation. What it hides under the rag is the how/why the vortex is formed in the first place, or worse - how it breaks. This is an annoying problem that is not solved neither analytically, nor numerically. It is not a problem of not understanding the physics - it is a problem of mathematical "instability" or chaos.

I hope this helped.

[bozon]

Three more remarks:

1. Die Hard's picture of airflow is completely un-physical. It seems to have some Re number being stated on it, so I guess it is from some numerical code? It is still nonsense.

2. Tell your physics teacher that supersonic lift is a different thing. In supersonic flow what happens at the trailing edge cannot affect the flow before it hits the shock front. This is a huge difference.

3. "Angle of attack creates lift" is a poor answer, as absolute AoA is poorly defined and not every surface will create lift if you place it against the flow, regardless of the angle. Personally, I'd define AoA=0 when the wing creates no lift, by I know engineers like to define it differently.

[Stoney]

My answer of "Angle of Attack" is consistent with Prandtl's Theory of Thin Airfoils.  This is an accepted method with which to model lift, even though it does it through using the concept of infinite aspect ratio, which we all know doesn't exist in real life.  I'm sure Mr. Prandtl knew that too, and yet, his theory is accepted.

Given that many assumptions are made in order to model aerodynamic forces, I chose to include this statement in my original post: "So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind...Its generalistic and brushes over a few other facets of airfoil theory, but..."

Sorry that the "Bubba" answer doesn't satisfy everyone.

[Die Hard]

1. Die Hard's picture of airflow is completely un-physical. It seems to have some Re number being stated on it, so I guess it is from some numerical code? It is still nonsense.

Your comment is unnecessarily confrontational, and also untrue. The picture is a visual representation of a temporarily reattached airflow due to the formation of vortexes at extreme angles of attack. I intended however only to use it to show how a change in AoA also change the effective shape of the airfoil due to the changes in airflow around it.

Here are some NACA wind tunnel tests on the phenomenon my previous picture illustrated:





These photos are from experiments using plasma to reattach airflow at high AoA's:



You can clearly see how the effective upper surface of the airfoil is larger than the effective lower surface despite the upper and lower camber being almost identical.



This is a simulation of both pressure and airflow changes while increasing AoA. It may help people to intuitively understand how lift is created.

[bozon]

Your comment is unnecessarily confrontational, and also untrue. The picture is a visual representation of a temporarily reattached airflow due to the formation of vortexes at extreme angles of attack. I intended however only to use it to show how a change in AoA also change the effective shape of the airfoil due to the changes in airflow around it.

No confrontation was intended. My gripe against that picture was that it it shows the airflow COMPLETELY undisturbed behind the wing. This cannot be a state in which lift is produced (assuming constant air velocity). The plots you posted now are much more realistic.

My answer of "Angle of Attack" is consistent with Prandtl's Theory of Thin Airfoils.

Perhaps, but it doesn't explain why my keyboard makes a lousy wing.

[Stoney]

Perhaps, but it doesn't explain why my keyboard makes a lousy wing.

Touche'...I suppose I should have made a caveat that my answer applied to inherently aerodynamic objects only.  Spheres, cubes, keyboards, et al excepted...

[TimRas]

It is the trailing edge that is responsible for the creation of lift. The flow around a corner creates a vortex, or a whirlpool if you like.

Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.

[Chalenge]

Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.

More inertia then anything else.

[bozon]

Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.

The tennis ball, unlike a static wing, generates vorticity by its rotation - no rotation, no lift. Instead of distributing vorticity in a "smart" way like the wing does, it adds vorticity to the system at the expense of its own angular momentum. The end result is similar - the global flow around the spinning ball resembles a superposition of a bulk flow and a rotational flow (in the ball's spin direction). In both cases the objects leave a wake of vortices in the flow behind them due to this sheer in the flow - the flow from above does not rush to "catch up" with the flow from below.

[bozon]

... double post

[B3YT]

Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.

what you may find is that tennis balls  , footballs (soccer) do in fact have a "trailing" edge in-flight . They do not travel as pure spheres   but are flattened  out .   then after the the first moments of flight the spin of the ball creates the lift that causes it to change direction  or flight path