Fundamentals of Lift: Differential Pressure & Wing Shape

[pivoxa15]

Is lift essentially created because of higher pressure at the bottom of the wing compared to the top of the wing, causing a postive net upward force?

With PA=F

And the difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it, achieved by the unique shape of the wing?

 

[Danger]

This has been dealt with fairly thoroughly. Here's a good start:
https://www.physicsforums.com/showthread.php?t=122128&highlight=airfoil+lift"

 

[Gokul43201]

I highly recommend the following thread, authored by arildno:
https://www.physicsforums.com/showthread.php?t=57710&highlight=lift

 

[Danger]

Good one, Gokul. I hadn't seen that before. Thanks.

 

[pivoxa15]

This has been dealt with fairly thoroughly. Here's a good start:
https://www.physicsforums.com/showthread.php?t=122128&highlight=airfoil+lift"

I am not after any detailed analysis. From this thread it seems that what I have posted is largely correct. I just need a confirmation of that and maybe some concise criticism if need be.

 

[Danger]

While the Bournoulli (sp?) effect has a part in it, most lift is in fact a result of air being deflected downward with the Newtonian reaction of the wing being deflected upward. Your best bet is to read the links whether or not you want to. You can't come to a serious science forum and then ignore the science behind the answer to your question.

 

[pivoxa15]

While the Bournoulli (sp?) effect has a part in it, most lift is in fact a result of air being deflected downward with the Newtonian reaction of the wing being deflected upward. Your best bet is to read the links whether or not you want to. You can't come to a serious science forum and then ignore the science behind the answer to your question.

I provided one scientific explanation for why lift exits and it seems to be a very worthy candidate. Your explanation of air being deflected downward hence wing deflected upward is completely complentary to (what I suggested) the Bournoulli principle because higher pressure (compared to the top of the wing) at the bottom of the wing results in more force per area. Hence more air molecules being deflected downward (then molecules deflected upward on top of the wing) after they hit the wing resulting in a net upward motion of the wing.

 

[Danger]

What I was trying to say is that the thing will lift even without a curved upper surface. That proves that the pressure differential isn't that important. I readily admit that when I was taking my flight training, they cited the Bournoulli principle. I didn't question it at the time, because I didn't care what held the damned thing up as long as I could play with it.
Anywho... if you've ever seen a hydroplane or F1 race car get airborne, you can see that it wasn't because of having a curved upper surface.

 

[rcgldr]

I created this thread as well:

https://www.physicsforums.com/showthread.php?t=107565&highlight=wings+lift

Is lift essentially created because of higher pressure at the bottom of the wing compared to the top of the wing, causing a postive net upward force?

Yes, when generating lift, the pressure below is higher than the pressure above.

And the difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it, achieved by the unique shape of the wing?

No, lift requires an effective angle of attack to accelerate air downwards, a flat board will generate lift. You can stick your hand out the window of a fast moving car and angle it so it produces lift. The unique shape of the wing just improves the lift versus drag ratio for an intended range of air speeds and wing loadings. In some cases, like civilian aircraft, efficiency is traded off for ease of manufacturing, which is why near flat bottom wings are used so often.

The speed of the air is relative to a frame of reference. Relative to the surrounding air, the air above a wing is traveling slower than the air below it.

As already mentioned, lift occurs because air is accelerated downwards, and the total force is simple force = sum of the mass of air molecules affected by a wing passing by, times the average acceleration of each air molecule.

At moderate AOA (angle of attack) even for a flat board, most of this downwards acceleration occurs from above the wing.

A simple explanation is that as a wing passes through the air with a moderate AOA, it deflects air downwards from below, and introduces a void as it passed through the air from above. This mostly downwards moving void creates a low pressure area that draws air towards it from all directions, except air can't flow upwards through the wing, so there's a net downwards acceleration of air towards this moving void.

Since the pressure above the wing is lower than the pressure below the wing, some of the air stream is "stolen" by the low pressure area, lowering the separation point of the air stream in front of the wing. This reduces the amount of air flowing below a wing, and adds some downwards component to the lower stream so that the wing has less deflection to do. The air that moves upwards from the separation point had to be accelerated downwards even more to fill in that void from above the wing, and this requires even more air from further above to help "push" the air into that sucking void. These are the main reasons that most of the downwards acceleration of air occurs from above a wing, even in the case of a flat board, as long as AOA is not extreme.

Getting back to air speeds above and below a wing, the speeds are different because air is being accelerated towards the low pressure area, and away from the high pressure areas. The speeds aren't constant either, but instead, the air is being accelerated, towards low pressure areas, and away from high pressure areas.

The classic Bernoulli theroem is a case of conservation of energy. The total engergy of a volume of air is it's kinetic energy (which is frame of reference relative), pressure, and temperature. A wing passing through the air peforms work on the air, accelerating the air mostly downwards and some forwards, and therefore it's changing the kinetic energy of the air. Using either the air or the wing as a reference, the downwards flow represents an increase in kinetic energy. Relative to the air, drag flow represents an increase in kinetic energy, and relative to the wing, a decrease.

curved upper surface

The most efficient airfoils used for gliders have curved upper and lower surfaces. If you draw a line from the leading edge the the trailing edge of a cross section of a wing, where the line is in the middle of the wing (equi-distant from upper and lower edges), the (downwards) curvature of this line is called camber. Gliders, both full scale and high end models is where most of the work in wing design is done these days. Wings are described as a basic airfoil shape, (the basic shape independent of camber), the thickness (% of the thickest part compared to the chord (distance from leading to trailing edge)), and the camber.

If you refer to the thread I posted, I included quite a few links on this subject.

 

[FredGarvin]

A glider's efficiency comes not so much from the aerfoil shape (it does play a big role though), but from the very high aspect ratio.

 

[rcgldr]

A glider's efficiency comes not so much from the aerfoil shape (it does play a big role though), but from the very high aspect ratio.

My point was that most of the current work is being done on gliders, specifically radio control gliders, since lift to drag ratio is important, especially for contest models. For powered models, it's not that important, many just use symmetrical airfoils, and in the case of full scale civilian aircraft, with a few exceptions with the "experimental" or kit aircraft, there's just not a lot of work being done, because new models aren't created that often.

As I mentioned, there is a NACA air foil replacement for the Cessna 182, that is cambered above and below, has a shorter wing chord, better lift to drag ratio, and more speed range, but it costs more to make than a flat bottomed wing, so it's rarely used except for experiements.

There's a lot of work being done on air foils, espeicially contest radio control gliders (since new models are made much more often than full scale models due to the cost). I refer to some of the airfoil designers, some of which are still very active, from a quote in my other thread below. I had the impression that the difference between a 50:1 glide ratio and a 60:1 glide ratio for a cross-country model has a lot to do with the airfoil and overall design of these gliders, it's gone beyond just aspect ratio.

The HQ airfoils by Dr. Helmut Quabeck are one example; he also makes actual rc contest gliders (most of this time is spent making the wing molds used to manufacture hollow modled composite wings). Selig / Donnivan teamed up to create the SD series of air foils, SD7037 is a popular air foil for rc gliders. Michael Selig / Ashok Gopalarathnam teamed up later to create the SA series of air foils. Rolf Girsberg made the RG airfoils, RG15 and the faster RG14.

 

[pivoxa15]

I created this thread as well:

https://www.physicsforums.com/showthread.php?t=107565&highlight=wings+lift

Yes, when generating lift, the pressure below is higher than the pressure above.

No, lift requires an effective angle of attack to accelerate air downwards, a flat board will generate lift. You can stick your hand out the window of a fast moving car and angle it so it produces lift. The unique shape of the wing just improves the lift versus drag ratio for an intended range of air speeds and wing loadings. In some cases, like civilian aircraft, efficiency is traded off for ease of manufacturing, which is why near flat bottom wings are used so often.

The speed of the air is relative to a frame of reference. Relative to the surrounding air, the air above a wing is traveling slower than the air below it.

As already mentioned, lift occurs because air is accelerated downwards, and the total force is simple force = sum of the mass of air molecules affected by a wing passing by, times the average acceleration of each air molecule.

You mentioned 'that most of the downwards acceleration of air occurs from above a wing'. If that is the case than there is there would be a net downward acceleration of air making the wing go downwards and not upwards.

Is this statement correct "The difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it created because of the angle of attack."?

 

[russ_watters]

You mentioned 'that most of the downwards acceleration of air occurs from above a wing'. If that is the case than there is there would be a net downward acceleration of air making the wing go downwards and not upwards.

No: action-reaction. Stand on a skateboard and throw a bowling ball in one direction and you go in the other. Throw air in one direction and you go in the other. It is easiest to see with a fan.

Is this statement correct "The difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it created because of the angle of attack."?

You have to be a little careful about how you define angle of attack. Effective angle of attack has the zero lift point as zero angle of attack, but geometric angle of attack (drawing a line from the tip to the trailing edge) is often negative at zero lift due to camber.

 

[rcgldr]

You mentioned 'that most of the downwards acceleration of air occurs from above a wing'. If that is the case than there is there would be a net downward acceleration of air making the wing go downwards and not upwards.

The air that's being accelerated downwards from above the wing is well above the wing, and is being accelerated towards a low pressure area immediately above the wing. As mentioned, the air is being accelerated towards this low pressure area from all directions but upwards, since the presence of the wing prevents upwards flow, so the result is a net downwards acceleration of air.

Is this statement correct "The difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it created because of the angle of attack."?

Without a change in velocity (or more accurately, a change in total energy), the air doesn't generate any force and/or peform any work, so it's not the velocities, but instead the change in velocties that matter. In the case of a normal wing, most of this change in velocity is downwards (lift) and a bit forwards (drag).

To summarize: a wing with an effective angle of attack travels through the air. This results in the creation of pressure zones that are different than the surrounding air, and the pressure above the wing is lower than the pressure below. Depending on the shape of the wing, a pressure zone may be created above the wing, below the wing, or both. Air is accelerated from higher pressure areas towards lower pressure areas. The presence of the wing prevents upwards air flow through the wing, so a net downwards acceleration occurs, corresponding to lift. The wing also prevents horizontal flow, so a net forwards acceleration of air occurs, corresponding to drag.

Afterwards, the air recovers by eventually returning to the velocity and pressure of the surrounding air, but relocated from where it started. In a closed system, a flying model will increase the pressure differential within the closed system, so that the net downwards force created by the pressure differential versus altitude within the closed system will be exactly the same as the weight of the air and the model within the closed system. (This assumes that there is no net vertical component of acceleration of the center of mass of the closed system.)

For an example of a wing that mainly produces lift from below, imagine a flat bottom wing inverted and traveling backwards, almost all of the lift is due to deflection, and the drag factor is high. NASA once considered a high drag lifting body with this type of shape as a space return vehicle.

 

[pivoxa15]

so a net downwards acceleration occurs, corresponding to lift.

Are you referring to the fact that the air pushes from below the wing causing an upward force (causing lift of the wings) and downward force on the air molecules.

 

[FredGarvin]

I had the impression that the difference between a 50:1 glide ratio and a 60:1 glide ratio for a cross-country model has a lot to do with the airfoil and overall design of these gliders, it's gone beyond just aspect ratio.

I never said it was just aspect ratio. There are a lot of aspects that are involved. However, the glaring, number one thing that all high efficiency aircraft have in common is a very large aspect ratio in an effort to reduce induced drag. Tip effects/losses tend to dominate on wings. I am sure that people are playing around with different sections along the spans and probably with varying wing twists as well. However, no matter what one does with the cross section, there is a reason why the U-2, Voyager and Global Flyer all have the designs they have.

 

[rcgldr]

so a net downwards acceleration occurs, corresponding to lift.

Are you referring to the fact that the air pushes from below the wing causing an upward force (causing lift of the wings) and downward force on the air molecules.

I think what I left out is the fact that in the typical case where most of the lift is due to air accelerated downwards from above the wing, that the air flows from above (and a bit in front) of a wing, and then flows downwards behind the wing as it passes by. There's a downwash of air at the trailing edge of a wing.

The "upwards" push occurs because the pressure above a wing is less than the pressure below a wing. It's possible for both these pressures to be less than the pressure of the surrounding air, or for the pressure below the wing to be the same as the surrounding air. All that is required is a difference in the pressures above and below the wing to produce lift.

Take the case of a flat bottom type wing. There' is a specific effective angle of attack where there is a low pressure area above the wing, but no change in pressure below the wing. If I remember correctly, a Cessna 182 can't fly this fast (200 to 300 knots, not sure), but a high powered radio control model with a flat bottom wing can.

So the result of this special case is a low pressure area just above the wing that moves with the wing, and no pressure change below the wing. Air accelerates from all directions towards this moving low pressure area, from above, from in front, from behind, from the sides, but hardly any flow from below, because the wing prevents upwards flow through the wing, except for the flow that goes around the wings edges. So the accelerations cancel except since the upwards flow is mostly blocked, the result is a net downwards acceleration of air from above the low pressure area, corresponding to lift. The wing also prevents some of the front to back flow, so there is also a net forwards acceleration of air, corresponding to drag.

As I just mentioned, the net flow starts from in front and above the wing, and ends up behind and below the wing. At the trailing edge of the wing, there's a downwash (lift) and some forwards flow (drag). At the leading and side edges, there's some upwards flow (which reduces lift somewhat). There's also an inwards flow from the sides, but these mostly cancel each other out.

The amount of air involved is huge. I've read that the mass of air affected per second by a small plane traveling at 100 knots is 2.5 to 5 times that of the plane. How much air is affected is somewhat subjective. Should you count the air that is only accelerated by .001g or less?

Now you could try to go through all sorts of complicated math, but Newtons laws aren't going to be violated, so we know that in level flight, lift will exactly equal the weight of an aircraft, and that the sum of the vertical component of forces from mass times acceleration of all the air molecules affected by the passing of the aircraft must be equal to the lift.

 

[rcgldr]

The most efficient airfoils used for gliders have curved upper and lower surfaces.

aspect ratio

I never said it was just aspect ratio

I didn't word my post clearly.

Generally in a contest glider, full scale or model, lift to drag ratios and adjustable camber for a range of air speeds is something that people involved are willing to pay for, regardless of the cost to manufacture such wings. This in turn, creates a lot of research activity into such airfoils.

In the case of most powered aircraft, the cost to manufacture is an important aspect of an airfoil, sacrificing some of the airfoils effeciency, with the exception for some experimental or special purpose aircraft, where again the cost to manufacture isn't as much of an issue. So most powered aircraft end up with airfoils that aren't as areodynamically efficient as they could be, because of the cost to manufacture aspect.

This is the reason I mentioned glider airfoils, because there is more research activity for them than there is for powered aircraft airfoils.

 

[rcgldr]

back to reality

In a typical aircraft, at normal cruise speed, angle of attack is high enough that there is significant higher pressure and downwards deflection from below a wing. However, the difference between this higher pressure and the surrounding air isn't as much as the difference between the surrounding air and the low pressure area above the wing. Part of this is because the air flows around the leading edge from below the wing to above the wing very close to the surface of the wing, but mostly because the low pressure area above the wing draws the air from in front of and below the wing, in effect stealing some of the air flow that would otherwise go below the wing, and the high pressure area below the wing, resists the air flow towards it, deflecting it upwards in front of the wing, directing even more of the air flow to over the wing.

So there's a significant upwash in front of and at the leading edge of a wing, which is the reason that most (but not all) of the lift is due to what is happening above the wing. In order for a wing to generate lift, it has to end up producing more downwash behind it than the upwash in front of it.

Other tidbits. Symmetrical airfoils are more efficient than cambered air foils at very low angles of attack, in other words at very high speeds. Cambered airfoils are more effcient at lower speeds. Since gliders generally fly at relatively slow speeds, they use airfoils with more camber for increased efficiency. One thing I'm trying to find again, is an aritcle that pointed out that cambered airfoils at low speeds reduced the upwash effect at the front of the wing by moving the center of pressure (differential) further back on the wing, increasing the distance between the pressure zones and the separation point of the air flow in front of the wing; ovbiously moving the center of pressure back helps, I'm not sure if it's the camber or if it's a more complicated aspect of airfoil design that accomplishes this.

 

[pivoxa15]

Take the case of a flat bottom type wing. There' is a specific effective angle of attack where there is a low pressure area above the wing, but no change in pressure below the wing. If I remember correctly, a Cessna 182 can't fly this fast (200 to 300 knots, not sure), but a high powered radio control model with a flat bottom wing can.

So essentially the difference in pressure between the top and bottom of the wing is a result of the angle of attack of the wing?

So the result of this special case is a low pressure area just above the wing that moves with the wing, and no pressure change below the wing. Air accelerates from all directions towards this moving low pressure area, from above, from in front, from behind, from the sides, but hardly any flow from below, because the wing prevents upwards flow through the wing, except for the flow that goes around the wings edges. So the accelerations cancel except since the upwards flow is mostly blocked, the result is a net downwards acceleration of air from above the low pressure area, corresponding to lift.

I see what you are getting at here. But I like to think about things from a slightly different persepective. I like to think that the fact that the air below the wing is blocked by the wing, the air molecules push against the wing hence creating lift (the air give the wing upward momentum). This way of thinking helps explain the mechanism behind lift and is more intuitive at least for me.

 

[russ_watters]

I like to think that the fact that the air below the wing is blocked by the wing, the air molecules push against the wing hence creating lift (the air give the wing upward momentum). This way of thinking helps explain the mechanism behind lift and is more intuitive at least for me.

More intuitive, but misleading, so it would be better to unlearn it. If a wing bottom is flat and the geometric angle of attack is zero, there will be nothing going on under the wing when it is moving that is any different from when it is stationary (from a pressure standpoint). And yet there will still be lift because of what is happening on the top surface of the wing.

Yes, it is true that the pressure under the wing is higher than above, so the positive force comes from under the wing, but it is important to understand that that is only true because the pressure above the wing has been lowered, while the pressure below has stayed the same in this case. And in every case of positive lift, the upper surface has a larger impact than the lower surface.

 

[rcgldr]

So essentially the difference in pressure between the top and bottom of the wing is a result of the angle of attack of the wing?

the effective angle of attack, and the air speed.

I like to think that the fact that the air below the wing is blocked by the wing, the air molecules push against the wing hence creating lift (the air give the wing upward momentum). This way of thinking helps explain the mechanism behind lift and is more intuitive at least for me.

The air molecules are pushing on the wing from all directions, including above and below, but in the case of lift, they molecules push with less force from above than they do from below. So this takes car of the pressure versus push issues.

But you don't get this for free, those pressure differences cause the air to accelerate, with the net result of a downwards and slightly forwards acceleration of air.

And in every case of positive lift, the upper surface has a larger impact than the lower surface.

Almost all cases, but not every case. As previously mentioned, there are some high drag airfoils that look like a very thick flat bottom wing that has been inverted and is flying backwards. As previously posted, NASA considered using such an airfoil as a lifting body for a returning space vehicle. The horrible looking thing could generate positive lift through deflection off the heavily angled bottom surface, usually with some negative angle of attack (at high speeds). The negative angle of attack mode of flight only occurred at high speeds. Once the speeds were slower, the vehicle would transition into a positive angle of attack, and in this case the lift from above may have had more affect than deflection from below.

Here's a link with some photos, ignore the first photo with the vehicles that looked like fat jets. It's the rest of the photos that show the lifting bodies with the steeply angled lower surface and a near flat backside.

http://www.nasa.gov/centers/dryden/news/FactSheets/FS-011-DFRC.html

I think the picture of the M2-F2 is the best one, showing the near horizontal upper surface and the angled lower surface while in flight. Note how thick the lifting body is compared to the fuselage of the much larger F104.

m2f2.jpg


Ultimately they dropped this idea (OK bad pun here), and ended up with a very poor glider in the form of the shuttle instead (it's needs something like 22 degrees nose down to maintain it's crusing speed while approaching it's landing area).

I'm not sure why it's important for these space return vehicles to have such a steep glide path, but almost all of them, including the shuttle, were designed to approach the landing area at a very nose down attitude, then flare out to slow to 200mph and then do a more normal type glide for landing.

 

[russ_watters]

Almost all cases, but not every case. As previously mentioned, there are some high drag airfoils that look like a very thick flat bottom wing that has been inverted and is flying backwards. As previously posted, NASA considered using such an airfoil as a lifting body for a returning space vehicle.

Fair enough - that's a pretty special case, though.

I'm not sure why it's important for these space return vehicles to have such a steep glide path, but almost all of them, including the shuttle, were designed to approach the landing area at a very nose down attitude, then flare out to slow to 200mph and then do a more normal type glide for landing.

Well, I'm not sure about "important", but I'd just say it is a biproduct of the facts that the shuttle is heavy and needs to be blunt.

 

[rcgldr]

need to be blunt

The website just mentions that blunt objects handle the heat of re-entry better, but one of the proposed vechicles had a sharp nose, and looked very aerodynamic. Maybe the purpose is that for re-entry into the atmoshpere, you want a significant amount of slowing at the edges of the atmosphere, so the overall amount speed and of heat build-up from re-entry is less as the vehicle descends.

Anyway, I've always liked the unusual look of those lifting body prototypes (they just don't look right), sort of the opposite of the flying wing concept, which was also cool.

 

[pivoxa15]

More intuitive, but misleading, so it would be better to unlearn it. If a wing bottom is flat and the geometric angle of attack is zero, there will be nothing going on under the wing when it is moving that is any different from when it is stationary (from a pressure standpoint). And yet there will still be lift because of what is happening on the top surface of the wing.

How does a flat wing with 0 angle of attack produce lift? I suppose it produces lift by pressure differences, so how is pressure differentials created?

 

[pivoxa15]

the effective angle of attack, and the air speed.

What about 0 effective angle of attack but with air speed in that the plane is moving? What is the mechanism that creates pressure differentials on top and below the wing?

But you don't get this for free, those pressure differences cause the air to accelerate, with the net result of a downwards and slightly forwards acceleration of air.

For lift to be maintained for some time, there must be new lower pressures on top of the wing. But the air accelerate downwards to equilibriate the pressure differences (plus the plane moves up to meet the downward air even faster) so how does the cycle of lower pressures on top of the wing be maintained during long lift periods?

 

[russ_watters]

How does a flat wing with 0 angle of attack produce lift? I suppose it produces lift by pressure differences, so how is pressure differentials created?

Well, this is why I said you should unlearn what you were thinking: the wing produces lift in that configuration because the pressure on the bottom surface is at 14.7 psi (atmospheric pressure) and the pressure on the top surface is significantly lower. Again, typically more lift is created by the lowering of pressure above the wing than is created by raising the pressure below it.

What about 0 effective angle of attack but with air speed in that the plane is moving? What is the mechanism that creates pressure differentials on top and below the wing?

Do you mean geometric angle of attack? By definition, "effective angle of attack" of zero is the angle of attack at which the wing produces no lift. For a flat-bottomed wing, a zero effective angle of attack sees the flat bottom pitched down several degrees.

Here is a graphic that shows how much more lift comes from the top surface than the bottom (not a flat-bottom, but still should be helpful): http://www.diam.unige.it/~irro/profilo4_e.html

Note: in that graphic, the vectors represent gage pressure, not absolute. So the small positive pressure on the bottom surface is above atmospheric while you have a large negative pressure (below atmospheric) pulling it up from above.

For lift to be maintained for some time, there must be new lower pressures on top of the wing. But the air accelerate downwards to equilibriate the pressure differences (plus the plane moves up to meet the downward air even faster) so how does the cycle of lower pressures on top of the wing be maintained during long lift periods?

Huh? Cycle? An airfoil is not static, it is moving. All of the conditions we are talking about are steady-state: the air is always accelerating downwards over the top of the wing because the wing is moving through the air and thus the pressure difference always exists.

 

[pivoxa15]

Well, this is why I said you should unlearn what you were thinking: the wing produces lift in that configuration because the pressure on the bottom surface is at 14.7 psi (atmospheric pressure) and the pressure on the top surface is significantly lower. Again, typically more lift is created by the lowering of pressure above the wing than is created by raising the pressure below it. Do you mean geometric angle of attack? By definition, "effective angle of attack" of zero is the angle of attack at which the wing produces no lift. For a flat-bottomed wing, a zero effective angle of attack sees the flat bottom pitched down several degrees.

Here is a graphic that shows how much more lift comes from the top surface than the bottom (not a flat-bottom, but still should be helpful): http://www.diam.unige.it/~irro/profilo4_e.html

Note: in that graphic, the vectors represent gage pressure, not absolute. So the small positive pressure on the bottom surface is above atmospheric while you have a large negative pressure (below atmospheric) pulling it up from above. Huh? Cycle? An airfoil is not static, it is moving. All of the conditions we are talking about are steady-state: the air is always accelerating downwards over the top of the wing because the wing is moving through the air and thus the pressure difference always exists.

I didn't know there were two different angle of attacks. What are their differences? This was the version of angle of attack I had alwasy been thinking, http://en.wikipedia.org/wiki/Angle_of_attack. Is it the effective angle of attack?

 

[rcgldr]

I didn't know there were two different angle of attacks. What are their differences? This was the version of angle of attack I had alwasy been thinking, http://en.wikipedia.org/wiki/Angle_of_attack. Is it the effective angle of attack?

No, the angle of attack as defined by wiki is relative to the chord of the wing. Other definitions of angle of attack use the line from the leading edge to the trailing edge as the definition. These definitions are based on the shape of the air foil, and in many cases you have non-zero lift with zero angle of attack. This is problematic, (at least for me). The solution is to define an angle of attack in such a way that the air foil doesn't matter, so ...

Effective angle of attack is defined as the angle relative to the angle of attack that produces zero lift. This definition is independent of the airfoil, and is more useful (in my opinion). By definition, zero lift means zero effective angle of attack, for any airfoil. Any lift at all means some amount of effective angle of attack, again, for any airfoil.

What about 0 effective angle of attack but with air speed in that the plane is moving? What is the mechanism that creates pressure differentials on top and below the wing?

At zero effective angle of attack, the (average) pressure above and below the wing are the same, so there's no lift, but the pressures may be different than the surrounding air. The only outcome is forwards acceleration of air, corresponding to drag.

For lift to be maintained for some time, there must be new lower pressures on top of the wing.

More correctly, a constant pressure differential where the pressure above the wing is less than the pressure below. The pressure zones being discussed here take into acount there is reduction of pressure differential because the air is responding to them.

The air accelerate downwards to equilibriate the pressure differences

But the air's momentum prevents it from accelerating fast enough to fully eliminate the pressure zones. The wing is moving fast enough that the air simply can never catch up, and the lag factor results in a pressure zone that is permanent as long as the effective angle of attack and air speed are maintained. Most of the downwash of air flows behind the trailing edge of the wing as it passes by.

the plane moves up to meet the downward air

We were discussing level flight, where there was only enough lift to keep a plane flying horizontal. However, the guys who do aerodynamics are one step ahead of you. They defined lift as the component of aerodynamic force perpendicular to the direction the plane is moving with respect to the air, and drag as the component in the direction the plane is moving (with respect to the air). In a steady climbing situation, the lift vector is tilted back a bit relative to the direction of gravity, and the drag vector is titled upwards a bit.

Now there's no rule that states lift has to equal the weight of the plane. When turning or looping, the lift has to be higher in order to generate enough centripetal force to keep the plane accelerating in a circular path. If the forces on a plane are not in balance, than you have acceleration. In order to increase the lift for a turn or loop, a higher angle of attack is required. The higher angle of attack also increases the drag, so more power is needed to compensate for the increase in drag. As an extreme example, a F16 fighter pulling 9 g's needs a huge angle of attack, and a speed of 440 knots or more. The drag from the huge angle of attack is so high that even full throttle and after burner only allows a speed of about 450 knots to be maintained.

so how does the cycle of lower pressures on top of the wing be maintained during long lift periods?

As stated, because the air's momentum prevents it from accelerating fast enough to eliminate the moving pressure zones created by a wing.

Have you ever witness the air flow behind a bus traveling on a highway? (Like leaves or debris being moved forwards by the air accelerated forwards due to the low pressure area behind the bus). The amount of air affected per second by the bus is the same after the bus has traveled 100miles as it is when the bus had only traveled 1 mile, as long as the bus is going the same speed and hasn't changed shape.

 

[russ_watters]

I didn't know there were two different angle of attacks. What are their differences?

I explained it in post 13 (and Jeff re-explained)

This was the version of angle of attack I had alwasy been thinking, http://en.wikipedia.org/wiki/Angle_of_attack. Is it the effective angle of attack?

Well, notice in the airfoil they show, the bottom surface of the wing is actually angled downward at zero geometric aoa. So both the bottom and the top produce a negative pressure region - the top just produces more.

 

[pivoxa15]

This is interesting stuff but I probably need to read a book with lost of pictures in order to fully understand the basics of this. Thanks for the introductions though.

 

[arildno]

First:
The "effective angle of attack" concept.
This comes from the flat-plate approximation, i.e, finding that angle a flat-plate wing would have, in order to produce the same lift as the actual wing in question.

A better concept, more in tune with reality, would be the "angle of escape"-concept (which, of course for the flat plate is the same as the angle of attack).

That is, a downwards angle of escape is directly seen to be related to the downwards deflection of air (and a corresponding lift, as in a normal action-reaction pair of forces).

Secondly:
Let us look at the proper setting of forces, namely Newton's 2. law of motion.
What is the ACCELERATION most readily associated with the lift force?

To answer that, simply look at the streamlines about the wing foil, as seen in the wing's rest frame.

Consistent with Newton's 3.law, the "majority" of these streamlines bend DOWNWARDS, i.e, i.e, the air has experienced a downwards force, and hence, the wing an upwards force (the lift).

BEND downwards..what sort of motion does this imply that the air has experienced?
Answer:
The air has undergone a CURVILINEAR motion; it was at the beginning moving strictly horizontally, but has, by passing by the wing gained a vertical component.

But, curvilinear motion is first and foremost associated with CENTRIPETAL acceleration, NOT tangential acceleration!

Thus, the force component properly related to the centripetal acceleration is the force component NORMAL to a fluid particle's trajectory, NOT the force component along the fluid particle's trajectory!

But Bernoulli's equation is merely the integral of F=ma ALONG a stream line (i.e, in the stationary case along a particle trajectory)...

But from this, it follows that the force as given by the pressure difference along the trajectory is not the force we should focus on!

Rather, we should focus on force given by the pressure difference ACROSS the streamlines, rather than along them (Crocco's theorem).

This is what I've done previously somewhere.

In general, we can replace ideally the effective angle of attack, with the effective CURVATURES of the wing foil, which display the intimate connection between foil shape and centripetal acceleration.
It should be emphasized that tipping a wing will change its EFFECTIVE curvature, even though its geometrical curvature remains unchanged.


As a simple illustration, consider what is done in the low-veløocity take-off phase:
Flaps go down, so that air following the underside of the wing experiences a centripetal acceleration with its centre of curvature way below the wing.
But that typically means that the pressure AT the underside must be GREATER than along the ground.
At the beginning of the take-off phase, we can say that the pressure ABOVE the wing is roughly equal to the GROUND pressure, that is, we have set up a lift-yielding presssure difference across the wing.

Once the plane is in the air, these flaps are no longer needed, since a low-pressure zone has been established on the upper side of the wing.