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Faro
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All Wing Lift = Net Downward Airflow?

Fri May 04, 2012 8:18 am

I am having difficulty conceptualising the lift force provided by a fixed-wing aircraft.

In the final analysis, do all and any fixed wing arrangements achieve lift by generating a net downward airflow which imparts an upward reaction force equal to the weight of an aircraft, as is the case with helicopters?

If yes, where and how can one ‘place’ this net downward airflow? The upper wing surface is being ‘sucked’ up by low relative air pressure as the lower wing surface is being pushed up by high relative air pressure. I have difficulty reconciling these two differential air pressure surfaces with the generation of a net downward airflow; how does this happen?

Finally, if we could somehow visualise such net downward airflow, what would we see?


Faro
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Mir
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RE: All Wing Lift = Net Downward Airflow?

Fri May 04, 2012 2:08 pm

Quoting faro (Thread starter):
In the final analysis, do all and any fixed wing arrangements achieve lift by generating a net downward airflow which imparts an upward reaction force equal to the weight of an aircraft, as is the case with helicopters?

Yes, minus whatever component of the engines' thrust is in the downward direction. Sometimes (when changing trajectory), the lift will be more or less than the weight, but in general in should be equal.

Quoting faro (Thread starter):
If yes, where and how can one ‘place’ this net downward airflow? The upper wing surface is being ‘sucked’ up by low relative air pressure as the lower wing surface is being pushed up by high relative air pressure. I have difficulty reconciling these two differential air pressure surfaces with the generation of a net downward airflow; how does this happen?

You can't have a pressure differential without downwash behind the wing - the way the air flows around the wing will necessarily create downwash in the same way it creates the pressure differential.

Quoting faro (Thread starter):
Finally, if we could somehow visualise such net downward airflow, what would we see?


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jetmech
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RE: All Wing Lift = Net Downward Airflow?

Sat May 05, 2012 1:42 am

Quoting faro (Thread starter):
In the final analysis, do all and any fixed wing arrangements achieve lift by generating a net downward airflow which imparts an upward reaction force equal to the weight of an aircraft, as is the case with helicopters?

According to J.D. Anderson jr. , in "Fundamentals of Aerodynamics," all the aerodynamic forces experienced by a heavier than air machine interacting with relative velocity to a body of fluid are due to pressure and shear stress distributions only. This is true no matter how complex the shape of the machine, and no matter what the flow regime is (supersonic, subsonic etc.).

Thus, lift would have to be primarily the result of pressure distributions over the top and bottom surfaces of the wing. If the average value of the pressure distribution over the top surface of the wing is less that that over the bottom surface of the wing, the will be a net force (lift) in the direction of the top surface of the wing.

Quoting faro (Thread starter):
If yes, where and how can one ‘place’ this net downward airflow?

As stated by Mir, the downwash is the result of the pressure differential. As far as the aerodynamic behaviour of the aircraft is concerned, it would be far more important to accurately "place" the centroid of the pressure distributions in relation to the chord of the wing. This location is known as the centre of pressure.

Quoting faro (Thread starter):
I have difficulty reconciling these two differential air pressure surfaces with the generation of a net downward airflow; how does this happen?

Have a read of the following thread;

AA-757 Wing Delamination In Flight (by Jetmech Aug 14 2010 in Tech Ops)

If the airflow follows the contours of an airfoil closely (without flow separation) whilst generating a net (upward) lift force, one result is a "downwash" off the trailing edge. What should be remembered however is that theories of lift such as the circulation and downwash theories of lift are theories only.

These theories gain acceptance and relevance as they allow us to calculate fairly accurate answers from physical quantities that are relatively easy to measure. However, these theories do not necessarily accurately describe the actual physics of the flow field.

Regards, JetMech
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RE: All Wing Lift = Net Downward Airflow?

Mon May 07, 2012 4:34 am

Quoting faro (Thread starter):
In the final analysis, do all and any fixed wing arrangements achieve lift by generating a net downward airflow which imparts an upward reaction force equal to the weight of an aircraft

Yes, in level or un-accelerated flight this holds true for any flying object, regardless of how it generates the downwash. This is a simple manifestation of Newton's laws of motion. The whole trick is of course generating the downwash in the most efficient manner possible!

Quoting faro (Thread starter):
If yes, where and how can one ‘place’ this net downward airflow?

One way to think about it is that the wing turns the incoming flow, in effect "shoving air down". Imagine a small control volume above or below the wing, which is being accelerated downwards due to differential pressure; you can think of this as the centripetal acceleration of the turning flow. In the far field (at infinity) the pressure must be equal to ambient, so the top surface of the wing is necessarily below ambient pressure and the bottom surface of the wing is necessarily above ambient pressure. This is not a totally correct explanation, but it's intuitive enough to reconcile the pressure picture with the downwash picture.

Quoting jetmech (Reply 2):
However, these theories do not necessarily accurately describe the actual physics of the flow field.

The downwash explanation is 100% correct regardless of how complex the flow field. It's not a theory, it's the law.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 12:03 am

Quoting WingedMigrator (Reply 3):
The downwash explanation is 100% correct regardless of how complex the flow field. It's not a theory, it's the law.

No doubt it is a real and measurable phenomenon, but I still think it is a by product rather than the cause of lift generation. It is a relatively accurate way to describe lift based upon a fairly easy to visualise quantity, but it is not the actual phenomenon responsible for the flow field behaviour.

I think the real mechanism at the root of lift generation would be that which causes the flow field to closely follow the shape of an airfoil, particularly the upper contour. This would probably have more to do with other mechanism such as the flow field behaviour in the boundary layer.

Regards, JetMech
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tdscanuck
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 2:35 am

Quoting jetmech (Reply 4):
No doubt it is a real and measurable phenomenon, but I still think it is a by product rather than the cause of lift generation. It is a relatively accurate way to describe lift based upon a fairly easy to visualise quantity, but it is not the actual phenomenon responsible for the flow field behaviour.

The "cause" of lift generation is differential pressure on the wing. That's it. Viscous forces almost exactly balance in the vertical direction so they contribute little/no lift force (they're much more relevant for drag).

This is *exactly* equivalent to the "cause" of lift generation is the wing shoving the air down. It's exactly the same physics applied in exactly the same way to exactly the same system. The only difference is where you draw the boundary for the integral and that's just a math trick that doesn't impact the physics or the result at all.

This is different from the question of *why* does the airfoil shove air down (equivalently, why is the net pressure lower on top than on the bottom). That's a much more complex and subtle question, but also a different question.

Any shape moving through a fluid that causes a net downwash will generate lift and it will have a differential pressure on it. The two statements are completely equivalent.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 8:27 am

Quoting tdscanuck (Reply 5):
The "cause" of lift generation is differential pressure on the wing. That's it. Viscous forces almost exactly balance in the vertical direction so they contribute little/no lift force (they're much more relevant for drag).

This is *exactly* equivalent to the "cause" of lift generation is the wing shoving the air down. It's exactly the same physics applied in exactly the same way to exactly the same system. The only difference is where you draw the boundary for the integral and that's just a math trick that doesn't impact the physics or the result at all.

This is were the confusion sets in for me: on the one hand there is i) a 'suction' force acting on the upper wing surface and on the other ii) a net downward 'thrust' generated off the wing trailing edge being the downwash. We all know that this suction force exists because wing upper skin surfaces are stressed upward and designed (and tested) to 'carry' such upward loading. The 'suction' force is to my mind a specific, distinct force from the downwash.

Are you saying that i) and ii) are exactly the same? I can understand that they both arise from a differential pressure phenomenon, yes. But *exactly* the same, that's a tougher sell...maybe there's another way of looking at the how one can reconcile them intuitively? Something like: 'suction' accounts for X% of total lift and downwash accounts for the remaining Y%?

I guess what exarcerbates the confusion is that all lay references to the mechanics of flight focus on the low-upper-wing-air-pressure vs high-lower-wing-air-pressure as the cause of lift. Hardly anyone ever mentions downwash even if it's an undissociable part of the equation...


Faro

PS: Assuming a wing at zero AoA, on what surface does the downwash exert its reactive, upward force? I just can't visualise this...

[Edited 2012-05-08 01:58:14]
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 9:22 am

Quoting faro (Reply 6):
Are you saying that i) and ii) are exactly the same?

Yes.

Quoting faro (Reply 6):
I can understand that they both arise from a differential pressure phenomenon, yes. But *exactly* the same, that's a tougher sell...maybe there's another way of looking at the how one can reconcile them intuitively? Something like: 'suction' accounts for X% of total lift and downwash accounts for the remaining Y%?

You can't split them apart because they're the same thing. The lift you get from adding up pressure contributions all over the airfoil is exactly the same (in physical origin, magnitude, and direction) as adding up all the downwash from the same airfoil. Suction accounts for 100% of the lift. Downwash accounts for 100% of the lift.

In some sense, pressure differential is "closer" to the airfoil since pressure and viscosity are the only interaction the airfoil has with the air (and viscosity has almost no contribution to the lift force). However, if you have a pressure differential you have downwash and if you have downwash you have pressure differential so you can't disconnect them.

Quoting faro (Reply 6):
I guess what exarcerbates the confusion is that all lay references to the mechanics of flight focus on the low-upper-wing-air-pressure vs high-lower-wing-air-pressure as the cause of lift. Hardly anyone ever mentions downwash even if it's an undissociable part of the equation...

It's considerably more abstract to think about; I suspect that's why it doesn't come up until you get into more advanced aerodynamics.

Quoting faro (Reply 6):
PS: Assuming a wing at zero AoA, on what surface does the downwash exert its reactive, upward force? I just can't visualise this...

If it's at zero AoA it's at zero lift, therefore no downwash.

If it's at non-zero AoA, you have a pressure differential and downwash. The pressure differential is what's actually "pushing the wing". This is exactly analogous to a jet engine, which is almost always described in lay references as being a momentum transfer (read "big downwash") but, just like an aifoil, the actual force transfer from the fluid to the device is pressure differential.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 9:50 am

Quoting tdscanuck (Reply 7):
If it's at zero AoA it's at zero lift

How can that be? An airfoil advancing through the airstream at zero AoA still generates lift because of the lower air pressure on its upper surface...am I missing something big here?

And how about this arrangement: zero AoA and I bolt onto the trailing edge of the wing a broad (and very rigid), flat metallic sheet that impedes net downward airflow and therefore kills downwash. I should still get lift because of the lower relative air pressure on the upper wing surface, no? Or will the downward force of the impeded air impinging on the sheet negate my lift generated off the upper wing surface?

Faro

[Edited 2012-05-08 02:56:40]
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 3:45 pm

Quoting tdscanuck (Reply 7):
It's considerably more abstract to think about; I suspect that's why it doesn't come up until you get into more advanced aerodynamics.

I'm not sure if downwash is all that abstract, since it's quite easy to visualize. And to conceptualize: action, reaction. It's an unfortunate quirk of the way aerodynamics are typically taught that things aren't better related to Newton's laws right from Aero 101. As a result, lots of pilots (pilots!) don't understand that no downwash = no flight, and lots of kids who ask about how airplanes fly get a complicated answer involving pressure and airfoils, when a simple "shoves air down" will do-- and relate very well to every kid's experience of swimming as "shoves water back".

Quoting faro (Reply 8):

How can that be? An airfoil advancing through the airstream at zero AoA still generates lift because of the lower air pressure on its upper surface...am I missing something big here?

Yes. At zero AoA the flow is exactly symmetrical and the pressure on the upper surface equals the pressure on the lower surface. The net result is zero lift.

A useful mental exercise is to *stop* thinking about airfoils. Airfoils are important because they are the most efficient shapes at doing what they do, but forget about them for a moment, get back to basics, and reflect on how a paper airplane (with flat wings!) achieves flight. Until you can understand and explain this, airfoils are a distraction.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 5:24 pm

Quoting WingedMigrator (Reply 9):

Yes. At zero AoA the flow is exactly symmetrical and the pressure on the upper surface equals the pressure on the lower surface. The net result is zero lift.

If it's a symmetrical airfoil, yes. A cambered airfoil can still generate lift (and equivalent downwash) at zero AOA.

Quoting faro (Reply 6):
This is were the confusion sets in for me: on the one hand there is i) a 'suction' force acting on the upper wing surface and on the other ii) a net downward 'thrust' generated off the wing trailing edge being the downwash. We all know that this suction force exists because wing upper skin surfaces are stressed upward and designed (and tested) to 'carry' such upward loading. The 'suction' force is to my mind a specific, distinct force from the downwash.

Are you saying that i) and ii) are exactly the same? I can understand that they both arise from a differential pressure phenomenon, yes. But *exactly* the same, that's a tougher sell...maybe there's another way of looking at the how one can reconcile them intuitively? Something like: 'suction' accounts for X% of total lift and downwash accounts for the remaining Y%?

Here's a quasi-analogy for you that is perhaps easier to grasp:

You're standing on a boat, and you throw a rock forward. You (and the boat) move backward in reaction. One could take the Newtonian view, and determine the mass and acceleration of the rock, and therefore the backwards force on you and the boat. Or, one could sum the pressure exerted on your hand by the rock while you're in the throwing motion, and arrive at the same result.

The Newtonian view says "there MUST be an equal and opposite reaction". But how is that reaction transmitted to you? By the pressure on your hand when you throw the rock.
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 5:31 pm

Quoting WingedMigrator (Reply 9):
Quoting faro (Reply 8):

How can that be? An airfoil advancing through the airstream at zero AoA still generates lift because of the lower air pressure on its upper surface...am I missing something big here?

Yes. At zero AoA the flow is exactly symmetrical and the pressure on the upper surface equals the pressure on the lower surface. The net result is zero lift.

That is probably the most amazing thing I have ever read in 5 year of a.net; I am simply astounded by that assertion.

I guess aerodynamics is indeed quite an intricate, if not abstract discipline. Not to mention that all that lay literature on lift/flight is just horrendously, heinously misleading.

Thanks Tom and Migrator, learned something quite significant today...


Faro
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tdscanuck
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 5:31 pm

Quoting faro (Reply 8):
Quoting tdscanuck (Reply 7):
If it's at zero AoA it's at zero lift

How can that be? An airfoil advancing through the airstream at zero AoA still generates lift because of the lower air pressure on its upper surface...am I missing something big here?

AoA is almost always referenced to the zero-lift angle. It gets kind of messy with cambered airfoils but, as WingedMigrator correctly notes, airfoils are really a distraction. Think about a symmetric airfoil or a flat plate...at zero AoA they just generate drag, no lift.

AoA is sometimes referenced to the chord line, in which case you can get some lift at "zero" AoA but that's a somewhat arbitrary choice of AoA reference. Every lifting shape always has some AoA where the lift is zero. For convenience, that's usually where you want to define AoA = zero. Put another way, you want to choose zero AoA so that the Cl vs. AoA curve goes through the origin.

Quoting faro (Reply 8):
And how about this arrangement: zero AoA and I bolt onto the trailing edge of the wing a broad (and very rigid), flat metallic sheet that impedes net downward airflow and therefore kills downwash. I should still get lift because of the lower relative air pressure on the upper wing surface, no?

No. If you actually kill the downwash you will have no net pressure difference and no lift. (which technically takes an finitely long board but a sufficiently large one will do in practice).

Quoting faro (Reply 8):
Or will the downward force of the impeded air impinging on the sheet negate my lift generated off the upper wing surface?

No. You simply won't have any pressure differential across the wing. You're not creating opposing forces, you're killing off the force in the first place.

Note: this is all about subsonic airfoils with respect to flat boards stuck to the trailing edge...supersonic is a totally different issue for the details, although the downwash/pressure relationship is the same.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 08, 2012 10:56 pm

This site is great for helping to visualize things... An oldie but a goodie, it's been up as long as I can remember.

http://www.av8n.com/how/htm/airfoils.html

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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 1:46 am

Quoting vikkyvik (Reply 10):
If it's a symmetrical airfoil, yes. A cambered airfoil can still generate lift (and equivalent downwash) at zero AOA.

Depends on what you're using as a reference for AoA. If you're using the chord line (as is common in basic flight training aerodynamics lectures), then you are correct. However, there's also something called "effective AoA", and that uses the angle at which no lift is produced as its zero point (if you're using the chord line as a reference, this would appear to be a negative AoA).

I find the first method easier to work with, because it's easier for people to understand, but I'm more in the flight education field than the engineering field, so I can understand how others would prefer to use effective AoA. It's like using different units of measurement - neither is any more correct or incorrect than the other, though you may have to do some conversion when comparing numbers with someone else.

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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 6:02 am

Quoting tdscanuck (Reply 5):
This is different from the question of *why* does the airfoil shove air down (equivalently, why is the net pressure lower on top than on the bottom). That's a much more complex and subtle question, but also a different question.

I agree! And this is the question that most interests myself, as it is this more fundamental aspect of fluid dynamic behavior that greatly enhances the production pressure distributions about an airfoil and downwash. I suspect that the most important physical aspect of the flow field leading to effective generation of lift (negligible flow separation) is that which causes the flow field to closely follow the upper contour of the airfoil.

Quoting tdscanuck (Reply 5):
Any shape moving through a fluid that causes a net downwash will generate lift and it will have a differential pressure on it. The two statements are completely equivalent.

Certainly, but is there an even more fundamental fluid dynamic phenomenon that causes the pressure distributions and downwash?

Quoting tdscanuck (Reply 5):
Viscous forces almost exactly balance in the vertical direction so they contribute little/no lift force

Sure, but the viscous action in the boundary layer is a major contributor to the effective generation of lift. We can certainly have lift and contour following without viscosity, but an airfoil operating in such a fluid would have a much lower maximum lift coefficient.

Quoting faro (Reply 6):
Quoting tdscanuck (Reply 7):

Further to Tom's explanation. An alternate explanation is to imagine an airfoil operating with a small positive angle of attack. It is fairly easy to see that the flow field interacting with the bottom surface of the airfoil has no choice but to be deflected downwards.

The situation for the upper surface of the airfoil may be harder to understand. If the flow field closely follows the upper wing contours, it will also leave the trailing edge of the airfoil with a downward deflection.

I suppose confusion can arise as the high pressure is on the lower surface of the wing, which tends to suggest that there should actually be an "upwash" from the trailing edge of the airfoil. However, in reality, I'd say the momentum contained in the flow field that is deflected downward by the airfoil can easy override this pressure differential.

Quoting WingedMigrator (Reply 9):
As a result, lots of pilots (pilots!) don't understand that no downwash = no flight, and lots of kids who ask about how airplanes fly get a complicated answer involving pressure and airfoils, when a simple "shoves air down" will do

Again, I feel that downwash is the end result of other more fundamental flow field behaviours. Lift generation may result in downwash, but the presence of downwash does not guarantee the presence of lift. I think the "shoves air down" will do is often part of what causes so much confusion.

Quoting WingedMigrator (Reply 9):
how a paper airplane (with flat wings!)

I think the only main difference is that a flat wing produces a large amount of drag for a given lift when compared to a cambered airfoil. If anything, I'd say the gross flow separation over the top surface of a flat wing would severely reduce the magnitude of downwash.

Quoting faro (Reply 6):
Assuming a wing at zero AoA, on what surface does the downwash exert its reactive, upward force? I just can't visualise this...

The downwash cannot exert force upon the airfoil. It is the differential pressure distribution that exerts the force on the airfoil. The downwash is merely a physical “side effect” of this pressure differential. The pressure differential is itself the result of more fundamental fluid dynamic behaviours

Regards, JetMech
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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 7:49 am

Quoting jetmech (Reply 15):

Again, I feel that downwash is the end result of other more fundamental flow field behaviours. Lift generation may result in downwash, but the presence of downwash does not guarantee the presence of lift.

The F=mA crowd would disagree - if you have net downwash you have lift.

The force on the airplane (its weight if in level flight) is equal the mass of the air deflected down times the acceleration of that air.

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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 8:15 am

Quoting ADent (Reply 16):

The force on the airplane (its weight if in level flight) is equal the mass of the air deflected down times the acceleration of that air.

But how is that force transmitted to the wings? By pressure differentials.

It's pretty meaningless (in my opinion) to say which causes which. They're both manifestations of the same thing, and one can't exist without the other.

Quoting jetmech (Reply 15):
but the presence of downwash does not guarantee the presence of lift.

If you're shoving air down (   ), then there HAS to be an equal and opposite reaction. In the case of wings, it happens to be lift.
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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 10:03 am

Quoting vikkyvik (Reply 17):
Quoting jetmech (Reply 15):
but the presence of downwash does not guarantee the presence of lift.

If you're shoving air down (   ), then there HAS to be an equal and opposite reaction. In the case of wings, it happens to be lift.

Perhaps one can think of it like this: a parcel of air being shoved down implies a like deficit of this same parcel somewhere else. That deficit of air (pressure) is located on the upper wing surface. Given this location, it has an upward, 'suction' effect on that upper wing surface. The role of the cambered upper wing surface is then to 'bend' the reactive, upward counter-force of the downwash so that it impinges on the upper wing and lifts it. Would this be accurate?

From what I understand, trying to ascertain which phenomenon gives rise to the other is impossible or, rather, not a logical question in aerodynamics. It's like Yeat's saying "How can we separate the dancer from the dance?".


Faro

[Edited 2012-05-09 03:04:00]
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tdscanuck
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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 2:45 pm

Quoting jetmech (Reply 15):
Lift generation may result in downwash, but the presence of downwash does not guarantee the presence of lift.

Yes, it does. Otherwise you don't have conservation of momentum.

Quoting faro (Reply 18):
From what I understand, trying to ascertain which phenomenon gives rise to the other is impossible or, rather, not a logical question in aerodynamics. It's like Yeat's saying "How can we separate the dancer from the dance?".

This is an excellent observation; it's not really a logical question in aerodynamics. Which view makes more sense in any particular problem can change (rockets and engines usually analyze with momentum, wings with pressure, but either works) but those are just different mathematical tools to look at exactly the same physical thing.

A good example of when you might want to change which tool you're using is measuring the drag coefficient of 2D airfoils. Rather than try to make a really nice wind tunnel balance that can precisely hold an airfoil in the tunnel right up against the wall without any contact, you just measure the speed of the wake parallel to the tunnel. The momentum deficit in the wake is exactly equal to the drag. You can very accurately measure the drag of any shape this way without ever having to look at pressure at all.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 4:17 pm

Quoting jetmech (Reply 15):
the presence of downwash does not guarantee the presence of lift.

As several have already pointed out, this statement betrays a fundamental misunderstanding of the mechanics of flight! The magnitude of the downwash (in momentum rate, i.e. how much downward momentum is generated in the fluid per unit time) is precisely equal to lift. To the last Newton. In subsonic, supersonic, or hypersonic flight. It's not just a rule of thumb, it's the law of physics.
 
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RE: All Wing Lift = Net Downward Airflow?

Wed May 09, 2012 6:42 pm

Quoting faro (Reply 18):

Perhaps one can think of it like this: a parcel of air being shoved down implies a like deficit of this same parcel somewhere else. That deficit of air (pressure) is located on the upper wing surface. Given this location, it has an upward, 'suction' effect on that upper wing surface. The role of the cambered upper wing surface is then to 'bend' the reactive, upward counter-force of the downwash so that it impinges on the upper wing and lifts it. Would this be accurate?

I don't think that's a good way to look at it, because you're still separating the two effects, while in reality, they're both part and parcel of the same flow field.

Maybe this is a good way to understand it: say you have air flowing around some objects. Here is your flow field. Here is the related pressure field, and here is the related momentum field. If you then change the arrangement of the objects, you get a new flow field, with a new momentum field and new pressure field. You can't change one without changing the other; it's not a physical possibility. You're just measuring different characteristics of the flow field.

Quoting tdscanuck (Reply 19):
A good example of when you might want to change which tool you're using is measuring the drag coefficient of 2D airfoils. Rather than try to make a really nice wind tunnel balance that can precisely hold an airfoil in the tunnel right up against the wall without any contact, you just measure the speed of the wake parallel to the tunnel. The momentum deficit in the wake is exactly equal to the drag. You can very accurately measure the drag of any shape this way without ever having to look at pressure at all.

We did exactly that in one of my college classes.
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jetmech
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RE: All Wing Lift = Net Downward Airflow?

Thu May 10, 2012 6:41 am

Quoting WingedMigrator (Reply 20):
As several have already pointed out, this statement betrays a fundamental misunderstanding of the mechanics of flight! The magnitude of the downwash (in momentum rate, i.e. how much downward momentum is generated in the fluid per unit time) is precisely equal to lift. To the last Newton. In subsonic, supersonic, or hypersonic flight. It's not just a rule of thumb, it's the law of physics.

I agree! As I have stated previously, theories of lift such as the momentum theory gain traction as they allow us to accurately calculate a quantity of interest (i.e. lift) from other quantities that themselves are easy to measure (i.e. downwash, momentum, pressure distributions etc.).

However, downwash - even though it may be equal to the lift force and vice versa - is a by-product or result of producing lift, lift is not the result of downwash. The detailed fluid dynamics that are the root cause of lift production have downwash as a by-product; it is not the presence of downwash that causes the detailed fluid dynamic behaviour.

Quoting ADent (Reply 16):
The F=mA crowd would disagree - if you have net downwash you have lift.
Quoting tdscanuck (Reply 19):
Yes, it does. Otherwise you don't have conservation of momentum.

If I allow a flat sheet of cardboard to fall from a height to the floor under the action of gravity, there is no doubt a downwash imposed upon the air underneath the sheet. However, the force experienced by the sheet from the air is not lift, it is pure drag. Momentum will also be conserved in this case, but the presence of downwash is not associated with lift.

Regards, JetMech
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tdscanuck
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RE: All Wing Lift = Net Downward Airflow?

Thu May 10, 2012 10:18 am

Quoting jetmech (Reply 22):
However, downwash - even though it may be equal to the lift force and vice versa - is a by-product or result of producing lift, lift is not the result of downwash.

Not exactly; lift is the result of producing downwash in exactly the same way that thrust is the result of accelerating air through the engine. If you accept the latter you have to accept the former because they're physically identical.

Quoting jetmech (Reply 22):
The detailed fluid dynamics that are the root cause of lift production have downwash as a by-product; it is not the presence of downwash that causes the detailed fluid dynamic behaviour.

This is true; downwash is a fallout of the physical solution to the Navier-Stokes equations. But *so is pressure*. So it's equally true to say "it is not the presence of pressure that causes the detailed fluid dynamic behavior" and we're back to square one.

Quoting jetmech (Reply 22):
If I allow a flat sheet of cardboard to fall from a height to the floor under the action of gravity, there is no doubt a downwash imposed upon the air underneath the sheet.

Yes, there's downwash (although it's mostly behind the sheet but that's not the point).

Quoting jetmech (Reply 22):
However, the force experienced by the sheet from the air is not lift, it is pure drag.

Correct. But your axes are rotated. Lift and drag are defined relative to the freestream. For a falling flat sheet, "lift" is sidways and "drag" is up. The sheet experiences a force in the direction of the downwash, just as a wing does.

It's important to note there that all airfoils experience a single aerodynamic force with (normally) both upward and backward components. We resolve the one force into perpendicular forces (lift and drag) for convenience. You can do the same to momentum change...we call the downward momentum change "downwash" and the horizontal momentum change "wake" but it works out the same way...the downwash equalls the lift, the wake equals the drag.

In the case of the falling flat plate it basically has no AoA so it's all drag, no lift, but the force on the plate still exactly equals the momentum change in the air.

Quoting jetmech (Reply 22):
Momentum will also be conserved in this case, but the presence of downwash is not associated with lift.

Yes but, in the normal aerodynamic coordinates that are centered on the body and "ahead" is the direction of motion, there's no downwash either.

Tom.
 
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Faro
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RE: All Wing Lift = Net Downward Airflow?

Thu May 10, 2012 2:10 pm

Quoting tdscanuck (Reply 23):
lift is the result of producing downwash in exactly the same way that thrust is the result of accelerating air through the engine

I suspect that a non-negligible part of the confusion arises from simple semantics: in terms of basic Newtonian action-reaction considrations, it is more accurate -though not complete- to talk of wing thrust rather than wing lift.

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vikkyvik
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RE: All Wing Lift = Net Downward Airflow?

Thu May 10, 2012 5:01 pm

Quoting faro (Reply 24):
I suspect that a non-negligible part of the confusion arises from simple semantics: in terms of basic Newtonian action-reaction considrations, it is more accurate -though not complete- to talk of wing thrust rather than wing lift.

We talk about "lift" because we're interested in the component of the wing force that is perpendicular to the freestream velocity. As Tom said, the wing actually produces an upward and backward resultant force. We just break it down into its components of lift and drag.

Saying "thrust" with reference to the wing would probably be more confusing, as thrust is typically taken as the forward force that is parallel to the freestream.
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jetmech
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RE: All Wing Lift = Net Downward Airflow?

Sat May 12, 2012 2:10 am

Quoting tdscanuck (Reply 23):
This is true; downwash is a fallout of the physical solution to the Navier-Stokes equations. But *so is pressure*. So it's equally true to say "it is not the presence of pressure that causes the detailed fluid dynamic behavior" and we're back to square one.

We could most certainly develop and apply a pressure differential to a body of fluid, and cause movement within that body of fluid (or even of the entire body of fluid), and this would probably invalidate my previous comments, but I feel the case of an airfoil may be different.

We do not directly develop and apply a pressure differential in order to get lift, we translate an airfoil shape through the air, and some fundamental aspect of fluid dynamic behaviour causes contour following, which result in a pressure differential. The result of this pressure differential is a net force we call "lift." We certainly do not need contour following to develop lift, but the exploitation of such a phenomenon is certainly desirable and one that aerodynamicists try to promote.

Quoting tdscanuck (Reply 23):
In the case of the falling flat plate it basically has no AoA so it's all drag, no lift, but the force on the plate still exactly equals the momentum change in the air.

Certainly. My example was trivial and it could be argued that it only succeeds if one is prepared to take a viewpoint that is either too pedantic or one that lacks sufficient rigour.

I hope you understand that I am not labouring the point for the sake of argument. My goal is a deeper more fundamental understanding only. I have the bad habit of thinking way too deeply about things   .

Regards, JetMech
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dakota123
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RE: All Wing Lift = Net Downward Airflow?

Sat May 12, 2012 4:41 pm

Quoting JetMech (Reply 26):
We certainly do not need contour following to develop lift

I'm thinking you meant in the physical wing shape sense, but isn't it that even when we're talking about a knife-edged wing, in effect we *do* have a contour, caused by flying it at more of an incidence to the relative wind (compared to a cambered wing)? Or am I missing something fundamnetal?

Thinking about circulation, obviously there is no net forward motion of air on the bottom of the wing (except from the stagnation point), but at what angle is the air coming off the trailing edge (for a typical case)? Is it a large deflection? Small? Or am I again missing something fundamental?

Great topic, BTW. It's been a terrific time waster at work...

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RE: All Wing Lift = Net Downward Airflow?

Sun May 13, 2012 4:50 am

Quoting JetMech (Reply 26):
we translate an airfoil shape through the air, and some fundamental aspect of fluid dynamic behaviour causes contour following, which result in a pressure differential.

The pressure picture is only one way to explain an airfoil's special properties. Remember both pictures (pressure and momentum) are equally valid. An airfoil is special because it shoves air down while dragging remarkably little air forward.

Quoting JetMech (Reply 26):
I feel the case of an airfoil may be different.

Nope!  
 
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 8:55 am

Quoting WingedMigrator (Reply 28):
The pressure picture is only one way to explain an airfoil's special properties. Remember both pictures (pressure and momentum) are equally valid.

Besides pressure and momentum schemes, how about the force scenario? Do we all agree that there is no unique, direct counter-force to the downwash which is 'felt' by the wing? So the only forces that the wing 'feels' are i) upward 'suction' force acting on the upper wing surface and ii) upward lifting force pushing on the lower wing surface.

This is part of what makes wing lift so counter-intuitive to understand: there is no direct counter-force to downwash 'felt' by the aircraft.


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keta
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 9:47 am

Quoting faro (Reply 29):
Do we all agree that there is no unique, direct counter-force to the downwash which is 'felt' by the wing?

The wing does feel the reaction to the downwash, and this force is *exactly* opposite to this downwash. The wing feels this force by means of the pressure all over its surface, which is the boundary between the two mediums.
If you have a given downwash, the wing will experience a pressure distribution which will exactly counteract the change of momentum of the air. It is really a one-to-one correspondence.

Quoting faro (Reply 29):
So the only forces that the wing 'feels' are i) upward 'suction' force acting on the upper wing surface and ii) upward lifting force pushing on the lower wing surface.

There is no suction as you picture it, I mean i and ii are the same. Recall that pressure is always positive, so the air will always be exerting some force to the wing or given body. The thing is that the pressure is not the same all over the surface, so in some points the pressure will push more that in others. In the wing, the lower surface feels more pressure than the upper one, so the overall effect is an upward force.

And yes, this pressure is the only thing the wing feels, but as said it is directly related to the downwash. Well, if you want to be more exact the wing also feels forces tangent to the surface, driven by viscosity. But this does not change the picture, the sum of all forces in the surface will equal the change in momentum of the air, with opposite direction.
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 11:01 am

Quoting keta (Reply 30):
If you have a given downwash, the wing will experience a pressure distribution which will exactly counteract the change of momentum of the air. It is really a one-to-one correspondence.

Agreed, from point of view of the *pressure* scenario. My post however was a consideration of the forces scenario only:

Quoting faro (Reply 29):
Besides pressure and momentum schemes, how about the force scenario?

One can have a pressure distribution around the wing that -taken in the aggregate- replicates a single unique counterforce to downwash. From a structural, 'what forces does the wing feel' scenario however, it has one force pulling on its upper surface and another lesser one pushing on its lower surface. Holistically and from a pressure point of view, they are/result from the same pressure distribution phenomenon; individually from a structures point of view, they are distinct forces acting on distinct wing surfaces. Make sense?


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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 1:32 pm

Quoting faro (Reply 31):
From a structural, 'what forces does the wing feel' scenario however, it has one force pulling on its upper surface and another lesser one pushing on its lower surface.

That depends entirely on what the static pressure is inside the wing. If you think about solid wings then it doesn't happen this way at all. The entire wing feels an inward-normal force from pressure. The pressure pushing down on the top surface isn't as big as the pressure pushing up on the bottom (all added up), hence net upwards force. Thinking about it as "suction" or "pulling" is somewhat dangerous because pressure can only push.

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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 1:56 pm

Quoting faro (Reply 31):
One can have a pressure distribution around the wing that -taken in the aggregate- replicates a single unique counterforce to downwash. From a structural, 'what forces does the wing feel' scenario however, it has one force pulling on its upper surface and another lesser one pushing on its lower surface. Holistically and from a pressure point of view, they are/result from the same pressure distribution phenomenon; individually from a structures point of view, they are distinct forces acting on distinct wing surfaces. Make sense?

Actually, what the wing feels is the pressure and shear forces on the skin, nothing more and nothing less. Let's say this is the physical way by which the force is transmitted to the wing.

Also, be careful when picturing the 'suction'. In the upper surface, the pressure is actually pushing down, while in the lower it pushes upwards. Well, if you have some internal pressure in the wing you might say that you have suction on the upper side, but this is only because the pressure inside is stronger than the one outside... Any suction is actually a stronger pressure in the other side.
So, just look at the pressure all over the surface, this is what the wing feels.
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 5:25 pm

Quoting tdscanuck (Reply 32):
The pressure pushing down on the top surface isn't as big as the pressure pushing up on the bottom (all added up), hence net upwards force. Thinking about it as "suction" or "pulling" is somewhat dangerous because pressure can only push.
Quoting keta (Reply 33):
In the upper surface, the pressure is actually pushing down, while in the lower it pushes upwards. Well, if you have some internal pressure in the wing you might say that you have suction on the upper side, but this is only because the pressure inside is stronger than the one outside... Any suction is actually a stronger pressure in the other side.

I agree that the pressure on the upper wing surface is pushing it relative to, say, a zero-Pascal pressure surface. Pressure like temperature will always be a relative measure.

Relative to the pressure inside the wing, my impression is that it should be causing a significant upward movement, regardless of whether we call it pull or suction or whatnot. This is surely over-simplistic, but for example in wing break tests there are pads placed on the upper wing surface which are pulled upwards to simulate this force, just as there are jacks pushing on pads on the lower wing surface. Structurally therefore, one is simulating a sustained upward 'tensile' force attaching to the upper wing surface, no? What is the flaw in with this viewpoint?


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vikkyvik
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 5:58 pm

Quoting faro (Reply 34):
Relative to the pressure inside the wing, my impression is that it should be causing a significant upward movement, regardless of whether we call it pull or suction or whatnot. This is surely over-simplistic, but for example in wing break tests there are pads placed on the upper wing surface which are pulled upwards to simulate this force, just as there are jacks pushing on pads on the lower wing surface. Structurally therefore, one is simulating a sustained upward 'tensile' force attaching to the upper wing surface, no? What is the flaw in with this viewpoint?

My guess is that the pads on the upper wing surface are not actually pulling, but pushing down, though I don't know that.

Quoting tdscanuck (Reply 32):
That depends entirely on what the static pressure is inside the wing. If you think about solid wings then it doesn't happen this way at all. The entire wing feels an inward-normal force from pressure. The pressure pushing down on the top surface isn't as big as the pressure pushing up on the bottom (all added up), hence net upwards force. Thinking about it as "suction" or "pulling" is somewhat dangerous because pressure can only push.

That drives me as crazy as when I heard a coworker at a previous company talk about lift being caused by "negative pressure" on the top surface of the wing.

Quoting faro (Reply 29):

Besides pressure and momentum schemes, how about the force scenario? Do we all agree that there is no unique, direct counter-force to the downwash which is 'felt' by the wing? So the only forces that the wing 'feels' are i) upward 'suction' force acting on the upper wing surface and ii) upward lifting force pushing on the lower wing surface.

No, we don't agree on that at all. See here:

Quoting vikkyvik (Reply 17):
Quoting ADent (Reply 16):

The force on the airplane (its weight if in level flight) is equal the mass of the air deflected down times the acceleration of that air.

But how is that force transmitted to the wings? By pressure differentials.

And here:

Quoting vikkyvik (Reply 10):
Here's a quasi-analogy for you that is perhaps easier to grasp:

You're standing on a boat, and you throw a rock forward. You (and the boat) move backward in reaction. One could take the Newtonian view, and determine the mass and acceleration of the rock, and therefore the backwards force on you and the boat. Or, one could sum the pressure exerted on your hand by the rock while you're in the throwing motion, and arrive at the same result.

The Newtonian view says "there MUST be an equal and opposite reaction". But how is that reaction transmitted to you? By the pressure on your hand when you throw the rock.

Regarding forces, a force is simply a pressure multiplied by an area. So if you sum the pressure distributions on the upper and lower surfaces, and multiply them by the areas, you'll get the net force.
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jetmech
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 8:15 pm

Quoting dakota123 (Reply 27):
I'm thinking you meant in the physical wing shape sense, but isn't it that even when we're talking about a knife-edged wing, in effect we *do* have a contour, caused by flying it at more of an incidence to the relative wind (compared to a cambered wing)? Or am I missing something fundamnetal?

I was primarily talking about the ability of the air to closely follow the upper contour of the airfoil. Because this usually represents a convex shape to the oncoming air, the tendency is for centrifugal forces to keep the air moving off in a straight direction without following the contour.

This centrifugal force is countered somewhat by the lower pressure area that would result if the air kept moving in a straight line. A significant entity that also counters the centrifugal force is the viscous action in the boundary layer. The resulting shear stresses tend to add a turning moment onto the air, which tends to turn the air into the convex contour of the upper airfoil surface.

The two entities mentioned in the previous paragraph are not essential to generating lift, but they do make the process far more efficient. Without them, we would have the flow separating from the upper contour of the airfoil, and any lift generated would have a very high amount of associated drag. The presence of the entities encouraging the flow to remain attached to the upper contour of the airfoil not only increases lift, but dramatically reduces pressure drag.

If you look at the pressure distributions of many airfoils, you will see that the lower surface only contributes a small part of the overall lift force. It is generally the upper surface that contributes most of the lift, with this contribution greatly enhanced by maintaining attached flow.

Quoting dakota123 (Reply 27):
but at what angle is the air coming off the trailing edge (for a typical case)? Is it a large deflection? Small? Or am I again missing something fundamental?

For conditions with attached flow, I'd say that for many airfoils, the downwash direction may be tangent to the mean camber line at the trailing edge. This would represent a small angle relative to the chord line. However, because the chord line can have an appreciable angle relative to the incident flow, the downwash direction relative to the free stream may be marked.

Quoting WingedMigrator (Reply 28):
Remember both pictures (pressure and momentum) are equally valid.

As reasonably easy to measure quantities that give an accurate measure of the lift force; I agree! As fundamental explanations for lift, I'm not so sure.

Quoting WingedMigrator (Reply 28):
An airfoil is special because it shoves air down while dragging remarkably little air forward.

This is only really true for cambered airfoils operating near their design lift coefficient, or symmetrical airfoils in the range of angles of attack near zero.

Quoting faro (Reply 29):
Do we all agree that there is no unique, direct counter-force to the downwash which is 'felt' by the wing? So the only forces that the wing 'feels' are i) upward 'suction' force acting on the upper wing surface and ii) upward lifting force pushing on the lower wing surface.

The pressure (and shear stress) distribution about the airfoil is the only thing nature has for communicating forces to that airfoil.

Quoting faro (Reply 29):
This is part of what makes wing lift so counter-intuitive to understand: there is no direct counter-force to downwash 'felt' by the aircraft.

As I have stated throughout this thread, I think it is not helpful to think of lift and downwash as interchangeable. The measurement of the magnitude of downwash and the lift force may give the exact same quantity due to Newton's laws, but I still think that downwash is a by-product of the fundamental fluid dynamics behaviour producing lift. I still can't really picture how downwash itself leads to behaviour such as the viscous action in the boundary layer etc..

A car travelling along a road is often the result of the chemical reaction known as combustion, but you certainly cannot say that combustion occurs as the result of the motion of a vehicle.

Regards, JetMech

[Edited 2012-05-14 13:22:01]
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tdscanuck
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RE: All Wing Lift = Net Downward Airflow?

Mon May 14, 2012 11:06 pm

Quoting faro (Reply 34):
I agree that the pressure on the upper wing surface is pushing it relative to, say, a zero-Pascal pressure surface. Pressure like temperature will always be a relative measure.

No, pressure isn't relative. Pressure is caused by molecules whacking into the surface. Zero pressure means nothing is running into the surface (perfect vacuum). Pressure "forces" in aerodynamics are almost always the result of differential pressure; two non-equal non-zero pressures acting on two surfaces of one object.

Quoting faro (Reply 34):
This is surely over-simplistic, but for example in wing break tests there are pads placed on the upper wing surface which are pulled upwards to simulate this force, just as there are jacks pushing on pads on the lower wing surface.

They're pulling up because they don't have actuators inside the wing. The air pressure is static and equal all over since the static test bird isn't moving. The pull that the pads are putting in simulates the *differential* pressure on the upper wing skin. It's actually more accurate to say that the upper wing skin is being pushed up by the air inside the wing than to say it's being "sucked up" by the low pressure above.

Quoting faro (Reply 34):
Structurally therefore, one is simulating a sustained upward 'tensile' force attaching to the upper wing surface, no? What is the flaw in with this viewpoint?

Because air can't apply a tensile force, only compressible. There is no such thing as negative absolute pressure. There is negative *gauge* pressure but that's really just differential between two positive pressures.

Quoting jetmech (Reply 36):
For conditions with attached flow, I'd say that for many airfoils, the downwash direction may be tangent to the mean camber line at the trailing edge.

This the Kutta condition. If you ignore viscosity you have to enforce this to get the right result; tons of experimental work shows that it's valid for attached flows.

Quoting jetmech (Reply 36):
Quoting WingedMigrator (Reply 28):
An airfoil is special because it shoves air down while dragging remarkably little air forward.

This is only really true for cambered airfoils operating near their design lift coefficient, or symmetrical airfoils in the range of angles of attack near zero.

It's true for all (good) airfoils in un-separated flow. Generating lift is easy; the point of airfoils is to drive the lift/drag ratio up. Cambered airfoils far away from their design lift coefficient still have excellent lift/drag compared to, say, a spinning cylinder or flat plate.

Quoting jetmech (Reply 36):
The measurement of the magnitude of downwash and the lift force may give the exact same quantity due to Newton's laws, but I still think that downwash is a by-product of the fundamental fluid dynamics behaviour producing lift.

This is true, but it's exactly and equivalently true to say that pressure is a by-product of the fundamental fluid dynamics behavior producing lift. A full picture of a fluid flowfield includes the velocity, density, and pressure at each point. Velocity and density are equivalent to momentum, pressure is pressure...neither is privledged measure that "causes" the other.

Quoting jetmech (Reply 36):
I still can't really picture how downwash itself leads to behaviour such as the viscous action in the boundary layer etc..

It doesn't. Downwash is a result of those kinds of behaviors...just like pressure.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 15, 2012 4:47 pm

Quoting tdscanuck (Reply 37):
This the Kutta condition. If you ignore viscosity you have to enforce this to get the right result; tons of experimental work shows that it's valid for attached flows.

Sure. The reason I used somewhat "open" phrasing when talking about downwash previously, was to account for the possibility that there was some situation I was unaware of that did not strictly adhere to convention. I find being a little less than adamant is a safer way to conduct research  !

Quoting tdscanuck (Reply 37):
Cambered airfoils far away from their design lift coefficient still have excellent lift/drag compared to, say, a spinning cylinder or flat plate.

No doubt, but in this situation, I'd hesitate to say that the airfoil was "dragging little air forward".

Quoting tdscanuck (Reply 37):
It doesn't. Downwash is a result of those kinds of behaviors...just like pressure.

If I'm interpreting this correctly, it seems to correspond to my thoughts on the issue, that is, for an airfoil at least, the pressure distributions and downwash are themselves the result of something else more fundamental.

Quoting tdscanuck (Reply 37):
Velocity and density are equivalent to momentum, pressure is pressure...neither is privledged measure that "causes" the other.

Sure. Downwash or pressure distributions may not take precedence and be the cause of the other, but would it be a reasonable statement to make that they were both the result of something else   ?

Regards, JetMech
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WingedMigrator
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RE: All Wing Lift = Net Downward Airflow?

Tue May 15, 2012 6:01 pm

Quoting jetmech (Reply 38):
would it be a reasonable statement to make that they were both the result of something else

Gas molecules bumping into each other and into a solid object (airfoil or otherwise) ?
 
dakota123
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RE: All Wing Lift = Net Downward Airflow?

Tue May 15, 2012 6:52 pm

Quoting jetmech (Reply 38):
Downwash or pressure distributions may not take precedence and be the cause of the other, but would it be a reasonable statement to make that they were both the result of something else ?

Trying to understand, not question... isn't downwash a (the?) result of circulation? i.e. if the wing hadn't left the scene, the downwash would turn forward.

Does the downwash act as a barrier of sorts to air rushing by, creating a higher pressure region under the wing (I mean, I understand there is a higher pressure region under the wing, but is that the way to think of the mechanism that produces it?) With pure circulation, as I understand it, it would be the forward motion of the stream under the wing subtracting from the rearward motion that produces the higher pressure, and downwash coming off the trailing edge at some slight angle to the free stream "blocking" the air entering the scene seems like the logical physical manifestation.

Or have I gone off the rails?

Mike
 
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RE: All Wing Lift = Net Downward Airflow?

Tue May 15, 2012 7:41 pm

Quoting tdscanuck (Reply 37):
Generating lift is easy; the point of airfoils is to drive the lift/drag ratio up. Cambered airfoils far away from their design lift coefficient still have excellent lift/drag compared to, say, a spinning cylinder or flat plate.

So this is the real role of camber: a flat plate tilted at a slight AoA would also produce lift but with a cambered upper surface this same lift is achieved with much lesser drag.

Question then: would a flat plate also create a low air pressure region above the wing (relative to the prevailing airstream pressure) or do you actually need the curvature of the cambered upper surface to establish such lower pressure?


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RE: All Wing Lift = Net Downward Airflow?

Tue May 15, 2012 8:09 pm

Quoting faro (Reply 41):
Question then: would a flat plate also create a low air pressure region above the wing (relative to the prevailing airstream pressure) or do you actually need the curvature of the cambered upper surface to establish such lower pressure?

Yes, you don't need the curvature to create low pressure above a flat wing. This is a common misunderstanding that arises from the broken way in which aerodynamics are taught (IMHO) by starting with airfoils, when there is nothing particularly special about airfoils other than low drag. Airfoils are not required for flight; they are only required for efficient flight.
 
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RE: All Wing Lift = Net Downward Airflow?

Wed May 16, 2012 3:56 am

Quoting jetmech (Reply 38):
for an airfoil at least, the pressure distributions and downwash are themselves the result of something else more fundamental.

Yes. Ultimately, it's all about bazillions of gas molecules elastically colliding with one another and the aircraft. Everything else is just different levels of abstraction.

Quoting dakota123 (Reply 40):
Trying to understand, not question... isn't downwash a (the?) result of circulation?

Not really. Circulation is a somewhat dangerous concept in this type of discussion because it's an artifact of ignoring viscosity...circulation is a good mathematical way to capture and apply the Kutta condition but it's not exactly physical. The physical manifestation is that an inviscid fluid is also irrotational (the curl of the velocity vector field is zero) but real world fluids always have some rotation (curl is non-zero in some places). The rotation actually comes about due to boundary layer shear but you don't get boundary layers in inviscid fluids so you have to make up for it some other way...ergo, circulation.

Quoting dakota123 (Reply 40):
Does the downwash act as a barrier of sorts to air rushing by, creating a higher pressure region under the wing (I mean, I understand there is a higher pressure region under the wing, but is that the way to think of the mechanism that produces it?)

No, the downwash isn't a barrier. It's just a result of the flowfield around the airfoil, as is the pressure field.

Tom.
 
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Faro
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RE: All Wing Lift = Net Downward Airflow?

Wed May 16, 2012 8:40 am

Just wondering about the interaction of downwash and wingtip vortices. Logically downwash will drag the wingtip vortex downward, won't it? Wouldn't a simple way of increasing wing efficiency be to make the downward angle of the vortex as great as possible so that one gets as much lift as possible from it? For example by increasing somewhat the angle of the wingtip vis-à-vis that of the main wing planform. Could this trade-off be beneficial?


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RE: All Wing Lift = Net Downward Airflow?

Wed May 16, 2012 10:53 am

Quoting WingedMigrator (Reply 39):
Gas molecules bumping into each other and into a solid object (airfoil or otherwise) ?

If we wanted to get to a very fundamental explanation, I suppose it would have something to do with explaining the microscopic details of particle motion. But an explanation that is more macroscopic, and one within the bounds of the subject area of aerodynamics may include significant parts of boundary layer theory and turbulence (theory?).

Quoting tdscanuck (Reply 43):
Circulation is a somewhat dangerous concept in this type of discussion because it's an artifact of ignoring viscosity...circulation is a good mathematical way to capture and apply the Kutta condition but it's not exactly physical.

This appears to fit in with my earlier thoughts on the issue.

Quoting jetmech (Reply 2):
What should be remembered however is that theories of lift such as the circulation and downwash theories of lift are theories only. These theories gain acceptance and relevance as they allow us to calculate fairly accurate answers from physical quantities that are relatively easy to measure. However, these theories do not necessarily accurately describe the actual physics of the flow field.
Quoting dakota123 (Reply 40):
Does the downwash act as a barrier of sorts to air rushing by, creating a higher pressure region under the wing (I mean, I understand there is a higher pressure region under the wing, but is that the way to think of the mechanism that produces it?)

I don't think so. Assuming we have attached flow, the pressure distributions of the upper and lower surfaces tend to "asymptotically" approach the same value at the trailing edge. The wake of an airfoil can be described as the region behind the trailing edge where there is a velocity decrement (local velocity less than free stream).

Again for attached flow, the wake will consist of the merged boundary layers from the upper and lower surfaces of the airfoil, and there will most definitely be a velocity decrement behind the trailing edge. However, there may not necessarily be a significant pressure variation across the wake. If there is a pressure variation, it would probably vary smoothly like the velocity decrement, thus, there would probably not be a sudden "step” change in pressure, and hence, no "barrier" as such.

A lot of pressure distributions I have seen reveal that the bottom surface of the airfoil often contributes little or nothing to the overall lift force. There may well be a region of increased (greater than static) pressure near the leading edge, but this can be followed with a region of mildly decreased (less than static) pressure over the rest of the bottom surface. Any pressure increase over static would most probably come about due to deceleration of the free stream. I don’t think the downwash would have much to do with this.

Quoting faro (Reply 41):
Quoting WingedMigrator (Reply 42):

Not only does the camber of an airfoil result in less drag for a given lift coefficient, it also allows the generation of a much higher maximum lift coefficient compared with a flat plate.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
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RE: All Wing Lift = Net Downward Airflow?

Wed May 16, 2012 1:35 pm

Quoting faro (Reply 44):
Just wondering about the interaction of downwash and wingtip vortices.

They're intimately coupled...the downwash *is* the downward flow between the two counterrotating vortices. It's very risky to try to separate the flowfield into different components because, at least for subsonic aerodynamics, they all influence each other.

Quoting faro (Reply 44):
Logically downwash will drag the wingtip vortex downward, won't it?

Basically, yes. That's why vortices descent (they're each in the downwash from the other vortex).

Quoting faro (Reply 44):
Wouldn't a simple way of increasing wing efficiency be to make the downward angle of the vortex as great as possible so that one gets as much lift as possible from it?

No. The vortex doesn't fully form until about a half-span back from the wing. You have basically no control over the vortex angle. The wing is actually shedding a vortex sheet, which is going to come off tangent to the trailing edge no matter you do. The vortex sheet rolls up to form the trailing vortices.

Quoting faro (Reply 44):
For example by increasing somewhat the angle of the wingtip vis-à-vis that of the main wing planform. Could this trade-off be beneficial?

Probably not, or it would have been done already. Increasing tip lift has some non-aerodynamic issues like stall performance and sturctural weight as well.

Tom.
 
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RE: All Wing Lift = Net Downward Airflow?

Thu May 17, 2012 4:37 am

Quoting tdscanuck (Reply 43):
No, the downwash isn't a barrier. It's just a result of the flowfield around the airfoil, as is the pressure field.

Well, that's what I get for losing sight of the fact that it's the wing moving through the air, and not the air streaming past the wing. Perspective is everything...

Quoting tdscanuck (Reply 43):
The physical manifestation is that an inviscid fluid is also irrotational (the curl of the velocity vector field is zero) but real world fluids always have some rotation (curl is non-zero in some places). The rotation actually comes about due to boundary layer shear but you don't get boundary layers in inviscid fluids so you have to make up for it some other way...ergo, circulation.

Thanks for that; I get it.

Mike
 
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RE: All Wing Lift = Net Downward Airflow?

Fri May 18, 2012 8:24 am

I know that it's very hard to define just what a supercritical airfoil comprises. However, are those supercritical airfoils with a pronounced downward cusp or curvature of the aft, lower wing surface better at producing downwash? Are they inherently more effcient due to the greater downward angle of the air coming off the aft lower wing cusp? If yes, why not adopt this cuspy aft section in all airfoils?


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RE: All Wing Lift = Net Downward Airflow?

Fri May 18, 2012 2:35 pm

Quoting faro (Reply 48):
I know that it's very hard to define just what a supercritical airfoil comprises.

It's like what a US judge once said about pornography (paraphrased): I can't define it, but I know it when I see it.

Quoting faro (Reply 48):
However, are those supercritical airfoils with a pronounced downward cusp or curvature of the aft, lower wing surface better at producing downwash?

Not really. The big benefit of supercritical airfoils is lower drag at high transonic Mach (they have a bigger spread between Mcr and Md), which means they're better at *not* dragging air forward with them. Their lift characteristics (i.e. downwash) aren't particularly different than other airfoils.

Quoting faro (Reply 48):
Are they inherently more effcient due to the greater downward angle of the air coming off the aft lower wing cusp?

No. It's the whole package, especially the nature of the shockwave on the upper surface, that makes them work.

Quoting faro (Reply 48):
If yes, why not adopt this cuspy aft section in all airfoils?

Because, for most airfoils, it would give you worse lift/drag ratios.

Tom.

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