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All Wing Lift = Net Downward Airflow?  
User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Posted (2 years 3 months 2 weeks 5 days 5 hours ago) and read 5184 times:

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


The chalice not my son
49 replies: All unread, showing first 25:
 
User currently offlineMir From United States of America, joined Jan 2004, 21554 posts, RR: 55
Reply 1, posted (2 years 3 months 2 weeks 5 days ago) and read 5101 times:

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?


-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinejetmech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 2, posted (2 years 3 months 2 weeks 4 days 12 hours ago) and read 4952 times:

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



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2212 posts, RR: 56
Reply 3, posted (2 years 3 months 2 weeks 2 days 9 hours ago) and read 4738 times:

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.


User currently offlinejetmech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 4, posted (2 years 3 months 2 weeks 1 day 14 hours ago) and read 4593 times:

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



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (2 years 3 months 2 weeks 1 day 11 hours ago) and read 4557 times:

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.


User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Reply 6, posted (2 years 3 months 2 weeks 1 day 5 hours ago) and read 4507 times:

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]


The chalice not my son
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (2 years 3 months 2 weeks 1 day 4 hours ago) and read 4488 times:

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.


User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Reply 8, posted (2 years 3 months 2 weeks 1 day 4 hours ago) and read 4481 times:

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]


The chalice not my son
User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2212 posts, RR: 56
Reply 9, posted (2 years 3 months 2 weeks 22 hours ago) and read 4403 times:

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.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9901 posts, RR: 26
Reply 10, posted (2 years 3 months 2 weeks 20 hours ago) and read 4379 times:
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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.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Reply 11, posted (2 years 3 months 2 weeks 20 hours ago) and read 4375 times:

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



The chalice not my son
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (2 years 3 months 2 weeks 20 hours ago) and read 4375 times:

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.


User currently offlinedakota123 From United States of America, joined Aug 2006, 116 posts, RR: 0
Reply 13, posted (2 years 3 months 2 weeks 15 hours ago) and read 4315 times:

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

Mike


User currently offlineMir From United States of America, joined Jan 2004, 21554 posts, RR: 55
Reply 14, posted (2 years 3 months 2 weeks 12 hours ago) and read 4290 times:

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.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinejetmech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 15, posted (2 years 3 months 2 weeks 8 hours ago) and read 4258 times:

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



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineADent From United States of America, joined Dec 2006, 1368 posts, RR: 2
Reply 16, posted (2 years 3 months 2 weeks 6 hours ago) and read 4251 times:

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.

http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9901 posts, RR: 26
Reply 17, posted (2 years 3 months 2 weeks 5 hours ago) and read 4247 times:
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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.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Reply 18, posted (2 years 3 months 2 weeks 4 hours ago) and read 4228 times:

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]


The chalice not my son
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 19, posted (2 years 3 months 1 week 6 days 23 hours ago) and read 4171 times:

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.


User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2212 posts, RR: 56
Reply 20, posted (2 years 3 months 1 week 6 days 21 hours ago) and read 4157 times:

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.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9901 posts, RR: 26
Reply 21, posted (2 years 3 months 1 week 6 days 19 hours ago) and read 4133 times:
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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.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinejetmech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 22, posted (2 years 3 months 1 week 6 days 7 hours ago) and read 4048 times:

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



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 23, posted (2 years 3 months 1 week 6 days 3 hours ago) and read 4014 times:

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.


User currently offlinefaro From Egypt, joined Aug 2007, 1540 posts, RR: 0
Reply 24, posted (2 years 3 months 1 week 6 days ago) and read 3985 times:

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.

Faro



The chalice not my son
25 vikkyvik : 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 w
26 Post contains images JetMech : 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 en
27 dakota123 : 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
28 Post contains images WingedMigrator : The pressure picture is only one way to explain an airfoil's special properties. Remember both pictures (pressure and momentum) are equally valid. An
29 faro : 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 wh
30 keta : 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 pre
31 faro : Agreed, from point of view of the *pressure* scenario. My post however was a consideration of the forces scenario only: One can have a pressure distr
32 tdscanuck : 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 enti
33 keta : 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 whic
34 faro : 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 al
35 vikkyvik : My guess is that the pads on the upper wing surface are not actually pulling, but pushing down, though I don't know that. That drives me as crazy as
36 jetmech : 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 sha
37 tdscanuck : No, pressure isn't relative. Pressure is caused by molecules whacking into the surface. Zero pressure means nothing is running into the surface (perf
38 Post contains images jetmech : Sure. The reason I used somewhat "open" phrasing when talking about downwash previously, was to account for the possibility that there was some situa
39 WingedMigrator : Gas molecules bumping into each other and into a solid object (airfoil or otherwise) ?
40 dakota123 : 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
41 faro : 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 a
42 WingedMigrator : 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 whic
43 tdscanuck : Yes. Ultimately, it's all about bazillions of gas molecules elastically colliding with one another and the aircraft. Everything else is just differen
44 faro : Just wondering about the interaction of downwash and wingtip vortices. Logically downwash will drag the wingtip vortex downward, won't it? Wouldn't a
45 jetmech : 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 mo
46 tdscanuck : They're intimately coupled...the downwash *is* the downward flow between the two counterrotating vortices. It's very risky to try to separate the flo
47 dakota123 : 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
48 faro : I know that it's very hard to define just what a supercritical airfoil comprises. However, are those supercritical airfoils with a pronounced downward
49 tdscanuck : 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. Not really. The big benefit of s
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