A "new" theory of lift ?

[rb1957]

i'd like some more informed aerodynamists' opinion on Mr (Dr?) Johnson's opinion that lift is not caused by circulation. From

http://knol.google.com/k/the-spell-of-kutta-zhukovsky-s-circulation-theory#

...

"it is shown that the lift you experience when you fly, comes without circulation, as displayed in the following figure showing the lift and circulation of a Naca0012 wing as function of the angle of attack, computed by solving the Navier-Stokes equations for the flow around the wing:

pic doesn't show, sigh

We see that the lift increases linearly with the angle of attack up to 16 degrees, while the circulation stays
basically zero up to 10 degrees: Lift and circulation are not equivalent as in Kutta-Zhukovsky's formula"

there is an impressive looking pic showing lift increasing with AoA, as expected, but "circulation" remaining constant, and close to zero. this sort of breaks the linkage between circulation and lift, but i'm smart enough not to take things I can't derive myself at face value.

As far as I've read Johnson doesn't propose a consistent new theory, but tries to explain lift and drag at near separation AoA.

opinions ?

 

[rb1957]

nah ... don't buy that "the center of lift is at the 1/4 chord point, for ANY airfoil producing lift" ... if i had an airfoil i could resolve the aero forces as a single vector (by integrating the surface pressures) and i wouldn't need a moment to balance the free body.

and as for tailplanes producing -ve lift (which of course they do), it is because they have -ve camber and -ve AoA.

 

[btrueblood]

"nah ... don't buy that "the center of lift is at the 1/4 chord point, for ANY airfoil producing lift" ... if i had an airfoil i could resolve the aero forces as a single vector (by integrating the surface pressures) and i wouldn't need a moment to balance the free body."

So all NACA airfoil reports are wrong? That's exactly what 2D airfoil section data (generated in a 2d tunnel) give, an integration of the chordwise pressure distribution. And, in better tunnels, a direct measurement of the pitching moment, about the center of pressure. So, you could resolve the forces to a single vector, yes - but the point of action of that force would be somewhere outside of the airfoils' surface - how can that be?

"and as for tailplanes producing -ve lift (which of course they do), it is because they have -ve camber and -ve AoA. "

Uh, yeah. But WHY do they have a negative camber and/or AoA?

"There is no physical mechanism for getting air to move forward against the flow of the atmosphere across a wing. You might get half of a circulation, but that's just the normal flow. A counterflow cannot physically exist. "

I can't quite follow your arguement. The atmosphere is quite big, and the counter flow does not have to happen at the wing, it can be distributed as a small flow disturbance some distance away. In real wings, the tip vortices carry some of the rotation. In 2d section wind tunnels, (or even in pipes with 90 degree bends, you can measure the axial vorticity carried in the fluid due to the turning imposed on the flow. Indeed, if you measure forces on airfoils in wind tunnels, you need to account for the upwash effect in order to account for differences seen from 2d pressure integration calculations.

 

[rb1957]

sorry, i believe that the NACA reports do exactly what i've said ... integrate the pressures, assume lift acts at 1/4 chord, and create the moment to balance the airfoil.

whay do you think the resultant force would act outside the airfoil ? the typical pressure distribution peaks towards the nose ... and see that it follows that the resultant force is beyond the airfoil ? and so what if it is ? if this is equivalent to the original cm data then what's the problem.

and tailplanes have ... oops i "mis-spoke" ... conventional tailplanes produce +ve lift (i typed without thinking); the lift of a conventional plane is W+Lh.

 

[IRstuff]

If you can buy the concept of counterflow at a distance, then you can buy momentum transfer, and it's a much easier concept to buy.

Otherwise, you have to accept that there are air currents, wherever, that are moving faster than the wing and going around it somehow. What's the physical mechanism and why hasn't it ever been observed? Moreover, you have to buy into the concept that the air ahead of the wing is somehow anticipating that the wing will move through it at a later point in time, and the air will move in anticipation of that.

TTFN

FAQ731-376

 

[rb1957]

as i understood it there is a pressure wave ahead of the wing, which gets steeper as the plane approaches mach 1, ultimating becoming the sonic shock for supersonic planes.

 

[IRstuff]

Yes, but none of that is going in the counterflow direction

TTFN

FAQ731-376

 

[KENAT]

So now the CoL isn't at quarter chord either?

I'm pretty sure I did labs at uni that demonstrated this to be true.

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[KENAT]

Oh, and the airflow doesn't have to 'move' against the airflow, just be retarded, so moving slower, on the lower surface as I recall.

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[vortexman]

aerofrix2:

I'm referring to the same Johnson referenced in the original post. I already commented that the spanwise lift distribution of a tapered wing is not necessarily constant. Beyond that, I didn't see any discussion on his pages about the lift distribution. After discovering that he was mistaking streamwise vorticity for circulation around the wing, I didn't read any more of his text. His 'theory' is not 'in order', it is clearly not valid.

IRstuff:

Counterflow, or upstream flow, is not necessary for non-zero circulation to exist. Circulation is the line integral of velocity around the wing. Aerodynamics textbooks often have an illustration showing the superposition of the non-lifting flow field (which has zero circulation) and an idealized circulating flow. The resulting flowfield doesn't need to have any regions of upstream flow, or counterflow, near or far, and yet the circulation is non-zero. The circulation of a lifting wing is physically measurable, and is very well known to conform (approximately) to the value predicted by "circulation theory", even though that theory is an idealized approximation. This is not a controversial topic in aerodynamics. Actually, the air ahead of the wing DOES know that the wing is coming, for subsonic flow, and behaves appropriately. Your old aerodynamics textbook will have all this stuff in it, nicely explained.

vortexman

 

[IRstuff]

I understood that it was slower; that's what was interpreted as due to circulation. It's mathematically a simple paradigm to account for the velocity differences, but that's what's physically not plausible.

TTFN

FAQ731-376

 

[KENAT]

So are you saying you don't believe the airflow underneath is slower, or just that the speed difference isn't due to circulation?

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[IRstuff]

the latter ;-)

TTFN

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[KENAT]

Phew.

Anyway, until someone comes up with a decent coherent explanation with diagrams to convince me otherwise, I'm sticking with bound vortexes/circulation theory and quarter chord lift.

I'll make the significant assumption that the various profs & doctors at uni who taught me aero/mechanics of flight & related subjects, knew more or less what they were on about.

However, given that some senior guy at BAe Airbus once said on TV that lift was generated because the surface of the top of the wing was longer than the surface of the bottom of the wing and so the air over the top had to go faster to get to the rear at the same time as that on the bottom, I may well end up being the donkey.

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[vortexman]

KENAT,

I'm not quite sure how you see the quarter chord point being involved, but the bound vortex is exactly what the circulation around the wing is. Again reaching back to Fluid Mechanics 101, the Helmholtz theory says that a vortex can't end in 3D space, so it has to be a ring vortex. When a wing starts moving from rest and generating lift, it starts to generate circulation around itself, which can be though of as a 'vortex'. This vortex is continued in the form of the tip vortices, and the ring is completed by an imaginary vortex at the starting point of the wing. That 'starting' vortex is imaginary, in the sense that the equivalent vorticity is actually spread out over a wider area, but the total amount of vorticity can be thought of, conceptually, as being a single vortex back where the wing started. You probably have a picture showing this in your old Fluids or Aerodynamics book. While this image is a bit idealized, the circulation is real.

IRstuff,

I guess I don't know what else to tell you. It always strikes me as odd when someone declares that something can't be true, when he could simply look it up in a readily available book, or with a trivial Google search. Suit yourself.

vortexman

 

[IRstuff]

Just because the air is slower on the bottom does not mean there is circulation. Correlation does not prove cause. Aside from the fact that one can come up with a number that makes the flows more balanced, that does not make for a physical reality or fact or phenomenon. There is no physical phenomenology that explains how air can be moving along the bottom of the wing in the same direction.

TTFN

FAQ731-376

 

[rb1957]

v'man, i think kenat was coupling a pair of thread that have arisen.

as far as i know, the CoP is not at the 1/4 chord. for early NACA airfoils it was a good approximation and it became a convention ... apply the lift at 1/4 chord, and add a moment term to complete the balance ... i'm going to have dig out my Abbott and von Doenhoff or the early NACA reports to find the reference.

ir, if the flow is slower under the the airfoil, then logically it must be faster above it (incompressible momentum requires this, no?). if it is faster above and slower below, that looks like a circulation flow to me. is that causing the lift, or caused by the lift ... i guess that is the question. my 2c is that if you start with fliud flows you're going to determine that thare is a net force (lift) from bernoulli, and if you start with the lift force which implies a pressure field then you'd determine the velocity field.

 

[vortexman]

IRstuff,

Actually, the air on the bottom of the wing moving slower than the air on the top DOES mean that there is circulation. Circulation is a well-defined quantity, and the situation you describe, slower flow on the bottom of the wing, is exactly what will cause that quantity to become non-zero. Again, it is easy to find good illustrations of this on a wide variety of websites.

This is not just a number, it is a physical reality. Air really does tend to depart the wing at the point of the sharp trailing edge as the angle of attack varies, until separation occurs (this is the Kutta condition). This is the physical mechanism that results in circulation, because there is a value of circulation that will result in that departure point of the air for each angle of attack. This isn't a mathematical expediency, it is a behavior of the air that is caused by the fact that it has nonzero viscosity. When the air departs the wing at the trailing edge, it results in a flow field that has the 'necessary' circulation. You can't really pin down the cause and effect; it doesn't work that way.

I don't understand what you're trying to say in your last sentence, so I can't respond to it. I recommend that you look at the Wikipedia page on the Kutta Condition; it is nicely written and correct.

vortexman

 

[IRstuff]

My bad, the original link used "circulation," particularly in the pictures to imply that there is a counterflow. "Circulation" to mean that there is a non-zero line integral of velocity around the object is consistent with the velocity being slower on the bottom.

TTFN

FAQ731-376

 

[btrueblood]

I'm gonna have go back and read up too, when I get the chance. I was taught, I thought, that you can't get the (correct, 3D) moment coefficient from a 2D wind tunnel test with pressure taps only...but that doesn't make sense now the way you put it, Rb. Whatever. I like vortexman's explanation, and agree with Kenat, I'll live comfortably with circulation theory.

Glad to have helped stir the pot.

 

[rb1957]

here's what wiki had to say on the Movement of center of pressure for aerodynamic fields ...
"The center of pressure on a symmetric airfoil typically lies close to 25% of the chord length behind the leading edge of the airfoil. (This is called the "quarter-chord point".) For a symmetric airfoil, as angle of attack and lift coefficient change, the center of pressure does not move. It remains around the quarter-chord point for all angles of attack and lift coefficients. The role of center of pressure in the control characterization of aircraft takes a different form than in missiles.

"On a cambered airfoil the center of pressure does not occupy a fixed location.[8] For a conventionally cambered airfoil, the center of pressure lies a little behind the quarter-chord point at maximum lift coefficient (large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves toward the rear.[9] When the lift coefficient is zero an airfoil is generating no lift but a conventionally cambered airfoil generates a nose-down pitching moment, so the location of the center of pressure is an infinite distance behind the airfoil. This direction of movement of the center of pressure on a conventionally cambered airfoil is de-stabilising, necessitating a horizontal stabiliser to provide the aircraft with longitudinal static stability. Aircraft tend to use cambered wings because they have relatively benign flights with preferred flight orientations as compared to missiles.

"For a reflex-cambered airfoil, the center of pressure lies a little ahead of the quarter-chord point at maximum lift coefficient (large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves forward. When the lift coefficient is zero an airfoil is generating no lift but a reflex-cambered airfoil generates a nose-up pitching moment, so the location of the center of pressure is an infinite distance ahead of the airfoil. This direction of movement of the center of pressure on a reflex-cambered airfoil is stabilising, and a horizontal stabiliser is not necessary. A tailless aircraft with a straight wing can be designed to have positive longitudinal static stability if the wing has reflex camber.

"The way the center of pressure moves as lift coefficient changes makes it difficult to use the center of pressure in the mathematical analysis of longitudinal static stability of an aircraft. For this reason, it is much simpler to use the aerodynamic center when carrying out a mathematical analysis. The aerodynamic center is a slightly more difficult concept to comprehend, but the aerodynamic center occupies a fixed location on an airfoil, typically close to the quarter-chord point.

"The aerodynamic center is the conceptual starting point for longitudinal stability. Providing the center of gravity of an aircraft lies forward of the aerodynamic center the aircraft will have positive longitudinal stability. The horizontal stabilizer contributes extra stability and this allows the center of gravity to be a small distance aft of the aerodynamic center without the aircraft reaching neutral stability. The position of the center of gravity at which the aircraft has neutral stability is called the neutral point."

but i'll look into NACA 'cause i'm sure using the 1/4 chord is a convention