The Mystery Of Lift

[Richard Miller]

There are, as we all know, two schools of thought concerning lift, the one based on Newtonian principles, the other on the Bernoulli equation. Are there any among you who would like, in a friendly way - and I stress that - to discuss the relative merits of the two? Or any others?

-Richard

[Sr71fan]

Hi Richard.

Here's MY theory about lift:

Lift=Deliberate Downward Displacement of Atmospheric Molecules

(DDDAM)

You get lift when you push air down. I guess I'll have to agree with Newton.:D

Gary

[Richard Miller]

Gary,

Can we ease into this, introducing one concept at a time and building carefully on what we accumulate?

If so, answer me this: Is there a net vertical movement of the air affected by a wing in the process of the production of lift?
Is it up? Down? Up and down? Down and up? Other?

-Richard

[Sr71fan]

Richard,

What part of down, did you not understand?:D

Gary

[Dave Robelen]

Hello Richard,
In short form, yes there is a net downward displacement of the air from an airplane in flight. As the wing moves throught the air there is first upwash where the air ahead of the wing begins to rise (rather like the bow wave of a boat) , then the air is accellerated to various degrees by the wing (depends on the airfoil section and angle to the airflow) and the the air moves downward behind the moving wing. With all of this going on, the net result is a downward displacement of the air the wing has passed through. This way too short an answer, but maybe it will get someone else going.
Regards, Dave Robelen

[Richard Miller]

Dave,

Proceeding on the one-thing-at-a-time program, what causes the upwash and what magnitude, in force terms, does it have?

Gary,

The part about conservation of momentum, as in what goes down must come up.

-Richard

[SteveH]

The geatest lift I ever experienced? My wife kicking me in the A**
to get me moving. :D :D

[SteveH]

What the heck are we dealing with here, rocket scientist's??
Oh!!! We are! Just teasing, I find your conversations interesting.

[Dave Robelen]

Hi Richard,
To preface my remarks, my carrer with NASA was in the stability and control department and not directly involving the research of airfoils and wings specifically. The term upwash was coined because the air can be observed to begin rising as the wing approaches in direct proportion to the amount of lift the wing is producing. This upward flow begins more than a chord length ahead of the wing and the amount of angularity is proportional to the distance from the LE.
The cause of this effect has to do with both the fact that air is compressible at subsonic conditions, and is also "elastic" in it's behaviour. There probably is a better term, but I can't think of it. This phenomenon is the reason that the Bernouli principle can be applied to a flat plate wing. The point where the air splits around the wing is not at the exact center of the LE, but behind and below the LE in proportion to the amount of lift being produced. This point is called the "stagnation point" and is the one location on an airfoil where there is no air pressure due to movement. Because the air must wrap up and around the LE, even the flat plate has some difference in the local air velocity on the upper and lower surface.
A bunch of words, and I doubt if I answered your question, but there you have it.
Regards, Dave Robelen

[paulriseborough]

I'm a guidance and control engineer by speciality (although autopilot designers require some aerodynamics to understand the interaction between the motion and forces acting on a flight vehicle), but when I was studying aerospace engineering, this was one question which we used to enjoy asking lecturers!

One question which was even better related to how Bernoullis theorem applies to lift of a flat plate, ie how does a flat plate generate a higher velocity on the top surface? This is a great question to ask when someone explains lift as due to the greater curvature on the top surface of the wing which forces the air on the top to speed up .............

Both Bernoullis and momentum theories are valid, that is if you measure the flow velocities distribution over a wing (outside the boundary layer), application of Bernoullis theorem will give the pressure distribution which can then be integrated over the surface to provide the lift.

Alternatively, measurement of the three components of air velocity behind the wing enables the rate of change of the airs momentum to be calculated which gives the lift and drag by applying Newtons theorem.

Bernoullis thoerom is more useful for doing calculations, but not always the best way to explain to someone how a wing (particularly a flat plate) generates lift.

Paul

[Richard Miller]

Dave and Paul,

Dave, I am scandalized, and there must be some :rolleyes: appropriate smilie for this, by your statement that "air is compressible at subsonic conditions." Would you like to think that over? [I'm new to smilies, but confess I like them.]

A horizontal rectangle, like the screen in front of you, with a line across the middle. If there were cold something in the top and hot something in the bottom, or any other such discontinuity, we could use it to do work.

If we make that an accelerated flow above and a decelerated flow below the horizontal dividing line, the work we can do is lift something. A force perpendicular to the direction of the flow.

Anything place on that line, an inclined plane, a palm stuck out a car window, a membrane between a leading and a trailing edge, a rotating cylinder, an airfoil, inverted if the angle of attack is great enough, a leaf in the wind, that initiates / generates a differential translational flow qualifies as an airfoil and will experience the designate force.

The separation, as you note, begins upstream, a fact, incidentally, that Newton was aware of, but didn't mention in Book II of the Principia. It does so because - the answer I was after from you - pressure propagates upstream in a fluid, from a source of resistance, at the speed of sound. The mean angle of the upwash is always the same; it is always parabolic and simply gets larger as the force increases.

So, having answered my own question, I leave you with a secondary part of it. What causes the upwash? Why is it parabolic? And Oh yes: Why call that locus of maximum dynamic pressure on the leading edge the stagnation point?

-Richard

[marcus]

Originally posted by Richard Miller
Dave and Paul,
The separation, as you note, begins upstream, a fact, incidentally, that Newton was aware of, but didn't mention in Book II of the Principia. It does so because - the answer I was after from you - pressure propagates upstream in a fluid, from a source of resistance, at the speed of sound. The mean angle of the upwash is always the same; it is always parabolic and simply gets larger as the force increases.

So, having answered my own question, I leave you with a secondary part of it. What causes the upwash? Why is it parabolic? And Oh yes: Why call that locus of maximum dynamic pressure on the leading edge the stagnation point?
-Richard

If you're going to pick on Dave's "compressible" then you'll have to do better than "the mean angle of the upwash is always the same; it is always parabolic", reference frames please.

The locus of of MDP is the stagnation point because ideally a particle will not move from there. The forces upward and downward and forward and backward are balanced there. The uplift is caused by the pressure differential between the top and bottom of the wing. IOW, the center of drag "a source of resistance" is not the stagnation point, nor is it on a line extended from the SP parallel to the flight path.

[paulriseborough]

Richard,

All wings generate what is called a 'bound vortex' which in simple terms is a rotating flow superimposed over the freestream velocity. This vortex actually forms a horse-shoe shape, because the vortex bound to the wing is shed as the wing loses lift towards the tip to eventually roll up into the trailing vortices which form behind the wing.

The velocity of a vortex is inversely proportional to the distance from its centre, so if we take a section through a wing and ignore the trailing vortices, you will have an upwash component in front of the wing and a downwash component behind the wing that vary proportional to the distance from the 1/4 chord.

Therefore the variation in upwash and downwash with distance from a wing is NOT parabolic, it follows a 1/R relationship.

In reality, the bound vorticity needs to be modelled as a number of smaller vortices which run spanwise at different chord locations.

This is the basis of the so called 'vortex panel' method of calculating flows over a three dimensional wing. This method was quite popular until the development of modern Computational Fluid Dynamics codes and powerful computers.

Regards,

Paul

[Richard Miller]

I am going to attempt, for the time being, to restrict discussion, if there is any more, to what happens ahead of the leading edge.
And I am going to confess to misspeaking myself: I affirm that the upwash has a parabolic profile; what I meant was that it begins little and gets bigger as the airspeed increases, that a line from its point of origin, however far ahead of the leading edge, to the stagnation point, remains the same.

Marcus,

Air is either compressible at Cub-flying speed, for example, or it isn't. "Pick on Dave..." What does that mean? Is this a friendly discussion of physical phenomena or are your shorts pulling up on you?

The greatest manifestation of dynamic pressure on an airfoil is, or may well be, at the stagnation point? I'll allow an ideal non-moving molecule at that point, but why the pressure, and if dynamic pressure is the business of the airfoil should its creation be referred to as stagnant in this instance?

Paul,

I am intimately familiar with the Lancaster-Prandtl theory as well as with its shortcomings. But I want to stay up front for the time being.

-Richard

[EUT]

I can't believe that you guys still belive Bernoulli, I bet you think the world is flat and revolves around the sun! Have you not heard of Coanda? He , and Newton have it all.

Eut

[paulriseborough]

Richard,

A scientist would argue that a gas can never be regarded as incompressible. However as an engineer I'm quite happy to ignore the change in density that occurs for flight speeds that our models fly at as there are many other approximations in out calculations that are of more significance.

In other words 'incompressible' means we ignore the density change because its effect is insignificant , whereas 'compressible' means that we can no longer do so.

Anyway, back to what happens in front of the leading edge, are you still maintaining that the streamlines are parabolic in a wing fixed coordinate frame?

EUT,

A flat earth model can work quite well as an approximation in a flight dynamics simulator, so don't go knocking flat earthers OK!!!!!!!!!

Oh and by the way, Bernoullis theorem does work.

Cheers,

Paul

[Blackhawk]

The rest of your readers ever wonder what happens when the worker bees takeover? This discussion is really obtuse for balsa benders.

Pat

[Dave Robelen]

Hi Pat,
I got lost in the dust quite a ways back, but I still am enjoying this exchange. Knowing that airplanes fly is easy, but really knowing why they fly is quite another matter! Thank goodness there is room for applied aerodynamics in model airplanes at various levels.
Why the next thing you know, these fine folk will start talking about the laminar bubble at low reynolds numbers and the merits of turbulators (I hope). Lets hang loose and see where the discussion goes from here.
Cheers, Dave Robelen

[SteveH]

It is definitely for a select crowd. I'm not one of them.
My theory of lift? Throw it, if it stays in the air....eureka! LIFT!!:D

[Blackhawk]

Steve

I agree-- it is a lotta theory and it can get overbearing at times..., Meanwhile, I build lotsa planes and win lotsa contests with French curve airfoils. But then, I guess arguing over what makes a plane fly is almost a much fun as building them and a whole lot easier! Just don't put them in charge of anything.

Pat

[EUT]

Originally posted by Richard Miller
There are, as we all know, two schools of thought concerning lift, the one based on Newtonian principles, the other on the Bernoulli equation. Are there any among you who would like, in a friendly way - and I stress that - to discuss the relative merits of the two? Or any others?

-Richard
At the time there were two schools of thought on whether the world was flat or round too!

[Richard Miller]

Paul, mainly,

Thanks for differentiating between an ideal and the engineer's incompressibility. There's a nit's difference there, and it's easy to get side tracked.

It was Huygens who first, to my knowledge, determined that resistance in fluid flow varied with the square of the velocity. Newton came to it later, and as he was wont, claimed priority.

Every aerodynamicist and aeronautical engineer uses that V\2 when he's calculating lift, or had better, but it rarely comes up in discussions of theory.:confused: If - when - you think it through you find the inverse-square law ruling at the source of resistance and the concept of the field ruling everything. [You following this Eut?]

Another thing you find is X plotted against Y\2 in any fluid dynamic situation, n'est-ce pas? What kind of curve does that give you?

And the rest of you guys. If this doesn't interest you, if its too abstruse or arcane, why do you bother? :confused: Not said in any mean way. I still in a :) mood.

-Richard

[paulriseborough]

Richard,

Could you please explain your statement "inverse-square law ruling at the source of resistance and the concept of the field ruling everything" It reads like gobbledygook, but then again I speak Australian english (as opposed to the Queens English).

"Another thing you find is X plotted against Y\2 in any fluid dynamic situation, n'est-ce pas? What kind of curve does that give you?"

I can't read French either, and you haven't defined what your X and Y are, but so far your haven't challenged the validity of the vortex panel method of predicting velocity distribution around a wing or the fact that a vortex induces a velocity that varies inversely with distance from the vortex core, both of which cause upwash to vary inversely with distance from the wing.

Keep on 'stirring the possum' as we say on Oz.

Regards,

Paul

[Richard Miller]

Paul,

Imagine a fan blowing a stream of air against a paddle. If the resistance to that flow at the face of the paddle is proportional to the square of the velocity [of the flow], it reads out as the inverse square. This much, incidentally, Newton recognized, although he didn't know where to go with it. In other words, these are the reciprocals: the structure of the resistance is the reciprocal of the flow.

Look at the accumulation of momentum on the face of the paddle. Imagine it as a hemisphere with the force, dynamic pressure substituted for gravity, manifest.

One is obviously dealing with a field, an area in which the designate force is dependent on the inverse square of the distance from the source. Don't look in the text books.

The X and Y are the first two dimensions of the Cartesian coordinates. The square is that derived from the law of resistance, aforementioned. Maintaining any such curve, or shear line, is dependent on dynamic pressures and it would have to be ruled by an exponent. A straight line wouldn't hold.

Is that more answers :( or more questions:confused: ?

-Richard

[Richard Miller]

One More Time

Let's define a field as the continuous distribution of some quantity or quality. It is obvious how readily gravity and the EM forces fall into this category. It is less obvious how aerodynamic forces do.

Whitehead, writing about fields, noted the confusion related to contiguous points, of which there are none in any field characterized as the continuous distribution of a quantity or a quality, and coincident bodies. These, with just about everybody else, Descartes and Leibnitz and Huygens and Newton, assumed were necessary for the transmission of momentum, and the alternative was action at a distance, which nobody wanted to champion, until Newton was forced to in The Principia.

There were two ways to address the problem. One addressed gravity and EM with the concept of a universal luminiferous aether, about which, as you know, there's been a lot of contention over the centuries.

In fluid dynamics appearances were saved by treating fluid as if it were a solid, as if there were a direct-contact transmission of momentum between air, or water, and the surface of the body comprising the source of resistance. Call this the justification of a facile intuition. One thing must impinge on another if momentum is to be transmitted. The Newtonian explanation of lift.

All fluid dynamics is field dynamics. [That's my mantra, one I broadcast repeatedly to a universe that never sends a signal back.]

The gravitational field strength at, say, 4,000,000 miles from the Earth would induce, in a given body, a [potential] velocity of X. At 2,000,000 miles the figure would be 4X, and at 1,000,000 miles, 16X. In this we see that [potential] velocity is the reciprocal of the field strength; the reciprocity of the square of the velocity and the inverse-square of the field strength.

Same thing in aerodynamics. The pressure on any point on the surface of a body in a flow = X, as up from the top surface of an airfoil, down from the under surface, and falls away in accord with the inverse-square law. That's obvious.

We draw picture of the presssure distribution, top and bottom, but never specify the immediate cause of that pressure, that it's dynamic presssure, and dynamic pressure is the disposition of momentum within a fluid. Despite all the obvious indications that we are dealing with field dynamics, no one acknowledges it. That's one of the things I keep saying, and keep puzzling over.

-Richard

















:( :( :confused: :( :(

[paulriseborough]

Richard,

I agree that the flow about a wing can be explained in terms of field equations and in effect that is what the Navier Stokes equations are. However for most practical purposes we are only interested in the pressure distribution over the surface of a wing or the upwash or downwash velocity at the tail or canard.

However it is not the 'dynamic pressure' that is measured at the surface of the wing, it is the static pressure which is equal to the total pressure minus the dynamic pressure (P_total - 0.5*density*V^2). We therefore have a chicken and egg situation whereby the static pressure field causes the flow to change speed and the change in speed results in a change in static pressure all of this occurring subject to constraint conditions imposed by the surface of the wing.

I accept in a potential flow field that as we move away from the surface, the static pressure approaches free-stream following an inverse square law (in practice some modification will occur because of geometry), however we were originally discussing the upwash component of velocity in front of the wing and that follows an inverse relationship.

Yes we are dealing with a field phenomena, but explaining the relationship between pressure and velocity to the unintiated is much easier when using Bernboullis theorem as it enables what occurs to be explained as an exchange between potential and kinetic energy.

Another reason for lack of acknowledgment is that although many aerodynamics courses cover potential flow theory that models the flow using field equations, the complex number algebra involved intimidates most undergraduates, and the problems that can be solved by hand are very limited, so most students will stash the notes away at the end of the course never to see the light of day again (like I did).

It's been an interesting discussin Richard, but it's brought me no closer to designing the perfect slo-fly aerobatics wing!!!!!!

Regards,

Paul

[Blackhawk]

Fellas

Uh-- I have this plastic French curve you might wanna use to design some airfoils--and they work pretty darn good.

Pat

[Richard Miller]

Paul,

First of all, by definition, this is just a discussion of theory, and thereby not science, but epistemology, and I do not see how any of it is going to contribute to the performance of any airplane, large or small. We work that out pragmatically, which is a good thing.

So - epistemologically - if fluid dynamics is field dynamics, why not call it that? If part of the population is incapable of understanding, so what? That's true of everything.

Navier Stokes. Very unreliable stuff. Quirky.

And, you say, it's STATIC pressure :confused: I say that the air in your tires, or any other closed space, may be considered static, that pressures in a free atmosphere are, by definition, of dynamic origin.

Please, if you will, define and describe what you conceive static and dynamic pressures to be, with references from text if possible.

-Richard

[paulriseborough]

Richard,

Static pressure is the pressure you would measure if you were a moving with the airflow. In the free-stream, static pressure is equivalent to barometric pressure at that flight condition. To measure static pressure you need to be able to make a pressure measurement without changing the velocity of the air being measured. This can be done over the surface of a wing by drilling fine holes and connecting them on the inside of the wing to pressure sensors.

Dynamic pressure is the increase in pressure above static obtained when the local flow is brought to rest (measured at a stagnation point).

Total pressure is the sum of Static and Dynamic pressure.

It is the Static pressure distribution around the wing which generates lift.

These terms get confusing due to the different reference frames that can be used. The way I have used the terms applies to a reference frame fixed to the wing.

Reference -
pp 46 - 47
Aerodynamics for Engineering Students, 3rd Edition
E L Houghton and N B Carruthers
Edward Arnold Publishing

I'm puzzled that you regard the Navier Stokes equations as unreliable. The numerical solution algorithms used to solve them are quirky, our understanding of how to model boundary conditions, etc is often unreliable, but the equations are still regarded by the computional aerodynamics community as representative of the physics. The numerical solvers and computing power have now reached a point where accurate predictions can be made of unsteady flows that were previously thought infeasible. I look forward to the day when I will be able to input the geometry of a new wing design and have software on my PC accurately predict what the lift, drag and stall characteristics will be.

Regards,

Paul

[Richard Miller]

Dear Paul,

I've enjoyed the exchange, and certainly bear you not a scintilla of ill will in consequence of our disagreement, but...

To claim that static pressure is the source of lift:( - well...

Here's a hint for you. It is not the movement of the fluid, but the momentum within the fluid that accounts for dynamic pressure. It can be accrued or extenuated and can move, does move in the case of life, at right angles to the direction of the flow of the carrier, the fluid.

-Richard

[paulriseborough]

Richard,

Two ways of looking at the same phenomena.

Regards,

Paul

[paulriseborough]

Richard,

Lets suppose that I want to design my own wind tunnel with a fan sucking air through a working section with a velocity of 10 m/s. I want to calculate how strong the walls of the working section need to be so I'm calculating the pressure of the air as it passes through the working section.

Pressure (Pa)

Static Dynamic Total
Air at rest in room 100000 + 0 = 100000
Air in working section 999939 + 61 = 100000

Now Richard, what do you suppose the force is acting on each sq metre of working section. Is it 61 Pa of dynamic pressure pushing outwards? NO, its the difference between the STATIC pressure of 100000 Pa outside and 999939 Pa inside which in effect is trying to collapse the working section inwards.

Same with a wing, to calculate lift I just integrate the static pressure distribution around the wing. What causes the change in static pressure? Bernoullis theorem says its the change in velocity. What causes the change in velocity? Momentum theory says its the change in pressure (static). Uh Oh.

Thats all from me,

Paul

[Richard Miller]

Paul,

You step out of the house on a windy day and are buffeted by a gust of wind. What kind of pressure is that?:confused:

-Richard

[BrokenChild]

I once had an extremely intelligent man tell me that the wind hits the L.E. and "bounces" up which sucks at the top of the 'hump' on the wing. The lift is on the top of the wing right? Im confused with the above responces, but I do understand. :)

broken

[Richard Miller]

Dear Broken Child,

Since at least 10 years before you were born I have been reading and researching this very simple matter of a wing in the passing flow. I've come to some conclusions about it from the point of view of phenomena, of the physics of the matter, but I think these matter are a lot less interesting than the nature of the mind that observes and thinks.

You might obtain a small notebook, date the first page, and enter your ideas as of today. Keep at it for five or ten or fifteen years. It would be one hell of an adventure and you'd have a chance to see how the phenomena change as you, and your thinking, mature. One day you might arrive at the truth.

-Richard

[BrokenChild]

I feel your responce was COMPLETELY uncalled for. First by using my age as a factor and not using your so called intelligence to read simple words as simple, you place fault. I was simpily asking a question. I stated what I was once told, and if that isnt acceptable in your world, then pay no attention to me. Thats how you end a problem.

You might as well start a "note book" of your own. :D God only knows what of since you claim to know it all. Your responce didnt even answer a simple question, it was a one word answer, yes or no. Thats all I was looking for, not advice on what to do with my mind or my life, but thanks for your false concern.

[Richard Miller]

Dear Broken Child,

Perhaps I should have been more explicit in my reply. What I meant to say was that I was, that I am, unable to bridge the gap between the sort of thinking that appears to be behind the words and concepts you use and the one I have in my mind. They simply don't translate.

I think back to when I began my study in a serious way, 25 years ago, and of the specific texts I struggled over, and how difficult they seemed. Today they are very clear to me. In other words, I exercised my mental powers, thought about the matter long and hard, drew innumerable diagrams for myself, in order to get from where I was then to where I am now.

This is the equivalent, in a way, of playing a musical instrument. I could do it poorly then, can do it well now. The reason is application.

If somebody hands you a trumpet or a violin you won't be able to make intelligible sounds. If you apply yourself for a while you'll acquire some degree of mastery.

I was not, as you seem to feel, being patronizing. I was simply saying that understanding something as complex as lift takes the same sort of investment in thinking that playing the violin does in drawing the bow.

Finally, I could not answer your question. It's as if it were in a different language from the one I speak.

-Richard

[BrokenChild]

I guess I just didn't try hard enough to properly word my questions... Thats as simple as I know to put it. Im sorry if it seems that I flew off the handle at you... I just get extremely iritated when someone looks at my age as my intelligence.

[Richard Miller]

Broken Child,

We seem to have cleared the air a little, so I'll try bridge the gap between what you were told and the ideas I have.

First, the air flows, and in the case as something as carefully designed as an airfoil it flows over and under. It is, remember, a fluid, so it cannot bump or bang into things, at least not at the velocities we're familiar with.

So the fluid accommodates to whatever object it finds in its way. Sit by the side of stream and watch how the water moves in relation to object such as rocks, or watch the water drain out of a sink in which there are plates and glasses.

Second, any resistance to a flow causes changes in velocity, some slowing and some speeding up, and the two must balance. Probably the most famous observation about the action of fluids is by Bernoulli: that the energy of the velocity component of a fluid and the energy of the pressure component are equal to a constant. That's not easy to get the first time you come across it.

Imagine a stream driving a mill wheel. The energy to drive the wheel and lift the water must come from the initial velocity of the stream. Whatever is taken out of the stream, as energy, added to what is left in the stream, as energy, add up to what was in the flow before it reached the wheel. They are equal to a constant.

Back to Bernoulli, and an additional step: the slower fluid has a greater [dynamic] pressure and the faster fluid a lesser [dynamic] pressure. Be alerted that dynamic pressure is hard to understand, even the columnist for AOPA Pilot gets it wrong and confuses static and dynamic pressures. Perhaps the best demonstratin of the lower pressure is to blow downward [thus getting rid of any gravitational influences] between a couple of pieces of paper [taped to pencils held parallel].

Finally, the wing is in the potential between the higher - bearing on the lower surface - pressure, and the lower - drawing on the upper surface - pressure. That's lift.

-Richard

[BrokenChild]

Its just been ages since I have been around ;) Reynolds numbers have always kinda stumped my toe... The Reynolds number is used in equations dealing with models of motion through resistive medium... Reynolds number (Re) is equal to (L*Rho*v)/Eta... L is a characteristic dimension of an object (like say for a sphere it would be equal to the diameter)... Rho is the density of the medium that the object is traveling in.... Eta is viscosity. v is the speed. If the Re value is small then it means slow motion through a stiff medium. Large values mean fast motion through a slippery medium. The drag coefficient depends on this number.....

For practical use, the Reynolds Number (Rn) is a reflection of the amount of "air stuff" (molecules, particles, parcels, whatever...) going past the airfoil at a given time. The main variables in calculating Rn include the airfoil chord length, the air density, and the speed with which the air is moving over the foil... Thinking about these variables, it should make sense that if you increaseany of them, you would dsimilarly increasse the amount of air passing over the airfoil.... The exact relationship of speed and chord are a little strange, but it all works well if you don't think real deeply about it.... Airliners have big wide wings (i.e. long airfoils) and high airpeeds, so the fly at high Reynolds Numbers (in the millions)... Model planes have short airfoils and low speeds and fly at low Reynolds Numbers (in the tens of thousands). As fate would have it, most of our airfoils have a very nasty habit of allowing the airflow to separate from the airfoil contour if their Rn drops into the range of around 60,000... The separation makes for lots of drag for the amount of lift generated. This is a problem if you fly powered planes since you usually fly slowest on landing approach where you can't really afford to let all that drag slow to to a stall, or if you fly thermal duration planes (sailplanes and old-timers) which exchange flight time and distance over ground for the energy they lose from the stall or high drag.... Performance curves for airfoils usually relate lift coefficient to angle of attack and drag coefficient to reynolds number... From there there are some fudge factors around that you can use to estimate Rn for a given airfoil, chord length, and airspeed -- using that with the performance curves will let you see how a chosen aairfoil should act at a given speed, what its stall speed should be, whether it should be faster than some other airfoil, etc..........But could you help me better understand HOW the small dimples on a golf ball actually reduce the drag profile?

~broken

[Mikerjf]

IMHO all you have to do is look at the airfoil plots to see where the two schools of thought have relevance. Bernouli handles the lift at negative angle of attack and the high CL max of a flat bottom or highly cambered airfoil, but falls apart for the non lifting (symmetrical) airfoils. I find it interesting that every time I see one of these discussions, symmetrical airfoils and ground effect are avoided like the plague. As a pilot and from a practical standpoint I know that no matter what the airfoil shape without velocity and pitch attitude it won't fly, and if you want to increase/decrease lift you change the velocity or angle of attack.

[Richard Miller]

Mike,

I'm not sure I understand your posting, but hasten to come to the defense of the lifting capacity of symmetrical airfoils. They're essentially no different than an inclined plane in which case the air ahead sees a surface coming towards it and air behind sees a surface receding from it, the result being that difference in velocities that we associate with the Bernoulli principle.

Have I addressed the matter?

-Richard

[BrokenChild]

Richard;

You seem like a very intelligent individual. Not very many people apply their minds to their surroundings in a way to understand them as well as you seem to be doing, or have done. Me, I have not been in this hobby as long as long as a lot of the people whom I associate with, but I try :D Richard, could I use your help in designing a biplane? Just a few questions: Does the top wing need to be set one dregee negative from the bottom to add pressure, or to add stability? <--that would mean having more positave incidence on the bottom wing to get the lift... I want to desgin a bipe that has around 45-50" on wingarea and 8-9 hundred inches of WA with a very light wingloading, to basicly put around...do you have any ideas?

Sincerely appreciated budd,

~Nate

[Mikerjf]

The air is not moving. The wing is moving (at constant velocity). The air in front is the air behind (not moving). The wing is a solid (the top and bottom sufaces move at the same velocity). Velocity difference?

[Richard Miller]

Nate, aka Broken Child,

Thank you for the conciliatory words. here's what they elicit.

I cut and glued my first balsa when your grandaddy was a kid, back in the early 30's. We poor kids didn't have much then, 10-cent kits and sheets of balsa and glue. Ha! Didn't have much? We had the world. Those WERE the good old days when you did everything but grow the tree and cut the logs yourself.

As a consequence we learned things from the ground up. I can't say how many little balsa gliders I made and sailed off the roof of the house, or wherever, nor what I learned from those simple models. Those led to larger gliders and other models, rubber powered and gas, and so on.

I feel genuinely sorry for the present generation, starting with an RC model, or equivalent, born on third base and headed toward home. You'll never know what you missed, and I DO NOT mean this in a mean way. I was present at the creation, and it was GOOD.

My recommendation would be to get a couple of sheets of balsa, 1/32" and 1/16", and build some models. Miller's first rule: Always build in the smallest practical scale. Solve the most basic problems there. You are going, I assure you, be surprised by what you can learn that way.

You should be able to throw together an 8 or 9" model in not more than an hour. Do one a day for a spell. Change decalage and stagger and sweep any other variables that occur to you. Glide and observe. This is hands-on stuff, and there is no substitute for it in books. And it's fun.

-Richard

[BrokenChild]

Sounds exciting, it really does! Ive designed and built a lot of airplanes, mostly lowwingers in the last 5 years of my hobby, but never a biplane. I just wanna see if I have in in my mind to build an extremely slow flying, large park flyer bipe... I think my biggest motivation would be my parents saying that i couldnt do it.. But then again, I dont know.. I took an old motor out of an electric screw driver, and built a speed control in 30 minutes and all workd great, plus it isnt very heavy and didnt cost me anything :D I seem to get along with the older pilots in the hobby... Bob Baker is a very good friend and good pilot who has helped me out more than I think he knows... Gots tattoos, he dips and talks the same language as my other teen ager friends, rides a motor bike, yet is 73 years old... A very neat person. I think we make a good pair... me being a few months away from legal age to buy snuff, and him outliving most people, still riding a motor bike :D Richard, you make me feel like I am talking to Bob all over again...lol

[Richard Miller]

Nate,

I'm pleased to find you receptive to the idea of building small test models. Meanwhile, to keep you from too much reinvention of the wheel, be apprised that a lot of work was done in olden times on biplane interference. The distance between planes and the amount of stagger are, of course, critical. If you search in old NACA papers n' like that I'm certain you'll find relevant material.

-Richard

[BrokenChild]

Thank you Richard

[Don Stackhouse]

Richard, your question about Newton Vs. Bernoulli belongs in the same category as downwind turns, etc.. It seems to stir up a huge firestorm whenever someone makes the least mention of it. Let me see if I can simplify things a bit.

If you'll indulge me a moment, this whole debate reminds me of the Hindu parable about the four blind men who met an elephant. The first grabbed the trunk and thought it was like a snake, the second grabbed an ear and thought it was like a tent, the third walked into a leg and thought it was like a tree, and the fourth grabbed the tail and thought it was like a rope. They argued all the way home, each insisting that he was right and the other three were wrong!

There is no conflict between Newton and Bernoulli. They merely describe different aspects of the same overall process.

A wing makes lift by grabbing chunks of air and shoving them downward. The air reacts by shoving the wing upward. It's Newton's third law, the one about action and reaction.

The catch is that air is rather slippery and uncooperative, and does not like to be grabbed, much less shoved. That's where Bernoulli comes in. The differential pressures between the upper and lower surfaces are the means by which the wing and the surrounding air manage to apply forces to each other. The "grabbing" is thanks to Bernoulli, the "shoving" and its attendant reaction ("lifting force") is thanks to Newton.

As far as flat plates and symmetrical airfoils are concerned, they only look flat and/or symmetrical to us, not to the air molecules. There is a "stagnation point" at the front of an airfoil in flight. All the air above this stagnation point goes over the top of the wing, and all the air below it goes under the wing.

As you increase the angle of attack, the stagnation point moves downward on the front of the airfoil, maybe even a little way onto the lower surface. The air above the stagnation point has to go all the way back around the leading edge and over the top of the wing, a much longer and curvier path than the air going under the stagnation point and across only the rear portion of the lower surface. Even though the airfoil might be symmetrical from our point of view relative to the chord line, the airflow sees a very non-symmetrical shape defined by the stagnation point and the trailing edge.

In deference to all the purists on this thread, yes, this is a gross oversimplification. However, in my defence, I had the impression that we were all arguing in such detail about the individual trees that we were beginning to lose sight of the forest.

[Mikerjf]

Don;

Pretty much the point of my original post. As an engineer with a passing interest in aviation I had accepted the generalized Bernoulli's based explanation as to why a wing flies, however once I got my license and got into aerobatics and then started with RC, Bernoulli's seemed to make less and less practical sense because when I get one of these things upside down the flat part's on the top and the curvy part's on the bottom and what they taught me about Bernoulli's says the lifts going the wrong way, but I know that if I push the stick forward enough and increase the angle of attack it flies pretty good. Then when I go to land this thing it doesn't like to come down when it's close to the ground, how come the wing works better when the air underneath has the ground to push on. Newton's got all the bases covered here.
Then I get a copy of IAC's Sport Aerobatics and the people that flip these things around all the time say the separation point changes as you change angle of attack. As an engineer I can't buy this because through a 15 degree range around the leading edge radius the difference in length between the top and bottom sufaces is changed minimally. But wait a minute, these Fun Fly guys fly these things at ridiculous angles of attack and these flight characteristics are attributed to the low aspect ratio, the thick chord and the large leading edge radius. With this profile the trailing edge stalls before the leading edge. Newton doesn't seem to be much help here. Looks like there's a lot going on with any wing and apparently both Bernoulli and Newton have merit.

[RichieRich]

"I was checkin' the uh...specs on the end line for the rotary girder."

Great posts guys.

[Richard Miller]

Don,

Thanks for joining the discussion. It is a continued source of fascination to me to read the various interpretations of the theory of lift. There must be hundreds.

The majority, as yours does, relies on Newton and the Third Law. Whenever it pops up, or almost always, I ask a question that never gets a satisfactory answer. It's about that downward moving air behind the wing.

Fast boats plane on the water and the evidence that they are doing so can be seen in the roostertail aft of the transom. How come air forced down, doesn't come up? How come it just keeps on going and going and going down? Huh?

-Richard

[Dave Robelen]

Gentlemen,
I have a question for the floor. Assuming a Reynolds no. in the range of 20K-30K and a wing with an aspect ratio of 7, what would be the best L/D and max lift that I might anticipate from a thin curved plate airfoil? I general terms of course.
Thanks, Dave Robelen

[Don Stackhouse]

Richard,

Regarding your question:

>The majority, as yours does, relies on Newton and the Third Law. Whenever it pops up, or almost always, I ask a question that never gets a satisfactory answer. It's about that downward moving air behind the wing.

Fast boats plane on the water and the evidence that they are doing so can be seen in the roostertail aft of the transom. How come air forced down, doesn't come up? How come it just keeps on going and going and going down? Huh? <

But it doesn't just keep on going and going... It spreads out to the sides, then upwards in an area outboard and aft of the wingtips, back towards the center, and then back down again in the middle, thereby becoming those infamous horizontal tornados trailing aft from the wingtips, those things we call the "tip vortices".

The rotation within each vortex is what eventually brings the displaced air back to where it started (more or less). The initial downward shove by the wing against the air first gets converted to rotation as the vortex forms, then to heat as the rotational energy in the vortex is eventually dissipated by friction with the surrounding non-rotating air mass.

This energy extracted from the airplane and eventually turned into heat through friction is what we see as "induced drag", i.e.: the drag that is the natural by-product of the lift-making process.

[Richard Miller]

Don,

There is indeed a vertical-upward [re]action at the tips, although the net movement is still downward. So, in order to get around the problem of disentangling them, let's just take the two-dimensional case.

-Richard

[Don Stackhouse]

Richard, when you arbitrarily decide to ignore the vortex circulation, you're artificially deleting the air's return path from your analysis. That's why you can't figure out where the air is all going. Simplifying assumptions often beget anomalous results.

[Richard Miller]

Don,

I won't say that your answer completely mystifies me, but it comes close.

Every serious treatment of lift I've ever read covers 2-dimension flow before it gets to 3-dimensional, and insomuch as the Third Law is working there, deflecting air downwards, the problem - my original question - remains.

-Richard

[Don Stackhouse]

Richard, it's because in 2-D flow the downwash velocity is zero. So is the induced drag.

Let's back up a little.

A wing makes lift by accelerating air downward. Bernoulli explains the mechanism that allows the wing to apply a force to the air to cause this acceleration, but Newton explains what amount of acceleration of what size chunk of air results in a given amount of force.

The key parameter in all of this is the size of those chunks of air. Imagine a cylinder of air, with diameter equal to the wingspan and length equal to the distance the plane flies in one second. The volume of this imaginary cylinder times the density of the air inside is a representation of the mass of those chunks of air that this wing is grabbing and accelerating in order to make lift.

Obviously the volume of the cylinder is proportional to the SQUARE of the diameter of the cylinder (in other words, the square of the wingspan), which is why wingspan is such a major determining factor of induced drag. If the wing grabs a larger chunk of air, the required downward acceleration of the air is reduced, which reduces the downwash and the resulting induced drag.

2-D flow represents a case where the wingspan is infinite. Therefore, the mass flow (the mass of air in the cylinder) is also infinite. In the formula F= Ma, for a given F (i.e.: lift force), as "M" (the mass of air inside the cylinder) approaches infinity, "a" (the downward acceleration required to generate that "F") goes to zero. If your wingspan, and therefore your mass flow, is infinite, the downward acceleration becomes zero, and there is therefore no downwash, and no induced drag. Induced drag and downwash are phenomena that exist only for finite span (i.e.: 3-D) wings.

[BrokenChild]

Very interesting topic here...

[Richard Miller]

Don,

I've been away, really away, for a couple of days and just got to your reply.

I continue to be mystified by your explanations. Because-

1) When 2-dimensional characteristics of a section are measured in a wind tunnel the downwash is obvious. I look at the airflow patterns in the book and see that on the one hand, read your explanation on the other. Winner: Streamlines.

2) If induced drag, so called, is the drag due to lift, and it only occurs in 3-dimensional flow, I guess a 2-dimensional section, as in a tunnel, doesn't generate lift!?

3) I love that part about the stream tube of air. The text in my books says, [as you do, in effect] "Take a stream tube of air..."
I fell for that at first, but there isn't any such thing. It's a fiction intended to save appearances, nothing more.

-Richard

[krisI.925]

hey im 16 and just about to get into this hobby. And im also majoring in aurotautical engineering. And iv noticed that some of you "rocket scientists" are confusing the bajesus out of me. Where have you guys mostly lerned most of this from. I mean out side of a college or school or something. Is there some place where all this could be explained a little simpaler. A understand most of what your saying for the most part but not all of it.

And from what i have lerned a flat plane will only create lift if the force is comming from underneath the wing. So the horizantil plane of the wing is always facing more than 1 degree up. Wich when moving thru the air creates more pressure under it becasuse of the drag and less on top there by creating lift. Is that what most of you are trying to say?

And a normal wing creates lift by creating less pressure on top and more pressure on the bottom because of its arced shape on top and flat on bottom. Where are you guys pulling all these other words from.

I would appresciate it if some on could explain some of this in simpler words. Thanx.

[Richard Miller]

Kris,

I was where you are now, more or less, in the mid-1970s, and for a very particular reason, which it is not appropriate to mention here, got REALLY interested in the theory of lift.

I happened to have a number of professional acquaintances, aeronautical engineers, aerodynamicists, who pushed texts under my nose, the same stuff Don is pushing under it now.

I have devoted, in the quarter century since then, an extraordinary amount of time to figuring things out, or trying to, and I can tell you one or two things for certain as a result.

ONE - I can hardly imagine more diversity in the opinions about any physical phenomenon that there are about the theory of lift. I can't say I've seen and read them all, but I've seen a lot, and you'll have to sort your way through them with the same care and discrimination you would through Biblical exegesis. It's a #@%^&*# jungle out there, buddy.

TWO - derived from ONE. There is no science, only epistemology, which is: Who is the knower, and what does he know, and what are the conceits of his knowledge?

THREE - What you learn courtesy of the establishment will probably be wrong. It almost always is. The good part about this is that you've got to think you way through these problems if you're serious about getting at - excuse me - The Truth.

FOUR - [I didn't think I'd get this far.] It's fun, and very rewarding. Just try to keep a cool head when you run into those who are vehemently in opposition to you.

FIVE - [Another one?] Be prepared for a circus of, let's call them: Fictive hypotheses. That stream tube of air Don brought up is an example. Look around an airfoil, visualize the air flow. Do you see anything resembling a stream tube?

That'll do for now.

-Richard

[Dave Robelen]

Hello Richard,
I do not have the pedigree you or Don posess, and you do have a way with words. My background involves a very long time working with NASA, wind tunnels and R/C models, wind tunnels of my own, and the same interest to get answers that match the measurements.
Generally in a two dimensional wind tunnel, the properties of an airfoil are measured as though it were a wing with infinite span. A fiction of course, but a place to start and build on. When this airfoil is incorporated into a wing that moves through open air, the "tube" concept is very helpful when sorting out the vortices, upwash, downwash, and interference effects from whatever the wing is mounted on. I have personally built vapor generators that allow visualization of the flow field in an environment where the air has room to move around the wing rather naturally.
It comes as no surprise that complete 3 dimensional wings never reach the L/D that is shown in the tables after viewing all these flow phenomenon. But for comparisons sake, the 2D data is a big help in comparing the airfoil performance alone.
I guess your degree of concern surprises me given the energy you have put into the search. Personally, I woukd rather believe in magic when I watch a C5A launch at what appears to be an impossibly slow speed, but somehow simple air has to get the credit.
Regards, Dave Robelen

[Richard Miller]

Dave,

Thanks for your thoughtful reply. I won't be able to say everything I want, because it's late, but...

Let us imagine a wing of infinite aspect ratio with an L/D of, say, 200:1. Let us then cut a 40 or 50 foot section out of it. It's going to leak at the ends, which leaks will have to be subtracted from its total efficiency. So maybe we have a wing with an L/D of 150:1.

As I visualize a wing there is outflow at the ends, a resultant vortex, and a good reason to lose 50 points of L/D. How this got transmogrified into "induced drag" - the "drag due to lift" is one of the abiding mysteries of life. I know the history: Prandtl and Betz and Munk, especially Munk, but how such a rediculous idea ever got into common currency dismays me. It is a great lesson in how embedded systems take up residence in human minds and how almost impossible it is to get rid of them.

You see the stream tube as helpful in understanding this. All well and good, but why not consider what's actually happening.

You are aware of scale effect. I know of few more fascinating subjects and would clone a self to study it for a long spell if I were able. It applies to visual matters as well as those of length, area, and volume. Very large diesel locomotives have run people down because the people were confused by the seeming slowness of the approach. Same with the C5A. They flew one of those big Antonovs at Brown Field [San Diego] a few years ago. it looked to me like it was barely moving.

My degree of concern about the theory of lift? That's because, as I see it, it's MY thing. Each of us - well, most of us have things. This is mine.

-Richard

[flyinhigh]

Hello
Ok Im new to this airfoil stuff,so could somebody explain the two laws of lift to me please?From what I read it all seems so simple (the theory of lift)and from what I gathered from the two it seems they are explaining the same exact thing,air is forced downward creating more pressure on the bottom of the wing and less at top
As for the plate that seems kinda simple too,depending on the deflection of the plate into the air (being the forward motion is deflected up) of course this is going to create lift.as it is creating a higher pressure on the bottom.
as for the guy flyin upside down your making your stick adjustments to get your wings deflected up a lil so you can still fly this goes back to the plate.As for you hittin flare during landings this is because your riddin on a cushion of air,and no matter how you look at it the pressure above the winf is goin to be lower thus helping to create lift...
did I make any sense,please correct me if Im wrong.but right now Im thinkin the hardest part of all this is creating the right airfoil
Shoot Me Down At Will :D
MIKE

[Richard Miller]

I guess there are laws of lift, although I never thought of it in exactly those terms before. I'll postulate Number One as -

The redirection of a force, that of the on-coming airflow, which is horizontal, to that of an aerodynamic force, which is vertical.

A couple of terms: "Translation" in physics means across your screen, left to right or v.v. "Common to.." means at right angles to this translational flow. So the airflow is translational, the lift force common to it.

The first thing to say about this 90-degree phase change is that all the force extracted by the airfoil from the translational flow is transformed to lift, except for an extra little bit. The "Resultant" or total force acting on the wing has a rearward slant, but that vector breaks down into the one that's vertical, designating lift, and the one that's horizontal, designating the system losses.

[These losses result from the principle qualities of the air. It has mass and it has viscosity. If you move it up or down, as happens, it costs something. If you cause shear forces, as in slowing the layers adjacent to a body in the flow, that costs something. The sum of those adds up to that rearward pointing vector.]
***************************************

The history of an inclined plate in a flow goes back several hundred years. The force attributed to it, derived from Book II of Newton's Principia, was based on what is [well] known as the sine-square law, which is to say, on trigonometry.

There is a reason for this which catapults us directly back into epistemology. Newton could not really grasp the idea of fluidity. His mind simply was up to it, so he treated fluids as solids. The fact that there was an exponent in the laws of fluids - resistance is proportional to the square of the velocity - and none is solid mechanics, slipped right by him.

It has continued to slip by everybody - well, almost everybody, since them. Aerodynamics has always been dominated by the idea of A DIRECT CONTACT EXCHANGE OF MOMENTUM. Like pool balls on a billiard table.

There's one big problem with this right at the outset. Triginometry wont take you around a 90-degree bend. Look at the table of sines. When you get to 90-degrees there no force. Zero. Thus far in my long adventure with the Establishment I've point this out 186 times. Total answers: Zero.

If you're going to pretend that fluids act/react as solids you should at least honor the lows pertaining to solids.

PART TWO

Now comes a time to pay special attention, because what follows is not just about the flow of air over a wing, nor about aerodynamics, nor even science, but about human nature, about those who hew to the straight and narrow, who are few, and the rest.

The issue is the validity of the confirmation of one's hypotheses.
This is something we are all involved in to some degree all day long, a hunch about this, a guess about that. Some of these hypotheses have a lot riding on them, and some little. Some prove true and some false. Those that are scarcely of any significance and prove wrong we can let go easily. As we go up the scale of significance, and the investment in ego becomes greater, we reach a point where push comes to shove. The stakes are so great that we can no longer just let it go. We lock in and commit ourselves.

That this happens in SCIENCE will only come as a suprise to the naive. In fact, it is rampant, and in ours as well.

It wasn't long after the fomulation of the sine-square rule that a number of independent investigators got around to measuring the force on an inclined plane in a flow. These proved to be 20 to 30 times as great as theory predicted. That's a big discrepancy. An ounce of beer in the can, or a morsel of meat in your Big Mac would get your attention. Time to regroup, find out in what particulars the threory is wrong.

This they did not do. The investment - the name of NEWTON - was too great. Failing that it was time to save apperances. "Take a stream tube of air..." The history of the theory of lift since those days, centuries ago now, has been a long rear-guard action, one still being fought today.

The downwash is the focal point. The wing deflects air downward, the theory tells us, by which action momentum is transmitted to the surface and lift is created. This exchange has three aspects - the rate of flow; the angle of the inclination; and the mass of air moved per unit time. What to do?

The rate of the flow is a known value and we can't mess with that. The angle of inclination is cast in concrete. No room to budge there. How about the total mass moving over the wing? Hmmm. So, "Take a stream tube of air..." The quantity of air that the wing actually works on is too small, by - I repeat - a factor of 20 to 30 to make the theory work. What we need is 20 to 30 times as much air, so "Take a stream tube of air..."

That's how they did it, and how they continue to do it. Try to get you mind around that before you get any deeper into the theory of lift

-Richard











-Richard