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# Theory on lift

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# Theory on lift

27th Jul 2012, 06:27

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Hi Owain

something like that, but as I say it is kind of an intuition. We can investigate this further.

The first part, the increased mass flow, I think it is clear: the faster the airspeed, the more mass of air is pushed, the more lift is generated. The other part, the increased acceleration due to increased airspeed, that is another matter, but seems logical: the stronger the push by the wing, the steeper the pressure gradients around the wing, the faster the air will flow as a result and the stronger the upwash and downwash effect.

that would explain speed squared

The truth is that the empirical approach is clearly the best.
27th Jul 2012, 08:33

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Hi Microburst,

As you say, it is almost intuition! I think that in reality everyone will go for an explanation that suits their mind set. For me, a one-time aerodynamicist, I tend to think non-dimensionally, so I seek an explanation in terms of flow angles - AoA and downwash angle off the trailing edge.

Classical (inviscid) theoretical aerodynamics couldn't explain either lift or drag until a chap called Kutta introduced a hypothesis that the flow should leave the airfoil at the trailing edge. To make that happen he had to introduce a new variable that he called circulation, so that the flow over the airfoil was a combination of freestream plus a "circular" flow which added velocity on the upper surface and reduced it on the lower. Trouble was, to get such circulation in real life you have to accept a 'starting vortex' when the air first starts to flow over the airfoil. But vortices are viscous phenomena so to make the (inviscid) theory work you have to rely on the existence of viscosity

Anyway, in my non-dimensional world the angle of the flow coming off the TE is only a function of AoA and wing camber. If you grant that, then the downwash velocity is proportional to freestream velocity as is the mass flow.
From that lift proportional to velocity squared follows directly.
27th Jul 2012, 09:05

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yes

Darroll Stinton was also based on the circulation, I think. I wish I had that book here...

sometimes I wonder If I would be a happier person just using the theory of bernouilli as given in the books...
27th Jul 2012, 09:06

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I think people should give some thought as to what use the various theories should be put to.

At the "why do wings fly" level, Newton clearly rules,to use the terminology of this forum. A wing is a machine for producing downwash, period. The fact that an unstalled wing deflects more of the air passing over its upper surface than that over its lower is only a detail, however important.

Unfortunately to get numerically correct results Newton's Laws should be applied to every air molecule passing over the wing, which is impractical even with today's computers. So there are the macroscopic theories, concepts of pressure and energy, Navier-Stokes equations, circulation, etc. But these theories are intended for calculation, not for explaining why a wing flies.

Finally, Bernoulli (and Bernoulli's equations are ultimately derived from Newton's Laws) is probably the best way of explaining the vital need of keeping the airflow attached to the upper surface, without going into the detail of either the atomic or the calculation-oriented approaches. For those whose day-to-day business it is to ensure that the airflow does stay attached, it's an approach that has worked for over a century - although how well it actually works may be questionable.
27th Jul 2012, 16:42

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Just integrate the pressure over a control surface and have done with it

. Potential flow plus circulation and diverting gaze from the odd singularity does it for me.
27th Jul 2012, 17:59

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fizz57, I agree with you

and I would add, for the indoctrination of those whose job is to keep the wing unstalled, what is the real problem of stall.

For most the stall is a problem of lift, when it is clearly not. Lift of a stalled wing is much greater that lift for level flight. the real problem is the greatly increased drag and the control problems. If an airplane had powerful enough engines and could control attitude even at very high angles of attack, flying while the wings were stalled would just be a problem of fuel. But airliners can't do that, and that is why they will sink if they stay stalled.

So yes, what all pilot should know, and it was supposed to know if from the first lesson, is that you have to unstall whe wing to eliminate the problem. Apparently many pilots did not have that very clear, as the whole industry has had to meet and embarrass the pilot community. Because it was embarrasing...
28th Jul 2012, 15:50

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At the "why do wings fly" level, Newton clearly rules,to use the terminology of this forum. A wing is a machine for producing downwash, period.
Usually? Maybe. "Period"? No. A wing in ground effect is not creating enough downwash to account for the lift.

I think you have to separate two questions.

1) How does one "explain" the empirical flow field that is observed around an aerofoil of a particular shape and at a particular AoA? (i.e. why does the air flow like that?)

2) How does one predict the lift produced by the aerofoil in that flow field?

"Newton" and "Bernoulli" as described here both help answer the second question. They don't help with the first. For that, one has to be satisfied with some waving of hands, or be comfortable with solving the equations associated with flow. I've never found much in between.
29th Jul 2012, 07:57

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Yes, indicative of the attention deficit that seems to apply once the general principles are believed understood and the crowd rush to other more sexy problems. Field problems are generally not that easy to interpret by cause and effect thinking. Easier to figure out steady state solution and not worry too much about the initial transient which was how the system got there . Think the unsatisfactory discussion on starting vortices begins here!
However for a structure in airflow midchord flow is forced down so momentum theory is happy and the interrupted flow causes a reduced pressure behind the wing which accelerates local air. So reduced pressure and higher speed above the wing and everybody is happy
29th Jul 2012, 10:06

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"Usually? Maybe. "Period"? No. A wing in ground effect is not creating enough downwash to account for the lift."

My thought on ground effect is that the wing functions in what is functionally denser air (ambient) due the piston effect of the wings. For a height of roughly one half the wing span, the air cannot escape the squeeze of the wing's downwash as quickly as in a fully 'open' region, the wing and ground proximity reduce the escape "area" between them; the local airmass is functionally, denser....
29th Jul 2012, 20:31

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For the really masochistic or obsessed Abbott and Van Doenhoff's 'Theory of Wing Sections' answers everything...it is probably my favorite book in aerodynamics though...but it's a very rough read and you really have to KNOW math and physics {engineering level}otherwise it makes absolutely no sense, it's mainly a reference book

J_T great explanation...

Last edited by Pugilistic Animus; 29th Jul 2012 at 20:33.
3rd Aug 2012, 12:00

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I use this website, it has excellent notes not only on aerodynamics but also on stability and control.

http://www.flightlab.net/Flightlab.n...rse_Notes.html

I struggled with the theory of lift for 3 years at uni.
Couple of things I have in my head off hand which help me to understand:

Inviscid fluids cannot produce lift. This has been experimentally proven. There has been experiments with superfluid helium which prove this is true. Viscosity is essential for the production of circulation, which is a function of the Starting vortex (which you can see in the bath, push your flat hand through water at an angle of attack and you can see it). There is a theory (von Karmen I think) which states there has to be an equal and opposite vortex attached at the wing to maintain balance of forces.

2D wing sections do not produce lift. only 3D wing sections produce lift. 3 Dimensional effects are very important and form the heart of the "Bound Vortex" Theory of lift. This also explains downwash experienced by the fuselage aft of the wing and at the tailplane, changing pitching moment across the wing section and changing angle of attack experinced across the wing.

Without drag, there can be no lift. This comes back to both viscosity and also induced drag as a result of angle of attack of the wing to the freestream flow.

Last edited by VinRouge; 3rd Aug 2012 at 12:02.
3rd Aug 2012, 12:09

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"You will never understand lift. Forget it. You haven’t got a chance."

And that is what makes me smile.... Whether from perversity or some other antisocial bent, I enjoy that man can not prove mastery over Nature. Some things are enjoyed in their mystery; it levels the field.

My motto regarding lift and it's tantalyzing resistance to explanation is,

Math hates RATE...... It's why pilots use their hands to explain things, and mathematicians use a pencil.
3rd Aug 2012, 13:11

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Abraham

I always like to compare lift equation with friction force equation, when talking about lift theory.

Cambridge, Gottingen or MIT scientists could pursue and find an equation for friction which could be pages long, using all kinds of variables such as molecular mass and reach a point where physics would meet chemistry and even quantum theory, why not...

But then it is so much easier to make some experiments, tabulate, find relations of proportionality and find the constants for each material...

The equation obtained is so simple and beautiful, and so innocent because it does not challenge the mistery of creation, it only stares one of its laws. We could have an endless debate about "why is ther a friction force?" similar to "why airfoils lift".

That is why I love the lift Equation. It does not explain why air behaves the way it does when going past a wing. it accept it behaves that way. Then it describes how it affects the wing an what to expect if a few variables are known.
3rd Aug 2012, 13:46

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Yes The quantum. The transition. I really do not hate Math...much. I don't know if there will be a meeting of the maths with Mother Nature. But I say, soldier on!

I 'll be patient. In no other pursuit I know, is Faith a component of Science. It is why the old ones were called back to fly the 747. Gray hair and four gold, something to believe in. If he thinks it will fly, maybe it will.....
5th Aug 2012, 20:08

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soldier on

@extricate:
if you prepare exam, flight licences, Atpl instructor qualification, etc. or if you want a well paid carrier in aerospace, stay on 1.
If you want to understand how birds fly, how unloading the wing works, how spacecrafts come back on earth, how butterfly flies, ...how nature allows lift goto 2!

1. If you want to get your exams, please, just write as it is written in YOUR books. Your teachers teach the lift as these crazy theories give them their salaries, to get famous, for some of them to get Nobel prices. And forget... (maybe you will finish like AF447 in the ocean, take a good insurance to help your family : all that is only statistics! "So many millions NM anf flights have not accident..." STOP. GOTO END

2. You want really fly like birds! Just look seriously the sky : can you really imagine the air is continuous, homogen, isotrope? NO NO NO. So forget ALL the continuous equations: Bernouilli, speed model, pressures equations like navier-stokes or Kutta,Lanchester :First learn Fractals,(it is difficult),dynamic systems (sampled fractals...), and plastic rupture! It is a nice story. Good kuck. And never forget. STOP. GOTOEND.

END.

Last edited by Jetdriver; 18th Aug 2012 at 19:03.
6th Aug 2012, 09:42

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For most the stall is a problem of lift, when it is clearly not. Lift of a stalled wing is much greater that lift for level flight. the real problem is the greatly increased drag and the control problems.
I do fully agree with the first sentence, but at the same time do fully disagree with the second.
The problem of stall is not lift, it is the slope of the lift-over-AoA Curve! The Airplane can fly at any Cl, given the right speed is applied. So a drop in lift can be compensated by more speed, which can be easily demonstrated in Gliders which have two Cl max peaks in the lift-over-AoA Curve. In full stall some of them fly fully stable, but with a higher speed than the lowest possible one. (You can easily demostrate that when flying in formation)
The equilibrum of forces perpendicular to the flight path (or the airflow) is only stable, if there is a positive slope of the lift-over-AoA Curve. Lift automatically equals weight as long as the slope is positive, mother nature controls this for us. Any lack in lift results in increase of (downward) vertical speed, which results in AoA increase which restores lift. If however the slope of the lift-over-AoA Curve is negative, then lack in lift results in increase of (downward) vertical speed, which results in AoA increase which further reduces lift, hence the airplane switches from the modern rules of aerodynamics to the ancient law of gravity.

Nobody worries about the theory of gravity, neither should we worry about where lift comes from. As long as we know how to loose it, and avoid that situation, we are fine.
6th Aug 2012, 14:37

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Volume

I don't agree with your disagreement.

Lift coefficient at very high angles of attack is very high, and in the stall, even well inside it, it is very high too, compared with typical cruise angle of attack.

Of course, if you keep pulling and pulling and your aoa increases a lot, then even CL will be low. We agree in that. But even in that case, if you have enough power and means to maintain a given attitude steadily, you can fly the airplane. However this is extremely difficult, specially the control part.

So if pilots were indoctrinated in this, maybe using modern fighter jets as an example, they could be convinced than pushing the stick is the right thing to do whenever they face a stall or impending stall situation.

In my flight school, long time ago, in the piper arrow we practiced a manoeuvre called something like "characteristic stall", which was a progressive stalling of the airplane in landing configuration. The exercise was successful if after the stall warning you recovered the stall... without losing more than 20 ft. Many times this exercise was carried out at merely 500 ft, over the runway!

I had lots of arguments for that with instructors, but I only had about 120 hours. Now I know I was right. The whole industry met only to clarify my point.

Negative training is intensive and extensive and it is everywhere. One day they should address it seriously, it deserves more attention.
6th Aug 2012, 21:47
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Many times this exercise was carried out at merely 500 ft, over the runway!

How quaint. How does one spell "silly" again ?

While specific techniques may vary between Types, the general rule with stalling is that alpha has to be reduced somehow in order to escape ..
8th Aug 2012, 12:48

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I'm really loving this discussion...I wish I had time to really join in the way I'd like to...

but I don't have the time really so if I make make two brief points
1...the best mind in aerodynamics who ever existed was SzQllQskizlaki Von Karman Todor...better known as Theodore Von Karman

2...a designer picks whatever equations s/he need to meet the design the theory of differential equations goes on seemingly forever...

One of the main reasons I don't have enough time to post what I'd really like to say sometimes is that my laptop (girlfriend) keeps hogging my desktop...confused?
well here's what I mean

ps I think she's doing a vg diagram

Last edited by Pugilistic Animus; 8th Aug 2012 at 12:51.
8th Aug 2012, 13:08

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what he has written on the board is what exactly i've been rambling about all along...