Lift Survey

[Flyandteach]

I'm doing a survey about pilot perceptions on various aviation concepts. First on the list is lift....


Please answer the questions to the best of your knowledge, based on what you have read and/or you have learned from your flight instructor(s).

Select only one answer for each question, unless otherwise noted.

1. Lift is best explained by:
a. Bernoulli’s equation (pressure differential between top and bottom of wing)
b. Newton’s laws (equal and opposite reaction)
c. Some lift is created via Bernoulli’s equation; some is created via Newton’s laws
d. Either Bernoulli’s or Newton’s can be used to explain total lift production

[JonWhitehouse]

As I know it, the lift force for a given aerofoil is directly proportional to the dynamic pressure, which is conserved in a laminar flow over a wing. this conservation is a form of bernoullis eqn, so i would answer a).
regards
Jon

[Henry Hallam]

b.
You cannot apply Bernoulli's law between the upper and lower surface of the wing since there is no streamline connecting them.

[JonWhitehouse]

b.
You cannot apply Bernoulli's law between the upper and lower surface of the wing since there is no streamline connecting them.

True, but you are not applyin Bernoullis eqn between top and bottom srfaces, your are simply looking at the pressure difference caused by the difference in airspeed which produces a resultant force upwards.
regards
Jon

[JonWhitehouse]

There is an argument for Newtons laws being used, in addition to bernoulli, as one Newtonian law states that an object will accelerate in the direction of a resultant force, but that was not the one refered to in the original question.
Regards
Jon

[Cool_Hand]

It's the horse shoe vortex. :)

[JonWhitehouse]

horse shoe vortex?! :suspect: :) ?

[Fg Off Max Stout]

There are lots of different ways of looking at and explaining lift generation, so the answer has to be d. The momentum change resulting from downwash can explain lift in terms of Newton's third law, the pressure difference over the wing surfaces can be used to explain lift in terms of Bernoulli's equation and vorticity is another way of accounting for lift. There are, I am sure, other techniques, but it's a long time since my Aero Eng degree. All different techniques are actually just looking at the same thing in different ways.

[Pitts2112]

Excellent! It's been at least six months since this knackered old thread has been given a repeat airing!! Posters please make sure we include a few of the standard jokes about lift being created by money, gremlins, etc, etc, etc.

But, if you're conducting a survey and want serious answers rather than an argument bordering on religious faith, I'd have to vote (d), though it's not as black and white as all that.

Pitts2112

[IO540]

Not that one again :O

Bernoulli can be derived from Newton; the two are different ways of looking at the same thing. Bernoulli just found a convenient way of working stuff out for the special case of fluids. If you did a simulation on every molecule around the wing, using Newtonian mechanics, you would (eventually) end up with Bernoulli.

So the answer is a) and b) and d). c) is meaningless.

[Henry Hallam]

But the "law of equal travel times" is nonsense and demonstrably untrue.

[IO540]

Not sure who your reply is aimed to, HH.

The explanation about equal travel times on top and bottom (i.e. particles separated at the leading edge meeting up again at the trailing edge) is false, yes.

To generate lift, you need to change the direction of the airflow. You can't change the direction of airflow without creating a pressure difference. Pressure difference gives you lift... The two are tied together. They are the same thing.

[Islander2]

And for a proper explanation of lift rather than the superficial knowledge implied by the four choices of answer you offer, an understanding of viscosity and circulation will be required.

[High Wing Drifter]

Here's a hamster wheel I made earlier: http://www.pprune.org/forums/showthread.php?t=207289

[IO540]

I am not a physicist but I think viscosity doesn't come into this (it plays a part but is not necessary to create lift through redirection of the fluid flow - all you need is for the fluid to have nonzero MASS), and circulation theory is just a handy way to look at the problem; it doesn't represent reality any more than anything else.

I doubt that X-Plane, when simulating the airflow over the airframe and the prop blades, is using circulation theory.

[Islander2]

I am not a physicist but I think viscosity doesn't come into this (it plays a part but is not necessary to create lift through redirection of the fluid flow - all you need is for the fluid to have nonzero MASS), and circulation theory is just a handy way to look at the problem; it doesn't represent reality any more than anything else.

Sorry, IO540, but that's fundamentally wrong on both counts.

Circulation is central to the production of lift and has nothing whatsoever to do with handy concepts. No circulation ... no lift, regardless of aerofoil shape or angle of attack.

And viscosity is central to the production of circulation. No viscosity ... no circulation and therefore no lift (or drag).

This is covered by all the main aerodynamics text books, but the mathematics can be rather daunting. The best non-mathematical treatise of lift generation that I've found is Volume 1 of Chris Carpenter's 'Flightwise - Principles of Aircraft Flight'. He is a former head of aerodynamics at RAF Cranwell. Some quotes from that book:

"Lift is generated by the production of circulation around the wings, by which some velocity is added to the stream-wise velocity over the top of the wing, and some is subtracted from the velocity beneath the lower wing surface."

"Viscosity is essential for the production of any aerodynamic force whatsoever."

Or, if that book is not your flavour, try 'The Design of the Aeroplane' by Darrol Stinton, a truly remarkable treatise that should be on the shelf of anyone with a deep interest in these matters:

"Viscosity is the property that makes a fluid 'real', as against mathematically 'ideal', and it only becomes important when fluids are set in motion. We could solve many problems if it could be disposed of, but we would banish lift as well."

"When a circulatory (bound vortex) motion is induced in a straight steady flow by an aerofoil, both motions can be added vectorially to produce the flow pattern seen around a lifting surface ....... the circulatory motion results in an aerodynamic cross force which can be resolved into lift, normal to, and drag, parallel with the flight path"

[Gnumann]

Every action has it's equal and opposite reaction right? Think about this--when your wing produces upward lift (due to Bernoulli's principle), it has to push down on the air. Your wing produces lift by continuously pushing (or sucking) air down!! If your wing produces 20,000 lbs of lift in the upward direction, then a mass of air must be accelerated in the down direction so that (mass of air) * (average acceleration of air) = 20,000 lbs. It's interesting to realize that as you go faster, the more air mass you run into, and the less you need to accelerate it downward to get 20,000 lbs. Thus the decrease in angle of attack required, etc.

I personally think that downwash is not what causes lift, just the natural by-product of a wing sucking air down. That's right, I said it--the wing sucks air down. I think I might be full of crap though, cause this stuff just came to me while falling asleep one night...

Same principles work for drag--you have to "drag" air along with you to get drag. Of course, you have to "thrust" air behind you to get thrust. These two cancel out if thrust = drag, so if you looked at the average movement of a body of air your airplane just flew through, it would just be moving down, not forward or back. Again, I could be full it, so don't flame me.

[IO540]

Islander - I maintain that all this is derived from Newtonian mechanics.

All the other stuff is just a way of looking at the problem on a bigger scale - no matter how many books contain the stuff.

[Genghis the Engineer]

Just butting in, Flyandteach was asking about pilot's perception, not about what actually happens.

G

[Islander2]

Islander - I maintain that all this is derived from Newtonian mechanics.



I have two problems with that:a) Saying that lift is governed by Newtonian mechanics, and that's that, is about as helpful in understanding the subject as .............!!b) Lift as a Newtonian reaction requires the airfoil to deflect the streamlines downwards. Unfortunately, without circulation - which requires viscosity - the streamlines aren't deflected downwards! Look in any text at a representation of inviscid flow around an aerofoil and tell me how you get a Newtonian reaction from that. So a knowledge of Newtonian mechanics alone ain't sufficient.




Genghis said:

"Just butting in, Flyandteach was asking about pilot's perception, not about what actually happens."

True enough, and I can see where you're coming from! But shouldn't the educational objective be to achieve some convergence of the two? :D

Edited to add one last observation before getting my hat and coat, never to be seen on this thread again! From Chris Carpenter's book referenced earlier:


"Interestingly, therefore, Newton's momentum theory of lift, although useless for the aerodynamics of conventional flight, actually comes into its own in modern hypersonic flight analysis." (my italics)

[rotorfossil]

I sometimes wonder if our perceptions are conditioned by visualising the situation in a wind tunnel. In the real world the air molecules are stationary until the wing/rotor etc has passed. Some of the molecules will be displaced downwards and therefore work has been done on them and there will be an equal and opposite reaction ie, lift. Equally the molecules will have been displaced in a horizontal direction creating drag. I suppose this means in the end the answer is Newton. By itself, Bernoulli doesn't explain how lift can be achieved by an angled flat plate.
However the endlees debate about how the displacements occur is always fascinating.

[bookworm]

Circulation is central to the production of lift and has nothing whatsoever to do with handy concepts. No circulation ... no lift, regardless of aerofoil shape or angle of attack.

And likewise, no chicken, no egg. ;) You don't need to know anything about circulation to understand lift, in the same way that you don't need to know anything about Gauss's theorem to understand gravity.

And viscosity is central to the production of circulation. No viscosity ... no circulation and therefore no lift (or drag).

"Viscosity is essential for the production of any aerodynamic force whatsoever."

Essential perhaps. Central? Not really. You need a trace of viscosity to see why the Kutta condition must hold. But beyond that, the magnitude of the viscosity plays only a small part in determining the lift.

It's a bit like balancing a billiard ball on the head of a pin and asking why it falls. In principle it could just sit there, but it doesn't, because there's some tiny asymmetry in position that causes it to start to fall. But the size of the asymmetry is irrelevant in determining how hard the ball hits your toe. Essential? Yes. Central? No.

[Islander2]

You don't need to know anything about circulation to understand lift, in the same way that you don't need to know anything about Gauss's theorem to understand gravity
And likewise, you don't need to understand the laws of thermodynamics to know how an internal combustion engine works. ;) Although in which case, do you really understand it?

Circulation produces the speed difference that results in the pressure difference predicted by Bernoulli's law. Circulation results from flow separation at the trailing edge, without which the flow can experience no 'action' for a Newtonian 'reaction'. And viscosity, however small in value, is a prerequisite for these conditions to occur. Furthermore, circulation is not just some academic concept (although it is mathematically valid), it is what actually happens. For these reasons, how can you possibly justify your assertion that no knowledge of circulation is required for an understanding of lift? I know you'll try, though.:D
Originally Posted by bookworm
Essential perhaps. Central? Not really.

A PhD in splitting hairs, by any chance? ;)

Chambers Dictionary: central: principal, dominant, most important, essential. :D

[porridge]

Surprised that Coanda Effect hasn't been mentioned here, this is an important alternative explanation to Bernouli.
Have a look at this reference and it subsequent argument: http://jef.raskincenter.org/published/coanda_effect.html
Would welcome some enlightened comment on this

[bookworm]

Circulation produces the speed difference that results in the pressure difference predicted by Bernoulli's law. Circulation results from flow separation at the trailing edge, without which the flow can experience no 'action' for a Newtonian 'reaction'. And viscosity, however small in value, is a prerequisite for these conditions to occur. Furthermore, circulation is not just some academic concept (although it is mathematically valid), it is what actually happens.
Circulation in everyday usage implies a motion around something. But that's not what's happening with an aerofoil -- the streamlines don't form loops. Only when you separate the flow field into a superposition of the freestream and various vortices do you see something that the man on the Clapham Airbus would recognise as circulation. That is a mathematical contruct. Without getting to that level of theory, circulation is just another way of saying that the air going over the top travels faster than the air over the bottom. Beyond that, it is not a prerequisite for understanding lift.

[Islander2]

Circulation in everyday usage implies a motion around something. But that's not what's happening with an aerofoil
So what? Call it something else, then, if that makes you happy! Why on earth, pray tell me, should the everyday general interpretation of a term invalidate its importance when applied to a specific scientific phenomenon?

Originally Posted by bookworm
Without getting to that level of theory, circulation is just another way of saying that the air going over the top travels faster than the air over the bottom. Beyond that, it is not a prerequisite for understanding lift.
Oh, I see. Rather like: without getting to that level of theory, fertilisation is just another way of saying that a sperm fuses with an egg. Beyond that, it is not a prerequisite for understanding procreation. :hmm:

[Woodenwonder]

Newton has it in my book. Bernoulli was a good chap, and deals with flow through pipes, but we don't fly in pipes.
Please see the spendid treatise (sorry I can't give you a link), a 13 page paper entitled "The Newtonian Description of Lift of a Wing", By David F Anderson and Scott Eberhardt.
I always used rate of change of momentum as my expanation for lift when instructing, ranging from John Farley sitting on a column of very fast moving hot air, through props, helicopter blades, fixed wings - and where span loading wins for greatest efficiency with a glider's wings gently working on a huge volume of air and imparting very little kinetic energy to it (would that be induced drag?)

[Islander2]

Newton has it in my book. Bernoulli was a good chap, and deals with flow through pipes, but we don't fly in pipes. Please see the spendid treatise (sorry I can't give you a link), a 13 page paper entitled "The Newtonian Description of Lift of a Wing", By David F Anderson and Scott Eberhardt.
Personally, I find it difficult to apply the word 'treatise' to this paper, let alone 'splendid'.

It follows the pattern of lots of other papers to be found on the internet and elsewhere that seek to debunk the explanations for lift that are given in non-scientific publications, e.g. pilot training books, science-for-the-layman texts, etc. I've no problem with that in principle, since many of those publications are full of tosh in their attempt to find easily understood explanations, including the oft-quoted: "air must speed up over the top surface of the wing because it has further to go", and sometimes even attributing at least a significant part of the lift to Newton's erroneous explanation that it is due to: "the bombardment of a wing's lower surface by air molecules at a shallow angle", an explanation that Newton himself apologised for in getting wrong!

What I do object to, however, is that having set out its stall as a paper that would correct some of these popular misconceptions, it goes on to dismiss, in passing, and without any scientific argument, the very basis of serious aerodynamics understanding as developed by Lanchester, Prandtl and Kutta, et al in the early part of the twentieth century; knowledge which is still used today throughout the aviation industry as a basis for modelling the flows around an aerofoil. It even comments rather scathingly on the aerodynamicists use of "a great deal of jargon, such as 'starting vortex' and 'bound vortices'. Duh, dumb-down time!

To round it off, in seeking a different 'simple' explanation, the paper introduces its own physics tosh and also (like some other recent popularist papers) seeks to give undue prominence to the Coanda effect, which it says is "the key to understanding flight." Sorry, but I'm not aware of a single leading academic or practitioner who subscribes to that view.

I note that the paper was submitted in 2001 to the American Journal of Physics, but has not been accepted for publication. I fancy the peer review would make for interesting reading!

Given that these disagreements have been raging for what seems like an eternity, it's a fair bet there is no simple explanation that can be widely understood! Indeed, I was grateful for, and rather amused by, porridge's earlier link. While we are busy arguing the "why does it happen?", that paper by Raskin highlights the confusion and misunderstandings that exist even about "what it is that happens" In five books from which he quotes that describe the deflection of a spinning ball, one, by a physicist, actually has the ball deflecting in the wrong direction! And the other four have various conflicting statements about the airflow velocities and pressures around the ball that give rise to the deflection, including a wonderfully wrong association of high velocity with high pressure in a book purporting to describe the physics of baseball.