Why airplanes fly: The Truth uncovered...happy reading

[mundaka]

Dear all,

enjoy the news...

http://iopscience.iop.org/0031-9120/38/6/001/pdf/0031-9120_38_6_001.pdf

:8:8:8:8:

[sky9]

Try this:
Airflow across a wing - YouTube

[DutchOne]

THanks man!!

[Flypro]

A prime candidate for Jet Blast methinks!

[Sciolistes]

Wolfgang Langewiesche told us this in the 1940's, so BS vanquished and truth rediscovered would be more accurate.

[KiloB]

So it's Newton 1. Bernoulli 0.
They LIED to me!

I presume the Cambridge guys will be having a look at downwind turns next?

[Genghis the Engineer]

Disregarding for a moment that Bernoulli died over a century before the aeroplane was invented, he was nonetheless right. It is certain instructors unnecessarily over-simplifying the theory, which has been very well understood since the early 1920s, who are lying to you.

It's a nice bit of video isn't it.

G

[Crabman]

Here we go again for the umteenth time. Even this article is an incomplete description of lift. For instance even a flat piece of plywood angled to the airflow will generate lift (no curves involved).

And I didn't see any mention of a frictionless conveyor belt .

Time for JB.

[John Farley]

You mention flat plates. I agree with you of course. For my students who cannot understand how a flat plate can ever produce low pressure above its top surface I talk along these lines:

Aero guys call the streamline that defines the boundary between the air which goes over a wing from the air which goes under a wing the ‘stagnation streamline’ and call the point where it hits the aerofoil the stagnation point.

The key thing to realise is that the only time the stagnation point (SP) is located right on the front of the wing leading edge is when the wing is at such an alpha that it is producing zero lift. If the wing, even a flat plate, is at an alpha that allows lift to be produced, the SP will be located on the underneath of the wing a small distance back from the leading edge. Furthermore the SP moves steadily back under the wing as alpha is increased until the stall – and even beyond. This effect can be seen in any simple wind tunnel where smoke is introduced to enable the flow past the wing to be visualised.

If you don't have a wind tunnel and to understand how this can also happen with a flat plate, I offer the following explanation. Imagine it is a windy day and you are carrying a large 8 x 4 ft sheet of chipboard out from the DIY store to your pickup. Being bright you take care to hold it flat and edge on to the wind so naturally the SP is right on the leading edge and all is well. Then out of the corner of your eye you see another chap who is carrying his sheet flat on to the wind.

Here the SP is right in the middle of the board with the air spilling round all four sides. He realises your way is easier and starts to rotate his board towards the flat position you used.

As the alpha is reduced from his 90° to your 0°, the SP has to move forward from the centre to the leading edge. Therefore is it not reasonable to accept that as the alpha of the sheet is reducing through say 10° (below that for the stall but not yet zero), the SP will be well on its way to the leading edge but it will not actually get there until the alpha for zero lift is reached? I apologise for that childishly simple analogy but I find such things useful so perhaps others will as well.

[bbrio1]

Interesting post, it inspired me (after having lurked for ages) to register. I am stupid, like the second man with the vertical plywood. But I'm a fast learner, so I gradually reduced my AoA to the right point, until eventually I was just pushing my 4X8 towards the trusty pickup with a couple of fingers (hoping at the same time that the wind didn't pick up and break my nose).

[italia458]

I posted on Reddit a few days ago talking about the inaccuracies of current lift explanations. I thought I would include the link instead of rehashing everything I said over there! My username is: 4fifty8

I don't believe this is a Newton vs Bernoulli fight. Both of them have contributed to the understanding of lift that scientists have in 2012. It's just that whoever taught you possibly taught you incorrectly that Bernoulli was the one and only reason why an airfoil produced lift and then stated the myth of equal transit times. Those are both wrong, but Bernoulli isn't!

Lift - still being poorly and inaccurately described : flying (http://www.reddit.com/r/flying/comments/ouum4/lift_still_being_poorly_and_inaccurately_described/)

[chris weston]

OK, head above parapet!

Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.

Hence the behaviour of sheets of 8x4 or upside down flight or flat wings or whatever.

The lower pressure above the wing is largely simply relative to the higher pressure beneath.

Shape helps but is not key.

CW

[italia458]

Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.

I don't think so. Imagine a flat bottomed airfoil with a positive camber on top that is positioned so the bottom is directly parallel to the relative airflow. There are no air molecules "hitting" the underside of the wing yet the wing is still able to produce a significant amount of lift in this position. There would be skin friction on the underside of the wing which would cause the molecules closest to the wing to slow down to a very slow speed and as you move further away from the bottom of the wing you'd notice the speed would increase progressively until you reached the free stream air speed.

When you're talking about a piece of plywood or a barn door flying, it would have to be at a VERY low angle of attack to fly by the same principles as an airfoil normally does. This is because the critical angle of attack would be very low due to the lack of a smooth and progressive path for the air to follow as it flows around the top. The change in velocity (direction in this case) would be so high that the surface tension would not be high enough to be able to impart enough force to keep the airflow attached to the upper surface of the plywood. It would break away at the critical angle of attack and it's now stalled. To make the plywood fly beyond this point, then you would be relying a lot on the molecules hitting the underside of the plywood and imparting a force based on Newton's laws that would cause the plywood to "fly". Important distinction is that this is not how airfoils normally fly!

[John Farley]

Your words sound eminently sensible however they do rather ignore the evidence from those wonderful aids to understanding wings - wind tunnels.

In tunnels it is easy to actually measure the pressures all over the top and bottom surfaces of a wing at different angles of attack. (This is done by making many small holes in the surface of the wing and connecting these 'pressure tapping points' with tubes inside the wing to a bank of manometers or U tubes containing fluid. Naturally the fluid gets 'blown' up at the places where the pressure is increased and 'sucked' up where the pressure is lowered. The height of each column is of course proportional to the local pressure at the tapping point and providing you know the specific gravity of the fluid in the manometer the actual pressure - positive or negative - can easily be calculated.)

You will have to take my word for it that at angles below the stall the suctions are relatively much larger than the positive pressures. If you doubt my word (and why not for goodness sake) you could examine any suitable reference work on such activities.

When it comes to generating forces you are however very right that shape is less important than angle of attack. A traditional 'aerofoil' shape has the edge of course over a flat plate (which is in turn much better than an aerofoil flying inverted) when it comes to the suctions produced on the upper surface. Of course a 'good' shape also wins hands down with that marvellous measure of aerodynamic efficiency - the ratio between the lift generated and the drag generated or the L/D ratio.

In simply recording the facts of this life I do realise that I have done nothing to explain the facts. Such an explanation requires more mental horsepower than I was issued with.

[italia458]

In tunnels it is easy to actually measure the pressures all over the top and bottom surfaces of a wing at different angles of attack. (This is done by making many small holes in the surface of the wing and connecting these 'pressure tapping points' with tubes inside the wing to a bank of manometers or U tubes containing fluid. Naturally the fluid gets 'blown' up at the places where the pressure is increased and 'sucked' up where the pressure is lowered. The height of each column is of course proportional to the local pressure at the tapping point and providing you know the specific gravity of the fluid in the manometer the actual pressure - positive or negative - can easily be calculated.)

That's an ingenious way of measuring I hadn't thought of!

You will have to take my word for it that at angles below the stall the suctions are relatively much larger than the positive pressures. If you doubt my word (and why not for goodness sake) you could examine any suitable reference work on such activities.

I don't believe I said anything that would contradict that. That's what I was meaning when I said that a piece of plywood would fly, based on the same principles as a normal airfoil, when below its critical angle of attack.

[sky9]

Of course my 6p balsa glider that I used to buy from Woolworths as a kid was nothing more sophisticated than your piece of 8 by 4 and used to fly incredibly well for the money.:ok:

[John Farley]

My post was not about what you wrote.

It was addressed to Chris.

[Mr Optimistic]

i thought cw was saying nothing more than that force is an integral of pressure which in turn is about the macroscopic consequence of all those little billiard balls. don't think he meant overpressure was more significant than overpressure.

[ft]

I think we're seeing a clash of the justified equal and opposite reaction to the classical "bullets impinging the wing and bouncing off the underside" flawed explanation of Newtonian lift, and a very similar-sounding explanation of lift through what the air molecules (not bullets) actually do in a strictly physical sense?

Edit: italia458, good example to counter the bullet theory. Adding that one to the repertoire. :)

Cheers,
Fred

[Mr Optimistic]

an explanation of pressure is not the same as trying to explain lift though a transfer of momentum argument. nevertheless conservation of momentum holds (clearly) and the actuality of force, including lift, drag, normal force, axial force is from the physical effects exerted on the wing.

[chris weston]

All,

I am no aerodynamicist, it is not my field - I am a mere chemist.

In truth I also lack the mathematical skills with which properly to support or to disprove my contention.

Mr Optimist summarises my contention nicely, thank you.

John F, I will always accept and respect experimental evidence and thank you for the correction, repeatable data must command respect!

I will lower my head from the slipstream.......

CW

[Mr Optimistic]

well to be honest this isn't my line of work either but i did study fluid mechanics in the von karman lab (if memory serves) and didn't care for it. there are pedagogic difficulties with the concept of lift which i think derive from experience of being ground bound. so lift becomes an anti gravity machine. its just a component of force but the theory gets bogged down in potential flow and inviscid simplifications and then linearised equations and what not. glad i don't have to teach it.

[KiloB]

Let's remember of course that Bernoulli was right! If not, carburettors and paint sprayers wouldn't work. JF's discription of low pressure areas above an aero foil also intimates that a Venturi doesn't have to have two sides to work. Are we agreed that Newton and Bernoulli have to share the credit for keeping us up there?

[italia458]

KiloB... I would say they both share some of the credit for UNDERSTANDING how/why we are able to fly - with the people who worked on Bjerknes' circulation theorem, Coanda, D'Alembert's principle, Navier–Stokes equations, Kutta-Zhukovsky’s Circulation Theory, Computational Fluid Dynamics, etc.

[bookworm]

I am no aerodynamicist, it is not my field - I am a mere chemist.

As a chemist, you're probably familiar with the idea of a mean free path in a gas, the average distance between collisions. If the mean free path is much greater than the distances of interest (e.g. the chord length of the wing), you can indeed treat the air molecules like bullets (Knudsen flow), in which case what lift there is will come from molecules hitting the underside.

But at the pressures we operate the mean free path is of the order of 100 nm (that's nanometres, not nautical miles!). That means that the interaction between molecules is also very important. The flow behaves in the way that we view macroscopically as a gas. Typically, the reduction of the number of collisions (crudely, the pressure) with the top surface of the wing is more significant than any increase below.

[cwatters]

For my students who cannot understand how a flat plate can ever produce low pressure above its top surface I talk along these lines:

Much easier way is to show them a flat plate at 90 degree AOA. It's rather obvious the presure is lower on the back side. Then show them one at 89 degrees, 88, 87 ...

[Basil]

In The Telegraph
Video: How aeroplanes' wings really work - Telegraph (http://www.telegraph.co.uk/science/science-video/9035320/How-aeroplanes-wings-really-work.html)
a contributor called low_flyer has posted THIS (http://www.aviation-history.com/theory/lift.htm),
From Sport Aviation, Feb. 1999
How Airplanes Fly: A Physical Description of Lift

I think it's very good.

p.s.
I am no aerodynamicist, it is not my field - I am a mere chemist.
IMHO two of the more difficult subjects.
Re lift: All Coandă and Newton :ok:

[bookworm]

While Babinsky's article is very good (I like the association of streamline curvature with pressure difference), there's one "observation" close to the end that is, I think, flawed.

For example, consider the difference between the streamlines over a thin and a thick aerofoil as shown schematically in figure 10 (determined from a computer simulation). Despite the difference in thickness, both have similar flow patterns above the upper surface. However, there is considerable difference in the flow underneath. On the thin aerofoil the amount of flow curvature below the wing is comparable to that above it and we might conclude that the overpressure on the underside is just as large as the suction on the upper surface — the two sides contribute almost equally to the lift. In the case of the thick aerofoil, however, there are regions of different senses of curvature below the lower surface. This suggests that there will be areas with suction as well as areas with overpressure. In this case the lower surface does not contribute much resultant force and we can conclude that thin aerofoils are better at generating lift. This is generally true, and birds tend to have thin curved wings. Aircraft do not, because of the structural difficulties of making thin wings, and also because the volume contained in the wing is useful, e.g. for fuel storage

I don't think that's the case. For aerofoils with the same camber (and the thin and thick ones in fig 10 do not have the same camber), the lift curve slope is almost the same for a thick aerofoil as a thin one -- see e.g. Abbott and von Doenhoff Fig 57 which shows a marginal decrease with thickness for the NACA 4 and 5 series, and a marginal increase for the NACA 6 series. What the thickness does, however is substantially increase the maximum attainable lift coefficient by increasing the critical AoA (Fig 58).

The early aviators like the Wrights and WW I designers made the wings thin. Later aircraft designers made the wing thicker, not because they couldn't make the wing thin, nor because of the useful space introduced. Rather, it was because a wing with a finite thickness is an aerodynamically better wing.

[BlueWolf]

So....those of us who have always thought that wings generated lift because the pressure above was less than the pressure below, have always been correct, but we have been wrong to believe that this was because the shape of the wing caused the air flowing over the top surface to cover more distance in the same time as the air flowing under the bottom surface?

....and we would be more correct in our knowledge if we understood that it is in fact the angle of attack which causes the upper flow to move faster, and the upper flow actually arrives at the trailing edge first, and therefore it is still the greater speed which generates the lower pressure, but the two flows do not arrive simultaneously?

or have I still missed something?

[John Farley]

Not sure if you were writing slightly tongue in cheek - but I would certainly agree with everything you say.

I always find it strange that some people ignore any relevant experimental evidence as they develop and discuss their theories (in the case of lift the pressures measured in tunnels).

The quote put up by bookworm being an example of this.

[rudderrudderrat]

Hi BlueWolf,

http://upload.wikimedia.org/wikipedia/commons/9/99/Karman_trefftz.gif
courtesy wikimedia.org/wikipedia

I concur.

[Basil]

rudderrudderrat,
That does look pretty extreme - and the little molecules never get to meet again, even way back.

[RVF750]

My Ground instructor put it more simply, before tearing into the best text book he could find. But then he did design Condorde's wings before he taught CAP509 ground school at Hamble then GBAA. Good old Alan. ( well I blame him for me getting 100% and walking out after 35 minutes of the CPL technical...)


Basically, it's all down to Viscosity. ;)

[DC-ATE]

I think you all are forgetting the ONE thing that keeps these flying machines in the air, and that is: MONEY !!

[Crabman]

In the videos, you can clearly see the "stagnation streamline" and "stagnation point" that John Farley mentioned. Now look at the video where they introduce the smoke puffs (in slow motion). Look at the puff in the first streamline just below the leading edge. See how it "splats" (how is that for a technical term?) against the wing and is then deflected down? That "splat" is momentum transfer. (i.e. a force). I can't think of a better intuitive view of lift. Everyone who has ever stuck his hand out of the window of a moving car understands this.

Concerning low pressure: It is strange how humans seem to intuitively give physical meaning to the opposite of what is happening. For example low pressure somehow gives rise to a "sucking" force (planes are sucked up into the air, vacuum cleaners suck up dirt, etc). Or we can "feel" cold (somehow we think that the cold flows from a cold railing to our hand). Sorry for the digression.

[BlueWolf]

Not tongue in cheek Mr Farley Sir, and thank you; I have always been happy with the pressure differential idea, but never pondered the arrival time difference.

Also I have never given much thought to the flat plane wing idea even though I probably had as many balsa wood gliders as a boy as anyone here....and never really contemplated inverted flight either.

Everything I have believed has not been a lie, but it appears it hasn't been complete either.

This makes sense and I am thankful for the clarification.

[fizz57]

Ah, the good old Bernoulli vs Newton argument. Fuelled by those who don't seem to realize that Bernoulli's equation is derived from Newton's Laws, as well as the implication of the equivalence symbols in the derivation.

To counter a comment made a few posts earlier, the momentum argument IS identical to the pressure argument. From kinetic theory, pressure results from the change of momentum of the gas atoms bouncing off a surface. Saying that there is a pressure difference across a wing generating lift is EQUIVALENT to saying that there is is more momentum change on the lower surface (pushing upwards) than there is on the upper (pushing downwards). Overall conservation of momentum requires that the airflow acquires a downwards change of momentum, also known as a downwash.

Equivalently, considering the wing as a machine for producing downwash, the existence of the downwash requires that there be a pressure difference across the wing. Conservation of energy then requires that the airflow on top of the wing is faster than that below, and therefore that a circular integral of the velocity field round the wing has a non-zero value (for those who believe in the magic word "circulation").

[EEngr]

That wikimedia animation (posted by rudderrudderrat (http://www.pprune.org/members/313106-rudderrudderrat), original at http://en.wikipedia.org/wiki/Lift_(force (http://en.wikipedia.org/wiki/Lift_%28force)) ) looks a bit odd.

While the timelines immediately above and below the wing's surfaces show the upper stream having a higher velocity (arriving at the trailing edge first), at some distance above and below the wing and behind it, the time relationship appears to be reversed. That is: the lower timelines reach the right hand side of the graphic first and are faster.

I would expect that at some distance from the wing (above and below) these timelines would remain vertical. But the animation doesn't show them converging on this condition. So, should I trust the animation completely? Or is something else going on?

Hey, I'm only an EE. All we have to do is understand how :mad: magnets work.

[error_401]

Maybe the slowing of the molecules below is of importance?

Could that be because force is transferred? (well no better way to say came to my mind)