"Why Planes Fly: What They Taught You In School Was Wrong"

[Miami Mike]

http://dmiessler.com/blog/why-planes-fly-what-they-taught-you-in-school-was-wrong

This is currently drawing some interest and discussion on Reddit (http://www.reddit.com/r/science/comments/7peai/why_planes_fly_what_you_learned_was_probably_wrong/). I'd be interested in reading evaluations of the article from the folks who frequent this forum.

[pda4you]

Flat foamies come to mind.... :)

[nmasters]

Yet another myopic view of the most complex problem in the physics of everyday mater.

Go here: http://www.arvelgentry.com/techs.htm
and read what somebody who actually understands it says

[slipstick]

Like all of these things you really first need to know the intended audience. The Anderson slide show that this article is based on seems to be aimed at pilots. Pilots just need an explanation that will help them make good flight decisions. I'd say the original works reasonably for that, but I don't know who this article is aimed at.

It's basically another attempt to debunk a theory that I doubt anyone with any involvement with aviation has held for many years. As I read it it's mainly trying to say that if anyone still believes in a simplified Bernoulli-based theory "proving" that only cambered airfoils create lift as an explanation of how planes fly then they're wrong. It's difficult to argue with that but then no-one has believed it for ages because we all know that planes with symmetrical airfoils fly and planes with cambered airfoils can fly inverted.

Always good to drop Coanda into the mix but the author doesn't seem to understand it too well (though the original Anderson slide show is rather better). But basically having debunked one out-of-date theory he then tries to propose another simple answer. Unfortunately no simple answer is ever going to be 100% correct because it's an intrinsically complex problem :(.

But the basic premise seems to be "Forget Bernoulli, just work from Newton's first principles and you'll get it". Can't really argue with that ;).

Steve

[Bg~]

The comments in that thread provide links to much better explanations. The author of that link doesn't really understand what he's talking about.

[nmasters]

Except that Newton himself couldn't predict how much force a sail would make in a given wind. IIRC me under estimated by a factor of 4. Newton's problem wasn't that he didn't have all the math, he did, it's that he didn't understand the problem and therefor didn't use all the appropriate formulae. Danial Bernoulli was the first person to apply Newtonian physics the fluids properly. Althoug Bernoulli's theorem doesn't work in a cause and effect argument for lift it certainly isn't wrong just misapplied. When somebody starts off arguing that Bernoulli was wrong it's a sure sign that the speaker is also wrong

[nmasters]

Von Kármán's fallacy
http://www.rcgroups.com/forums/showthread.php?p=9045024&highlight=loaded+gun+monkey#post9045024

[nauga]

Pilots just need an explanation that will help them make good flight decisions.Accepting the Coanda effect as the 'cause' of lift for this reason is akin to saying the Sun revolves around the Earth. It can be misinterpreted to explain the effects we observe but it is fundamentally incorrect.

But the basic premise seems to be "Forget Bernoulli, just work from Newton's first principles and you'll get it". Can't really argue with that ;).So many people try to differentiate (no pun intended) between Newton and Bernoulli when Bernoulli's equation is equivalent in the abstract to Newton's second law, roughly F=ma, for an incompressible, inviscid fluid.

The comments following the linked article, at least the initial ones I read, have a better grasp of the fundamentals than the author.

Nauga,
and the flowfield all-stars

[bwalt822]

The only time I ever really heard the Coanda effect mentioned in my aeronautical engineering course work was when a group was making a flying saucer looking thing based on the Coanda effect. Needless to say it didnt fly...but that was because they didnt know how to build anything right.

[nauga]

As I read it it's mainly trying to say that if anyone still believes in a simplified Bernoulli-based theory "proving" that only cambered airfoils create lift as an explanation of how planes fly then they're wrong.You touched on this, but anyone who thinks using Bernoulli's equation to estimate lift will result in zero lift for a symmetric airfoil is applying the principle incorrectly. It can be used to show that symmetric airfoils produce no net lift at zero angle of attack, but produce lift conventionally at any nonzero angle of attack.
Of course that lift also results in a transfer of momentum producing downwash...and measuring the momentum transfer also results in a similar prediction of lift. What some people seem to miss is that it's not one or the other (Bernoulli or Newton), it's the same idea expressed from different points of view...and one (Bernoulli) can conveniently be derived from the other (Newton's conservation of momentum).

Nauga,
who doesn't cross the streamtubes

[Brandano]

Amen!

[bwalt822]

Does anyone actually use upwash and downwash to calculate lift? I know its theoretically possible but what would your control volume look like?

Is it possible to calculate up/downwash without knowing/calculating the pressure or velocity distribution around the airfoil first? Our dynamics book had equations to estimate up/downwash but those were based on variables that needed to be obtained from wind tunnel testing or CFD. They were also used to figure out the actual angle of attack of the horizontal stab or a canard that was in the up/downwash

It seems like to calculate lift using the momentum transfer method you need to calculate lift based on pressure first maybe?

[mnowell129]

How come there's no mention of the Kutta condition? This always made the explanation simple for me.

[Montag DP]

There are some people who actually know what they're talking about in that discussion, but at the same time there's a ton of misinformation going around too. People just throw out sciency words like "Bernoulli" and "Coanda effect" and pretend they know what they're talking about, when they really have no clue.

The funniest is when they try to say that "30% of the total lift is due to Bernoulli and 70% is due to Newton," as if they are two separate concepts.

[nmasters]

Does anyone actually use upwash and downwash to calculate lift?
Yes, it's called circulation theory. It should be mentioned in one of your textbooks somewhere. Basically you have a starting vortex and a bound vortex which are connected by the tip vortices. The starting vortex is how mother nature enforces the Kutta condition. Without viscosity there is no vorticity and the aft stagnation point will be somewhere forward of the TE on the top surface. Inviced flow solvers introduce a vorticity to force the aft stagnation point to the TE. When the vorticity is sufficient to push the aft stagnation point to the trailing edge then the program can tell you how much lift you're going to get. Simple huh?

But don't take my word for it, after all, I'm just an innumerate art school dropout :p See what this guy has to say: http://yarchive.net/air/lift.html

--Norm

[bwalt822]

Yeah we were taught lifting line/circulation theory but that doesnt predict lift based direclty on the mass and velocity of air that is deflected upwards and downwards, it just uses the strenth of the vorticies.

Well i guess the vorticies could be considered the total integrated effect of up/downwash

[nmasters]

Find a copy of "An Album of Fluid Motion" by Milton Van Dyke. It's a collection of B/W wind and water tunnel pictures ranging from creeping flow to supersonic. Pgs 110 and 111 show waves with neutrally buoyant particles to show the motion of individual molecules in the wave. Now think of it this way: a wing sets up a traveling wave in the air, it stays at the center of this wave with ascending air in front and descending air behind with accelerated flow above and decelerated flow below. If there were no tips the upwash would equal the downwash but since there are tips the wave is slightly wider than the wing and some of the recirculating flow spills off the tips. The upwash part of this spillage is absorbed back into the atmosphere but the downwash part curves around toward the trailing edge and adds to the wing's downwash. That extra downwash is your induced drag and that's what the picture of the Citation over the fog is showing.

--Norm

[Brandano]

Perhaps it is a bit of a simplification, but I agree fully with Norm. In my opinion, a blunter description is: air has been moved about once the plane has passed. This took some energy, and that energy had to come from somewhere.

[vintage1]

Why has no one answered the question?.

You have all answered HOW planes fly.

Why they fly, is because someone smart intends them to, and the laws of Nature, or God, permit them to. Whichever.
:D

[nmasters]

Why has no one answered the question?.

You have all answered HOW planes fly.

Why they fly, is because someone smart intends them to, and the laws of Nature, or God, permit them to. Whichever.
:D
Oh...
Money ;)

Unless it's built by an Anglo-French consortium, then it's pride

[nmasters]

BTW here's that Cessna Citation over lake taho

[nmasters]

air has been moved about once the plane has passed. This took some energy, and that energy had to come from somewhere.

Yes but in 2D there is lift without a net downwash. Non other than Al Bowers explained this to me and it still sounds like magic

[JetPlaneFlyer]

Also WIG (wing in ground effect) vehicles are a real world example of near 2D, infinite wing, airflow where there is no, or very little, net downwash.

Steve

[Brandano]

Yes but in 2D there is lift without a net downwash. Non other than Al Bowers explained this to me and it still sounds like magic
I agree, it's a bit hard to visualize the concept, it's a bit like pulling yourself up by your shoelaces. Seeing it as a macro-system the complex of engine and airframe (and any turbulent flow areas that travel with the plane) creates a pressure wave that coincides with the plane. Less pressure above and behind the plane, more pressure ahead and below the plane, and this is what keeps the plane up (and drags it back!). The difference in pressure has a net cost, air was in a steady state and now it has an energy potential, energy that was "stolen" from the propulsion. Once the plane has passed and energy isn't put any more in the air volume the pressure will even out releasing this accumulated energy as sound and heat.

[vintage1]

Yes but in 2D there is lift without a net downwash. Non other than Al Bowers explained this to me and it still sounds like magic

Why should there be? there is no momentum change to the aircraft, why should there be to the air?

Any net change of momentum to the air equates to loss of momentum of the aircraft essentially. I.e. drag.

What you are trying to do is induce a net low pressure above the wing and a net high under it. Without making turbulence. To that end anything that forces the air to speed up above, and maybe slow down below,works.

That's why an angled flat plate works..it creates a lump of stillish air on the back side of the top, and the moving air has to skip over it.

All a curved airfoil section does, is fill in that lump of stagnant air with wing...reducing turbulence and drag.

[eflightray]

Please write quietly.

Bumble bees may be listening. They're going to hate you if they end up having to walk everywhere. ;)

[bwalt822]

I understand how and why planes fly. The problem is the people that say its not the pressure of that gives lift, and the bottom surface doesn't matter. Its the Coanda effect and downwash.

I understand that this is practically not feasible but COULD it be possible to somehow measure the velocity and direction of hundreds or thousands of points in front of and behind a 2D airfoil in a wind tunnel to try to measure lift. Yes I realize it would be hundreds of times easier to just use a load cell.

[nmasters]

That's sort of what they do now. You get a reading of the free stream veloisity compare it to the velocity and direction of the wake. Here's a picture of prof Dezso George-Falvy and some one I don't recognize installing an integrated wake rake. I got it from nurflugel.com (http://www.nurflugel.com/Nurflugel/Horten_Nurflugels/ho_iv/Falvy_Pics/body_falvy_pics.html) .

--Norm

[markdrela]

Yes but in 2D there is lift without a net downwash. Non other than Al Bowers explained this to me and it still sounds like magic It gets even weirder. A lifting 3D wing doesn't create a net downwash either, at least not if you account for the entire atmosphere. There is downwash directly behind the wing between the tip vortices, and upwash outside the tip vortices. The net vertical momentum change caused by the wing's passage is zero. This is shown in the bottom left figure in the second PDF.

The reason there's no net vertical momentum change is because while the airplane is pushing down on the airmass (with force F1=-Lift), the airplane's pressure field is pushing up on the airmass with an opposing force F2 (see first PDF). For some control volumes (second PDF), which must be chosen to do the accounting, there may be an imbalance between F1 and F2, which will then show up as a net momentum change DeltaM. The most general "infinite" control volume in the bottom left has zero momentum change.

One interesting factoid is that when an aircraft flies overhead at any altitude, there is a very slight overpressure at ground level, which sums up to F2 and is equal to the lift. So the real answer to the common question
"What holds up the airplane?" is
"The ground holds up the airplane(!)".
It sounds like a joke, but it's technically correct:
The upward force of the ground is transmitted to the airplane via the airplane's own air-pressure field.

[bwalt822]

That's sort of what they do now. You get a reading of the free stream veloisity compare it to the velocity and direction of the wake. Here's a picture of prof Dezso George-Falvy and some one I don't recognize installing an integrated wake rake. I got it from nurflugel.com (http://www.nurflugel.com/Nurflugel/Horten_Nurflugels/ho_iv/Falvy_Pics/body_falvy_pics.html) .

--Norm

I believe these are for measuring drag, not lift. We did it in a lab with a cylinder in a wind tunnel. The results are pretty accurate.

[nmasters]

I believe these are for measuring drag, not lift. We did it in a lab with a cylinder in a wind tunnel. The results are pretty accurate.
Yeah... I realized that after I sent it. I was a little distracted and you may have also noticed that I'm not the brightest guy on this BBS. Anyway Dr Drela answered your question about calculating lift from a momentum change in a control volume.

--Norm

[M Scherrer]

Unless it's built by an Anglo-French consortium, then it's pride

LOL ! :rolleyes:

& don't forget German !

Matthieu (From Toulouse ;) )

[biber]

In case of the germans it fly's by order of Colonel Manfred von Holstein. ;)

biber

[kcaldwel]

One interesting factoid is that when an aircraft flies overhead at any altitude, there is a very slight overpressure at ground level, which sums up to F2 and is equal to the lift. So the real answer to the common question
"What holds up the airplane?" is
"The ground holds up the airplane(!)".
It sounds like a joke, but it's technically correct:
The upward force of the ground is transmitted to the airplane via the airplane's own air-pressure field.

Thanks very much again Mark, for your clear and understandable explanations of a very confusing concept.

I wish I'd had a prof like you! This makes more sense today than it has for the last 30 years.

So in an infinite atmosphere with no ground surface, would the pressure field just eventually dissipate as heat due to viscosity?

Kevin

[Brandano]

It already does, don't forget that this is not a theoretically perfect environment, and the 2nd law has to be respected. Incidentally, in an infinite atmosphere with no ground surface, would you have gravity? If you consider a gas giant, then the denser layers of gas, then liquid gas and then plasma would effectively act as ground. If you had no gravity then you wouldn't need lift to keep airborne, though you could still use aerodynamic surfaces for control.

[eflightray]

I started looking for an article i saw recently on the spinning cylindrical wing attempts of past, couldn't find it, but.........

Here's one 'flying device' I wouldn't want to go in - Cyclogyro (http://en.wikipedia.org/wiki/Cyclogyro), a paddle wheel 'helicopter'?

This link also lead to the - Magnus effect (http://en.wikipedia.org/wiki/Magnus_effect)

the - Kutta–Joukowski theorem (http://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem), this is where my two remaining brain cells said, "Enough is enough".

Though I did also come across quite a good one on, Stepped airfoils (http://users.acsol.net/~nmasters/vortex-lift/step.html), (did notice a 'nmasters' in the URL, if it's you know who, well done.)

[nmasters]

I started looking for an article i saw recently on the spinning cylindrical wing attempts of past, couldn't find it
Like this: http://www.rcgroups.com/forums/showthread.php?t=54802&page=2&highlight=flettner

(did notice a 'nmasters' in the URL, if it's you know who, well done.)
Yep that's me. Thanks. The real value of that whole vortex lift project is the bibliography. If you're interested in that stuff I'm sure you've noticed the numerous KF airfoil threads in the foamies (scratch built) forum. Those guys mostly concentrate on ease of building. Steve Morris has a thread in this forum that is a bit more technical: http://www.rcgroups.com/forums/showthread.php?t=979409

--Norm

[pyrobus]

Planes fly because someone pulled back on the stick. Pull back more and they come down. They all fly the same.

Pyro

[Mtn_Commando]

The myth of lift is that its not Bernoulli and it is really Newtons laws. Another variation of the myth is that its XX% Bernoulli and YY% Newtons laws. This leads people to say crazy things like the bottom surface of an airfoild does not matter and its the Coanda effect and yadda yadda yadda. They will then cite that flat foamies fly and therefore Bernoulli does not matter. The real answer is that its 100% Bernoulli and 100% Newtons laws. They are the same thing just rearranged or with different viewpoints.
post#1034 (http://www.rcgroups.com/forums/showpost.php?p=11398363&postcount=1034)


Not wanting to derail dag's thread, I'm answering here. Just want to clarify that while its true that both Bernouli and Newton are different perspectives for the same explanation, its incorrect to say that "lift occurs due to Bernouli's principle" for the following reasons:

1) equal transit times myth. There is nothing to force this, and therefore it doesn't occur. The air over the top suface does move faster, and in fact a lot faster than it would IF equal transit times was true. It has to, because IF this was the only factor creating lift, the top wing surface would need to be 50% longer than the bottom.
2) The faster velocity of top surface stream is due to the differential pressure, which is caused by the downwash of both the upper and lower streams (the former due to aerofoil, the latter due to angle of attack). So it is Bernouli's principle in action, but in corollary/backwards.

What's incorrect, is advocating BP as 'the reason' which 'causes' lift, and waht makes it worse is the equal transit times explanation, which as we all know is staple.

Cheers.

[biber]

You could rather say if there is a reason for the wing lifting, it is the fact that you drag the wing through the air and the fact, that it is properly shaped for the kutta condition to apply.
Newton and Bernoulli are both fine with that, no reason for any contradictions there.

There is no accounting possible as to how much of the lift is due Bernoulli or Newton.
They are just two points of view on one and the same thing, really.

biber

[bwalt822]

i dont think anyone has argued for the equal transit "theory" so thats done.

I also dont think you can say low pressure on top of the wing is due to up/downwash or vice versa. They all work together and one wont exist without the other.

Here's the link which pretty much settles it from the other thread

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

So when people argue about a prop being a rotating wing or a momentum tranferrer, they are both right. Its a rotating wing that causes a tranfer of momentum to the air.

[Rickn816]

Ha. You beat me to it.

Nicely done. :D