Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...

[JB77UK]

OK so here is a frustrating situation for a new Helicopter CFI working on how to teach lift theory:

The 2012 FAA Helicopter Flying Handbook explains the creation of lift as a combination of half a venturi (the bottom half is the airfoil) and air pushing up on the bottom of the airfoil (like a water skier).

The 2012 FAA Helicopter Instructor's Handbook explains lift as the upper surface having a longer distance for the air molecules to travel than the lower surface and so causing the airflow above to speed up to make sure the molecules meet at the trailing edge.

Two different theories in books released within a month or two of each other!!! To make things even more frustrating, both explanations are completely wrong according to NASA:

Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html)
Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html)
Incorrect Lift Theory (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html)

So apart from a moan at the FAA, the question is what do you teach a new helicopter student:

What the Helicopter Instructor's Handbook says you should teach - 'Equal Transit Time' theory
What the Helicopter Flying Handbook teaches - a combination of 'Skipping Stone' & '1/2 a Venturi' theories
What NASA (who put a man on the moon using a pocket calculator) says - ' Turning of a fluid'

I WANT to teach option 3, but will they suffer in written tests and at the hands of DPEs because of it?

Note to the FAA - the NASA theory has been established for a long time - even longer than the amount of time ADFs have NOT been used in the average cockpit... ;-)

[paco]

I know someone who was trying to revise the Rotorcraft book but was bombed out because they just didn't want the changes. Did they finally get the tail rotor drift diagram right? I had to point out that the arrows were going the wrong way.

We have the same problem in Europe - according to EASA, the only reason aerofoils produce lift is because of the upper surface reduction in pressure, in the first 25% of chord.

So tell me - based on that, where do helicopters get downwash from? :) They should read Stick and Rudder!

Like you, I find it frustrating to have to say "this is what they want in the exams, but this is what really happens!" Which is what we do, otherwise they will not pass the exams in the first place (with the lies), or the tech interviews after that (the truth).

The truth is that all those theories work in combination. None of them work entirely alone.

[SilsoeSid]

Whenever I see the geese migrate south at this time of year, flying in close formation the following birds using less energy than the leader due to the airflow disruption, I wonder how this transfers to rotor blades.

The V formation greatly boosts the efficiency and range of flying birds, particularly over long migratory routes. All the birds except the first fly in the upwash from the wingtip vortices of the bird ahead. The upwash assists each bird in supporting its own weight in flight, in the same way a glider can climb or maintain height indefinitely in rising air. In a V formation of 25 members, each bird can achieve a reduction of induced drag by up to 65% and as a result increase their range by 71%.[1] The birds flying at the tips and at the front are rotated in a timely cyclical fashion to spread flight fatigue equally among the flock members.

V formation - Wikipedia, the free encyclopedia (http://en.m.wikipedia.org/wiki/V_formation#section_1)

[jymil]

@JB77UK: thx for sharing the links. I haven't really spend much time thinking about it, but the half venturi never added up for me. It's not like the atmosphere is a gigantic other half of a venturi. Any why should there be some magic force requiring the molecules to meet again at the end of the flow ? In reality, they don't. The NASA explanation does absolutely make sense for me. It's basically Newton's second law of motion combined with the physical properties of a fluid.

Here's the whole "turning of a fluid" explanation:
Lift from Flow Turning (http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html)

[RVDT]

Try this -

hQ99JkaOwEk

[Thomas coupling]

Voila:

How wings really work - Research - University of Cambridge (http://www.cam.ac.uk/research/news/how-wings-really-work/)

The end.................:ugh:

[RVDT]

TC,

Thats hilarious - the University of Cambridge doesn't understand it either!

Amazing. :{ Maybe Holger has too much "tunnel vision" - pun intended.

[army_av8r]

I use a good mix of "all" the reasons why helicopters fly. but if someone doesn't get it, i just have them put there hand out a car window. start at zero pitch, and slowly increase. they can then feel the upward force on their palm. i also use symmetric airfoils to help explain why the upper/lower stuff isnt the only thing at play.

[lelebebbel]

All models are wrong - some are useful.
George E. P. Box

Teach what will get them through the exams. Mention that "this explanation works for practical purposes but is not fully accurate, and here are some links to NASA with more information if you are really interested in the subject. Now lets move on in the syllabus. What is next, ah yes, LTE.." :rolleyes:

[paco]

Don't you mean overpitching? ;)

Phil

[Thomas coupling]

RVDT: would you care to point out where they got it wrong at Cambridge Uni?

I'm sure their all ears listening to another aerodynamics research scientist:sad:

For those of us who have been teaching it for decades, I for one would be most grateful for further pointers to the holy grail.

[jymil]

The cambridge website says "it’s the curvature that creates lift". NASA says the deflection of air creates lift. Therefore, a flat wooden board also produces lift at a positive angle of attack. The shape does have an influence on how much the air is deflected, but it is not the source of lift. The coefficient of lift from the lift formula is a combination of both the angle of attack and the effects of the geometry of the airfoil, which may have either a positive, negative, or no effect on the deflection.

[[email protected]]

Well, whilst it is clear from the Cambridge video that the air doesn't meet again at the same point after the trailing edge of the wing, it is also clear that almost no air is deflected downwards - the streamlines only a short distance below the wing are completely undisturbed!

How does the Newtonian 'deflects air downwards therefore produces lift upwards' school of thought explain that?

The 'flat plate' effect at least is understandable since it relates exactly to what happens when you stick your hand out of a car window - the air pushes against your hand and forces it upwards.

I have yet to listen to the whole of the video RVDT has posted but the first 10 minutes are not that enthralling - he tries to rubbish benoulli lift theorem by saying that an aircraft in a 2G turn doesn't increase its speed so it cannot increase the acceleration over the top of the wing - I don't think he has heard of angle of attack.

[puntosaurus]

I should imagine the Newton 'school of thoughters' would say F=MA, and they would be wondering about why you were concerned about the displacement of air rather than the acceleration applied to it.

I think they would also consider that they'd moved on from the 'school of thought' phase.

[jymil]

it is also clear that almost no air is deflected downwards


If you compare the height of each line on the left with the right side, you'll see that air is indeed deflected downwards. And we all know this phenomenon from reality as downwash.

[Shawn Coyle]

I finally realized that it's a bit of Bernoulli and a bit of Newton. Mostly Newton, but the curvature of the upper surface does help to deflect the mass of air down.
You might also look at 'See How it Flies', a Fixed wing book freely available online. Great physics of flight stuff there.

[Ascend Charlie]

It's a combination of straight flow and rotational flow - the rotation goes from under the wing, around the leading edge, over the top surface, and down off the trailing edge - see the beautiful attachment point on those smoke lines, with the initial part of the airflow over the wing coming from underneath, not in front.

Try it yourself - blow up a ballon, and try to push it forward.

Now try it again with some backspin, and watch how it climbs and moves forward, having deflected the air down and back. Add frontspin and the balloon dives at the ground big time.

Sticking your arm out the window is a simple example of deflecting flow, without the rotation- the air is deflected down, and your arm goes up and back, the total reaction. This can be split into lift, and drag.

The rotation comes from the aerofoil shape, which makes the straight Newtonial deflection into a rather prosaic rotational flow with streamlines, inflow, downwash and angels singing. Mr Coanda joins in the chorus.:}

[canterbury crusader]

Clicking on this link will compel you to part with some of your cash.

Then you can make up your own minds on the deflection from the top surface. Worth every cent (check the other versions before purchasing).

https://itunes.apple.com/au/app/wind-tunnel/id381971296?mt=8&ls=1

[[email protected]]

Any downwash caused by the curvature of the wing is preceded by an upwash. Anyone done the maths to see how these might cancel each other out? The streamlines show no net downward movement of the air, so where is the lift for a 747?

Any acceleration imparted by the curvature of the wing would (if you ignore benoulli and think Newton) impart a downward force onto the wing itself as the centre of curvature (somewhere below the wing) is where that force must be trying to push towards (like centripetal force).

The air on the underside of the wing pushes the wing up.

[RVDT]

the streamlines only a short distance below the wing are completely undisturbed!

Well it is in a tunnel so where is it going to go?

A WING IS NOT A HALF VENTURI
You might find it amusing to note that ground school courses
often introduce the venturi as an example of how wings fly.
The presentation includes moving one of the walls of the
venturi so far away as to not influence the other wall. What is
left is a wall with a hump.The instructors
tell the students that because of Bernoulli’s principle this “half
venturi” has lift. But you now know that this is wrong.The
wall blocks the downwash, so there can be no lift. After
leaving the hump, the air is traveling at the same speed and in
the same direction as before the hump. As we know, if there is
no net change made to the airflow there cannot be lift. So,
what do you do when you see this on the FAA written exam?
Well, if you want to pass, you will have to give them the
answer they want to hear, even though it is wrong!

Forget Bernoulli’s theorem
WOLFGANG LANGEWIESCHE, STICK AND RUDDER, 1944

In brief, the air bends around the wing because of the Coanda effect.

When the air bends around the surface of the wing it tries to separate from the airflow above it. But since it has a strong reluctance to form voids, the attempt to separate lowers the pressure and bends the adjacent streamlines above. The lowering of the pressure propagates out at the speed of sound, causing a great deal of air to bend around the wing.This is the source of the lowered pressure above the wing and the production of the downwash behind the wing.

This reduction in pressure causes acceleration of the air via the Bernoulli effect

The acceleration of air over the top of a wing is the result of the lowered pressure and not the cause of the lowered pressure.

As is popularly quoted.

For me it goes -

Coanda -> Newton -> And some interesting little Bernoulli effects along the way.

The lift of a wing is proportional to the amount of air diverted per
time, times the vertical velocity of that air.

The Popular Description of Lift
Most of us have been taught what we will call the “popular description
of lift,” which fixates on the shape of a wing. The key point of
the popular description of lift is that the air accelerates over the top
of the wing. Because of the Bernoulli effect, which relates the speed
of the air to the static pressure, a reduced static pressure is produced
above the wing, creating lift. The missing piece in the description is
an understanding of the cause of the acceleration of the air over the
top of the wing. A clever person contributed this piece with the introduction
of the “principle of equal transit times,” which states that the
air that separates at the leading edge of the wing must rejoin at the trailing edge.
Since the wing has a hump on the top, the air
going over the top travels farther. Thus it must go faster to
rejoin at the trailing edge. The description is complete.
This is a tidy explanation and it is easy to understand. But
one way to judge an explanation is to see how general it is. Here one
starts to encounter some troubles. If this description gives us a true
understanding of lift, how do airplanes fly inverted? How do
symmetric wings (the same shape on the top and the bottom) fly?
How does a wing flying at a constant speed adjust for changes in load,
such as in a steep turn or as fuel is consumed? One is given more
questions than answers by the popular description of lift.One might also ask
if the numbers calculated by the popular description really work.
Let us look at an example. Take a Cessna 172,
which is a popular, high-winged, four-seat airplane. The wings must
lift 2300 lb (1045 kg) at its maximum flying weight. The path length
for the air over the top of the wing is only about 1.5 percent greater
than the length under the wing. Using the popular description of lift,
the wing would develop only about 2 percent of the needed lift at 65
mi/h (104 km/h), which is “slow flight” for this airplane. In fact, the
calculations say that the minimum speed for this wing to develop
sufficient lift is over 400 mi/h (640 km/h). If one works the problem
the other way and asks what the difference in path length would have
to be for the popular description to account for lift in slow flight, the
answer would be 50 percent. The thickness of the wing would be
almost the same as the chord length.

Understanding Flight, Second Edition
By David Anderson, Scott Eberhardt

David Anderson is a pilot and a lifelong flight enthusiast. He has degrees from the University of Washington, Seattle, and a Ph.D. in Physics from Columbia University. He has had a thirty-year career in High Energy Physics at Los Alamos National Laboratory, C.E.R.N in Geneva Switzerland and at the Fermi National Accelerator Laboratory.

Scott Eberhardt - Aerodynamics Engineer - Boeing (current) Professor University of Washington July 1986 – April 2006 (19 years 10 months)
Engineer NASA Ames Research Center - September 1994 – June 1996 (1 year 10 months)

Scott Eberhardt's Education - Stanford University Ph.D., Aeronautics and Astronautics 1982 – 1985 - Massachusetts Institute of Technology
M.S., Aeronautics and Astronautics - 1980 – 1981 - Massachusetts Institute of Technology - B.S., Aeronautics and Astronautics - 1976 – 1980

Available on Google Books (http://books.google.com/books?id=wmuPXQuZnGoC&printsec=frontcover#v=onepage&q&f=false) or Amazon in a range of digital or hard media.

Me? I'm sticking with the physicist and the Boeing dude!

[puntosaurus]

Crab.

Look at Jymil's post #15. The streamlines ARE deflected down. They are at least 1/2cm lower on the right than the left.

Why would accelerating air downwards create a downward force on the wing ? Is that what you observe on a pool table or in a rocket engine ?

Your point about the underside of the wing is absolutely correct, but the point about Newtonian mechanics is that they do not attempt to deal with why or how the wing does what it does, which part of the wing does what, and less still the best shape to achieve it. They simply describe the aggregate consequences of it.

You can still have long arguments about how the wing actually achieves the acceleration without calling into question Newton laws.

[[email protected]]

Punto - I am just playing devil's advocate. In order to deflect the air to follow the curved path along the wing an acceleration downwards (relative to the wing) is required - if the air is in contact with the wing, it must push down on it (as the wing will push up against it)

However, jymil's post with the turning of fluids is a graphic which does show downward displacement - the cambridge video is actual streamlines which show negligible downwards displacement but a significant upwards displacement.

Which of these is the bad science?

If all the clever scientists in the world can't agree, what hope have us mere pilots got?

It would be nice to have a definitive explanation - probably a combination of all the current theories.

[oxo]

if the air is in contact with the wing, it must push down on it (as the wing will push up against it)

Depends on the pressure actually. I see no reason for claiming a fluid in contact must exert a force in any particular direction.

However, jymil's post with the turning of fluids is a graphic which does show downward displacement - the cambridge video is actual streamlines which show negligible downwards displacement but a significant upwards displacement.

Well, no. You need to look at the video again without a preconceived conclusion.


You cannot beat physical law, and Newtons second law is the only possibility here, unless you wish to claim that it all works by magic or other supernatural forces.

[RVDT]

Crab,

Part with the 12 Quid and see for yourself. :p

[Thomas coupling]

RVDT: you don't really know who you believe do you? You've stuck with your preferred theory for some time and refuse to budge when the truth comes along:D
"KISS": stick to cambridge and you won't ever get disproven.

What is a travesty is that for so long, so many mil pilots were taught bo**ocks and went along with it. Shame on you all.

How many others believe the low pressure upper surface definition?

[Hank195]

Peter Garrison had a recent article in Flying Magazine which discussed the subject (and its history). His final conclusion certainly has some truth in it - "we do not understand lift; we merely talk about it".

You Will Never Understand Lift | Flying Magazine (http://www.flyingmag.com/pilots-places/pilots-adventures-more/you-will-never-understand-lift)

[jymil]

@[email protected]: watch this video, it shows actual streamlines with varying angle of attack. This should make it clear air is indeed deflected downwards.
Airfoil in a wind tunnel at varying angles of attack - YouTube

If this doesn't convince you, I'll hover 10ft above you to prove some significant amount of air is indeed deflected downwards :-)

[jymil]

The video from RVDT's post has also an interesting point at around 21 min:

The speed and direction of the downwash from an observer on the ground would be more or less straight down. Wind tunnels are actually showing reality in an inverted way: it's not the air flowing around a static wing, but the wing plowing through the air. So if you picture the wing going from right to left and the air being still, you see the air is mainly deflected downwards.

[[email protected]]

I see no reason for claiming a fluid in contact must exert a force in any particular direction. surely by the definition of pressure, it exerts a force in all directions.

The wings in the tunnel are deflecting air downwards, I don't dispute that but the amount of downwash is very small - even at high AoA the streamlines even a short way below the wing get closer together but no more so than those a short way above it. How can you emphasise the downwash and ignore the upwash?

Strange how everyone is such an instant expert in this field.

[puntosaurus]

Quiet day at the office crab ?

PS. TC what's the low pressure upper surface explanation ?

[Cows getting bigger]

I find it rather bizarre that a bunch of helicopter pilots are talking about aerodynamics. :)

PS. I'm a Newton man.

[407 too]

This link, from post #316, by RDVT in Vortex ring / settling with power says it all :DHow Airplanes Fly: A Physical Description of Lift (http://www.aviation-history.com/theory/lift.htm)

[RVDT]

TC,

[What Newtonians and Bernoulians dont say is that the path that the air has to travel is longer on the upper surface and hence the air has to speed up to join its counterparts flowing under the aerofoil]

Oh really? Or don't you believe that any more?

[Thomas coupling]

Punto: If you look at BOTH the bernoulli and the newton explanations, they both discriminate in favour of reduced air pressure on the upper surface - low pressure - to produce lift?
Have you not recognised this yet? :rolleyes:

[puntosaurus]

The reduction in air pressure over the top of the wing is an observed experimental fact, but I wouldn't try and stretch Newtonian mechanics to explain it.

[[email protected]]

So the only thing wrong with the Bernoulli explanation is the flawed concept of equal transit times to explain why the air should accelerate over the top of the wing.

But the Newton explanation doesn't actually explain this either.

If you want something to travel in anything other than a straight line you must apply a force to it - what is the force that makes the air follow the wing?

Coanda is an effect, not a force and was first noticed during WW1 when observing the smoke from the guns and engines of the old fighters as it tracked round the fuselage so it isn't new.

I don't have the right answer - I'm neither a physicist nor a mathematician but Bernoulli (apart from equal transit time which I don't think was part of his theorem anyway) seems to be the closest to explaining all the observable phenomena.

The pretty picture of the downwash over the fog bank is fine but the vertical penetration of that downwash is limited - is there really enough air moving downwards at enough of a rate to hold up a multi-ton jet??

At least with a helicopter downwash you can see a column of air being displaced downwards (and feel its effects if you are underneath it)to balance the mass of the aircraft

[[email protected]]

From the same NASA pages that JB77UK used at the beginning of this thread

Bernoulli and Newton (http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html)

Summary - it is complicated and only really clever people can understand it:ok:

The last line To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations. well that counts me out:)

[Thomas coupling]

Punto: Please find time to read 407too's post where he offers a link about Newton's theory. At the very end, where the 'experts' actually describe the effect they specifically describe the formation of a low pressure area above the wing. :ugh:

Crab: The Bernoulli theorem " equal transit times" aspect has well and truly been disproved, many years ago. IF one was to adopt the BT solution (as opposed to the other camp belonging to the NT solution), then it is a fact that the air over the surface with the greater cadence (in most cases this refers to the upper surface - but it could be the lower surface if the a/c was inverted!!)...does speed up and accelerate past its counter molecule transitting the other side of the wing. This increase in speed causes the reduction in air pressure and voila. Job done. :ok:

BT and NT come to the same conclusions:
(a) Low pressure is formed on the upper surface.
(b) Lift is formed as a reaction.

Next........................................................ .:)

[canterbury crusader]

The wing occupies a certain amount of space. Fact.

When the wing moves from that space something must replace it. There cannot be nothing. Fact.

Solution = air.

That air is moved into place by the ambient airpressure. It flows smoothly down following the surface of the wing that is constantly moving out of the way. This movement of air that was stationery now has kinetic energy that has come from the plane moving. The plane keeps moving because of thrust provided by the powerplant. The wing doesn't pull it down, it is pushed down by surrounding air. No powerplant, potential energy from height.

Please note this is only addressing the top surface. The bottom surface is so obvious it shouldn't need explanation. Also it's my theory and not proven but I'm sticking with it until someone comes up with something simpler.

[puntosaurus]

TC.

Newton may have had a theory at one point in time but now he has a law, in fact three of them. When I quote Newton, I'm not doing it through anyone's intermediation (eg 407too's post), nor am I supporting a Newtonian theory of lift (whatever that might be). I'm simply setting out what his insights offer by explaining lift as a reaction to air being accelerated downwards.

From a pilot teaching perspective it's perfect, because it builds on existing universal knowledge (rockets, pool tables, school physics), and avoids the pitfalls of other approaches by leaving the mechanism out of it.

If other people have chosen to bend Newton to their aims as many have tried with Bernoulli, then good luck to them, but they're nothing to do with me other than standing on the shoulders of the same giants.

[Thomas coupling]

Punto: for the sake of the industry take time out to read the links. You genuinely might learn something rather than re-inventing the wheel.
You've only started Instructing since 2006 - how many studes have you contaminated with your ad lib theories?

[puntosaurus]

Whoa Buddy. Wind it in. This is not about me.

I've read How Airplanes Fly: A Physical Description of Lift, and like these guys I start with Newton. Since I'm not qualified to work at the National Accelerator Laboratory Batavia IL, or the Dept. of Aeronautics and Astronautics University of Washington, that's where I stop. And when I stop I make it clear that what I've told them is the beginning of the story, not the end. I then point them at google if they're interested in finding out more.

You might like to note that the article in question is entitled A Physical Description of Lift, not A Newtonian Theory of Lift. And BTW I think it's an excellent article.

[[email protected]]

Punto - please explain how the acceleration of air upwards - ie UP and over the top of the wing is conveniently ignored when you talk about the air being accelerated downwards to produce so much lift.

[puntosaurus]

It's not conveniently ignored, it's part of the story and on it's own would produce a downforce. The aggregate effect of the wing though is a downward acceleration of air.

[Camp Freddie]

All this was discussed in series 1, programme 1 of the BBC radio 4 comedy "Cabin Pressure" with Arthur the cabin attendant asking Martin the pilot all the right questions ! The pilot didn't know then either :)

[ShyTorque]

Air molecules are very small, very sensitive, herding creatures. But they are surprisingly strong for their size.
If you upset them they run away.
Clonking them with an aerofoil gets them very upset.
In their rush to get away they push the aerofoil.
The aerofoil is connected to the aircraft so up it goes.

Simple, really.

(Best told in the theme of Peter Cook and Dudley Moore).

hvQq_tqB0jA

;)

[Thomas coupling]

Shy: love it:ok:
Punto: I would have thought that leaving your stude with half a story is worse than not telling them in the first place. Either you as an individual would want to know what is really happening, or you as a professional MUST know what is happening. Believe either Bernoulli or Newton but take your pick and follow thru!:ugh:

[Peter3127]

I can't believe nobody has mentioned the role of the bound spanwise vortex, circulation and the role of the shed vortices ..... :8

While I think of it .... regardless of how that flowfield near to the airfoil is displaced in actuality ... the nett result (think of a fixed wing here) is a spanwise vortex shed at rotation and left on the runway (flow at top towards flight path) , two trailing vortices from the tips of the wings (of opposite sign, flow "leaks" from bottom to top), and a bound vortex that travels with the wing creating the "circulation" (greek symbol Kappa from memory) that can be thought of as a superposition on the free air velocity and kind of fitting the Bernouilli theory (faster on top).

So you have a "box" of vortex threads with a nett vorticity of zero that make a pump that forces air down through the middle of the "box", exchange of momentum equals a force that is the lift etc. Bigger lift = bigger vortex, hence the wake behind "heavies".

Confused yet? I am and I only just opened the bottle.

By the way, when a fixed wing lands and the lift is reduced to zero, a big-ass vortex rolls off the front of the wing (because it left one behind at takeoff and they have to cancel to zero). Don't believe me? Accelerate your spoon through the crema on a long black or espresso at an angle of attack, then abruptly stop it. You will see the vortex shed at the start (takeoff) and another one of opposite sign roll off when you stop the spoon.

Now ... lets take that fixed wing, make it 3 of them, rotate them around an axis, and then ..... wait ... :mad:

[ShyTorque]

and the role of the shed vortices

I sometimes get these after lunch, whilst working in my garden shed.

Come on guys, lighten up a little.

[puntosaurus]

Now that is funny TC. Do you know what is happening ? Really ?

I can't do better than to extend the quote from the NASA article (http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html) that crab referred to in Post 37 (http://www.pprune.org/7530073-post37.html):
The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations (http://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations) which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations (http://en.wikipedia.org/wiki/Euler_equations_(fluid_dynamics)). I notice that there's $1m on offer from the Clay Mathematics Institute for solving the existence and smoothness problems in the Navier-Stokes equations, so maybe you should take some time out of aviation management and put us straight. For the benefit of the industry, you understand.

Best,
Punto

[Camp Freddie]

Arthur: Why does the air on the top have to keep up the air at the bottom? Why don't they just... split up?
Douglas: For the sake of the kids?

[[email protected]]

Punto - but the wind tunnel streamlines seem to show pretty much an equal and opposite effect with little evidence of a net downwash (certainly not enough to hold up the aircraft).

[Shawn Coyle]

Wind tunnels, while good, are not the real world. In the real world, the air is still, and the blade/wing is moving through it.

[puntosaurus]

Well since I made that assertion, I thought I'd better check, and if you follow each individual streamline, by my reckoning the average deflection down is just over 1/2cm. Only the top three actually go up. I don't think the displacement in a bounded system like a wind tunnel tells you much anyway, and besides, the Newtonian issue is not the displacement but the acceleration imparted to the air.

The article in post 32 does a good job of both explaining exactly how much (a lot !) of air is involved, and presenting a picture which illustrates it.

[Camp Freddie]

If you stand underneath a AW139 in a high hover it seems quite windy, isn't that downward deflection / downwash :)

[RVDT]

Wikipedia - Lift (force) (http://en.wikipedia.org/wiki/Lift_(force))

The explanation seems reasonable and the "Reference and Notes" also has
most of the players quoted.

Of note -

In short, Bernoulli’s equation is a straight-forward application of Newton’s laws, often
misinterpreted. There are other details that must be included in a full treatment of
aerodynamic/hydrodynamic flow, including viscous drag and turbulence, but there should be no
surprises in Bernoulli’s equation.
The difficulties are removed by recognizing that Bernoulli’s equation tell us that a pressure
difference causes a change in speed, and pressure differences are caused by curvature of flow,
interpreted locally as producing a centrifugal force.
For those who wish to avoid the details, it is only necessary to point out that where there is
curved fluid flow, there is a pressure difference (i.e., lift), and from Bernoulli’s equation (simply
and properly interpreted) the existence of the pressure difference tells us there must be a speed
difference.

Robert P. Bauman
Professor of Physics Emeritus
University of Alabama at Birmingham

[Thomas coupling]

RVDT: Perfect - thanks.

Punto: "KISS".........your perambulations are too melifluous!

[[email protected]]

So does the change in pressure cause the speed to increase or does the increase in speed cause the pressure to drop?

Either way the lift is a combination of the suck at the top of the wing and the curving of the airflow - possibly;)

[Thomas coupling]

The latter crab. Speed change first. Henceforth the reduced pressure allows the object to backfill this space which is offering less resistance than was previously there. The "wing" in this instance moves upwards.

[407 too]

TC, you are moving into the vacuum (suck theory) (Bernoulli).
The jist of the article in post #32 is that the air molecules have had work (force) applied to them, drawing them down, an equal and opposite force is applied to the wing (the item applying the force), lifting it up. It is not moving to fill a void.

[RVDT]

Coanda -> Newton -> And some interesting little Bernoulli effects along the way.

I stick to my original guns. And the premise that Bernoulli is only an extension of Newtons laws.

BTW The USAF have been experts for years with "Heavy Hot Smoke" from the worlds only coal fired aeroplane.

The physicist and the Boeing dude still get the vote.

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This 380 seems to be leaving a big hole in the sky!

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Slightly different perspective than a wind tunnel.

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