Lift - what mechanism?
 

I've been having a discussion on Wikipedia recently about an edit I made to a page concerning lift. We have come to an agreement, but I wanted to ask a couple of questions to clear something up in my mind.

I have an Aeronautical Engineering Masters from London and came across a bit on Wikipedia that talked about the equal transit time fallacy for air travelling above and below and wing surface. It was quite rightly saying this isn't true, but the said "lift is actually produced because air is deflected downward". I changed that bit and it resulted in a tit for tat reversion of what each other had written. Our discussion hinged over what caused lift and Newton's laws.

My edit said that lift is caused by the pressure field around the wing, caused by pressure changes in the air both above and below the wing caused in turn by the fact that air is "turned". I backed it up with references to Anderson's Fundamentals of Aerodynamics and some other excellent books such as Kermode and Aerodynamics for Naval Aviators.

The chap in question kept saying that lift could be calculated by working out the downwash, and that lift was equal to the downward momentum transferred to the air.

I'm quite happy with the fact that Bernoulli's equation is initially derived from Newton's Laws and that Newton's laws can be applied to a control volume to derive the Euler equations and the Navier-Stokes equations. These are of course approximations to actual flow. However, I don't think its as simple as downwash = lift.

Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash? Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash.

So have I missed something? Is lift = downwash = integrated pressure around the whole body?

I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution. I reckon this is wrong - but have I missed something?

Looking forward to a discussion.

I am sure I read somewhere many moons ago that a proportion of the lift from an aerofoil was generated by the downthrust it gives to the air as a result of A of A, about 20% rings a bell. For instance if you hold a perfectly flat board horizontally into an airflow & gradualy tilt it towards the vertical it will give give an upward lift. Obviously it is a very crude wing & drag will soon take over.

Taking another example. if you hold the back of a spoon into a flow of water from a tap it will move further into the water flow(coander effect). No water flows on the underside of the spoon so no lift is as a result of the example above.

A wing will produce lift even with a negative A of A ( about -4degrees in the case of a typical light aircraft wing) so surely this must be entirely generated by the wing camber & not downwash.

This may be an over simplification, but that is my understanding.

This is all a bag of worms, depending on how accurate you want your explanation to be.

The current ATPL (JAA) explanation, aka the 'air deflected downwards / equal and opposite reaction makes lift' explanation is (a) a step in the right direction (versus the 'equal transit time' fallacy) and (b) at least doesn't violate Newton 3. However, it does violate momentum conservation.

Just say to the bloke something along the lines of: "In your explanation, consider level flight. The aircraft hasn't changed its vertical velocity component, but if the air has been deflected downwards, so momentum has not been conserved"

I've spent a lot of time thinking about this over the last couple of years. Over my career I've worked as a ground school instructor teaching PofF, a school teacher of Physics in addition to my flying. What I do not have though is a degree level of understanding regarding fluid dynamics.

In general, for explaining anything mechanical, my yardstick for something to be a good model is: Newton 1,2,3 not violated. Momentum conserved. Energy conserved.

Also, everything else being equal, scalar based explanations are simpler than vector based explantions, e.g. Energy rather than forces (doesn't help much with explaining lift, but helps a lot with explaining induced drag).

As far as I can see , it is impossible (by my yardstick) to give a good model for lift production if you are only looking at a 2d aerofoil. However, once you go to 3d the world is your oyster.

Anyway, I'm short of time to post further at this point.

pb

I like to think of it as displacing a mass of air, like a boat displaces a mass of water.

Recently in a discussion with a ex Royal Navy test pilot he said that it was explained to him in the way that if the differential pressure theory was correct, how the hell could a jumbo get off the ground? i.e. with a takeoff weight of I guess here at 220t the pressure differential required would suck all the panels off the top of the wings etc!

I reckon it's magic.

It is definitely magic.

Sucking the wing panels off is not a problem. The actual force per unit area is not all that much as the wing surface is large. In Mechanics of Flight by Kermode, it says that a typical "suck" required is less than a baby sucking a nipple. Now I've never had my nipples sucked by a baby, but I'm sure it wouldn't take the panels off a jumbo.

According to the Boeing website

Jumbo wing area = 524.9 m^2
Max weight for ER version 412,775Kg = 4,045,195N

Therefore average pressure = 7700 N/m^2 = 0.77N/cm^2

So thats like a load of 1 apple per postage stamp. Hardly going to break the wing. (I realise that calculation is a bit simplistic).

Jetstream Rider: Not disagreeing with the point made in your post #7, but you have a factor of 10 error in the calculated average pressure.

Try one apple per postage stamp, which illustrates your point even better.

Now corrected - thanks very much - early morning this morning. Thought it was bigger than I had imagined....

If you can wade though 160 posts, here is a prior thread

Thanks - I'll have a look. I'd love to hear from some of the TPs and FTEs on this forum though, unless of course they have posted on the previous thread.

I'm just your average (maybe below) bog aviator and the way I have always conceptualised "lift" is as follows. I think of a wing as a propellor blade. The blade produces lift (thrust) to pull/push the aircraft forward. How does the lift manifest itself? By throwing a quanity of air to the back. No air being thrown to the back, no lift (thrust). This view of lift may stem from my helicopter background. No downwash from the rotor blades, no lift.
As I infer above I think of lift as being the result of a mass of air being thrown at the ground, and the means by which that is brought about is by the pressure distribution about the wing. So the two go hand in hand, can't have downwash without the pressure distribution. Of course with an airfoil at its zero lift angle of attack you still have pressure distribution but no downwash.
My understanding is that reflex in the trailing edge is introduced to control the pitching moment of the airfoil. You still need downwash to get lift (lift, as in to keep a body flying).

In my opinion lift is produced the way I have allways learned it was, and the way it is still explained in the books I have.

So lift is produced because the air moving on the top of the wing moves faster than on the bottom, causing a drop in pressure, causing an upward lift.

How else can you explain that a cambered wing at at angle of attack of 0 degrees can produce lift.

I do not believe in all these new fancy theories like lift being produced by donwash or by turning the flow or by little eddys on top of the wing.

Bart

There is a big difference between the mathematical modelling of the flow and the actual flow. The wing can be modelled as a sheet of little vortices - but we know in reality a wing doesn't work like this.

How can pressure distribution and downwash be the same thing?

If it were just about "throwing air downward", then why don't curved plates that generate downwash but a poor pressure distribution generate so little lift?

Have a look at FoilSim II 1.5a beta

By being a bit creative with your entires to the boxes, you can generate lift with more upwash than downwash. OK, its a sim and not a real windtunnel, but I doubt it is so inaccurate that it is not representative.

For instance, try angle -20 deg, camber 24.5, thick/cord 16.725. You will have a lift of 797 llbs, so its not very efficient as a lift producer, but the air is visibly going up more than it is going down, so there is no net downwash.

Similarly, set the angle 9.56 deg, camber -8.1, thick/chord 12.775. The flow from the back of the wing is slightly upward (see the jink in the pixels on the line) and it is generating a small amount of lift (again, albeit inefficiently).

If downwash does not provide a significant contribution to total lift in straight and level flight, how do the deniers of this explain the well-established phenomenon of ground effect? This "floating" tendency which extends the landing run unless pilot input counters it, can only be caused by the downwards-moving mass of air below the wings coming into contact with terra firma.

This effect is visible when landing on dry bush strips, or in ultra-low flying by the reckless aviator, when a continuous cloud of dust is raised by the aerodynamic "squeezing" or "cushioning" between wing and ground.

To my straightforward (simple?) engineer's mind, it seems blindingly obvious that total lift is a variable combination of pressure-distribution and downwash effects, based on distant memories of fluid mechanics lectures and lab work. This "controversy" about the components of lift all seems a bit artificial to me, due to the apparent tendency to polarise opinions in an "all or nothing" approach. In other words, I think both sides are right, to some degree!

Lift usually means downwash.

However, I think pressure distribution is more important. We were taught in ATPL exams that a third of the lift comes from downwash and 2 thirds from pressure distribution. While this is necessarily a gross simplification, there may be a grain of truth in it.

Certainly the lower surface accounts for more lift than upper surface only theories would have you believe.

I think what I am seeing with no downwash lift is purely the pressure distribution, and with flat plates turning the flow is almost purely downwash created lift. A real wing being a mixture of the two. It still means "lift is produced by deflecting the air downward" is no the right way to describe lift.

Because when you come into ground effect the wingtip vortices are being reduced because there is no more air below to let them propagate, therefore drag reduces, hence the floating of the aircraft.

If lift would be caused by downward deflection of the air, why would there be wingtip vortices

From Wikipedia (and referenced on that page):

"The phenomenon of 'wing in ground effect' is caused by the ground 'interrupting' the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the performance of the aircraft while it is experiencing the ground effect."

Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".

Be careful about "downwash" here. There are two definitions, one the downward motion of air behind the wing, the other is due to induced drag of the wingtip vortices which reduces local angle of attack.

I think we are in fairly close agreement, with one reservation, regarding flat plates and the "turning" effect.

Up to a moderate angle of attack, at a moderate airspeed, airflow over the upper surface soon separates and becomes turbulent, i.e. the flat "wing" section stalls, experiencing a drastic loss of lift at the upper surface. However, at high angle of attack, at a low airspeed, vortex lift can develop on the upper surface. This can restore the uplift proportion of total lift such that downwash no longer makes up most of the lift.

If this effect did not occur, neither Concorde nor the Vulcan could have been the practical successes that they were. These aircraft, and indeed delta planforms with slender wing sections generally, would need to have much higher landing speeds and/or highly complex wing slat and flap systems, and land at much lower angles of attack. These factors could well have rendered them impractical, I understand.

Dug out my old study notes which explains it thus. The only forces that can act on a body moving through a fluid are those produced by friction (shearing stress in the fluid which is a function of viscosity) or those produced by pressure. Except when minimum drag is considered, the pressure forces are by far the most important, and completely responsible for the production of lift. The ambient or static pressure existing around a body moving through the air, cannot produce a resultant force, so dynamic pressure is left as the fundamental source of aerodynamic forces.

The maximum force that can be produced by dynamic pressure would seem to be,

Force = Dynamic Pressure x Area

Most airfoils are capable of producing a total reaction considerably greater than that suggested by the above formula.

Consider a mass of air travelling at constant velocity that is turned through an angle of 90° by a vane whilst maintaining its constant velocity. A vector diagram of the velocity’s will show that that the resultant (acceleration or change in velocity) will be 1.414 x Velocity (being an equilateral triangle with included angles of 90° and 45°). Newton’s second law suggests then that the force acting in the direction of the resultant (on the vane) will be,

Force = Mass x Change in Velocity

Mass/Unit Time = Rho x Area x Velocity

Therefore Force = Rho x Area x Velocity x 1.414 Velocity

= 2.828 x ½ Rho x Velocity^2 x Area

= 2.828 x Dynamic Pressure x Area

Although an airfoil is unlike the vane in that it is immersed in the airstream, it still produces lift by changing the momentum of the air, and producing a greater force than predicted by the simple “pressure x area” relationship. This is equivalent to saying that the airfoil produces an aerodynamic mechanical advantage. Although the forces may be magnified by mechanical advantage, the total energy remains constant. The dynamic pressure produces the resultant force by altering the value of the local static pressure, with the total pressure remaining constant. Until the airfoil produces a change in momentum to the airflow, the dynamic pressure will not affect the static pressure, or will affect it equally, and no resultant force will exist. Once the angle of attack is set so that the momentum of the airflow is changed, the resulting dynamic pressure will produce an out of balance of the static pressures, and a resultant force will act on the airfoil. The ability of an airfoil to change the momentum of the airstream is a function of airfoil camber and angle of attack.

A helicopter with rotor blades that are free to flap give as good an illustration as you could possibly get of the downwash/lift relationship. With blades at minimum pitch there is no noticeable lift and little downwash and the tip path plane of the blades will be at a given position. As you progressively add pitch you increase both lift and downwash and you will see the tip path plane increase in height (that is, the rotor blades increase their coning angle as the lift, and downwash, progressively increase).

To sum up, the wing throws air at the ground. No downwash, no lift.

Why are you saying the wind deflected downwards?

I'm doing my PPL and have a fascination with how a wing produces lift...

I'll say it more simple as I love taking technical words and giving simple explanations...

The wind can do two things I think...:
1) Wind going over the top creating low pressure then getting sucked upwards like the paper example.
2) Wind pushing against the underneath of the wing (is that where you keep saying wind deflection)
I just stick my hand out the window when driving to see that I can feel wind pushing the underneather of my hand...

So I think it can be a mixture of both 1 and 2 :)

The truth is that there is no requirement for nature to understand our definitions, our formulae, or even basic mathematics. We can fly because of how nature behaves, not because of how we describe it. We make up a bunch of words to describe what we have observed, then realize it is a bit wrong and fix it. Then repeat.

The descriptions that start with single molecules of air and Newtonian Law are the most accurate place to start. When the system is real-sized, this description becomes far too complex to solve and offers no intuition. The descriptions that deal with airflow shapes, multiple engineering terms, pressure fields, etc. are attempts to simplify the Newtonian calculations using assumptions and bounds with the caveat that the accuracy may reduce. and may in fact be completely wrong (especially true in earlier models). The descriptions that are given to pilots typically are lacking scientific accuracy, contain very few formulae, and don't yield useful results. However, they are the ones that are most intuitive and tend to be the easiest way to generate sufficient understanding to fly the airplane.

In the end, every explanation is inaccurate to some extent. The most accurate explanation is the Newtonian one. The complex explanation is just a derivation from Newtonian physics. Talking only about pressure gradients lies between these other two methods, and I think is just confusing.

This is how I like to think of it:
- First, think of just one molecule coming towards a wing. There is only one thing the wing can do to help the wing counter gravity, that is to deflect the molecule downwards.

- If you continue to deflect more molecules downwards, you will have more molecules below the level of the wing than you have above, so you will have greater pressure below.

- The molecules will move from highest pressure to lowest pressure. We've seen this, it seems intuitive and we've even added words to the physicist's vocabulary to explain why. Entropy, Brownian motion, etc.

- The move from high to low manifests itself in a number of ways, most of which are important for aerodynamicists to understand. Flow behind a wing can be upwards. Wing tip vortices will develop. There is a lot of flow disruption behind the wing, but in the production of lift, the wing only causes the initial effect. Everything else is a result of the disturbances to the air that have already been caused.

- The pressure gradient (created by moving air molecules downwards) will also change the flow in front of the wing. I think this is a key point to acknowledge, because this one seems to conflict with the starting point of considering just one molecule. It doesn't conflict, but does require that we now think of one molecule in front of the wing that already has some vertical velocity.

- If the single molecule starting point is still causing some concern, then consider a group of molecules, bumping into each other with a net vertical velocity. What can the wing do to that group to help the wing counter gravity? Would just compressing them help at all? Or would you have to change the net vertical velocity as well?

Sorry, no formulae to punch into your calculators. Just a description of what I believe is going on.





Capt Pit Bull, according to your understanding of physics, falling bodies don't accelerate, because otherwise momentum wouldn't be conserved. Include gravity into your consideration of downwash, and you will find that momentum and energy are conserved.

Thanks for that explanation and the others too...I suppose I need to be able to see the aerodynamics in a tunnel too to appreciate it :)

Sorry, have to disagree with that.

There's a definite shift in the lift-curve slope, with increased CL at a given AoA (at least at low AoA), in ground effect. It's not just a drag effect.

:D :D

If we could fully describe flight by math & physics, all the test pilots would be out of a job!

I stand corrected.

The particles bouncing off the lower surface theory does not account for the fact that around 4/5 of the lift comes from the top surface (see Kermode, early pages). Have a look here:

Incorrect Lift Theory

Forwards and backwards on the arrows at the bottom give more good info.

I disagree. Look at the flow sim and lift over a spinning cylinder. Up = down, especially on the cylinder, yet you get a large amount of lift. We only don't use that for aeroplanes as the drag is very high.



The maths of flow is by nature an approximation, or a "trick" to get the world to work. Bernoulli's equation is a description, not the whole text.

Jetstream Rider, that link you provided only discusses one idea on how the downwash is created. The "4/5 of lift [that] comes from the top surface" is also generated by deflecting air molecules against gravity, its just that its not doing it through a collision with the lower surface of the wing.

Interestingly, areas of lower pressure don't pull. Instead, areas of higher pressure push. Granted, these are definitions we have made, but intuitively, what is attaching the area of low pressure to the surface it is supposedly pulling on? With this in mind, I'd find it interesting to see how Kermode has decided that the top surface is producing more lift than the bottom surface.

The spinning cylinder redirects airflow. Creates downwash in the production of lift. This is consistent with momentum theory (and inconsistent with "skipping stone" theory). Not sure why you brought this up.

Indeed there is no "pull" from sucking, pressure can only push.

Most of the lift is due to the top surface geometry, hence 4/5 coming form the top of the wing. Take away that geometry and it just doesn't work the same way.

The spinning cylinder directs as much air up as it does down - we can't choose to ignore the upwash and only look at the downwash, so yes the air is re-directed, but as much up as down. Resolving the entire flow, as much momentum goes up and goes down, so the skipping stone theory doesn't add up. There are further links on the NASA page I linked to above.

There is no need to "attach" low pressure to the wing - the wing is pushed in to the air by the high pressure below, but its the top surface that creates the low pressure that makes the pressure below the wing the higher of the two.

Certainly the skipping stone theory is incorrect, although along with the equal time fallacy, there may be truth mixed in with the error.

From Index of Aerodynamics Slides

Fallacy Number 1. The "Longer Path" theory, or the "Equal Transit Time" theory. This theory is one of the most widely circulated, incorrect explanations. The theory states that airfoils are shaped with the upper surface longer than the bottom. The air molecules have farther to travel over the top of the airfoil than along the bottom. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from Bernoulli's equation, the pressure is lower. The difference in pressure across the airfoil produces the lift.

Fallacy Number 2. The "Skipping Stone" theory is often seen on web sites and in popular literature. The theory is based on the idea that lift is the reaction force to air molecules striking the bottom of the airfoil as it moves through the air. Because this is similar to the way in which a flat rock thrown at a shallow angle skips across a body of water, it is called the "Skipping Stone" theory of lift. It is sometimes called a Newtonian theory of lift, since it involves Newton's third law.

Fallacy Number 3. The “Venturi” theory is based on the idea that the airfoil upper surface is shaped to act as a nozzle which accelerates the flow. Such a nozzle configuration is called a Venturi nozzle and it can be analyzed classically. Considering the conservation of mass, the mass flowing past any point in the nozzle is a constant; the mass flow rate of a Venturi nozzle is a constant. The mass flow rate m dot is equal to the density r times the velocity V times the flow area A:

m dot = r * V * A = constant

For a constant density, decreasing the area increases the velocity.

Turning to the incorrect airfoil theory, the top of the airfoil is curved, which constricts the flow. Since the area is decreased, the velocity over the top of the foil is increased. Then from Bernoulli's equation, higher velocity produces a lower pressure on the upper surface. The low pressure over the upper surface of the airfoil produces the lift.

The Real Story. There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning
From a Newtonian perspective, lift is generated by turning a flow of air. The flow turning creates a downwash from the wing.

Ground Effect. The proximity of the ground reduces the downwash. At a height equal to the span the induced drag is reduced by 2%. At a height equal to 1/10th the span induced drag is reduced by 50%. The reduction in drag will only occur where the induced drag is predominant. At high speeds where parasite drag is predominant ground effect will not result in a drag reduction. Ground effect will only be significant during take off and landing. As the ground is approached during landing the induced drag progressively decreases and a constant angle of attack will produce an increasing CL, and the reduced downwash produces a change in longitudinal stability and a change in trim. The reduction in induced drag and increasing CL will cause the aircraft to “float”.

Am I naïve to think that Dr. Dick Whitcomb, inventor of area rule, the supercritical wing and winglets, and his colleagues may know what they are talking about? :eek:

if you can do both differential and integral calculus--and have some background in mechanics---I highly recommend the following book listed below---trust me it's a light book in terms of the mathematics of aerodynamics:\--but it give a lot of the true basis of section lift-though geometric 2D airfoil analysis---as well several--{hundered} or so pages of data on various NACA airfoils---very good for beginning aero--injunirs and such:}



Ira H. Abbott and Albert Von Doenhoff ---Theory of Wing Sections [1959]:ok:

PA

Oh yeah for Pilots--- wing pushes air down -- air pushes wing up---except of course when on knife edge when your rudder is your elevator anyway:}


PA:)

Well, that might be your understanding of my understanding!

Falling body acceleration conserves momentum quite happily, you just need to consider the other mass involved in the gravitational attraction. You don't need any air around to explain it either.

Like I said, expand the view to a 3 dimensional model and conserving momentum become easy. Two vortices with equal and opposite angular momentum (for unaccelerated flight of course), job done.

pb

pb, you are correct, that was my understanding of your thread. However, you did say that momentum was not conserved when the air goes down. However, the free falling mass increases its momentum and you contend that momentum is conserved in that case because the earth moves.

What if the free falling mass started hurtling small bundles of mass to the ground with sufficient effort that the large mass reduced its momentum to zero? Get my point?


This sounds like a gyro-levitation device (those don't work). If creating two equal and opposite vortices is the action, then the reaction is going to be two equal and opposite moments. That would sum to zero as well.

See here:
Lift from Flow Turning

The last bit:
Summary

Lift and drag are mechanical forces generated on the surface of an object as it interacts with a fluid. The net fluid force is generated by the pressure acting over the entire surface of a closed body. The pressure varies around a body in a moving fluid because it is related to the fluid momentum (mass times velocity). The velocity varies around the body because of the flow deflection described above.

I'm happy that mathematical descriptions for lift are far from exact, are necessarily approximations and sometimes are quite wacky (yet still yield good results). I'm also happy that we don't have an analysis toolkit that is good enough to describe flow and lift adequately (especially not turbulence), regardless of computer size or complexity of model. What I was after by starting this thread was to improve my conceptual understanding.

Anderson says "...the aerodynamic forces and moment on the body are due to only two basic sources: 1. Pressure distribution over the body surface 2. Shear stress distribution over the body surface".

My current understanding therefore is that in a flow, energy, mass, and momentum are conserved. In doing so and by passing flow around a closed surface the pressures and therefore forces within that fluid change. This causes lift, by a number of mechanisms including viscosity and we can roughly
describe it. The force on the closed body is caused by pressure changes and shear stress distribution, these pressure changes are caused by "turning" the flow and by the fact it has momentum. By turning it carefully and with experience we can approximate what nature will give us.

In short, lift is complex and hard to understand and we've not got that near describing it exactly by using mathematics. We do know however that it is caused by pressure changes within a fluid, caused by flow turning and the fact it has momentum.

Any disagreements?

Matthew,

You seem to be extrapolating my posts beyond what I've stated.

My position is simply that lift can not be adequately explained by a simple 2d model, and that the 'air deflected downwards' is not a complete model.

Sure. I don't dispute it.

But extend your model. Where are these lumps of mass coming from, and where do they go after they've impacted the other body?

Weather systems aside, if you took a large fleet of aircraft and just flew them around all day, would the atmosphere get gradually compressed in the lower levels as the wings deflected air downwards?

No mention of Gyro theory from me. I'm talking about a wing, and wake vortices.

The moments sum to zero, (which is why our aircraft isn't undergoing any angular acceleration about its longitudinal axis.) However, given that the two wings are on opposite sides of the aircraft, this is consistent with an upwards force on both wings.

Bottom line is most of this comes into the 'picture paints a thousand words' category, so I'll leave it there for tonight.

pb

Interesting to explain wing lift scientifically/mathematically/physically or whatever but the bird flying around in the pressurised cabin of an aircraft nevertheless transfers its weight to the aircraft by air pressure waves and the aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves.

Boats/ships do the same thing. Space things don't.

Bring on anti-gravity!!

Really?

I can deal with the aircraft exerting the same force on the air as the air exerts on the aircraft, but is the force of the aircraft on the air not taken up by viscosity and eventually heating?

I'm not saying you're wrong, but could you explain it a bit more please?

To extend the example a bit:

The hydrofoil at rest is a displacement vessel (Archimedes and all that).

But as it gathers speed, its weight is taken up by the foils, and the water reacts to the dynamic load of the downwash of the foils. If it were a model hydrofoil in a long tank aboard an aircraft, its wing still carries the same load.

No. Because momentum is conserved.

If you accelerate, say, 1 kg of air to 1000 m/s every second, you are exerting a force of 1000 newtons - and spending 500 kW energy to do so.

If that 1 kg of air then exerts force on and entrains 9 kg air then momentum is conserved. 10 kg air per second will be moving at 100 m/s. And it carries only 50 kW of energy.

The other 450 kW energy was lost to viscous heating of the air. But the force and momentum is still there in its entirety.

As the air goes on moving, its energy decreases as increasing amount of air is made to move at increasingly low speeds. But the momentum remains there until it is transferred to ground.

The link provides an explanation by two learned gentlemen in simple easy to understand language. Lift is the result of throwing air at the ground.
How Airplanes Fly: A Physical Description of Lift c