Lift - what mechanism?

[Jetstream Rider]

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.

[cessnarepairman]

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.

[Capt Pit Bull]

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

[Shawn Coyle]

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

[SpamCanDriver]

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!

[Knight Paladin]

I reckon it's magic.

[Jetstream Rider]

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).

[603DX]

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.

[Jetstream Rider]

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

[barit1]

If you can wade though 160 posts, here is a prior thread (http://www.pprune.org/forums/private-flying/295168-debunking-lift-theories.html)

[Jetstream Rider]

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.

[Brian Abraham]

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.
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.
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.
Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without 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).

[bArt2]

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

[Jetstream Rider]

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 (http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html)

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).

[603DX]

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!

[Jetstream Rider]

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.

[bArt2]

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.


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

[Jetstream Rider]

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.

[603DX]

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.

[Brian Abraham]

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.

[Phil1980's]

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 :)

[Matthew Parsons]

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.

[Phil1980's]

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 :)

[Mad (Flt) Scientist]

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

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.

[barit1]

We can fly because of how nature behaves, not because of how we describe it. :D :D

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

[Jetstream Rider]

Quote:
Originally Posted by Jetstream Rider View Post
Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".
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.

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 (http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html)

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


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

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.

[Matthew Parsons]

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.

[Jetstream Rider]

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.

[Brian Abraham]

From Index of Aerodynamics Slides (http://www.grc.nasa.gov/WWW/K-12/airplane/short.html)

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:

[Pugilistic Animus]

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

[Pugilistic Animus]

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:)

[Capt Pit Bull]

Capt Pit Bull, according to your understanding of physics, falling bodies don't accelerate, because otherwise momentum wouldn't be conserved.

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.

Include gravity into your consideration of downwash, and you will find that momentum and energy are conserved.

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

[Matthew Parsons]

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?


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.

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.

[Jetstream Rider]

The real story...lift is generated by turning a flow of air. The flow turning creates a downwash from the wing.

See here:
Lift from Flow Turning (http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html)

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?

[Capt Pit Bull]

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.

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?

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?

This sounds like a gyro-levitation device (those don't work).

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

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.

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

[Milt]

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!!

[Jetstream Rider]

...aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves.

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?

[barit1]

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.

[chornedsnorkack]

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?


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.

[Brian Abraham]

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 (http://www.aviation-history.com/theory/lift.htm)

[Double Zero]

Just as I suspected, with one or two illustrious exceptions -

I've been putting my life in the hands of pilots for years, and you still haven't agreed how the wings work ! :)


I've always gone for a mix of the ' airflow deflected down ' idea- I've been under low flying fast jets, and that's how it seemed to me ! Mind you they were at relatively high alpha, flaps & power on, for chase work.

Also the ' baby sucking on a straw ' pressure theories, but have only studied aeronatics slightly compared to most posting here.

I once attended a lecture by Bill Bedford, when he was asked by an idiot ( who naturally qualified later as a BAe 'manager' for about 5 minutes ) " what new developments are in the pipeline, and will we see the P-1154 ?"

This was 1978 ! Bill gave him a withering look, and replied " well we haven't got anti-gravity paint worked out yet ! "

[Jetstream Rider]

So what I wrote above would be correct - forces are transferred to the wing by pressure forces, caused by turning the air. These pressure forces are as a result of conserving momentum, energy and mass. The body feeling the reaction of the force imparted to the flow.

I'm quite happy with that, with two exceptions.

How does one explain lift without downwash (spinning cylinder and reflex trailing edge) and lift without power for infinite spans?

I don't think "lift is produced by throwing air at the ground" is quite as accurate as that. Surely "lift is produced by turning air and feeling the reaction" would be better - albeit with the ground (or other solid objects) eventually taking the momentum away from the air.

[Double Zero]

As I said I am no expert ( did play with a wind tunnel a few times ) but surely what you're describing is ground effect, not true wing lift - velocity, chord & all that ?

[603DX]

Jetstream Rider (Post #40):

I don't believe that lift can occur without downwash. The first two cases you quote, "spinning cylinder and reflex trailing edge", both turn the wake downwards.

A cylinder spinning backwards and travelling forwards has a delayed boundary layer separation point on the upper surface, where the boundary layer is moving in the same direction as the free stream flow, and a premature separation point on the lower surface where it is moving against the free stream flow. This deflects the whole cylinder wake downwards, i.e. it creates downwash, and hence lift.

This is the same effect as a spin bowler uses to curve the flight of a cricket ball - it works for spheres, too!

An aerofoil wing section with a reflex trailing edge will still generate downwash and lift, simply less of each because it is not an optimum shape for high lift coefficient.

I'm afraid I don't understand what you mean by "lift without power for infinite spans", so cannot comment.

[gardua]

As one who has had the experience of standing beneath a Sea Stallion Helicopter as it turned through 90 degrees at 50ft altitude just before landing,
I can vouchsafe that it felt to me as if the air mass equivalent to the whole weight of the machine plus a percentage more to allow for the turn was washed down by the rotors as cyclic pitch plus power was applied by the pilot. Not very scientific, I agree, but a practical demonstration I shall never forget.

[Brian Abraham]

How does one explain lift without downwash
You can't have lift without downwash. If you are able to produce lift without downwash you have your fortune made, helicopter manufacturers for one will beat a path to your door. A couple of examples I hope may illustrate.

Example 1 Consider the fin/rudder of an aircraft flying without any induced sideslip from other causes. The rudder will be faired with the fin (0° deflection) and the angle of attack is zero. In this condintion we have a symetrical airfoil and on either side of the fin/rudder will be a pressure field induced by the airlow being accelerated over the curved surface. The pressure fields on either side of the fin/rudder are equal and are thus unable to effect any turning of the airflow to create a downwash. The instant rudder is applied a positive angle of attack is created which then creates an imbalance in the pressure field on either side of the fin/rudder. The imbalance in the pressure field causes downwash and so you then have lift. You may have seen the computer modelling of the pressure fields about an aircraft where the pressure level is depicted by different colours. An aircraft experiences pressures of varying levels on all external surfaces, but unless there is an ability of the pressure field to induce a downwash there is no lift.

Example 2 A propellor is nothing but a wing that rotates, rather than travelling through the air in a straight line as does an aircrafts wing (for all intense purposes). As a pilot you intuitively know that unless the propellor is blowing air back you have no thrust. Thrust and lift are exactly the same and is the result of an airfoils movement through the air at an angle of attack sufficient to induce downwash, thrust being the lift produced by a propellor, and lift the thrust produced by a wing. In the propellors case the downwash is seen as a cylinder of air being thrown to the rear, and in the wings case, as a mass of air being thrown towards the ground.

As kids we used to live on a small hill directly under the approach path. The heaviest aircraft we used to see was the DC-3 and they would pass over the top at a couple of hundred feet. On a calm day a party trick with school mates was to place a sheet of newspaper on the ground and shortly after the -3 passing over the paper would move as if by an invisible hand.

[Jetstream Rider]

"lift without power for infinite spans" comes from the link above

here
How Airplanes Fly: A Physical Description of Lift c (http://www.aviation-history.com/theory/lift.htm)

"There is a misconception held by some that lift does not require power. This comes from aeronautics in the study of the idealized theory of wing sections (airfoils). When dealing with an airfoil, the picture is actually that of a wing with infinite span. Since we have seen that the power necessary for lift is proportional to one over the length of the wing, a wing of infinite span does not require power for lift."

This is the bit I'm struggling with. The rest is quite comfortable.

I've looked again at the spinning cylinder and its clearer now.

[barit1]

Since we have seen that the power necessary for lift is proportional to one over the length of the wing, a wing of infinite span does not require power for lift

An infinite span - It's an absurdity on the face of it, right?

Simply postulating a mathematical asymtotic extrapolation doesn't mean we have to build the damn thing. :}

Matter of fact, if we build a wing with slightly over a 40 million meter span, the left tip will meet the right tip, there will be no tip vortices, and it can float at zero IAS. :rolleyes:

[Jetstream Rider]

Its just hard to understand why that would be. Surely a wing needs some force on it to keep it floating, and without the mechanism that produces that force, surely it would not have a force acting on it.

I realise it fits the maths well, but it doesn't fit reality surely?

[Algy]

Milt says: "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."


OK, so when the bird is flying its weight is transferred by "air pressure waves" - maybe - and when the bird sits in seat 18B it is effectively part of the aircraft. So far, maybe, so good.

So how much does the combination weigh while the bird is descending inside the aircraft with its wings folded? There are then no "air pressure waves", and it is not "part of the aircraft".

Or try this. Suppose the bird is in its seat on the aircraft sitting stationary on the ground. We weigh the combination and get a figure. Then the bird takes to the air inside the aircraft. How much do the scales now register? Does it matter if the cabin has a roof or not? And if we (highly hypothetically) performed the same experiment with the aircraft in flight, now does it matter if there is a roof or not?

[barit1]

Surely a wing needs some force on it to keep it floating, and without the mechanism that produces that force, surely it would not have a force acting on it

Think of the rings of Saturn. Now think of all those millions of ring particles connected to each other in a solid mass, with the centre of mass of this ring object coinciding with the planet's centre.

Now freeze the ring so it's stationary WRT the planet, not orbiting around it. Gravity will keep the two centres of mass together!

(What does this have to do with aerodynamics? gee--- duh--- )

[Jetstream Rider]

The maths behind zero power for an infinitely long wing don't take account of the fact that the Saturn analogy is a closed loop. Imagining a flat plane with an object above it, infinitely long in all directions, then surely if there is a gravitational force, the object and the plane attract each other. Unless they orbit each other, (like Saturn's rings orbit Saturn), then they will move toward each other. Surely the particles in Saturn's rings are in perpetual freefall, just like satellites orbiting earth? Without that motion around each other, the infinite wing would still need something to keep it in the air?

Obviously with a wing around the earth as a single body, the pull on one side would be counteracted by the pull on the other and it would float, or in other words the earth would be pulled toward each part of our ring equally. The difference being that this is a single body and that Saturn's rings are many bodies.

I realise it works mathematically, I'm just saying I find it hard to conceptualise how it would work in reality.

[chornedsnorkack]

So how much does the combination weigh while the bird is descending inside the aircraft with its wings folded? There are then no "air pressure waves", and it is not "part of the aircraft".

Or try this. Suppose the bird is in its seat on the aircraft sitting stationary on the ground. We weigh the combination and get a figure. Then the bird takes to the air inside the aircraft. How much do the scales now register? Does it matter if the cabin has a roof or not? And if we (highly hypothetically) performed the same experiment with the aircraft in flight, now does it matter if there is a roof or not?

Consider it this way: look at a freighter with a crane installed in its ceiling (I think several freighters have them) and a heavy item suspended from that crane.

So long as the item remains hooked at the crane, the weight that works on the landing gear is that of the plane with all its loads - including the item.

Now, a clumsy loadmaster drops the item.

So long as the item is in free fall inside the plane, the weight of the plane decreases and is the weight of the plane and load without the item in free fall.

When the item hits the floor, the weight of the plane increases, and becomes bigger than the weight of the plane and load including the item. This lasts until the item comes to a stop (or breaks through the bottom of the plane). After the item has come to a stop, the weight of the plane is again exactly the weight of the plane and load including the item.

[Algy]

Yup, that sounds right. But I don't think it's analogous to the bird in the cabin. Surely the bird hanging (getting slightly surreal here) from the cabin roof is not the same as it being in lift-generating flight inside the cabin?

But if it's not, then when it's in flight does it add to the weight of the 'aircraft' as suggested earlier. And if it does, then does that weight go away from the 'aircraft' while the bird is in non-lift-generating descending-flight?

[chornedsnorkack]

But I don't think it's analogous to the bird in the cabin. Surely the bird hanging (getting slightly surreal here) from the cabin roof is not the same as it being in lift-generating flight inside the cabin?

But if it's not, then when it's in flight does it add to the weight of the 'aircraft' as suggested earlier. And if it does, then does that weight go away from the 'aircraft' while the bird is in non-lift-generating descending-flight?

Imagine having a small helicopter, hovercraft and other vehicles in the cargo hold of a big freighter plane - whether the plane is resting on tarmac, in steady taxying or in steady cruise.

When a vehicle is inactive and just sits in the hold, its weight is transferred to the cargo hold through its landing gear.

The weight is actually already carried by air. A vehicle is supported by air trapped in its suspension. And the same vehicle is also supported by air supported inside its tires. (No power is needed to keep it there, because it cannot escape).

Now, put power on the hovercraft. It rises up on air cushion. A hovercraft in steady hover has precisely its entire weight supported by the same cargo floor, because of the extra pressure of air cushion to the floor. The hovercraft does spend some power to hover because the air in air cushion can escape, unlike that inside tires or suspension.

Now, power up your small chopper. It creates downwash. Its weight is no longer supported by its landing gear - but the downwash transfers exact same weight on floor, and perhaps ceiling. So long as it is a steady hover.

If your helicopter is in unsteady flight them the weight of the plane will change. When the copter is in free fall, accelerating downwards, its weight goes away. When the helicopter is accelerating upwards, the weight of airplane will be more than that including the helicopter. Although you should remember to count the acceleration of air inside - the plane does not acquire the weight as soon as the helicopter increases lift, but only when the extra downwash so created hits floor (or ceiling).

[Algy]

Hmm, well maybe it's because of the three glasses of riesling I just had, or maybe it's because I just find it hard to believe anyone called chornedsnorkack :) but I still have problems with this.

However, maybe we stumbled across something interesting. Think of the aircraft on the ground and with the bird (or hovercraft or helicopter) being suspended inside the aircraft from a cable which is hanging from a totally separate crane outside the aircraft. If the crane lowers the bird onto its seat again then the aircraft will weigh more. But if it picks it up then clearly it will weigh just the weight of the aircraft.

But, you say, and other people in the thread say, that if the bird is flying (at least in an enclosed cabin) then the downwash from the bird will make the aircraft weigh more. So that seems to go back to the original nice and simple question that started the thread - aerodynamic lift is a different force from the tension in the cable attached to the crane. So it must be the downwash that results in lift.

But suppose the downwash is insufficiently strong to reach the floor of the cabin (or it's blown horizontally by a fan for example) then is there no lift anymore?

Don't feel obliged to answer this - perhaps you have a busier life than mine over there in Estonia.

[Jetstream Rider]

Correct me if I'm worng, but.... The reaction of the ground isn't anything to do with the creation of lift at the source, its just that the momentum has to go somewhere eventually.

If your bird on your outside crane started flapping, the tension in the wire would decrease, leading to a decrease in the total weight of the crane reacted through its wheels.

This lift would then have to be reacted somewhere and it would be the floor of the aeroplane the bird is in (making it heavier). If a fan blew the downwash away, the downward momentum would still be there and would have to be reacted somewhere, perhaps further down the cabin, or if the aeroplane had an opening tail, further down the airport.

[Capt Pit Bull]

its just that the momentum has to go somewhere eventually.

Watch out. Momentum conservation is not like a set of accounts that has to balance at the end of the tax year. It is balanced instantaneously, as a consequence of Newtons second and third laws.

pb

[chornedsnorkack]

its just that the momentum has to go somewhere eventually.
Watch out. Momentum conservation is not like a set of accounts that has to balance at the end of the tax year. It is balanced instantaneously, as a consequence of Newtons second and third laws.


Both are right.

Returning to the example of an item dropped from the ceiling of a plane - yes, the momentum transferred to it by force of gravity balances instantaneously, by third law. It balances by accelerating the item, in free fall, by second law.

But the item cannot get out of the plane. When it hits the floor and comes to stop, its momentum returns to zero. That´s when the entire weight and momentum goes back to the airframe.

So, when you count the momenta of moving parts, the momentum must be conserved instantly. But since internal momentum cannot be nonzero in long term, eventually the momentum must be conserved without any allowance for moving parts.

[Capt Pit Bull]

Returning to the example of an item dropped from the ceiling of a plane - yes, the momentum transferred to it by force of gravity balances instantaneously, by third law. It balances by accelerating the item, in free fall, by second law.

No.

The momentum is not "balanced by the item accelerating".

Gravitational attract is occuring between two masses, one very big (the Earth) and one very small (the item you dropped). So, not only is the item accelerated towards the earth, but an equal and opposite gravitational attraction (Newton 3) is pulling the Earth towards the item. As a result, both accelerate towards on another.

However, since the Mass of the Earth is very very Huge, the resulting acceleration (newton 2) is very small. The item though has a much smaller mass and therefore accelerates a lot more.

Newton 3 tells us the gravitational attraction is equal and opposite.
Newton 2 tells us each item experiences a rate of change of momentum equal to the force.

The upwards momentum of the earth is equal in size to the downwards momentum of the item at all times. Added together as vectors they cancel one another out, and the total vertical momentum of the item and the Earth is zero, at all times.

[Algy]

...it's kept me awake for three nights running.

That bird in the cabin again. So it's argued above that in an enclosed/pressurised cabin the bird, when it's flying, still contributes to the weight of the aircraft.

I'm still not sure that's true, but what about the other bit of my question - does it contribute to the weight if the cabin is not enclosed/pressurised (not sure which one of those is the significant one.) Clearly not, I'd suggest, as a moment's reflection on a bird sitting on your lap in a Tiger Moth will make obvious.

So what exactly is the difference when the cabin is enclosed/pressurised? I guess supporters of the bird-weight theory will say it's still the downdraft, but I have to say I found the response to my question as to what happens if the downdraft is artifically blown away by a fan unconvincing.

I also suspect that for the same reasons, some of you will say the Tiger Moth situation described above is not so simple. OK, but then the question arises as to how far does the bird have to go before it no longer contributes to the aircraft weight.

(Presumably this could in fact be tested on the ground - I wonder if it's ever been done?)

[Lord_Flashheart]

(Presumably this could in fact be tested on the ground - I wonder if it's ever been done?)


Google is your friend

MythBusters (season 5) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/MythBusters_(season_5)#Birds_in_a_Truck)

[chornedsnorkack]

I'm still not sure that's true, but what about the other bit of my question - does it contribute to the weight if the cabin is not enclosed/pressurised (not sure which one of those is the significant one.) Clearly not, I'd suggest, as a moment's reflection on a bird sitting on your lap in a Tiger Moth will make obvious.

So what exactly is the difference when the cabin is enclosed/pressurised? I guess supporters of the bird-weight theory will say it's still the downdraft, but I have to say I found the response to my question as to what happens if the downdraft is artifically blown away by a fan unconvincing.

I also suspect that for the same reasons, some of you will say the Tiger Moth situation described above is not so simple. OK, but then the question arises as to how far does the bird have to go before it no longer contributes to the aircraft weight.

The significant part is the cabin being enclosed. Pressurization is not important.

Consider two aircraft in formation flight, or one flying and the other on ground. If the downdraft generated by one aircraft hits the other craft, the weight of the plane flying/located in the downdraft increases, because some of the weight of the other plane rests on it. But so long as both craft are in external airflow and not in enclosed cabin, both are generating some lift and their propwash or jet blast is escaping. Now, when an aircraft flies into the cabin or cargo hold of another plane and the cargo door is closed, its downwash no longer escapes the cabin, so exactly its whole weight is transferred to the other craft, whether the inside aircraft is hovering or resting on cabin floor.

[CirrusF]

Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash?

If the cylinder is spinning in still air, there is no lift. Lift is only developed if there is relative airflow over the cylinder - and in which case there will be downwash at the "trailing edge" of the cylinder - check your Navier-Stokes notes...


Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash.

Yes, but only in the two dimensional cross section of the wing that you are examining. There will also be an axial flow towards the wingtip (in the case of a non-infinite aspect ratio!) and at the wing-tip there will be vortices with very large downwash.

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

Yes, this is always the case (ignoring the vertical component of engine thrust of course - which is far from negligible especially during take-off).

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?

The only thing that you are missing is the vertical component of engine thrust, but otherwise you are correct in straight and level flight, that the aerodynamic component of lift is equal to aerodynamic downwash (conservation of momentum) and that the weight of the aircraft is equal to the net vertical force created by the pressure distribution around the entire airframe

[Jetstream Rider]

CirrusF and everyone else who has replied to my initial question - thanks very much.

I understand the thrust bit, but didn't want to complicate things initially.

[bookworm]

Here's an analogy that helps me.

Imagine a rubber ball thrown against a wall. The ball exerts a force on the wall, for a time, during its collision. Why?

A) The ball deforms when it is in contact with the wall, causing pressure to be applied to the wall by the rubber where it is in contact.

B) The momentum of the ball changes between the time it is moving towards the wall and the time it is moving away. Therefore there must be an impulse applied to the ball and consequently a force applied both by the ball to the wall and the wall to the ball.

Which explanation is "correct"? Well they both are correct. They're just models at different scales. If you were able to make appropriate measurements of deformation of the ball, you'd find the total impulse applied to be consistent with the change in momentum.

The lift models are a little like that. You can look at the pressure acting on the surface of the wing at every point, a bit like A. Or you can look at the momentum change of the air that has been turned by the wing, a bit like B. The lift predicted by either model will be the same. The only difference is in the practicality of the calculation. In the case of the rubber ball, it's obviously simpler to measure the velocity before and after and calculate the impulse that way. With wings, it tends to be easier to look at the pressure distribution on the wing than to calculate the momentum change of every relevant element of airflow. Hence aerodynamicists tend to think of lift as being "caused" by the pressure distribution, which is in turn "caused" by velocity differences in the airflow around the wing.