Aerodynamics - why wings create lift - current vs historical discussions

[russ_watters]

Doesn’t the word “principle” imply the status of Bernoulli’s calculations? It doesn’t appear to claim to be a Law.

No, words like "principle" and "law" do not convey any sort of status in science. The word principle is used because Bernoulli's Principle is a verbal description. Why Bernoulli's Equation isn't Bernoulli's law I don't know, but it doesn't really mean anything. Being a "law" doesn't make Newton's Law of gravity absolutely correct, for example.

 

[sophiecentaur]

Newton’s gravity law can be observed to work accurately in local ‘space’ (ideal) conditions. Bernoulli never has ideal conditions. Electric forces always mess with it. I would say the two models are very different in that respect.

 

[russ_watters]

Newton’s gravity law can be observed to work accurately in local ‘space’ (ideal) conditions. Bernoulli never has ideal conditions. Electric forces always mess with it. I would say the two models are very different in that respect.

Fair enough - I didn't mean for this to be a competition, I'm only pointing out that Newton's Law isn't exactly correct either, though it may have been believed to be when it was named. There is no governing body that scores them and assigns labels as awards.

Both have known limitations yet both are highly useful when used within those limitations.

 

[rcgldr]

simply insert a term for the energy gain/loss. Then you can apply it as needed.

Which is similar to how it's typically stated for propellers (or rotors), the flow before and after the air crosses the "disc" of interaction, Bernoulli applies, but the flow across the "disc" of interaction, "violates" Bernoulli, because of a jump in pressure (increase in energy).

https://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

it would mean that as airplanes flew, they would eventually force the atmosphere down into a liquid ocean on the surface of the Earth. And that's not a joke; the air has to end up back where it started.

Not quite, the air can't be pushed beyond the ground below, but it can be compressed (eventually it will recover from this as well). Consider the air and a plane flying through the air as a closed system. Over the affected volume of air and area of the earth, the downforce of the air onto the Earth corresponds to the weight of the air and any objects "supported" by the air, (including something like a helium balloon). It's easier to visualize this in a hypothetical closed system, consisting of a large box, filled with air. There's a pressure differential within the box, lower at the top, higher at the bottom, and the net downwards force exerted by the air onto the box equals the weight of the air. Now add a model aircraft flying in a horizontal circle inside the box. The net downwards force exerted on the box now equals the weight of the air and model aircraft (as long as there is no net vertical acceleration of the center of mass of this system), via an increase in the pressure differential within the box.

 

[gmax137]

cue up the "truckload of birds on a bridge" threads

https://www.physicsforums.com/threads/weight-of-the-flying-bees.170218/

https://www.physicsforums.com/threads/pigeons-flying-in-a-truck-on-mythbusters.166425/

 

[FactChecker]

Why lift is predictable? That's more of a philosophical question. Why should anything be predictable?

Why it has the magnitude it has? That's again a quantitative question about how much not about why. Run a CFD stimulation, look at the results, that's your "why".

For an initial aircraft design, it's predictable precisely because of the CFD results and because of preliminary wing theory. These days, those estimates are all done before an airplane is built. But for a particular airplane that is already built and flying, the lift is very predictable. It is not just because the lift is not likely to be exactly zero, as you stated.

 

[Klystron]

Pitot or static? I'm familiar with an array of static ports on a wing surface, and that's a distinctly "Bernoulli" approach. I suppose measuring the velocity at the surface would be as well, but I don't see how that would work, because it changes rapidly as distance from the surface changes (and at the surface itself should be zero due to friction in the boundary layer). Previously you mentioned measurement of the downward airflow - for Newton - and that's what I was asking about.

Recent PF threads discussed the type and quantity of air sensing instruments on commercial transport aircraft such as Boeing's 737 series. Wind tunnel tests help decide these design questions.

I have helped collect data from actual valves mounted on sections of aircraft fuselage in full-scale wind tunnels then confirmed by data collected in free flight of a prototype. Earlier experiments might use scale-models of the prototype and an analog of the actual (pitot) valve(s) to optimize placement and minimize distortion once in production.

You may be interested in ground effect experiments conducted in large (12 foot and up) subsonic wind tunnels. Different "ground plates" are added to the configuration that simulate runway conditions at landing and take-off producing fascinating downward and upward air flows. Ground plates are also common in automotive wind tunnels.

My more general point remains that wind tunnel CFD data compares favorably with free flight data and need not be dismissed out of hand because tunnel walls do not dissipate air flow identically to the open atmosphere. Thanks.

 

[cjl]

It's also worth noting that the conservation of momentum model also doesn't say why the flow is bent downward, and oddly people don't seem to get bent out of shape over this limitation or its obvious associated misconception:

Often the first time people think about lift is when they stick their hand out the window of a car and make it fly. The simple explanation: the air hits the bottom of your hand and is deflected downward. Heck, you can FEEL it. Momentum transfer! But it is a lot harder to deal with the fact that the air moving over the top of the wing is much more important to lift than the air bouncing off the bottom. The reasons for the air accelerating and curving down over the top of the wing present the same problems/complications for Bernoulli and Newton.

Agreed. The details of exactly why the flowfield is the way it is are equally complicated no matter what your preferred method of looking at it is, and neither of the simple explanations really gets to the root of why airfoils look the way they do to make lift efficiently.

 

[cjl]

I am confused by the Bernouli adherents’ total attention to the effect on the Plane by the air flow and the curves. The pressures on the wing work in all directions. I know it’s a lot harder to predict the overall flow of the atmosphere (and the losses due to viscosity). But the net downward momentum will meet up with the ground.

I am confused by what you even mean when you say "bernoulli adherents'". As I stated above, all forces on the wing are transferred through pressure, and (for flows slow enough to be considered incompressible, below about mach 0.3 or so) Bernoulli accurately describes the relationship between pressure and local velocity at all points around an airplane. That's not a statement because I'm a "bernoulli adherent", that's simply a fact of the physics involved. Similarly, there will be a downwash after a wing where the momentum flux is directly related to the lift created, but that doesn't make me a "Newton adherent" any more than my pressure statement above makes me a "bernoulli adherent". Both methods of approaching the problem can fully describe the lift, and (as russ said above) neither is very enlightening when it comes to learning why the flow behaves the way it does.

 

[cjl]

Newton’s gravity law can be observed to work accurately in local ‘space’ (ideal) conditions. Bernoulli never has ideal conditions. Electric forces always mess with it. I would say the two models are very different in that respect.

Electric forces are pretty much always completely negligible when talking about fluid flow, and Bernoulli is very close to perfectly accurate when talking about any low-speed aircraft flight (less than about mach 0.3).

 

[sophiecentaur]

Electric forces are pretty much always completely negligible when talking about fluid flow, and Bernoulli is very close to perfectly accurate when talking about any low-speed aircraft flight (less than about mach 0.3).

Doesn't viscosity have an effect on the behaviour of the air? Perhaps I should have used the term frictional and turbulent losses instead. Gravitational forces between celestial bodies don't have a similar effect to mess them up. Planetary orbits are the nearest thing to Perpetual Motion that we had, before superconductivity, I think.
I don't know enough to have a serious opinion about Bernoulli but my point is that Bernoulli only considers the air flow very near to the plane and doesn't particularly care about what happens to all the displaced air in the wake of the aircraft.
Bernoulli can be as accurate as you like but it doesn't consider (and probably doesn't need to) the bigger picture. But there is clearly something relevant when you consider ground effect, when lift is very much influenced. The pressure on the air around the wake of an aircraft doesn't have to be building up 'below' it. The pictures higher up in the thread show very large scale motion with air going up at the edges and down near the centre.

If you think of a balloon envelope resting on the ground with a Helium cylinder and the Helium expanding into the balloon until it rises, the total force on the Earth will be the weight of the ballon, it's just that the pressure is very small over a huge area. A similar thing is going on with a heavier than air craft. The total weight of all flying craft is an incredibly small fraction of the weight of the whole atmosphere.

 

[cjl]

Doesn't viscosity have an effect on the behaviour of the air? Perhaps I should have used the term frictional and turbulent losses instead.

For most aircraft, and really a surprising amount of fluid flow in general, viscosity can be completely neglected except in the boundary layer. In general, you will get a very accurate lift value around an airfoil even if you simulate a completely inviscid flow, though the drag will of course be wrong (since a decent amount of drag does indeed come from the boundary layer).

I don't know enough to have a serious opinion about Bernoulli but my point is that Bernoulli only considers the air flow very near to the plane and doesn't particularly care about what happens to all the displaced air in the wake of the aircraft.

No, Bernoulli is generally true throughout the flowfield, except where there is energy addition or loss (which is pretty much just in the boundary layer and in any engine wakes). Of course, as stated above, Bernoulli doesn't tell you enough to know what the flow will actually do, just how the velocity and pressure will be related at any point in the flow, but the "Newtonian" flow deflection explanation suffers from the exact same problem. If you actually want to know what your streamlines will look like and get quantitative data on how much lift you'll make, neither explanation will cut it and you need to get into some much more complicated models to figure that out.

Bernoulli can be as accurate as you like but it doesn't consider (and probably doesn't need to) the bigger picture. But there is clearly something relevant when you consider ground effect, when lift is very much influenced. The pressure on the air around the wake of an aircraft doesn't have to be building up 'below' it. The pictures higher up in the thread show very large scale motion with air going up at the edges and down near the centre.

Again, no simple explanation actually tells you what the flowfield looks like. Bernoulli absolutely can tell you about the lift increase in ground effect, because the flowfield around the wing (and the velocities next to the wing surface) are different in ground effect than they are when the plane is far from the ground. Neither Bernoulli nor the downwash explanation do a good job explaining why ground effect is a thing though. As far as simple explanations go, I prefer to refer to the wingtip vortex formation. The presence of the ground interferes with the full development of tip vortices and decreases the induced downwash angle that the wing sees. This means that for the same global angle of attack (for the airplane relative to freestream), the local angle of attack seen by each wing section is slightly increased, and the lift vector is tilted slightly forwards (due to the inflow angle being more parallel to the flight path). This causes a reduction in induced drag, and an increase in lift, and also explains why the stall angle for an aircraft in ground effect is reduced (which does not make sense if you just think of it as the downwash impinging on the ground).

If you think of a balloon envelope resting on the ground with a Helium cylinder and the Helium expanding into the balloon until it rises, the total force on the Earth will be the weight of the ballon, it's just that the pressure is very small over a huge area. A similar thing is going on with a heavier than air craft. The total weight of all flying craft is an incredibly small fraction of the weight of the whole atmosphere.

Sure, but I don't really know what this has to do with the rest of the post. This also doesn't depend on only the downwash explanation - it works just as well if you think of the aircraft as applying a local pressure step across the wing that eventually propagates down to the ground.

 

[sophiecentaur]

Sure, but I don't really know what this has to do with the rest of the post. This also doesn't depend on only the downwash explanation

I did wonder about adding that para but the same thing applies against the argument about 'what happens to the pressure on the ground?' The Earth is massive enough to ignore its Momentum change and the overall CM stays fixed, of course.

The downwash (thanks @cjl for reminding me of that excellent term - I had forgotten it) will transfer Energy to large scale air movement and what goes down must come up - to reverse a well known saying. If you include all that motion then you have a full enough explanation which doesn't clash with Momentum Conservation. Outside of a large enough region of atmosphere, the motion of air will be small enough to ignore but that is well beyond all the local airflow diagrams that are used to explain lift. Without the bigger picture, it relies on sky-hooks, in the end. Bernoulli works ok and that's the main concern for aircraft engineers but why do people reject the idea that something else is at work?

The parallels with simple Antenna Theory are everywhere to see. Assumptions are made which allow antennae to be designed successfully. I can't remember coming across an equivalent conflict between the big and small views. But perhaps it's because EM is so non-intuitive that people don't feel competent to have strong opinions as they do with fluid dynamics.

The presence of the ground interferes with the full development of tip vortices and decreases the induced downwash angle that the wing sees.

Interesting. Sounds a good explanation (to someone who knows little about the topic). Could it involve wavelength of the pressure waves and the separation between plane and ground?

 

[FactChecker]

Again, no simple explanation actually tells you what the flowfield looks like. Bernoulli absolutely can tell you about the lift increase in ground effect, because the flowfield around the wing (and the velocities next to the wing surface) are different in ground effect than they are when the plane is far from the ground. Neither Bernoulli nor the downwash explanation do a good job explaining why ground effect is a thing though. As far as simple explanations go, I prefer to refer to the wingtip vortex formation. The presence of the ground interferes with the full development of tip vortices and decreases the induced downwash angle that the wing sees. This means that for the same global angle of attack (for the airplane relative to freestream), the local angle of attack seen by each wing section is slightly increased, and the lift vector is tilted slightly forwards (due to the inflow angle being more parallel to the flight path). This causes a reduction in induced drag, and an increase in lift, and also explains why the stall angle for an aircraft in ground effect is reduced (which does not make sense if you just think of it as the downwash impinging on the ground).

Just to clarify, the plane does not need to be very high to ignore ground effects. The rule of thumb is that ground effects start when the plane altitude is half of its wingspan or lower. (I personally enjoy the feel of the ground effect during a landing. The plane scoots forward noticeably. I'm not sure why I like that, but I do.)

 

[russ_watters]

Could it involve wavelength of the pressure waves and the separation between plane and ground?

There really isn't a wavelength. If the plane is straight and level, you have a static (unchanging) pressure field surrounding the plane and moving with it. If anything changes that pressure field (throttle, control inputs), those changes propagate away from the plane at the speed of sound. But they aren't coherent waves with a definable wavelength.

 

[sophiecentaur]

they aren't coherent waves with a definable wavelength

Not well defined but the circular motion is periodic with a spectrum of ω. wouldn't that involve a delay and hence a wavelength of sorts? A reflection at a boundary plus the delay would produce a temporal filter, perhaps(?) and produce an amplitude / ω response in the vortices. But that idea may well be fruitless, of course.

 

[Master1022]

You can edit your prior post if you want.

it wasn't letting me, but maybe I am missing something

 

[russ_watters]

Not well defined but the circular motion...

What circular motion?

 

[sophiecentaur]

What circular motion?

The motion inside any vortex?

 

[russ_watters]

The motion inside any vortex?

Oh, well having rotation doesn't mean there are emitted waves or a wavelength.

 

[cjl]

The motion inside any vortex?

The vortex is superimposed on the bulk flow, so there's not really a bulk circular motion. In addition, a potential vortex doesn't have a single period, since the rotational velocity is inversely proportional to radius (unlike a solid body rotation).

 

[sophiecentaur]

The vortex is superimposed on the bulk flow, so there's not really a bulk circular motion.
In addition, a potential vortex doesn't have a single period, since the rotational velocity is inversely proportional to radius (unlike a solid body rotation).

Which is why I used the word 'spectrum'.
I think you are implying that the ground effect is resistive rather than reactive so it just damps the vortex, rather than reflecting it. That sounds very reasonable.

 

[Jordi H]

What about a very, very thin wing, just a curved plane. The air would travel the same distance over and under the wing, yet it will still produce lift.

 

[cjl]

The travel distance doesn't matter though. In my very first post on this thread, back on page 1, I even said "the common "longer path length" explanation is completely bogus".

 

[Lnewqban]

What about a very, very thin wing, just a curved plane. The air would travel the same distance over and under the wing, yet it will still produce lift.

A thin curved plane is just a cambered airfoil that has a poor aerodynamic shape; hence, its lifting capability is lower and its drag is higher than those of an equivalent streamlined airfoil.
The Wright brothers experimented with that shape first, but moved to more rounded leading edges, where most of the magic happens.

 

[russ_watters]

What about a very, very thin wing, just a curved plane. The air would travel the same distance over and under the wing...

No, it wouldn't, just as circles of different radius's have different circumferences so too the underside and top of a thin, curved airfoil have different path lengths. The closer you get to the wing the smaller the difference may be, but there is a difference nonetheless, and the airflow attached to the wing can't be viewed in isolation. The wing disturbs/re-directs air far away from its surface.

 

[russ_watters]

A thin curved plane is just a cambered airfoil that has a poor aerodynamic shape; hence, its lifting capability is lower and its drag is higher than those of an equivalent streamlined airfoil.
The Wright brothers experimented with that shape first, but moved to more rounded leading edges, where most of the magic happens.

That's a really good point -- when discussing this issue people often bring up sailboats, but if sails made good airfoils planes would have airfoils as thin as they could be made. Sailboats have thin airfoils not because they are good, but because they are easy.

Having thickness to the airfoil helps, and it takes away the "same path length" argument.

 

[russ_watters]

The travel distance doesn't matter though. In my very first post on this thread, back on page 1, I even said "the common "longer path length" explanation is completely bogus".

I wouldn't want to argue about a strawman, so please describe exactly what this argument you consider to be "completely bogus" is.

 

[profbuxton]

Had a read through the "arguments " here re air flow and wing lift. Not being an expert on such in depth matters, but I would like to query the "downwards" force explanation versus the pressure differential( if that how I am understanding the issues. If it was simple matter of "downward" force why would a layer of ice (specially on leading edges)on wings reduce/effect wing lift to the extent of causing aircraft to lose lift and crash(refer to "Air crash investigations)?. Happened on a number of occasions Why would it be nesessary to deice wings? All one would need would be increase the "angle of attack" to force more air downwards.
I also note that excessive "angle of attack" will cause loss of lift.
I would expect there to be some element of both involved since aircraft use flaps when taking off or landing and these seem to direct airflow directly(as per the hand out the car window example)

 

[jbriggs444]

I would like to query the "downwards" force explanation versus the pressure differential

Then you should start by giving your current understanding of both explanations.

The version of the "downward force" explanation that I get from your post sounds like the "air bounces off wing" explanation that everyone agrees is incorrect.

 

[profbuxton]

jbriggs444, sorry if I gave the wrong impression. I don't subscribe to the "air bouncing off wings" explanation of wing lift. If it were that simple one would only need a flat piece of sheet for a wing (like sticking ones hand out of a car window at speed, although there may be an element of aero foil effect over fat fingers). As I mentioned if it were only "air bouncing off wing" there would be no need for aero foil design and no need for deicing of wings to ensure lift is maintained in icy conditions.

 

[gmax137]

I always thought de-icing was to remove weight from the airplane. I have been wrong about many things, maybe this is another?

 

[russ_watters]

I always thought de-icing was to remove weight from the airplane.

A quick check; a 1/4" coating of ice on a Cessna 172's wings and tail would be a lot of ice, but would only weigh about 250lb, about the weight of a passenger and bag. It's enough to matter but should not crash a plane, especially one that's already in the air. For big jets, the impact is smaller.

No, the main issue with ice is the disruption of the airflow. The shape and texture of the wing is precise, and even small changes in a bad spot can have a big impact. And it is worse at low speed and high angle of attack, when the airflow stability is much more fragile.

 

[gmax137]

No, the main issue with ice is the disruption of the airflow.

Thanks, @russ_watters & @profbuxton , I try to learn something new every day.

 

[cjl]

I wouldn't want to argue about a strawman, so please describe exactly what this argument you consider to be "completely bogus" is.

The common (and entirely wrong) "bernoulli" description of lift goes as follows:

1) The top of a wing is longer than the bottom of the wing due to the airfoil shape
2) The air going around the wing will all arrive at the back simultaneously (often with some handwaving about "continuity" or something)
3) The air going over the top must travel farther, therefore it must travel faster
4) Therefore, by the Bernoulli relation, the pressure is lower above the wing, creating lift

This makes a nice "just-so" explanation of lift, but it's actually entirely wrong. Specifically, step 2 is where it completely falls apart. There's no reason to expect the flow around both sides to arrive at the back of the wing simultaneously, and in fact, it usually doesn't.