Aerodynamics - why wings create lift - current vs historical discussions

[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.

 

[cjl]

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)?.

To be clear, wings do direct air downwards. You can entirely explain the lift from a wing through downwash. However, the air deflection actually mostly occurs above the wing, not below it, and the upper surface is more critical to both maintaining lift and keeping drag low. If you have ice, the flow is prone to separating from the upper surface, and this both slightly (or dramatically, depending on how far past stall you are) reduces lift and substantially increases drag.

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.

It's worth noting that even if the airplane is able to generate enough lift through extra angle of attack, the drag rise is also a huge problem unless you have enough engine power to deal with the extra drag. A lightly stalled wing is actually making pretty similar lift to one just before stall, but it'll have many times the drag. This can lead to a vicious cycle too, since as the aircraft slows down due to the excessive drag, the required Cl climbs, and therefore the wing stalls more deeply.

Finally, just as a fun bit of extra trivia, many airfoils do produce peak lift at around 45 degrees, or at least many thinner, lower-camber airfoils do. They do stall at 10 or 12 degrees, but the lift climbs back up at 30 or 40 degrees and matches or exceeds the lift performance at 10 degrees. Of course, the drag is much, much higher, so overall efficiency is terrible, but purely from a lift standpoint, it's better than you might think. Here's an example lift polar for a NACA 0012: https://i.stack.imgur.com/jpvEl.jpg.

I also note that excessive "angle of attack" will cause loss of lift.

Yes, and this is because as I mentioned above, most of the flow redirection happens above the wing rather than below it. When you reach an angle such that the flow over the top is unable to continue to follow the curvature, the flow separates and the flow over the top stops being directed downwards, losing you much of your 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)

Again, it's not either-or. The bernoulli relation holds everywhere around a wing in incompressible flow (below about mach 0.3), and even in compressible flow, it holds with some minor modifications (as long as you stay subsonic). If you cover a wing in pressure transducers, you'll see that 100% of the lift can be explained through differences in pressure around the wing.

In addition, you can measure downwash in the wake of the wing. If you have a good way to measure downwash quantity and velocity, you can also find that 100% of the wing's lift is explained through the amount of air it is pumping downwards as it travels through the air.

Flaps don't counteract this either. They provide a way to enlarge areas of low pressure above the wing and dramatically increase cl_max, which also increases downwash. Even a hand out a window probably makes a better lift coefficient than you might think, and even in the case of a flat plate, most of that lift does come from the suction side of the airfoil rather than the pressure side.

 

[russ_watters]

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.

Fair enough. For my part, I don't like saying something is "entirely wrong", when it contains multiple points and one at least (the last one) looks right as stated. I don't generally subscribe to the idea that one a line of logic fails everything else associated with it (or at least after it) must be wrong. In this case it gives the false impression that Bernoulli is of no help when describing lift. That's exactly the problem that led to the creation of the thread.

 

[sophiecentaur]

Fair enough. For my part, I don't like saying something is "entirely wrong",

Point two is the weak link, surely. Where can you define the effective trailing edge of the wing? This actual point only needs to be slightly below the physical trailing edge and things are more believable. Is the argument that laminar flow has to be occurring?

 

[cjl]

Point 2 is entirely wrong, regardless of where you define the "effective" trailing edge. The air passing above the wing will arrive at the back entirely misaligned with air that passed beneath. It also doesn't matter whether you're talking about a laminar or turbulent boundary layer - the air outside the boundary layer will still arrive misaligned.

 

[A.T.]

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.

It is correct that all the momentum transferred by the wing to the air (and vice versa) is accounted for by the direct collisions between the wing and air molecules.

Whether this qualifies as an explanation is not a matter of correct or incorrect, but of the expectations one has of an explanation.

 

[sophiecentaur]

Point 2 is entirely wrong, regardless of where you define the "effective" trailing edge. The air passing above the wing will arrive at the back entirely misaligned with air that passed beneath. It also doesn't matter whether you're talking about a laminar or turbulent boundary layer - the air outside the boundary layer will still arrive misaligned.

I an believe that 2 is just wrong but it's certainly what the elementary diagrams seem to tell us. I guess the diagrams all imply that there should be continuity of 'something' across the boundary between upper and lower airstreams. But there's nothing to say that the neighbouring regions of air where the streams come together should be the same adjacent regions when the streams come together. So there is no requirement for speed over the top to be higher than underneath - in the steady state - just different path lengths.

What would need to be continuous? Presumably the velocity profile across the boundary. But the vertical component of velocity would be greater over an aerofoil because the air has to move further up and down as the wing passes through it. That would cause a lower pressure on the top.

Explanations all seem to use the Wind Tunnel frame but a flying wing is going through stationary air and it must be pushing air forward causing drag and downward causing lift. The air way out in front and well to the rear is stationary. (I know it's just a choice of reference frame but some frames make more immediate sense than others, at times.

 

[profbuxton]

if a wing just pushes air forward(drag) and downward for lift, why not just use a flat wing,no aero foil needed. Note that wings have different aero foil shapes depending on application and speed. Also it appears to me that the shape of the leading edge in fairly critical for generating lift.
If a wing just relied on thrusting air downward surely it wouldn't matter what shape the leading edge was as long it didnt increase drag by excessive thickness.

 

[cjl]

I an believe that 2 is just wrong but it's certainly what the elementary diagrams seem to tell us. I guess the diagrams all imply that there should be continuity of 'something' across the boundary between upper and lower airstreams. But there's nothing to say that the neighbouring regions of air where the streams come together should be the same adjacent regions when the streams come together. So there is no requirement for speed over the top to be higher than underneath - in the steady state - just different path lengths.

This is one area where it can be misleading to rely too much on basic diagrams. You're right that there's no requirement for them to come together at the same spot, and a more accurate diagram would show that the air over the upper surface actually outruns the air beneath, despite the longer distance (so an "equal transit time" calculation would severely underpredict the lift). You can see this effect in this video, especially around 40-60 seconds.

What would need to be continuous? Presumably the velocity profile across the boundary. But the vertical component of velocity would be greater over an aerofoil because the air has to move further up and down as the wing passes through it. That would cause a lower pressure on the top.

There's actually not even a requirement that velocity be continuous, though in practice it will be because the pressure does have to match, and outside of the viscous dissipation region, there's no mechanism for a mismatch of velocity that wouldn't also result in a mismatch of pressure. However, behind a supersonic airfoil, you can get a so-called "slip line", where the velocity does have a mismatch off the top vs bottom surface of the airfoil. All that is actually required is that the flow coming off the back has to be parallel (flow off the top and bottom surface can't diverge or converge), and there can't be a vertical pressure gradient, so pressure just behind the trailing edge has to be the same both in flow from the bottom and top.

Explanations all seem to use the Wind Tunnel frame but a flying wing is going through stationary air and it must be pushing air forward causing drag and downward causing lift. The air way out in front and well to the rear is stationary. (I know it's just a choice of reference frame but some frames make more immediate sense than others, at times.

Physically, there's no difference between the two frames of course. The misalignment behind the wing in the wind tunnel frame corresponds to the wing actually pushing some of the air traveling over the suction side in the opposite direction as the flight direction, while dragging some air on the pressure side with it. So, after the airplane passes, some of the air that traveled above the wing is displaced slightly backwards and some of the air below the wing is displaced slightly forwards, even though you're right that they both end up stationary.

 

[cjl]

if a wing just pushes air forward(drag) and downward for lift, why not just use a flat wing,no aero foil needed. Note that wings have different aero foil shapes depending on application and speed. Also it appears to me that the shape of the leading edge in fairly critical for generating lift.
If a wing just relied on thrusting air downward surely it wouldn't matter what shape the leading edge was as long it didnt increase drag by excessive thickness.

Because a cambered, nonzero-thickness wing is better at redirecting flow downwards with less drag. As I've been saying, most of this redirection actually happens above the wing, so the way the flow behaves on the suction side is very important. A flat plate doesn't do a good job creating a nice large low pressure region on the suction side to redirect flow downwards, and it also has a bit of a tendency to stall at a significantly lower angle of attack than an airfoil, both of which make it a poor choice.

 

[russ_watters]

It's worth noting that even if the airplane is able to generate enough lift through extra angle of attack, the drag rise is also a huge problem unless you have enough engine power to deal with the extra drag. A lightly stalled wing is actually making pretty similar lift to one just before stall, but it'll have many times the drag. This can lead to a vicious cycle too, since as the aircraft slows down due to the excessive drag, the required Cl climbs, and therefore the wing stalls more deeply.

Another angle ( ) to the vicious cycle is that when the plane that is flying level stalls, it starts descending fairly rapidly, and that causes the relative wind to rotate down, further increasing the angle of attack and deepening the stall, unless the pitch-angle is rapidly lowered. Stalls can be pretty violent, and that cascade/cycle happens rapidly.

 

[russ_watters]

Point two is the weak link, surely. Where can you define the effective trailing edge of the wing? This actual point only needs to be slightly below the physical trailing edge and things are more believable. Is the argument that laminar flow has to be occurring?

I'm not sure I understand what you mean. The trailing edge is always in exactly the same place and is the most easily identifiable point in the flow. The whole point of making the trailing edge of the wing sharp is to enforce that that's the trailing stagnation point.

This is unlike the leading-edge stagnation point, which moves up and down as the pitch changes, and isn't even aligned with the freestream direction of flow (it's below it).

 

[russ_watters]

Because a cambered, nonzero-thickness wing is better at redirecting flow downwards with less drag. As I've been saying, most of this redirection actually happens above the wing, so the way the flow behaves on the suction side is very important. A flat plate doesn't do a good job creating a nice large low pressure region on the suction side to redirect flow downwards, and it also has a bit of a tendency to stall at a significantly lower angle of attack than an airfoil, both of which make it a poor choice.

Other side of the...coin... a sail (zero thickness, large camber) doesn't do a good job of creating a good high pressure region under the wing. I think it was you who pointed out that that was one of the first things the Wright brothers realized in studying their kites and testing airfoils in a wind tunnel.

 

[cjl]

That wasn't me, but it does provide an interesting case study. I would suspect the reason it doesn't work as well as a blunt nose is largely because at high angles of attack, the leading stagnation point actually moves around underneath the airfoil, so you get flow wrapping around the nose of the airfoil from beneath. If you have a large leading edge radius, that allows the flow to stay attached while this happens, allowing for better lift production at high angles of attack. With a cambered but very thin wing, the stagnation point really can't migrate beneath the wing, since the flow will just separate at the thin leading edge, and that causes it to be ineffective at creating high lift at high angles of attack.

(if it's unclear what I'm talking about, here's a diagram)

 

[Lnewqban]

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

 

[rcgldr]

leading stagnation point - separation of flow

Regardless of the airfoil shape, the separation point is below the leading edge of an airfoil making lift because the lower pressure above draws the air from in front and below the leading edge backwards and upwards to flow over the surface.

icing

Icing tends to fill in the reduced pressure zone on the upper surface, effectively changing the airfoil into one that produces less lift at the same AOA, requiring more AOA and associated drag to produce the same lift. In an extreme case, the wing may reach a stall state before it produces sufficient lift.

flat or thin wings

These work fine at low Reynolds numbers (small wing chord and slow speed). A small balsa glider with about a 30 inch wingspan glides reasonably well with barely any camber.

http://www.ericbrasseur.org/glider2.html

a more accurate diagram would show that the air over the upper surface actually outruns the air beneath, despite the longer distance

The whole point of making the trailing edge of the wing sharp is to enforce that that's the trailing stagnation point.

There are cases where the longer distance is on the bottom, and where the trailing edge isn't sharp. For example, the M2-F2 (glider) lifting body prototype re-entry vehicle. The trailing edge is not sharp because that is where the rocket engines were place in the later M2-F3.

 

[russ_watters]

There are cases where the longer distance is on the bottom, and where the trailing edge isn't sharp. For example, the M2-F2 (glider) lifting body prototype re-entry vehicle. The trailing edge is not sharp because that is where the rocket engines were place in the later M2-F3.

View attachment 259020

You've posted that claim and that image several times before, without evidence that your claim is true -- the picture isn't evidence because you can't see the flow. I'm not inclined to accept your claim, without evidence that a lifting body works any different from a normal wing. Specifically, I bet the stagnation point is far under the chin and the airflow does indeed take a longer path over the top than over the bottom. The video of it landing at very high AOA compared to the F-104 flying next to it implies that should be true.

Further, even if true I think it is unhelpful to use an outlier when trying to teach the basics. It's a great way to create wrong impressions of what is typical or best. I'm sure a brick generates lift at a small positive angle of attack, but we shouldn't be generalizing that or listing it as a noteworthy exception.

 

[cjl]

Regardless of the airfoil shape, the separation point is below the leading edge of an airfoil making lift because the lower pressure above draws the air from in front and below the leading edge backwards and upwards to flow over the surface.

It's not clear to me that this is always true, and at low Cl (such as in cruise), the stagnation point is effectively at the leading edge on pretty much any airfoil. Certainly any reasonable airfoil operating at higher Cl though will have the stagnation point well below the leading edge.

Icing tends to fill in the reduced pressure zone on the upper surface, effectively changing the airfoil into one that produces less lift at the same AOA, requiring more AOA and associated drag to produce the same lift. In an extreme case, the wing may reach a stall state before it produces sufficient lift.

The increased surface roughness, and associated higher drag and earlier stall angle is a much larger problem than gross airfoil shape changes. In general, adding to the top surface of the airfoil won't decrease the achievable Cl_max (unless you get to some fairly large changes there) - that tends to be more dependent on leading edge radius instead (sharply curved leading edges lead to lower Cl_max).

These work fine at low Reynolds numbers (small wing chord and slow speed). A small balsa glider with about a 30 inch wingspan glides reasonably well with barely any camber.

http://www.ericbrasseur.org/glider2.html

It's not just the low reynolds number helping him there, it's also the low angle of attack and required Cl. You're right that flat airfoils do perfectly fine as you go down to low reynolds numbers though (or more accurately, "traditional" airfoils do really badly - you'd be hard pressed to get a flat plate above a L/D of 10 or so, while 100:1 is pretty readily achievable with a more normal airfoil, but only at Re>10^5 or so)

There are cases where the longer distance is on the bottom, and where the trailing edge isn't sharp. For example, the M2-F2 (glider) lifting body prototype re-entry vehicle. The trailing edge is not sharp because that is where the rocket engines were place in the later M2-F3.

View attachment 259020

For the purposes of my statement, a flatback and a sharp trailing edge both achieve the same thing: they prevent flow from wrapping around and effectively enforce a rear stagnation point. There are some additional interesting effects when it comes to flatback airfoils that make them a really useful choice for very high thickness airfoils, especially when you also consider structural properties of your airfoil section on top of just the aerodynamic effects.

Also, it's not necessarily useful to use your M2-F3 example as an airfoil at all. It doesn't behave like an airfoil, because the flow isn't even close to 2D. I would bet that a large part of the lift on that is actually related to vortices around the sides onto the upper surface, similar to a delta wing at high AoA, and there are so many 3d effects impacting the flow that I don't even know that you could meaningfully talk about "path lengths" for the flow except right on the centerline.

 

[A.T.]

...the airflow does indeed take a longer path over the top than over the bottom.

Is this key to generating lift?

 

[FactChecker]

EDIT: Although the statements in this post are reasonable, I think it is safe to say that having a longer airflow over the top of the wing is essential to obtaining lift.

Is this key to generating lift?

There are multiple things adding to the total lift and they are all inter-related. It is hard to know how much lift to credit to each cause. I would not call any single cause "key". The total lift of a wing has a very messy explanation when you look critically at it. That is why CFD calculations are used.
The only exception is Newton's Third Law, where the lift of the wing is equal and opposite to the total downward force on the air. But that is a bottom-line number with messy intermediate details that again require CFD. It is almost a platitude.

 

[russ_watters]

Is this key to generating lift?

It's probably not a coincidence.