Wings, Propellers, and Lift

[KerikBalm]

In another thread, a dispute came up about the nature of wings, propellers, and lift.

I think its best to discuss it here. So I will try to present the argument without naming names, and let others weigh in before arguing my position.

Basically, someone was arguing that wings and propellers do not work the same way, and that the lift force cannot be explained in terms of Newton's 3rd law/ that the wing deflects air downward, such that the air is effectively "reaction mass".

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It seems to me that rotor-based props don't actually make "thrust" by shoving a fluid atmosphere backwards like real props do.  Instead, because they're made of wing panels or control surfaces, they function the same way wings normally do in KSP.  That is, if they move forward through the atmosphere with a positive AoA, a lift force magically appears on their upper surface and pulls them in that direction.

... here a distinction is made between wing lift, and prop thrust (which is said to behave in terms of using air as reaction mass, made explicit more below)

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IRL, props do function like wings. And wings do push air down as they go by, so this isn't saying all that much

Somewhat, but it's not their main thing.  Props (and jets and rockets) function in real life by shoving fluids to the rear so Newton's 3rd Law moves the vehicle forward.

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The two are one and the same.

Wings produce lift by Newton's 3rd law as well. The primary difference between the concepts you seem to have, is that one moves a large masd of air at low velocity (thus low total KE), and one moves a smaller mass of air at higher velocity.

The aerodynamic principles are exactly the same.

no, that's incorrect.  There are 2 totally separate things going on with REAL props.  Yes, there's a little bit of lift, but the main thing is "paddling". 

Lift intentionally has very little to do with the 3rd Law because it does not rely on reaction mass.  That is, wings do not shove air down to react the plane up.  If that was how lift worked, you'd be flattened by the weight of a plane passing low overhead. Instead, the object of wing design is to move as little air as possible because imparting movement to the air sucks energy away from moving the plane (aka drag).   As a wing moves forward, its shape and angle of attack creates a pressure differential in the surrounding fluid, higher on the bottom and lower at the top.  Thus, the partial vacuum on the top sucks the wing upwards (or, the higher pressure on the bottom tries to expand into the partial vacuum, pushing the wing ahead of it).  That is lift.

Props (and turbofan blades), however, create propulsive force by using reaction mass, so are all about the 3rd Law.  The blade physically shoves a wad of air backwards and the 3rd Law pushes the plane forward.  It's exactly the same thing as rowing a boat or how a paddlewheel ship works.  The oars/paddles shove water backwards and the reaction force moves the boat forward.  Thus, REAL props have more in common with rocket engines than wings. 

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Again...1 and the same, not 2 separate things.

That pressure differential causes air to move down. Lift can be entirely explained by action and reaction. There would be a serious problem if it couldn't.

You don't get flattened when a plane flies over you because the force is very widely distributed.

And yes, it takes energy. Contrary to what you say, its designed to move as much air as possible, as slowly as possible.

The energy lost is 1/2mv^2, while the force made is proportional to mv.

Thus you minimize energy losses by moving more air, but slower 

...

Yes, a plane moving through the air necessarily causes the air to move somewhat.  The plane and the air can't occupy the same space so the plane pushes the fluid air out of the way.  But only to the least extent possible, because moving the air wastes energy,  ...

HOWEVER, this minimal movement of the air can be put to use to create lift.  Lift comes from fluid dynamics, specifically Bernoulli's Principle, which can be derived from Newton's SECOND Law, not the 3rd.  It's a conservation of energy thing.  The air atop the wing moves a bit faster than that below, causing it to have lower pressure and creating a partial vacuum.  The higher pressure below attempts to re-establish equilibrium and carries the wing with it.  That is how aerodynamic lift is made.

If lift worked via Newton's 3rd Law as you mistakenly believe, then there'd be no difference between airplanes and helicopters.  But they're demonstrably different, as I shall now demonstrate....

I believe we can agree that helicopters are all about the 3rd Law, "paddling" to use air as reaction mass, right?   Helicopters hover and maintain altitude by countering gravity head-on with the 3rd Law reaction force, same as a jet or rocket VTOL.  And I'm sure you're familiar with the strong, often dangerous winds under a low-flying helicopter.  That wind is the reaction mass, accelerated by the rotor, and the volume of air and the speed it's moving are all necessary to counter the gravitational pull on the mass of the helicopter as a whole.  It's exactly like being in the exhaust stream of a rocket or jet with the diameter of the helicopter's rotor disk.  Agreed?

OK, then if wings worked the same way as helicopter rotors (using the 3rd Law), then to lift a plane of the same mass as the helicopter, the wings would have to shove down just as much air just as fast.  But an airplane's wing area is only a small fraction of the rotor disk area of a helicopter of the same mass (rather than a larger area as you posited above).  Therefore, to generate the same gravity-countering reaction force from a much smaller area, the airplane's wing would have to accelerate the air to a much higher downwards velocity than the helicopter's rotor to get the same mass-flow rate as the helicopter.  Therefore, the wind felt under a low-flying plane would be even more intense than that under a helicopter of the same mass.

But this is not the case.  In fact, there is no perceptible downdraft under a low-flying plane, because wings DO NOT use the 3rd law to generate lift.  They use fluid dynamics.  Totally different animal.

So, would anyone care to weigh in on this discussion before I invite the other party (rather than continuing the off topic discussion in another thread), and continue the argument?

 

[mikegarrison]

This is silly.

Propellers are small, rotating wings. This is both obvious and factual.

There are methods of analyzing propellers that ignore the physical nature of propellers (disk actuator theory), but these are convenient approximations. Blade element theory is also valid, and it treats the propeller as individual blades.

As for the whole "how do wings create lift" thing, it's been argued so often that it's total cliche. Most people who argue about it have no clue how wings actually work.

The argument is stupid because if you consider the pressures on the wing, the total sum of all the pressure normal to the skin of the wing times the surface area of that skin will add up to equal the lift. AND the total momentum change of the air that is flowing past the wing will add up to equal the lift. Both ways of looking at the the problem are physically valid, so both ways must always add up to equal each other.

ps. The quoted argument that claims wings don't deflect a significant amount of air down is simply physically wrong.

Some flow vis. Notice how the streaklines bend down and all have some downward velocity after passing by the wing. (The whole flowfield is affected by the wing, not just the part closest to it, because this is subsonic flow.)

 

Edited June 24, 2019 by mikegarrison

[KerikBalm]

46 minutes ago, mikegarrison said:

This is silly.

Propellers are small, rotating wings. This is both obvious and factual.

There are methods of analyzing propellers that ignore the physical nature of propellers (disk actuator theory), but these are convenient approximations. Blade element theory is also valid, and it treats the propeller as individual blades.

As for the whole "how do wings create lift" thing, it's been argued so often that it's total cliche. Most people who argue about it have no clue how wings actually work.

The argument is stupid because if you consider the pressures on the wing, the total sum of all the pressure normal to the skin of the wing times the surface area of that skin will add up to equal the lift. AND the total momentum change of the air that is flowing past the wing will add up to equal the lift. Both ways of looking at the the problem are physically valid, so both ways must always add up to equal each other.

ps. The quoted argument that claims wings don't deflect a significant amount of air down is simply physically wrong.

Well, since no one else has yet chimed in, I'll cease a pretense of not trying to bias the responders.

I am in complete agreement with you:

"Propellers are small, rotating wings" - from the very start, this was the issue, even though it seems obvious to me. He drew a distinction between helicopters and planes, and somehow thinks "rotary wing" for a helicopter is just an imprecise name.

"Both ways of looking at the the problem are physically valid, so both ways must always add up to equal each other." Yup, as  I said "Again...1 and the same, not 2 separate things.

That pressure differential causes air to move down"

And as I said to someone else who weighed in by private message: "for every physical system, newtons 3 laws must be obeyed (ignoring relativistic and quantum effects).

Bernoulli's principle is one way of explaining the force acting on the wing. Often the same effect can be explained multiple ways/there are multiple ways to derive a formula from different starting points.

When the air generates the lift force on the wing, the wing must be generating an equal force on the air. This means that it will be accelerating air downward. 

You can't just throw out 'Bernoulli's principle', and get to ignore laws of physics."

I like that picture. I pointed them towards this site, but I'm not sure they read it:

http://www.aeroman.org/html/newton_s_laws_and_lift_rev.html

 

[Meecrob]

A picture is worth 1000 words, as they say.

Prop cross-section:
https://images.app.goo.gl/zP5DWJqjExyDzmGdA

 

Wing cross section:

https://images.app.goo.gl/uTZeLTXgxxRX81AC8

 

As an aside, one of the reasons the Wright Brothers were so successful is because they were the first to realize that you can get flight-viable thrust to weight ratios out of the turn-of-the-century internal combustion engines if they used small wings as the propellers, as opposed to flat paddles.

 

Edited to add: Perhaps this discussion would benefit from refraining from using the word "lift". All airfoils, when moved in a certain way through a fluid such as air produce "thrust" (which can be further broken down into mass*acceleration). We call this thrust "lift" if it is acting to oppose gravity, but it acts the exact same way, with the same force, no matter which direction it is acting.

Edited June 24, 2019 by Meecrob

[Shpaget]

This is a good place to apply my favorite approach to conceprual thinking. Just go to extremes.

Imagine a huge propeller, infinitely big. At some point the blade of the propeller is traveling in pretty much a straight line - just as a wing does.

Edited June 24, 2019 by Shpaget

[SRB]

AFAIK, KSP just calculates the generated lift using its lift formula, not caring how the lift is made or what direction it goes-lift from a wing/rotor or thrust from a propeller.

[Bill Phil]

(I feel like we've been here before...)

It's been said that even a flat board would have some lift. 

So the question is what causes it?

There's a whole string of theories about it but generally some kind of momentum exchange is the accepted cause. 

Of course propellers aren't strictly identical to wings - they have different geometries. I also suspect that using a simple wing shape for a prop would lead to inefficiencies. However the prop geometry can be optimized for thrust - both are airfoils but they are in different environments. Since it's an object moving in a fluid it likely derives force from similar sources as wings. Newton's 3rd is likely at play at here since force is a product of momentum changing, but just saying that's the case doesn't strictly describe the mechanism by which thrust is produced. For example there are multiple theories about how Newton's 3rd law provides lift.

Edited June 25, 2019 by Bill Phil

[Nightside]

 

23 hours ago, KerikBalm said:

Some unknown person who is not @KerikBalm said: “There are 2 totally separate things going on with REAL props.  Yes, there's a little bit of lift, but the main thing is "paddling". “

And I’m off to KSP to build a turbo-paddlewheeler airplane.

Edited June 25, 2019 by Nightside

[Shpaget]

1 hour ago, Bill Phil said:

Of course propellers aren't strictly identical to wings - they have different geometries. I also suspect that using a simple wing shape for a prop would lead to inefficiencies.

Of course props have variable geometry. At the root the propeller the cord ratio is much higher than at the tip, and the entire aerofoil is twisted. That is to compensate for differences in airspeed near root and near tip of the blade. Using a simple wing geometry for prop would lead to inefficiency because wing is designed for one single airspeed - cruising. Each "slice" of propeller crossection is designed for different airspeed because at the designed standard angular speed of the propeller different parts of the blades travel at different speeds. But there is not fundamental or conceptual difference.

[OHara]

A significant fraction of people, even of those active in aviation, say adamantly that lift happens 'because' of Bernoulli's theorem rather than 'because ' Newton's third law. 
I have tried, but never really understood the distinction they had in their minds (maybe due to indoctrination to the conventional thinking via a Physics PhD).  My mind is used to the idea that physical laws describe, not cause, the behavior of things.

Trying to avoid semantics, I have had success with "whether the cause of lift or not, Newton's third law still holds true, for times when that is the simpler way to understand some aspect of flight."

Gravity pulls a 1-tonne aircraft down with about 10kN of force, so either the aircraft will start accelerating down (falling) 10 m/s², or 5000 kg/s of air will get thrown down at 2 m/s, or some such product to balance 10 kN = 10'000 kg m/s².    An airplane spreads that thrown-down air in a long wake, with a cross section near one wingspan in diameter near the craft, spreading out with time.  A 10-meter wingspan at 50m/s might catch 5000 m³/s of 1kg/m³ air  and throw it down 2m/s.  You certainly feel the wake if you are following a heavy slow airplane too closely, or if you circle tightly in a glider to fly through your own before it spreads too wide.

A hovering helicopter doesn't spread the wake out at all, just throws it straight down.  A 1-tonne helicopter might only grab 500 kg/s of air (a 25m² disk throwing 1kg/m³ air at 20m/s) down at 20m/s all on the same spot, to make the required product.

[KerikBalm]

1 hour ago, Nightside said:

And I’m off to KSP to build a turbo-paddlewheeler airplane.

Please modify your post, you quote me quoting someone else, but present it as if the source was me. If it wasn't clear, I was not the one arguing that position.

I posted a link to this thread in the other thread, I thought the guy would have shown up by now... he was quite adamant that he was right. I was omitting the more stubborn sounding parts like:

"Nope, nope, nope, nope, nope.  You are completely wrong here, I'm afraid.  If you can't see that, then you should go do some more research "

" (heavy sigh).  You've been building rockets too long, my friend.  You can't see the differences between being in vacuum and being immersed in a fluid medium,.  There are more ways to generate useful forces than simply hurling reaction mass astern.  I'm sure you've heard of electromagnetism?  OK, if you believe in that, then you should be open to the possibility that maybe, just maybe, creating a pressure differential in a fluid can also create a force without reaction mass.  You know, kinda how suction cups work?  Lift and suction cups both rely on normal atmospheric pressure trying to restore equilibrium, all without reaction mass.  Open your mind "

but maybe I shouldn't have expected him to show up, when I suggested moving the discussion about lift mechanism here, he replied:

"I see no point in that.  You can't think in any terms but the 3rd Law, even when shown that's not even what's happening here."

34 minutes ago, OHara said:

A significant fraction of people, even of those active in aviation, say adamantly that lift happens 'because' of Bernoulli's theorem rather than 'because ' Newton's third law. 
I have tried, but never really understood the distinction they had in their minds (maybe due to indoctrination to the conventional thinking via a Physics PhD).  My mind is used to the idea that physical laws describe, not cause, the behavior of things.

Trying to avoid semantics, I have had success with "whether the cause of lift or not, Newton's third law still holds true, for times when that is the simpler way to understand some aspect of flight."

Agreed, as I said earlier "When the air generates the lift force on the wing, the wing must be generating an equal force on the air. This means that it will be accelerating air downward. 

You can't just throw out 'Bernoulli's principle', and get to ignore laws of physics." and as the 2nd page of a link I posted says: " In brief, the air bends around the wing producing downwash. Newton’s first law says that the bending of the air requires a force on the air, and Newton’s third law says that there is an equal and opposite force on the wing. That is a description of lift. The pressure difference across the wing is the mechanism in which lift is transferred to the wing due to the bending of the air. " Now that pressure difference related to Bernoulli's principle, but these are just different ways of describing the same physical thing that is happening. In other words, "They are one and the same"

 

[mikegarrison]

28 minutes ago, KerikBalm said:

but maybe I shouldn't have expected him to show up, when I suggested moving the discussion about lift mechanism here, he replied:

"I see no point in that.  You can't think in any terms but the 3rd Law, even when shown that's not even what's happening here."

[Hotel26]

A couple of observations:

• this debate has been going on (heatedly) in aviation/aeronautic circles for decades which ought to be a clear signal to the alert that this is not a black-or-white proposition.
• therefore being "correct" does not mean the other guy is entirely incorrect -- unless all you care about is "being correct".
• Newton's Third Law predates powered flight by quite some time.  It is certainly true that momentum will be conserved, but perhaps that was not enough knowledge for Man to fly?
• Wilbur Wright uttered the metaphor, "a propeller is nothing but a twisted wing", which was a novel idea then to be communicated but it may be more helpful these days to think that both are just airfoils -- similar as airfoils but different in other ways.
• I personally guess that it is probable that you cannot deflect air efficiently (without undue turbulence) without understanding less obvious effects such as Bernoulli's and I wouldn't be at all surprised if a modern aeronautics 4-year program starts with 3 days to verify a student is sufficiently familiar with Newton's Laws and their implications and then spends the rest of the time on fluid dynamics and more.

When the definition of a wing includes rubber-band powered balsa wings you can see that deflection of air is the purpose but when you look at the particular shape of a modern wing (an airfoil) and you ask "well, what makes this an airfoil and not a plank?", you get the first glimmer that what is going on on the way to producing the final result (including displacement of air) is quite a bit more subtle.  (If you don't think this is important in modern aeronautics then consider that some of the first prototype parachutes were made of balsa wood -- and they did not work...)

Being "correct" is fine but not so much fun when you have to accept that you are also being simplistic -- and dating yourself back in a pre-flight era.

Finally, if you don't think there isn't something a bit more surprising going on than simply sticking your hand out of a car's open window, then think about a sailing dinghy tacking upwind.  You might be certain that the sail is somehow deflecting air but you will have little hope of explaining how it does it unless you know[*] a lot more than Newton's Third.  This is where "a propeller is nothing but a twisted wing" (in the 21st) equals a closed mind.

* Thought experiment: predict what happens if you place spoiling devices on the windward side of the sail and then on the leeward and measure the respective differences in performance of the yacht??

Edited June 25, 2019 by Hotel26

[KerikBalm]

41 minutes ago, Hotel26 said:

this debate has been going on (heatedly) in aviation/aeronautic circles for decades which ought to be...

There's a lot of incorrect information circulating around on a lot of topics. Being able to fly a plane doesn't mean you understand the mechanism by which if flies. It means you understand what you need to do, not why you need to.

You yourself posted a link (in the other thread) that used an often repeated, and quite wrong demonstration of Bernoulli's principle:

https://en.wikipedia.org/wiki/Bernoulli's_principle#Misapplications_of_Bernoulli's_principle_in_common_classroom_demonstrations

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being "correct" does not mean the other guy is entirely incorrect

Most of what he was saying was fine, but on the specific point of the debate, there is a clear right or wrong.

The aerodynamic principles of a propellor, a wing, and a rotary wing, are the same. To claim otherwise is just wrong, clear-cut.

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It is certainly true that momentum will be conserved, but perhaps that was not enough knowledge for Man to fly?

Nobody was claiming that it was.

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you cannot deflect air efficiently ... without ... less obvious effects such as Bernoulli's

Nobody was claiming otherwise. As I've said repeatedly, Bernoulli's is just describing a mechanism by which air is deflected.

[Hotel26]

13 minutes ago, KerikBalm said:

Bernoulli's is just describing a mechanism by which air is deflected.

And this is what I think causes the endless debate about either Bernoulli or Newton as the cause for flight; that, in some flight regimes, there are a chain of "causes" and any can be cited even though none can be dispensed with.

 

[KerikBalm]

In every regime, Newton's 3rd law holds. Air is being deflected down. To say that is not the case is to be wrong. Whether or not air is being deflected down because of Bernoulli's principle is irrelevant to the earlier statement.

[wizzlebippi]

https://en.m.wikipedia.org/wiki/Kutta–Joukowski_theorem

[mikegarrison]

3 hours ago, Hotel26 said:

A couple of observations:

• this debate has been going on (heatedly) in aviation/aeronautic circles for decades which ought to be a clear signal to the alert that this is not a black-or-white proposition.

There are still people who say the Earth is flat. The fact that they still debate this does not make it a clear signal that they may have a valid point.

11 minutes ago, wizzlebippi said:

https://en.m.wikipedia.org/wiki/Kutta–Joukowski_theorem

Yes, a mathematically equivalent and convenient way of calculating the lift, but it doesn't change the physics. It's usually harder for people to understand "circulation" than to understand other, more intuitive, ways of describing the situation.

On the other hand, circulation does make it easier to understand why there are vortices that trail behind an airplane in flight.

[Hotel26]

@wizzlebippi Thanks for the citation.

Of note for me, is this:

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Kutta and Joukowski showed that for computing the pressure and lift of a thin airfoil for flow at large Reynolds number and small angle of attack, the flow can be assumed inviscid in the entire region outside the airfoil provided the Kutta condition is imposed. This is known as the potential flow theory and works remarkably well in practice.

and also the distinction about camber vs angle of attack. 

I actually don't know which side this comes down on (and you can annotate that for me), but it does give me the opportunity to elucidate my "in some flight regimes" qualification.  Passengers today spend the great majority of their time airborne at cruise altitude/speed in a regime optimized for very low angle of attack.  At that speed, lots of lift can be produced but little (comparatively) is needed.  (The article I cited in the other thread was criticized for being wrong in one point but it made the point quite well that a small pressure differential over a large wing area can easily generate the required lift for level flight at high speed.)  At the same time, drag at high speed is a killer and the design effort needs to absolutely minimize it, which means eliminate turbulence and minimize any net deflection of the air.  In this regime, if Bernoulli's effect actually exists at all, it may be that it is completely sufficient in and of itself to provide the required lift.

(The reason this is particularly interesting to me can be given by example: I build a twin Panther machine that cannot break the sound barrier and @swjr-swis reworks the machine and it does Mach 2.5.  He's done this enough times now that I fly the result and see that he has made small changes and the machine flies with some discernible inefficiencies in certain regimes, but by the time it has reached its top speed, the angle of attack is Zero and the drag penalty is as low as it can go.)

 

Edited June 25, 2019 by Hotel26

[Phreakish]

4 hours ago, Hotel26 said:

A couple of observations:

• this debate has been going on (heatedly) in aviation/aeronautic circles for decades which ought to be a clear signal to the alert that this is not a black-or-white proposition.
• therefore being "correct" does not mean the other guy is entirely incorrect -- unless all you care about is "being correct".
• Newton's Third Law predates powered flight by quite some time.  It is certainly true that momentum will be conserved, but perhaps that was not enough knowledge for Man to fly?
• Wilbur Wright uttered the metaphor, "a propeller is nothing but a twisted wing", which was a novel idea then to be communicated but it may be more helpful these days to think that both are just airfoils -- similar as airfoils but different in other ways.
• I personally guess that it is probable that you cannot deflect air efficiently (without undue turbulence) without understanding less obvious effects such as Bernoulli's and I wouldn't be at all surprised if a modern aeronautics 4-year program starts with 3 days to verify a student is sufficiently familiar with Newton's Laws and their implications and then spends the rest of the time on fluid dynamics and more.

When the definition of a wing includes rubber-band powered balsa wings you can see that deflection of air is the purpose but when you look at the particular shape of a modern wing (an airfoil) and you ask "well, what makes this an airfoil and not a plank?", you get the first glimmer that what is going on on the way to producing the final result (including displacement of air) is quite a bit more subtle.  (If you don't think this is important in modern aeronautics then consider that some of the first prototype parachutes were made of balsa wood -- and they did not work...)

Being "correct" is fine but not so much fun when you have to accept that you are also being simplistic -- and dating yourself back in a pre-flight era.

Finally, if you don't think there isn't something a bit more surprising going on than simply sticking your hand out of a car's open window, then think about a sailing dinghy tacking upwind.  You might be certain that the sail is somehow deflecting air but you will have little hope of explaining how it does it unless you know[*] a lot more than Newton's Third.  This is where "a propeller is nothing but a twisted wing" (in the 21st) equals a closed mind.

* Thought experiment: predict what happens if you place spoiling devices on the windward side of the sail and then on the leeward and measure the respective differences in performance of the yacht??

The debate 'between' bernoulli and newton with respect to aviation is not a debate. It's a conflict between people of varying degree of knowledge and scientific credentials. 

Wings, propellers, rotors, or anything which moves through a fluid can have it's behavior and performance predicted by physics. The term "bernoulli" gets thrown around, but all Bernoulli taught is that pressure drops linearly with an increase in velocity. That knowledge can be combined with the unwaveringly true 3rd law, which states that momentum must be conserved. 

A flat plate makes lift the same way an airfoil does - when you tilt a flat plate, the stagnation point changes which acts on the fluid (usually air) in such a way as to cause a pressure drop above the surface. This pressure drop is responsible for the creation of the lifting force. It is also responsible for an increase in drag - usually called induced drag.
An airfoil is far more complex a shape, but for good cause - the geometry of an airfoil is intended to streamline the wing area upon which the reduced pressure in the fluid is acting. With the reduced pressure comes an increase in the speed of the fluid flow over the top of the wing. Pressure and Velocity are linked linearly, but momentum goes up with the square of velocity. As a result, the mass moving over the top of the wing has greater momentum.
Since a wing must have a trailing edge, the fluid cleaved by the wing will eventually rejoin - when it does, the fluid which has been acted upon over the top of the wing will have a greater momentum (coming from the work done which is propelling the wing through the fluid). The fluid flow will have a velocity vector with a downward component. If one were to integrate along the length of the wing and determine the magnitude of the momentum exchange, it would agree with the results of "bernoulli's principle". 

Bernoulli just helped develop a way to calculate it without having to take direct measurements of the fluid flow at all points along the chord of an airfoil. 

Edited June 25, 2019 by Phreakish

[Phreakish]

1 hour ago, Hotel26 said:

@wizzlebippi Thanks for the citation.

Of note for me, is this:

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Kutta and Joukowski showed that for computing the pressure and lift of a thin airfoil for flow at large Reynolds number and small angle of attack, the flow can be assumed inviscid in the entire region outside the airfoil provided the Kutta condition is imposed. This is known as the potential flow theory and works remarkably well in practice.

and also the distinction about camber vs angle of attack. 

I actually don't know which side this comes down on (and you can annotate that for me), but it does give me the opportunity to elucidate my "in some flight regimes" qualification.  Passengers today spend the great majority of their time airborne at cruise altitude/speed in a regime optimized for very low angle of attack.  At that speed, lots of lift can be produced but little (comparatively) is needed.  (The article I cited in the other thread was criticized for being wrong in one point but it made the point quite well that a small pressure differential over a large wing area can easily generate the required lift for level flight at high speed.)  At the same time, drag at high speed is a killer and the design effort needs to absolutely minimize it, which means eliminate turbulence and minimize any net deflection of the air.  In this regime, if Bernoulli's effect actually exists at all, it may be that it is completely sufficient in and of itself to provide the required lift.

 

Reynolds number relates to the viscosity of a fluid flow, and increases with scale and speed (high Re means low viscosity). Kutta/Joukowski is good for approximating things  at very high speeds and of very large size (think airliners instead of paper airplanes). Boundary layers are thicker where fluid flow is slower, and thinner where it is faster - Kutta allows a simpler means of determine a 'true' performance analysis of an airfoil by helping approximate an effective shape of a given foil (assuming the analysis parameters hold to all of Kutta's constraints) such that it can be analyzed using inviscid principles. 

In a nutshell, Kutta/Joukowski is typically used for very niche applications such as cruise regimes of large craft. It is based, however, on the very root principles of the physics of flight which can help illustrate the concepts of aerodynamic lift. 

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(The reason this is particularly interesting to me can be given by example: I build a twin Panther machine that cannot break the sound barrier and @swjr-swis reworks the machine and it does Mach 2.5.  He's done this enough times now that I fly the result and see that he has made small changes and the machine flies with some discernible inefficiencies in certain regimes, but by the time it has reached its top speed, the angle of attack is Zero and the drag penalty is as low as it can go.)

One consequence of lift is drag. Induced drag is the component of drag which increases linearly with lift, and the square of velocity. Lift can be determined using Coefficient of lift and velocity, using the lift equation: L=A*Cl*q, where A is the wing area, Cl is the coefficient of lift, and q is dynamic pressure (1/2 *density*velocity-squared). Induced drag can be calculated using the equation: Cdi = (Cl^2) / (pi * AR * e) where Cl is coefficient of lift, pi is... pi (3.1415926....), AR is the aspect ratio and e is an efficiency factor. Now, I'm pretty sure KSP doesn't calculate AR or e, but maybe it has an assumed e and does calculate AR? AR is a ratio of span versus area - longer spans have a higher AR and better efficiency too. This is why airliners and gliders have very long wings.

At any rate, from these equations we can see that an increase in wing area will increase lift without increasing the drag except by the amount of inherent parasitic drag the wing member creates. Parasitic drag is usually orders of magnitude lower than induced drag. The more wing area a craft has, the less angle of attack it needs, and as a result the Cl can remain low. A low Cl means low induced drag.

From what I've observed, KSP doesn't technically calculate induced drag, but drag definitely increases with angle-of attack. What I've found with craft in KSP that have a hard time reaching high speed is that they either: A) lack enough wing wing area to remain aloft in the subsonic range without a significant angle-of-attack and thus create more drag that prevents more speed, B) have a high parts with lots of drag cubes that keeps total drag too high to achieve supersonic performance. Remember that any angle of attack, positive or negative, means the entire fuselage is also exposed to the relative wind (airflow around the craft) which means that the various fuselage parts being to make their own drag. By keeping the angle of attack, of both the wings and fuselage, as low as possible - drag is minimized. As a result, short fuselages with minimal wing tend to do best. Think F22 instead of SR71. Long fuselages can contribute lots of drag unless the wing area and incidence angle (rotation of wing relative the body) are tweaked just-so. 

[Bill Phil]

11 hours ago, Shpaget said:

Of course props have variable geometry. At the root the propeller the cord ratio is much higher than at the tip, and the entire aerofoil is twisted. That is to compensate for differences in airspeed near root and near tip of the blade. Using a simple wing geometry for prop would lead to inefficiency because wing is designed for one single airspeed - cruising. Each "slice" of propeller crossection is designed for different airspeed because at the designed standard angular speed of the propeller different parts of the blades travel at different speeds. But there is not fundamental or conceptual difference.

Both are airfoils optimized for different environments. I believe I mentioned this in my post. 

[mikegarrison]

4 hours ago, Hotel26 said:

I actually don't know which side this comes down on (and you can annotate that for me), but it does give me the opportunity to elucidate my "in some flight regimes" qualification.

When it says "in some flight regimes", think airplanes versus bumblebees. Generally speaking, with regard to the concept of boundary layers and potential flow, all subsonic airplanes are in the same "flight regime". (If you go slow enough, staying below about Mach 0.3, you can simplify your calculations a lot by ignoring the effects of compressibility, but it's still potential flow as long as you isolate the effects of viscosity to a small boundary layer near the object.)

[RCgothic]

Yeah, propellors and wings are both aerofoils. They both create thrust by deflecting quantities of air by Newton's third law. An understanding of Bernoulli just allows you to design shapes that do so efficiently.

There's no special distinction for propellors or turbine blades. They just have twist and chord variation to compensate for the varying effective velocity vector of the incident airflow over the aerofoil due to the circumferential speed of the blade varying from the root to the tip. 

Edited June 25, 2019 by RCgothic

[Hotel26]

Well, on reflection overnight, I'll yield on Bernoulli due to the following in the Wikipedia article itself on Bernoulli:

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 Modern writings agree that both Bernoulli's principle and Newton's laws are relevant and either can be used to correctly describe lift.^[12][27][28]

It is not the Bernoulli principle itself that is questioned because this principle is well established (the airflow above the wing is faster, the question is why it is faster).

An impediment to understanding is that Bernoulli's principle is said to be based on Newton's Second Law whereas action/reaction is the Third Law -- one might think they are independent and separate.  In fluid dynamics with numerous molecules in action, a treatment with the Second Law might be much more tractable, but the Third Law still applies overall.  And thus Bernoulli and Newton III can be interchangeably employed; the implication of which is that air is deflected as a reaction to the action of the wing.

Hmm, one other thing, I think, was an impediment.  The conservation of momentum may have been mentioned along the way.  At first glance, thinking particularly of an airplane in level, un-accelerated flight, its momentum is constant.  However, it is harnessing lift to counteract gravity.  So for this force (under the circumstances which it is generated), some (less visible) reaction ought to be incurred.

In 2003, I started a new job and a new colleague opined that science did not have a full explanation for modern flight and, in particular, Bernoulli wasn't particularly any explanation.  I thought he was nuts, actually, because we know how to fly.  The open question above is whether we do now understand the "why [it works]" well enough to have a complete consensus.  But in any case, Bernoulli seems very restricted.  And maybe that colleague was not nuts, either?

(Thanks to those above who offered reasoned explanations.)

Edited June 26, 2019 by Hotel26