The Topic That Wouldn't Die: Bernoulli's Principle and airfoils

[DaveC426913]

Recently, I was discussing airfoils with a friend. I thought I was being clever and said "No, Bernoulli's Law isn't what's keeping it in the air - that's a fallacy. If it were Bernoulli's Law keeping it in the air, then airplanes would not be able to fly upside down. What keeps a plane in the air when flying upside down is the angle of attack."

He came back with "No, what keeps a plane in the air when flying upside down is the fact that planes that fly upside down are so overpowered that simply pointing the thrust in the desired direction (say, up) will cause the plane to go whereever the engine takes it, wings or no - and despite Bernoulli's Principle."

I had to concede right then and there.


So: can a plane fly upside down using airfoil-generated lift, as opposed to thrust-lift? Evidence?

 

[Mech_Engineer]

A symmetric air foil would be a good example of one that can fly upside down or right side up without a degradation of it's properties. For non-symmetric air foil, all it takes is a small adjustment of the ailerons to give it lift in the upside down direction.

If the plane's lift were all thrust generated, it would be pointing straight up! Angle of attack, ailerons etc. can all explain it however.

There's an old thread about this as well: https://www.physicsforums.com/showthread.php?t=130414

 

[russ_watters]

For non-symmetric air foil, all it takes is a small adjustment of the ailerons to give it lift in the upside down direction.

...or just some angle of attack!

 

[russ_watters]

Recently, I was discussing airfoils with a friend. I thought I was being clever and said "No, Bernoulli's Law isn't what's keeping it in the air - that's a fallacy.

Bernoulli's law is an important part of how airplanes fly - it isn't a fallacy. The fallacy is the 'equal transit time' argument, which in a double-fallacy is often fallaciously connected to Bernoulli's principle. The two are not connected to each other.

 

[RandomGuy88]

A cambered airfoil that is upside down can still produce lift as long as it is at a positive angle of attack. It will likely need a higher angle of attack and will produce more drag but it will still generate lift.

 

[JaredJames]

Well firstly, are all aircraft that can fly upside down "overpowered"?

Secondly, whether it's through thrust or aerofoil, AoA is the key. Level flight, with all the thrust possible and no aerofoil just isn't possible - there's a b*tch called gravity that kicks in - think bullet.

 

[DaveC426913]

Well firstly, are all aircraft that can fly upside down "overpowered"?

That is the question I'm asking, yes.

- there's a b*tch called gravity that kicks in - think bullet.

Um. Thrust can oppose gravity. Think rocket.

Level flight, with all the thrust possible and no aerofoil just isn't possible

'tis. One of the tricks they do at airshows is having an F-15 stand on its tail.


Actually, a more on-point question is this: on an airplane flying upsidedown, does the lift from angle of attack significantly* exceed the downward force generated by the camber of the wing?

If so, AoA is a dominant lifting factor.
If not, then overthrusting is the dominant factor that keeps the plane aloft.


* I'm sure that, at a high enough AoA, the low pressure above the wing will likely exceed the pressure under the wing despite any camber. The question is: is there enough to lift the plane, or is it only enough to help?

 

[JaredJames]

Um. Thrust can oppose gravity. Think rocket.

Um. If all thrust is directly horizontal, there is nothing providing lift so no, it isn't possible. (Well I suppose if we start talking orbital velocity and rockets... but I don't know if aircraft count in that domain.)

'tis. One of the tricks they do at airshows is having an F-15 stand on its tail.

Is that considered level flight? I'd think the attitude indicator and artificial horizon would disagree and say you've got one hell of an angle of attack. But that's up to you.

Perhaps I should have been clearer. When I said "level flight" I was referring to a trajectory with 0 AoA (just to make sure I'm perfectly clear here, AoA in relation to the horizontal).

 

[JaredJames]

Actually, a more on-point question is this: on an airplane flying upsidedown, does the lift from angle of attack significantly* exceed the downward force generated by the camber of the wing?

If so, AoA is a dominant lifting factor.
If not, then overthrusting is the dominant factor that keeps the plane aloft.

This is easily answered by checking if there's an aircraft that can fly upside down that isn't overpowered (I'm having a look).

 

[russ_watters]

Well firstly, are all aircraft that can fly upside down "overpowered"?

Well what does that even mean? What does one thing have to do with the other? The F-16 has a ridiculously high thrust to weight ratio - does that make it "overpowered"? That ridiculously thrust to weight ratio has nothing at all to do with what makes it good at inverted flight: what makes it good at inverted flight is its symmetrical airfoil.

An airplane with a cambered airfoil will perform worse upside-down than right-side-up. How much worse depends on the plane. How big of an impact the engine thrust has, both on the ability to overcome the higher drag of the higher angle of attack and its direct contribution to the lift depends on the plane. But only fighter jets, stunt planes and models are capable of getting the vast majority of their lift from their engines and standing on their tails.

 

[russ_watters]

Actually, a more on-point question is this: on an airplane flying upsidedown, does the lift from angle of attack significantly* exceed the downward force generated by the camber of the wing?

If so, AoA is a dominant lifting factor.
If not, then overthrusting is the dominant factor that keeps the plane aloft.

That's a nonsensical question: a wing either produces positive lift or negative lift, not both at the same time. Trying to separate what the top and bottom surface of the wing does is pointless because even in normal level flight, the bottom surface typically doesn't make a positive contribution to the lift unless the angle of attack is very high and/or the airfoil very cambered.

What actually matters is lift to drag ratio. An inverted cambered airfoil needs a higher angle of attack than a symmetrical airfoil (or an upright cambered airfoil) in order to produce lift and with that higher angle of attack comes higher drag. Higher thrust is required to counter that higher drag and keep the plane moving forward.

But again, that's still different than saying that the engine itself is supporting the plane. I'd say that in neither case (even in the "overpowered" case) is the engine itself providing a substantial fraction of the lift. An F-16 can stand on its tail with a 90 degree angle of attack, but at a 20 degree angle of attack (inverted), the engine isn't providing anywhere close to enough lift to keep the plane in the air (roughly 1/3 of what is required), but it flies just fine. The wings provide the lift. Your friend is still wrong.

* I'm sure that, at a high enough AoA, the low pressure above the wing will likely exceed the pressure under the wing despite any camber. The question is: is there enough to lift the plane, or is it only enough to help?

For most planes flying upside-down at moderate angle of attack, the wings provide the vast majority of the lift to make them fly. I'd even say that a plane standing on its tail strains the definition of "fly" for an airplane - as you said, a rocket stands on its tail. Right: not an airplane.

 

[JaredJames]

Well what does that even mean?

I've actually been considering that. My first thought was when supplying something with 'excessive thrust'. But then, what is excessive? Those aircraft are designed with it so it's not excessive, it's perfectly normal.

 

[russ_watters]

A possibly even bigger and again separate issue is other design features that may prevent inverted flight such as fuel flow control and structural integrity. I remember from flying WWII flight simulators that some of the airplanes would fly just fine inverted...until the engine cut out after a few seconds due to failure of the fuel supply!

 

[russ_watters]

I've actually been considering that. My first thought was when supplying something with 'excessive thrust'. But then, what is excessive? Those aircraft are designed with it so it's not excessive, it's perfectly normal.

Dave's friend seems to define "excessive" as the engine providing amost all of the thrust required for inverted flight. That's what's required for an engine to take an object where-ever it wants, wings or not.

 

[Q_Goest]

He came back with "No, what keeps a plane in the air when flying upside down is the fact that planes that fly upside down are so overpowered that simply pointing the thrust in the desired direction (say, up) will cause the plane to go whereever the engine takes it, wings or no ...

That doesn't make sense. Draw a freebody diagram of the plane. In order for this to be true, the weight of the aircraft (assuming wings are not contributing to upward force) has to be balanced by the verticle component of thrust produced by the engine. When I see aircraft flying upside down, the angle of attack is relatively slight, say much less than 45^o from horizontal, probably more like 10 to 25 degrees. So now you can calculate the thrust the engine must produce by assuming the verticle thrust is equal to the weight.

T_e = F_v / sin (A)
Where
T_e = engine thrust or force produced by the engine in the direction the aircraft is heading
F_v = Verticle force produced by the engine's thrust and is equal to the aircraft weight assuming the wings don't produce any upward force
sin(A) is the angle from horizontal that the aircraft's axis of thrust is pointed, say on the order of 10 to 45 degrees.

If we have a 5000 pound aircraft flying inverted at an angle of 15 degrees to the horizontal, the thrust produced by the engine has to be a whopping 19,300 pounds! This aircraft could easily accelerate straight up like a rocket. In that case, you'd be pinned to your seat with nearly 4 G's of force flying straight up. So no, there MUST be some upward force produced by the wings in any aircraft flying inverted.

 

[minger]

T_e = F_v / sin (A)
Where
T_e = engine thrust or force produced by the engine in the direction the aircraft is heading
F_v = Verticle force produced by the engine's thrust and is equal to the aircraft weight assuming the wings don't produce any upward force
sin(A) is the angle from horizontal that the aircraft's axis of thrust is pointed, say on the order of 10 to 45 degrees.

If we have a 5000 pound aircraft flying inverted at an angle of 15 degrees to the horizontal, the thrust produced by the engine has to be a whopping 19,300 pounds! This aircraft could easily accelerate straight up like a rocket. In that case, you'd be pinned to your seat with nearly 4 G's of force flying straight up. So no, there MUST be some upward force produced by the wings in any aircraft flying inverted.

Seen this topic at lunch and didn't get a chance to respond. . . this is what I was planning on typing though.

 

[DaveC426913]

Um. If all thrust is directly horizontal, there is nothing providing lift so no, it isn't possible. (Well I suppose if we start talking orbital velocity and rockets... but I don't know if aircraft count in that domain.)


Is that considered level flight? I'd think the attitude indicator and artificial horizon would disagree and say you've got one hell of an angle of attack. But that's up to you.

Perhaps I should have been clearer. When I said "level flight" I was referring to a trajectory with 0 AoA (just to make sure I'm perfectly clear here, AoA in relation to the horizontal).

I didn't say anything about thrust being horizontal. All of this is based on your addition of that factor.

That's a nonsensical question: a wing either produces positive lift or negative lift, not both at the same time.

It's prefectly sensical. The contribution of the components can be separated for the purpose of comparing what is doing the contributing, even if it's abstract. If I have a single wing where 3/4ths of it has a +ive AoA, and 1/4 of the wing has a -ive AoA, I can certainly break its components out to see how they contribute to the overall lift.

If we have a 5000 pound aircraft flying inverted at an angle of 15 degrees to the horizontal, the thrust produced by the engine has to be a whopping 19,300 pounds! This aircraft could easily accelerate straight up like a rocket. In that case, you'd be pinned to your seat with nearly 4 G's of force flying straight up.

Thus, overpowered.

So no, there MUST be some upward force produced by the wings in any aircraft flying inverted.

OK, so you're saying that

1] if we were to remove the wings from the plane, in order for the plane to remain in the air at a 15 degree angle from horizontal, it would have to have so much thrust that, should it turn vertical, that much thrust could accelerate vertically at 4gs.

2] Since no plane can do 4gs straight up, that means no plane could keep itself in horizontal motion simply with thrust angled up at 15 degees.

3] Since real planes do maintain horizontal motion while angled only 15 degrees up, the only way this is possible is if the (inverted) wings are providing the bulk of the lift.

Hm.

 

[JaredJames]

I didn't say anything about thrust being horizontal. All of this is based on your addition of that factor.

I said "level flight". I was trying to imply the aircraft was horizontal with the thrust directly horizontal. I obviously wasn't clear on that and clarified it later (a lot). I do apologise for my hazy description.

 

[JaredJames]

It's prefectly sensical. The contribution of the components can be separated for the purpose of comparing what is doing the contributing, even if it's abstract. If I have a single wing where 3/4ths of it has a +ive AoA, and 1/4 of the wing has a -ive AoA, I can certainly break its components out to see how they contribute to the overall lift.

Why break it down though? It's either producing lift in the required direction or it's not. If it's not, then the engine is obviously the supporting device.

Thus, overpowered.

Only overpowered if it isn't designed to have that much power. If I design an aircraft to put out that much thrust, for some reason, then it isn't overpowered.

 

[DaveC426913]

Why break it down though? It's either producing lift in the required direction or it's not. If it's not, then the engine is obviously the supporting device.

Because the claim is that an inverted wing is producing lift. I wanted to examine the components to see what's doing that.

Only overpowered if it isn't designed to have that much power. If I design an aircraft to put out that much thrust, for some reason, then it isn't overpowered.

I am not using a strict definition of overpowered. I am simply saying the plane will go where the engine pulls it, wings or no.

 

[RandomGuy88]

Because the claim is that an inverted wing is producing lift. I wanted to examine the components to see what's doing that.

An inverted wing can produce enough lift to lift an aircraft. It just won't be as efficient.

 

[DaveC426913]

An inverted wing can produce enough lift to lift an aircraft. It just won't be as efficient.

Please back up your claim. I can't very well claim that I've made my case because some random guy said so.

 

[AlephZero]

Recently, I was discussing airfoils with a friend. I thought I was being clever and said "No, Bernoulli's Law isn't what's keeping it in the air - that's a fallacy."

I suspect you are both misunderstanding what "Bernoulli's Law" actually is. Certainly your friend is misunderstanding it.

It is misleading to call it a "law" or even a "principle". Bernoulli's EQUATION is simply a re-statement of Newton's third law of motion for a fluid, using pressure and density as variables instead of force and mass. The is no new "principle" involved in it. It's doesn't say anything specifically about airfioils.

He came back with "No, what keeps a plane in the air when flying upside down is the fact that planes that fly upside down are so overpowered that simply pointing the thrust in the desired direction (say, up) will cause the plane to go whereever the engine takes it, wings or no - and despite Bernoulli's Principle."

I had to concede right then and there.

Just take him to an airshow and listen to some propellor driven planes flying straight, level, at constant speed, and inverted. The engine will be making the same amount of noise as when they fly straight, level, at constant speed, and non-inverted. So the engine power is about the same either way up.

Even better, watch a sailplane doing aerobatics. They can fly inverted just as well as non-inverted, with no engine at all.

 

[Mech_Engineer]

Here's a good example of the same airfoil being utilized right-side up and upside-down at the same time. Looks to me like airfoil's generating lift in both directions (small aileron and AoA changes), so the OP question has been answered. Thrust is definitely not the only thing keeping these planes in the air.

 

[Q_Goest]

OK, so you're saying that

1] if we were to remove the wings from the plane, in order for the plane to remain in the air at a 15 degree angle from horizontal, it would have to have so much thrust that, should it turn vertical, that much thrust could accelerate vertically at 4gs.

2] Since no plane can do 4gs straight up, that means no plane could keep itself in horizontal motion simply with thrust angled up at 15 degees.

3] Since real planes do maintain horizontal motion while angled only 15 degrees up, the only way this is possible is if the (inverted) wings are providing the bulk of the lift.

Hm.

Yes. But the 15 degrees is just an example. Plug the numbers into the equation and you find that to maximize the amount of verticle force produced by the engine, the aircraft has to be turned verticle. The verticle component of "lift" produced by the engine alone (without wings) is a function of the sine of the angle. Knowing that only military aircraft have a thrust that might exceed the weight, and that typical acrobatic aircraft have a thrust that is far less than the weight, it is impossible for any aircraft that doesn't have a thrust to weight ratio greater than 1 to fly inverted without a wing. For military aircraft with a thrust to weight ratio of slightly more than 1, such as the http://en.wikipedia.org/wiki/Thrust-to-weight_ratio" , that aircraft couldn't fly at less than a 66^o angle from horizontal without having some contribution of lift from the wings. We've all seen F-16's at air shows such as shown in the photo above, flying aproximately level. Therefore, one can rest assured that the math is telling us that the vast majority of the lift is being produced by the wings.

 

[russ_watters]

Here's a lift calculator you can play with, Dave: http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html

The graph at the lower right shows the pressure profiles along the upper and lower surfaces, though it doesn't integrate for lift per surface, just total lift.

Try these values (all others default):
Camber: 5.0
Thickness: 12.5

Then picking a few AoA values yeilds the following:

AoA: ... Lift: ... Drag:... L/D
-10.1 .. -1506 .. 166 ... -9.0
-10 ... -1484 .. 163 ... -9.1
-8 ... -902 ... 86 .. -10.4
-6 ... -298 ... 43 ... -6.9
-4 ... 324 ... 35 ... 9.0
-2 ... 928 ... 63 ... 14.6
0 ... 1508 ... 122 ... 12.3
2 ... 2065 ... 209 ... 9.9
4 ... 2601 ... 319 ... 8.1

Lets say your aircraft weighs 1508 lb (this is all arbitrary, of course). At 0 degrees AoA, it can fly straight and level at whatever speed that applet is using, providing lift equal to its weight. It also produces 122lb of drag, for an L/D ratio of 12.3:1. To fly straight and level, inverted, requires -10.1 degrees of angle of attack and produces 166lb of drag (36% more) for a ratio of 9.0:1. So that's a pretty substantial difference in angle of attack required for inverted vs upright flight and somewhat substantial difference in drag, but at 10 degrees, the engine still doesn't contribute much to the lift and at 36% more drag, no plane would have trouble maintaining speed (otherwise it would never be able to fly with it's gear down!).

 

[DaveC426913]

It is misleading to call it a "law" or even a "principle". Bernoulli's EQUATION is simply a re-statement of Newton's third law of motion for a fluid, using pressure and density as variables instead of force and mass. The is no new "principle" involved in it. It's doesn't say anything specifically about airfioils.

Typing Bernoulli into Google autocompletes with Bernoulli's Principle, the first hit of which is the Wiki page of the same name, sooo... take it up with google and Wiki.

Just take him to an airshow and listen to some propellor driven planes flying straight, level, at constant speed, and inverted. The engine will be making the same amount of noise as when they fly straight, level, at constant speed, and non-inverted. So the engine power is about the same either way up.

Well, if that were true, and if it were easy to confirm all those parameters - including engine speed - I'd agree. But I am very dubious about how reliable this is as a test. Too dubious to consider it a valid experiment.

 

[DaveC426913]

Try these values (all others default):
Camber: 5.0
Thickness: 12.5

Then picking a few AoA values yeilds the following:

AoA: ... Lift: ... Drag:... L/D
-10.1 .. -1506 .. 166 ... -9.0
-10 ... -1484 .. 163 ... -9.1
-8 ... -902 ... 86 .. -10.4
-6 ... -298 ... 43 ... -6.9
-4 ... 324 ... 35 ... 9.0
-2 ... 928 ... 63 ... 14.6
0 ... 1508 ... 122 ... 12.3
2 ... 2065 ... 209 ... 9.9
4 ... 2601 ... 319 ... 8.1

Lets say your aircraft weighs 1508 lb (this is all arbitrary, of course). At 0 degrees AoA, it can fly straight and level at whatever speed that applet is using, providing lift equal to its weight. It also produces 122lb of drag, for an L/D ratio of 12.3:1. To fly straight and level, inverted, requires -10.1 degrees of angle of attack and produces 166lb of drag (36% more) for a ratio of 9.0:1. So that's a pretty substantial difference in angle of attack required for inverted vs upright flight and somewhat substantial difference in drag, but at 10 degrees, the engine still doesn't contribute much to the lift and at 36% more drag, no plane would have trouble maintaining speed (otherwise it would never be able to fly with it's gear down!).

I haven't downloaded the app to run it but those numbers confuse me - probably because I don't really know a lot about aerodynamics.

Why would an AoA of zero allow a plane to fly straight and level? I thought AoA always had to be +ive. Is that lift just due to the camber?

Also, why does L/D go +ive while AoA is still -5, and why does L/D max out when AoA is still -ive?
I think I see. Because the wing has camber, it means that lift can begin increasing before drag does (since it's at AoA = 0).

Seems to me, the conclusion to be had is that, to get the best L/D ratio, a plane's wings should actual have a slight negative AoA.


Finally, I still see how to the chart helps undetrstand inverted flight.

Maybe if I play with the app...

 

[boneh3ad]

Typing Bernoulli into Google autocompletes with Bernoulli's Principle, the first hit of which is the wiki page of the same name.

Wikipedia has a lot of dumb crap on it. It isn't really a valid source all the time. Still, I would contend that Bernoulli's Principle is a perfectly valid way to describe it, but it is not common to see it referred to that way in text or by aerodynamicists as it is misleading; in some ways, it really is a misnomer. Most aerodynamicists just call it Bernoulli's equation since it is, after all, just an equation relating pressure and velocity for incompressible, steady flows.

The reality is that Bernoulli's Principle is not what governs lift in any case, per se. It is just a mathematical tool that can be used to show the differential pressure on the top and bottom of the airfoil, which is the most simple way to explain lift. Of course, these differing pressures are related to differing velocities by Bernoulli.

The different velocities themselves are a much more complicated issue and arise from a combination of camber, angle of attack and a sharp trailing edge. The sharp trailing edge is quite important, as there is a mathematical and physical singularity at that point if certain conditions are not met. That condition, known as the Kutta condition, is that the trailing edge stagnation point (also a misnomer) is always at that sharp trailing edge, otherwise an infinite (or near infinite) velocity would be required.

So, with the rear velocity fixed in such a way, as you increase angle of attack up until the point of separation, the upper air moves faster to satisfy the governing equations while still maintaining that same trailing edge stagnation point. In this way, you can fly upside down with certain airfoils and, with a combination of aileron and angle of attack, counteract any of the negative lift you would be getting through the shape of the airfoil. Of course, as mentioned before, this comes with serious drag penalties and performance issues, which is why you only see it in air shows.

 

[DaveC426913]

This pic is actually pretty much impossible to refute. Because we can see both craft, we know the inverted one is indeed in virtually level flight, and its thrust is also virtually level. No amount of overpowering from that engine could possibly keep that jet flying parallel with the lower jet.

The craft must be getting lift via its wings.

 

[AlephZero]

Well, if that were true, and if it were easy to confirm all those parameters - including engine speed - I'd agree. But I am very dubious about how reliable this is as a test. Too dubious to consider it a valid experiment.

It's easy to confirm the engine power of a sailplane

There is a sailplane aerobatics duo in the UK who regularly display at airshows. I (and tens of thousands of other people) have seen them fly 360 degree vertical loops exactly the same way as powered aircraft (except a bit slower, and a lot quieter). By your friend's logic (and your scepticism), woudn't they drop like a stone when they were upside down at the top of the loop, with no engine thrust to keep them up there?

 

[JaredJames]

There is a sailplane aerobatics duo in the UK who regularly display at airshows. I (and tens of thousands of other people) have seen them fly 360 degree vertical loops exactly the same way as powered aircraft (except a bit slower, and a lot quieter). By your friend's logic (and your scepticism), woudn't they drop like a stone when they were upside down at the top of the loop, with no engine thrust to keep them up there?

That's not quite the same as flying in a straight line upside down in a glider.

Your vertical velocity hits zero at the top of the loop and your horizontal velocity is constantly dropping rather sharply. If you don't pitch down and complete the loop, yes, you will drop like a stone.

 

[AlephZero]

This pic is actually pretty much impossible to refute.
...
The craft must be getting lift via its wings.

Actually it's quite easy to "refute" that picture. This is the USAF "Thunderbirds" display team. If you look closely at the inverted aircraft, you will see the number "5" on the engine intake is also inverted (i.e. it is the right way up, in the picture). As the display commentators usually mention, that is because #5 spends pretty much the entire display flying upside down.

So prove to me that #5 isn't a specially built aircraft that is designed to ONLY fly upside down, and the "#5" that you see taking off and landing at the start of the display is just an "identical" decoy, to fool you. It is obvious that they wouldn't show you the secret military technology that let's the "real" #5 take off and land upside down, if that is the only way it can fly

(Or, there is a simpler explanation: both aircraft were actually flying vertically upwards, and somebody rotated the picture 90 degrees in photoshop).

 

[JaredJames]

and the "#5" that you see taking off and landing at the start of the display is just an "identical" decoy, to fool you. It is obvious that they wouldn't show you the secret military technology that let's the "real" #5 take off and land upside down, if that is the only way it can fly

A good point about number 5, but then surely it wouldn't fly so well right way up?

Let's stay in the realms of reality shall we.

 

[boneh3ad]

A good point about number 5, but then surely it wouldn't fly so well right way up?

Let's stay in the realms of reality shall we.

I think his point is that it is not irrefutable, and on that he is absolutely right. It could easily be photoshopped (though I think we all know that it wasn't).

 

[Mech_Engineer]

Here, this picture is twice as irrefutable

Another good one:

 

[russ_watters]

I haven't downloaded the app to run it but those numbers confuse me - probably because I don't really know a lot about aerodynamics.

Just click the link - it runs in a browser.

Why would an AoA of zero allow a plane to fly straight and level? I thought AoA always had to be +ive. Is that lift just due to the camber?

Yes - AoA is typically meausured geometrically, with the chord line going from the leading edge to the trailing edge. A cambered airfoil produces lift at zero (even slightly negative!) AoA.

Also, why does L/D go +ive while AoA is still -5, and why does L/D max out when AoA is still -ive?

L/D is simply the ratio of lift to drag. If lift is positive, L/D is positive. A cambered airfoil can produce lift at negative AoA and the drag remains low because it's still not disturbing the air much with a pressure wake...

I think I see. Because the wing has camber, it means that lift can begin increasing before drag does (since it's at AoA = 0).

Yes.

Seems to me, the conclusion to be had is that, to get the best L/D ratio, a plane's wings should actual have a slight negative AoA.

Yes, and it may well be that a plane with this L/D flies at cruise speed at negative AoA - for efficient cruise flight, you'd want to design it that way. But note also that lift is a function of speed, so as speed drops, lift drops, so the AoA has to rise to keep the plane aloft.

 

[russ_watters]

A good point about number 5, but then surely it wouldn't fly so well right way up?

Let's stay in the realms of reality shall we.

Yes, please. Have you people never been to an airshow?

Also, something to note from that first Blue Angels pic (looking back, it is in the F-16 pic too...): it may just be an illusion of perspective, but the missile rail on plane 3 is clearly pointing slightly down while the missile rail on #1 is pointed up, with a greater AoA. This would fit with a slightly cambered airfoil similar to the trial I just ran: AoA of -2 for the right-side-up plane produces the same lift as +8 for the inverted plane.

 

[DaveC426913]

Actually it's quite easy to "refute" that picture. This is the USAF "Thunderbirds" display team. If you look closely at the inverted aircraft, you will see the number "5" on the engine intake is also inverted (i.e. it is the right way up, in the picture). As the display commentators usually mention, that is because #5 spends pretty much the entire display flying upside down.

So prove to me that #5 isn't a specially built aircraft that is designed to ONLY fly upside down, and the "#5" that you see taking off and landing at the start of the display is just an "identical" decoy, to fool you. It is obvious that they wouldn't show you the secret military technology that let's the "real" #5 take off and land upside down, if that is the only way it can fly

(Or, there is a simpler explanation: both aircraft were actually flying vertically upwards, and somebody rotated the picture 90 degrees in photoshop).

I'm pretty sure you're goofing around. But in case you're not, what you propose isn't really a refutation. Your suggestion is less plausible and requires more hoops to jump through to make sense than merely the assumption that the pic is accurate.

By analogy, we don't assume a signature on a document has been be forged unless there's good reason to think so.

Also, something to note from that first Blue Angels pic (looking back, it is in the F-16 pic too...): it may just be an illusion of perspective, but the missile rail on plane 3 is clearly pointing slightly down

I noticed that in the first pic, yes.

 

[Q_Goest]

Here, this picture is twice as irrefutable

We all KNOW those pictures are rotated 90^o!

 

[viscousflow]

I know I'm a late jumper but when designing an aircraft one must account for negative lift load factor in the event that an aircraft flies upside down whether the airfoil is cambered or not. This is mandated by the FARs. So as stated before the power of the aircraft really has nothing special to do with it. The engines simply need to maintain the required velocity to keep the aircraft aloft. C_L = \frac{L}{.5\rho V^2S} Where, \rho = density and V = relative wind velocity L=lifting force, S= reference area. (reference Russ' table on page 2)

 

[AlephZero]

I'm pretty sure you're goofing around.

Indeed. That's what the "big grin" was for. I've already learned the hard way that humo(u)r isn't always an international language, though.

 

[JaredJames]

Well, good humour is.

 

[DaveC426913]

Indeed. That's what the "big grin" was for. I've already learned the hard way that humo(u)r isn't always an international language, though.

I know what you mean. I too have learned that deadpan humour does not translate so well to the intertoobs.

 

[murphyr]

Look up the Coanda Effect. It is named after Nicholas Coanda, a Romanian Physicist from the early 30's.This is the best explanation I have ever seen and the one that NASA likes the best. If you have a stream of water, like from a faucet and you put a cylinder in the stream such that the water strikes the side of the cylinder; the water doesn't just bounce off. The water tends to stick to the side of the cylinder, following it around almost to the bottom of the cylinder.


Air is considered a fluid and behaves in the same manner. With respect to a wing, the air contacts the leading edge and follows the shape of the foil and continues downward as it leaves the trailing edge. Obviously, a steeper angle of attack and lowering of flaps causes the air to be forced downward at a much steeper angle, producing even greater lift but at the cost of increased drag. Therefore, it is Newton's Third Law, for every action there is an equal and opposite reaction, that creates lift.

The same can be said of the bottom of the wing. It, too, generates lift by directing air downward as do both the front and back sides of a sail.I am no Physics Guru, I'm just a curios PE teacher who flys R/C planes and this explanation makes sense.

It is also the same reason sailboats sail in directions other than directly down wind. The mass of air is redirected behind the boat creating an equal and opposite forward force.

Bernouli's Principal is why curve balls curve but not why airplanes fly.

 

[boneh3ad]

The Coanda effect only explains how the flow stays attached to the wing, not why it generates lift. Yes it comes into play and it is what keeps the flow moving along the wing instead of just separating instantly, but it doesn't in any way explain why there is lift. In fact, if you think about the Coanda effect and ignore the Kutta condition, you will end up with an impossible situation.

With respect to a wing, the air contacts the leading edge and follows the shape of the foil and continues downward as it leaves the trailing edge. Obviously, a steeper angle of attack and lowering of flaps causes the air to be forced downward at a much steeper angle, producing even greater lift but at the cost of increased drag. Therefore, it is Newton's Third Law, for every action there is an equal and opposite reaction, that creates lift.

This isn't true. This works if you blow over a piece of paper, but it is only true because of the fact that there is no fluid motion on the bottom and so when the air leaves the trailing edge of the paper, there is nothing to stop it from going downward like it wants. With an airfoil, there is fluid motion on the top and bottom of the foil, and the air is not directed downward like you describe.

Bernouli's Principal is why curve balls curve but not why airplanes fly.

Curve balls curve because there is a velocity difference on each side of the ball, producing a pressure differential and thus a net force. You can relate this velocity difference to the pressure difference through Bernoulli's principle. This is, in fact, still true on an airfoil. The difference (and difficult part) is explaining just why the air above the airfoil moves faster than the air below it. I already described that earlier.