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

[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.