This may not be news to some of the rivet counters that inhabit these parts, but apparently the generally accepted theory of lift (using the bernoulli principle) is a complete fallacy.

Don't take my word for it. NASA says so.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

As I (don't) understand things, it seems that flow turning (is this the same as the coanda effect??) is now considered the accepted theory for how an aerodynamic surface creates lift. Or is NASA going crackers? As a teacher I HATE teaching the wrong thing, and teaching my students the various theories is out of the question.

Cheers

I don't think they're saying that Bernoulli's principle is wrong. Really all Bernoulli's principle says is that increasing the speed of a fluid decrease's its pressure.

I read it as taking issue with texts that claim airfoils generate lift by having the top surface longer than the bottom surface. They're pointing out symmetrical airfoils (same length top and bottom) can generate lift as well as designs where the bottom surface is longer.

Of course, I'm still half asleep right now, so I could be misreading something.

Agreed. The issue here isn't Bernoulli's principle, it's the theory ((longer path/equal transit theory) which uses bernoulli's principle) to explain lift that NASA is debunking. This theory is THE theory that almost everyone learns, it's in just about every sim manual I've read and the number 1 theory carried by websites and encyclopedias.

What I'm interested to know is which theory is now considered to be the 'correct theory'. From a teaching perspective coanda effect is infinitely neater and simpler to teach aswell as being fun to demonstrate.

Cheers

The 'debunking' is a bit of a nit pick. Although the original theory of air travelling faster over the top to meet the air coming from the bottom has been extremely widespread I don't remember specifically learning that when I studied Aeronautical Engineering at college in the 80s. In fact we used a rotating cylinder to represent an aerofoil in most theoretical situations.

The nit pick comes from the fact that they are now saying that the air going over the top doesn't meet up exactly with the air coming over the bottom, so that's not the exact reason that the air goes faster.

Basically the air goes faster over the top because it's got further to go than the air at the bottom, that's still true, they're just saying that it's not racing to catch up with the air on the bottom.

Hope that makes sense, typing in a bit of a hurry

You can continue to teach based on Bernoulli's principle as it remains perfectly valid, just drop the additional assumption of equal transit time on top and bottom (for which there quite simply is no justification). As Ant says, the latter merely provided a convenient, though unscientific, explanation of WHY the flow on the airfoil suction side is faster but has no basis in fact - it does not follow from any established physical principle (including Bernoulli's).

So the air DOES go faster on top and you CAN calculate the correct lift force from the resulting pressure difference according to Bernoulli, but equal transit time is NOT the reason why said difference in velocity manifests.

The use of Bernoulli's theory is inappropriate for explaining the creation of lift, such as why the suction surface is *so* much stronger than the lower surface, AoA effects, assumption of equal transit times, not to mention the lack of viscous effects.

AD, as you mention, it IS the *turning* of air (or rather, accelerating it around the curvature of the aerofoil) which creates lift through changing the surface pressure. If you think back down to Newton's laws, F=ma - by turning the mass of air, you are accelerating it in a different direction, which exerts pressure on the aerofoil surface, resulting in lift.

If it was as easy as applying Bernoulli, we wouldn't need (so many) supercomputers to analyse the aerodynamic design of aircraft.

I have not believed the oficial old theory since i heard it for the first time, some 35 years ago.

But then gases can be compressed and thinned out. Think about it.
The classical form is good for letting the air flow as undisturbed and turbulence-free as possible, and thinning the air at the upper side.
You need energy. "With enough energy, even a barndoor will fly" (Ernst Heinkel)

I don't know why this would be a "new" theory. Wolfgang Langewiesche wrote what makes lift work (Newtonian physics) in Stick and Rudder back in 1944.

What makes the air flow around and then downward on the trailing edge of the wing (and forcing the plane "up" on the airfoil) is aided by Bernoulli, but I always knew the whole "sucking the airplane into the sky" thing didn't make sense long before I read the science on it.

Mathematical physicists had a decent and general formula long before then as well, so none of the underlying knowledge is new at all. I blame media ignorance for this whole Bernoulli thing.

Does this mean my airplane will no longer fly ?