Airflow clarification- there's a lesson here

Comments on Cecil's Columns/Staff Reports
#1

In Cecil’s answer to how stunt planes fly upside-down, he mentions potential lift from two sources: angle of attack (explains upside-down flying) and the Bernoulli effect (conventional airfoil lift).

I was surprised to learn–and perhaps you will be too–that conventional airfoil lift was a point of controversy. While Cecil’s use of the term “Bernoulli effect” does not imply endorsement of incorrect assertions used to justify this effect in analyzing lift, I think use of the term can be misleading to minds of much lower calibre than the Master’s (which would include probably 99.99999% of the population, i.e. everyone not named Cecil Adams).

Aerodynamic lift is a force caused by the turning of an airflow; if air is deflected in one direction, by Newton’s third law the deflector is pushed by an equal and opposite force. This quite easily explains the angle-of-attack contribution to lift (air molecules bounce downward off the wing, pushing the wing upward), but what many fail to realize is Newton’s third law also applies to the air passing over the top of the airfoil. As the airflow bends around the top of the airfoil, the downward force that causes this bending must be balanced by an equal and opposite upward force on the wing. The Bernoulli effect explains why this airflow bending takes place, based on the relationship between velocity and pressure.

I belabor this point not to lecture on aerodynamics (there are many here more qualified to do so), but to illustrate the dangers of oversimplification. Both the Bernoulli effect and an understanding of Newton’s laws are necessary to explain lift (which is itself better explained by understanding the Navier-Stokes equations, but we can’t all be versed in vector calculus, can we?). Ignoring one leads to assumptions (e.g. air goes faster over the top of the wing so that it can catch up to the air below the wing) that are flat-out wrong. The controversy is akin to arguing which law of physics we intend to obey today, when in fact all reality is absolutely and permanently subject to them.

#2

I wrote a Staff Report last year which addressed this very controversy. Of course, it’s no surprise that it was subject to its own controversy, in this thread.

The basic idea is that Bernoulli isn’t wrong, it’s just not good as a predictive tool. The real cause of lift can be understood by working the physics from the ground up…it’s just really hard.

All the different ways of describing lift, from simplified “equal-and-opposite reaction” models to the complete application of the Navier-Stokes equations, are all expressions of the same underlying physics. The thread I linked to included a lot of debate about flow-turning vs. pressure distributions. And as I said in my article, they’re all manifestations of the same F = ma physics we all know and love. To have one is to have the other, and it’s just a matter

You are correct about the dangers of oversimplification. People tend to isolate one of the noticeable aerodynamic behaviors (be it flow turning, pressure changes, etc) and tend to call it a cause. It then leads into half-assed wannabe theories which get built on that tunnel vision, but without any basis in reality…like Equal Transit Times (see the article).

#3

That was supposed to read, “To have one is to have the other, and it’s just a matter of what you choose to pay attention to.”

#4

I had a different problem with the article.

Drag is bad, and stall is bad, but too much drag does not cause stall.

#5

And, indeed, a stall does not cause a plane to drop like a rock.