[twirligigue]

As a child I used to stick a school ruler out of the back window of the car and rotate it slightly to make it move upwards, like a plane's wing. Intuitively I felt that this happened because it was pushing some of the horizontal airflow downwards and the air was pushing back up on the ruler. Yet the books I read about aeroplanes referred to something called Bernoulli's principle which was pretty demoralising because I couldn't understand it.

[lowkey]

I suspect, like many other things that didn’t make sense - the reason was that it wasn’t actually true.

The Bernoulli effect explains that lift is due to the design of the wing such that the path above the wing is longer than the path below the wing.

This coupled with the fact that due to the Bernoulli effect an air particle just above the wing would reach the back of the wing at the same time as an air particle just below, and that since the upper particle would therefore have to travel faster than the lower particle the pressure differential would cause lift.

The problem is the theory doesn’t hold up under testing because it isn’t true.

[adtac]

Isn't Bernoulli's principle only applicable when talking about the same flow? I've always found the "above path is longer than the lower path" explanation to be unintuitive because we're not talking about the same flow. They're separate flows.

[lowkey]

That may be the correct answer here. The important point to note is that there is no physical reason why the two separate upper and lower streamlines would collude to arrive at the back of the wing at the same time and in fact they do not.

[Serow225]

Bernoulli's principle, the actual thing, has very strict criteria* to be applicable. People usually neglect this entirely in casually throwing the term around.

- points 1 and 2 lie on a streamline,

- the fluid has constant density (note effects of height difference > gravitational potential energy between point 1 and 2),

- the flow is steady, and

- there is no friction.

[drwiggly]

Is it possible to think of.. the roundness on the front disrupting the airflow over the top causing air to become turbulent and less dense on the top. Where as the air flow under the wing high higher relative density and the wing will rise to the less dense position?