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I'm trying to explain Bernoulli's principle (in the context of flight) to an audience that doesn't have a physics background. So I'd like to avoid mentioning technical concepts as much as possible.

The part I'm specifically struggling to explain is why an increase in air velocity necessarily results in a decrease in air pressure. I could go down the path of explaining that an increase in velocity results in an increase in dynamic pressure which, due to the conservation of energy, must be met by a corresponding decrease in static pressure. But I'm looking for a more practical explanation that is easier to conceive.

So instead, I want to say the following:

The air over the wing travels faster than the air under the wing (I have already explained why this must be the case - Continuity Equation). There are two consequences of this:

  1. The molecules flowing over the wing hit the wing at a shallower angle since they have more forward motion. Since the pressure exerted by a fluid on a container is basically the sum of the forces by all the small collisions of the fluid's particles, this results in less pressure being exerted on the top of the wing.
  2. The molecules spend less time over the top of the wing (because they're traveling faster) and therefore have less opportunities to strike the upper surface of the wing. For the same reasoning above, this results in a less pressure being exerted on the top of the wing.

To play devil's advocate, someone might say "Yes, but you're missing one point which is that the molecules traveling above the wing are traveling faster. So while they may hit the wing at a shallower angle and less frequently, when they do hit the wing, they will hit harder and therefore exert more pressure".

So two questions:

  1. How do I address that counter argument?
  2. Are there any glaring holes or flaws in my overall explanation?
faridghar
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  • You can also add some points from the answers in this question https://physics.stackexchange.com/q/290/243162 – Jdeep Jul 20 '20 at 13:30
  • Invoking the BP to explain flight is an old canard as the link in the previous comment shows. So I'd like to avoid mentioning technical concepts as much as possible Physics is physics is physics. It ain't magic. – Gert Jul 20 '20 at 14:48
  • @Gert I'm not denying the role physics is playing. I'm just trying to spike interest and curiosity in my audience and I think bringing in a bunch of technical physics concepts (e.g. Continuity Equation, Bernoulli's Principe, dynamic pressure, conservation of energy etc.) complicates things and could lead to a loss of interest, especially with a non-technical audience. So I'm trying to distill these concepts to more relatable/conceivable ideas and concepts. The link is helpful, I'll have a look at it. – faridghar Jul 21 '20 at 12:03
  • Sorry but I'm a hardliner on this. Explaining physics without the parts that make physics so beautiful but also so hard leads to 'popularisations' that often have very little meaning. That serves no one, least of all those who come away believing they've understood something when in reality they haven't. If you can't take the heat, stay out of the kitchen! Even among physics student misconceptions remain rife, due often to oversimplifications in well-meaning textbooks. – Gert Jul 21 '20 at 14:18
  • An SUV roof rack covered with a polyethylene sheet and driven fast on a freeway shows a bulge.. due to higher static pressure below the sheet, higher dynamic pressure above the sheet. – Narasimham Jul 22 '20 at 15:32

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There are so many great explanations of this topic, such as this and this, and many more. But if I have to explain this to my pre-school great-nephews, here's how I put it.

First, you can feel it. Just stick your hand out the window of a moving car. If you turn your hand up or down you can feel the force. Air is heavy stuff, and if you push it down, it pushes your hand up.

Second, why does it work with your hand, but not with your fist? Because your hand is sort-of flat. It has a back edge. And the air wants to get to that back edge, not turn sharply around it. This is related to the fact that you can blow out birthday candles, but you can't suck them out. Air doesn't like to make sharp turns.

Don't let anybody tell you that the air divided at the front has to meet up again at the back. It doesn't. In fact the wing wouldn't work if it did. (You're going to get an argument on this, because people have been taught the wrong thing for a long time. If they like big words, it's called the equal time fallacy.)

In the same vein, don't let anybody tell you that the wing works because it's longer on top. Planes can easily fly upside-down. The curve in the wing is just there to help. Aerobatic airplanes do without it.

The Bernoulli stuff is just a technical detail about how the air gets pushed/pulled down. The main thing is, the air gets pushed/pulled down.

Mike Dunlavey
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