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Q & A: Lift of Aircraft Wings

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Most recent answer: 09/21/2009
Q:
I’m an airline pilot with daughters in 3rd and 4th grade and I’m looking for a simple demonstration/experiment pertaining to Bernoullis Theorem and the creation of lift over aircraft wings.
- Bill Munch (age 45)
Marston School, Hampton, NH
A:
Bill,

I think one of the coolest (and simplest) ways to demonstrate Bernoulli's Theorem is to blow over a sheet of paper and watch it rise. The kids can do it themselves, which is good too. Hold one end of the page horizontally, just below your lower lip (the other end will be sagging straight down). Now blow, and the sagging end of the paper will rise up and flap around due to the movement of air over it.

Mats

(published on 10/22/2007)

Follow-Up #1: Bernoulli?

Q:
The answer you gave is wrong. The reason the paper rises is not due to Bernoullis Theorem but due to the fact that the first portion of the paper is curved. The air going over this curve goes straight on causing a pressure reduction in the next section of paper. You can easily disprove Bernoullis Theorem by putting a sheet of paper horizontally on a flat surface and bloeing over it. Even with a powerful blower it will nor rise one iota. Bernoullis theorem has nothing to do with the lift in aerofoils either.
- Roger Pendleton (age 63)
United Kingdom
A:
It's true that in the simple paper-blowing experiment Mats mentioned other effects besides Bernoulli's effect could get mixed in. Also, unlike what one reads in many sources the main lift source of modern airplanes comes from the tilt of the wings, with the Bernoulli effect making a smaller contribution. That does not mean that the Bernoulli effect is false- it's very real and plays a major role in many fluid flow situations.
A nice summary of examples, and a discussion of the conditions under which the Bernoulli effect is found, may be found at

Mike W.

(published on 03/07/2008)

Follow-Up #2: Bernoulli

Q:
Roger, you yourself are quite wrong in trying to disprove Bernoulli's Theorem! Everyone seems to forget Bernoulli gave us just that - a Theorem! And it describes pressure static) with velocity. If your paper sheet is horizontally flat, with no curviture at all, then there is no change in the flow of air, therefore no change in the pressure differential, so of course it will not rise "one iota"! No one disputes pressure equalisation, so as long as there is a pressure differential between the top and bottom surfaces(like blowing over the top of a sheet of paper) you will get a reaction. Sure, there are other ideas involved in the total production of lift on an aerofoil, but you did not disprove Bernoulli one iota. And the pilot father was, afterall simply trying to explain an idea to a child!
- Simon (age 27)
Brisbane, Australia
A:
You're absolutely right that we should have pointed out that this is a theorem, not a speculation. The theorem does involve some assumptions which are not exactly true of real materials (incompressibility, lack of viscosity) but adding in those complications doesn't make the Bernoulli effect vanish.

Mike W.

(published on 09/21/2009)

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