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Name: Jerry P.
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Question:
Many sites I go to about kites say that a kite creates lift due to Bernoulli's Principle. I just cannot buy it--to me it seems like it is Newton's 3rd Law of Motion...air hits the underside of the kite and is deflected downward, the kite goes upward. Can you explain which is correct in a non technical way that could be used to teach 3rd-8th graders?



Replies:
Jerry,

Most kites I have ever seen have little to no airfoil on them, making your second suggestion an easier explanation.

With or without an airfoil though, what lifts a kite or any other form of wing is the presence of greater air pressure on one side than the other. If a normal wing is tilted far enough down into the wind, that deflection will still cause it to drop, regardless of how much curvature is on either side.

As for the functioning of airfoils, the easiest explanation I have ever heard involved a picture of one, with lots of dots to represent air molecules passing over it. Dots along the bottom were spaced the same as dots ahead of or behind the airfoil. Dots over the top were more widely spaced, with the explanation that the same number of dots had to cover a greater distance. (make sure you draw the same number of dots, or you invite skeptics!) It may not be the most technically accurate description in the world, but it does get the job done without having to build a wave tank or wind tunnel.

Ryan Belscamper


You are right about the kite, and even airplane wings. But, explaining how airfoils and sails generate lift from the reaction force, if presented as nothing more than wind pushing up from the bottom, will probably lead to problems for the younger students, to say nothing of their parents!

I expect explaining the Bernoulli effect is the purpose of your lesson and not introducing them to the real subtleties of Newton's Laws. If I

Have the students try this old standby:

Give each pair of students rectangular strips of newsprint about 1 1/2 by 6 inches. Have then hold the small end of the strip (with one finger) down on the table and blow along the length of the strip, as close to the table as they can. The strip will rise up and try to "fly".

The exact explanation of the effect, of course, will vary with the grade level. For the higher grades, you could also try smoothing the papers down and then let the kids blow - the paper should not fly (if it does, substitute a less porous paper such as photocopy paper), proving it takes a reaction force to make the lift.

Bob Avakian


Jerry -

You are asking a question that is debated by many. The answer is that the kite flies (and an airplane flies) for both reasons. The amount of force (lift) created by each principle is dependent on the angle of attack - the angle between the chord line (similar to the longitudinal centerline) of the lifting surface and the relative wind.

In the case of your kite, the higher the angle that the kite flies at (compared to the ground), the lower the angle of attack.

At low angles of attack the percentage of lift created by Bernoulli's principle is greater. Less comes from Newton's Third Law. It is the opposite at high angles of attack. (Like when the kite is closer to the ground in front of you.)

Even people who design airliners debate the importance of the two principles in describing lift. Your 3rd graders are not the only ones to ask the question.

Good examples of each -

Newton's Third Law - Water skiing (try to do it standing still!)

Bernoulli's Principle - blowing over a strip of paper... or try having your student turn over a sheet of paper on the table by blowing under it.

Larry Krengel



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