Symmetry -- An All Important Element

when you think of a particular design for a kite, you must always consider the adverse effects of any asymmetry. this applies not only to errors in shape and size between the right and left sides of the kite, but also to the type of fabric and the direction of fabric grain. in short, the right and left sides of a kite should be mirror images of each other in every aerodynamic aspect.

Collecting Pieces

the examples i have given demonstrate the general principles that allow kites to operate and can serve as a starting point for kite design. note that i have only described the force component that is directly resisting the force of the wind. because as this force is deflected, it produces lift on a kite. ultimately the kite achieves a static condition where the weight of the kite is precisely balanced with its lift.

Maxwell Eden has written an excellent book on kite building--THE MAGNIFICENT BOOK OF KITES, Black Dog and Leventhal Publishers,Copyright 1998. on p. 201 Eden gives the following formula for the minimum wind speed required to produce lift.

w = wt. in ounces
A' = effective area in square feet
v = minimum wind speed in mph

v = 7*(w/A')^1/2

note the similarity to the equations we have used so far.
the coefficient of 7 is generally applicable but smaller coefficients ~5 are used for kites designed for light wind.

Simplifying Things

now that we have the basic equation for a force wind exerts on a surface it blows directly against, it's time to illustrate a simple principle. if the surface we're talking about is at an angle to the wind -- as shown above -- the equation F = 1/2(d*A*v^2) becomes
F = 1/2(d*A'*v^2). A' is perpendicular to the wind. this gives you the component of force F opposite to the wind. note that the wind passing through the area A' includes all the wind that will strike surface A. it can be called the "effective area."

A wee bit of physics

the equation for the force on the perpendicular surface should be F=1/2(d Av^2). the pressure created by this force is known as the velocity pressure, as described on p. 9-61 in Esbach, Handbook of Engineering Fundamentals, John Wiley & Sons, co. 1936.

Kite Making Project

i'm diving right in here and suggesting an interesting and challenging project that can help students understand some basic aerodynamics and structural mechanics. kite-building involves both of these subjects. a kite must produce enough lift to overcome its own weight by deflecting a stream of air. a kite must also be a structural entity that maintains its shape in the presence of the forces placed upon it. this project can be done by groups of students at almost any grade level and can be as simple or as complex as needed. I will continue posting on this subject.

Mission Statement

the purpose of this space is both boring and useful.
i'm trying to help students and instructors with ideas for
applied physics curricula as well as some solutions to
posted problems.