*
A derivation of the formula. *

After the publication of my article"Mass-stab" in Drachen Magazin (2/93) and English version of it in rec.kites archive ("Bending spars with a Coke can"), some kite designers have contacted me and asked:

The frame of a kite is made of separate spars or beams. In the following we have a look at a loaded beam, supported at two points.

According to technical handbooks

Where

**E = modulus of material**

and

**I = moment of inertia.**

If we make (our kite larger/smaller = ) the beam longer/shorter by a factor of S :

but want (the kite behave similarly) then the relative deflection should be the same:

Substituting (EQ1) into (EQ3):

which simplifies to:

The area of a kite changes by the square of scale

substituting (EQ2) and (EQ6) into (EQ5):

which simplifies to:

Let's define:

For practical purposes we select a reference spar (e.g. K-75), which has the relative stiffness

From (EQ11) and (EQ12):

Smooth Winds & Scaling.