Fig. 52. The tetrahedral kite
The tetrahedral form invented by Dr. Graham Bell is unique and interesting. Based on the geometrical figure, it has a remarkable strength of frame, and possesses a surprising lifting power. The principal difficulty in the construction is in fastening the sticks, as three of them meet at every point. The frame consists of six pieces of equal length. Drill a 1⁄32-inch hole in each end of all the pieces, about 1⁄4 inch from the end. Place the pieces on the floor as shown at 1. Pass a piece of soft iron or brass wire through the three holes at a and bind lightly. Do the same at angles b and c. Now raise loose ends d e f until they meet over the centre, as at 2. Join with wire and tighten all the joints with a pair of pliers. ([Fig. 52].)
Each face of the frame is an equilateral triangle, and the covering is to be on only two sides, as shown at 3. The shape of the piece to be cut is shown at 4. This forms a single cell, and the large sizes are broken up into many small tetrahedral cells. The line may be tied at c or d.
The designing of fancy figure kites is a fascinating occupation, but unless certain fixed principles are kept in mind may end in much experimenting and many disappointments. The question of steadiness or stability seems to be summed up in the mathematical expression—"dihedral angle."
A kite having a stiff, flat surface presented to the wind will often cut up queer antics, while the same frame covered with a more flexible covering will fly beautifully. The reason is that the flexible covering will be bowed back by the wind, forming an approximate "dihedral angle."
In the triangular box and tetrahedral kites this bowing back is not so necessary, because the dihedral angle is provided in the construction.
In these kites, when a sudden gust of wind presses harder on one side than on the other, the first side is pressed back, reducing the resistance, and the other side is brought forward until both sides receive equal pressure, or the kite is in equilibrium, facing the wind; and the shifting of the breeze is constantly provided for. The bowing back of the covering of an Eddy kite takes care of sudden changes in the same way. Double Malay kites or two tetrahedral kites, fastened together, tandem fashion, will be found stable, especially if the rear one be slightly smaller than the forward one. ([Fig. 53].)