Physical aspects - Aspects Physiques

Author : Glenn Davison.  the book, “Kites in the Classroom.”

A single-line kite forms a stable and self-adjusting system while in flight. The moving air creates lift and the kite rises. A kite flies because it is spilling air downward to give it lift, and outward to give it stability. Think of the air like lots of small pellets bouncing off the kite or like waves of water being pushed away by the kite, just like the hull of a boat pushes away water.

Forces at Work on a Kite

Forming a Stable System
How does a kite become stable? To provide stable flight, the kite must have some aerodynamic shape that can equalize opposing forces: lift vs. gravity, roll/pitch/yaw, and lift + drag vs. line tension.

Lift vs. Gravity
There is an important relationship between the area of the sail and the weight of the kite. A large boat can carry hundreds of passengers but a small boat will sink if there are as many passengers as the large boat. The same is true for kites. The kite must be light enough to float on the current of air.

A longer tail will often help to stabilize a kite by adding drag below the kite without adding much weight. The tail will help keep the kite pointed into the wind.

In geometry, an angle formed by two planes is called the "dihedral." On kites, it is the bow or "V" shape of the kite. The typical dihedral angle is about 30 degrees, or 15 degrees on each side. The dihedral gives the kite roll stability. When a kite with a dihedral rolls left or right, the wind exerts greater force on the side flattest to the wind therefore pushing it back into equilibrium. This gives roll stability and yaw stability.

Roll vs. Pitch vs. Yaw

Roll - When a kite twists, flutters, or rolls, it is usually caused by a kite that is not symmetric. Make sure left and right are the same size and that the bridle or tow point are in the center.

Pitch - The angle that a kite is tilted into the wind is also called the pitch or "angle of attack." This can be adjusted by moving the tow point up or down on the bridle. The lift generated by the kite is related to the pitch and the sail area. In light winds the kite should be adjusted to have a large angle of attack and in strong winds the angle of attack should be reduced.
Yaw - If the kite turns or spins left or right, lengthen the tail to add drag or bow the kite to increase the dihedral.

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