Why Does a Kite Fly?

Why Does a Kite Fly?

A kite acts as a wing, adhering to the same physical principles that govern an airplane wing. When observing a kite in cross-section, you will notice it’s shaped by two distinct lines: the curvature of the upper contour is greater than that of the lower one, resulting in a larger area on top compared to the bottom.

As the oncoming airflow meets the leading edge of the kite, it divides into two streams—one flowing above the kite and the other below it. The airflow above, due to its convex shape, has to travel a longer distance than the airflow below. To maintain continuity in the fluid medium (in this case, air), the upper flow must move faster to cover this greater distance.

According to Bernoulli’s principle, an increase in the speed of the fluid results in a decrease in pressure. Consequently, the pressure underneath the kite, pushing upwards, becomes greater than the pressure above it, pushing downwards. This difference in pressure creates a lifting force on the kite. When this lift exceeds the forces of gravity (the weight of the kite), the kite takes off.

This same mechanism allows the kite to remain airborne. The lift generated by the airflow (whether from the wind or the motion of the kite itself) balances out the force of gravity and the kite’s drag force. Thus, the kite naturally tends to fly in the direction where its leading edge is oriented.

THE CONCEPT OF A WIND WINDOW

A wind window refers to the area relative to the rider where the kite can fly. If you stand with your back to the wind, the projection of the sector of the sphere in front of you (bounded on one side by land or water and on the other by a vertical plane behind you, perpendicular to the wind direction) represents your wind window.

The kite can always fly downwind below you but cannot fly behind your position.

ZONES OF KITE POSITION IN THE WIND WINDOW

1. Maximum Traction Zone – This zone is centered directly in front of you within the sphere segment. In this position, the kite’s wing is maximally closed relative to the wind direction, providing maximum traction.

2. Minimal Traction Zone – Located along the boundary of the sphere segment that is perpendicular to the wind direction, this area sees the kite’s wing maximally open to the wind, resulting in minimal traction. (In practice, this corresponds to positions of 0 and 90 degrees.)

3. Average Traction Zone – Positioned between the maximum and minimum traction zones, the kite’s wing here is partially closed concerning the wind direction and generates a moderate amount of traction. (In practice, this corresponds to a position of 45 degrees.)

When explaining the position of the kite in the wind window, two coordinate systems are commonly used:

1. Clock Face Principle – This is represented from 9 to 3 o’clock.

So, where is the optimal position for the kite? The optimal position lies within the average traction zone.

Let’s explore the kite’s movement from the minimal traction zone to the maximum traction zone. Initially, in the minimal traction zone, the kite is fully open to the wind direction, resulting in a minimal angle of attack and consequently minimal thrust. As the kite moves toward the average traction zone, its projection into the wind direction increases (the kite begins to catch the wind). The angle of attack gains significance, resulting in lift (the kite begins to fly). However, this also introduces a harmful drag force that tends to push the kite backward.

At the medium traction zone (45 degrees position), the kite reaches an optimal state. If it continues to move toward the maximum thrust zone, the drag force will increase significantly, while the lift force decreases, exceeding the critical angle of attack. Practically, this leads to a highly discharged turbulent flow behind the kite, which can cause the kite to flip over.

KITE STRUCTURE

The kite is composed of a dome, dome lines, power lines, control lines, a control bar, and a release system. The power lines are attached to the leading edge of the kite (the central inflatable cylinder) through some of the dome lines and then pass through the center of the bar, connecting to the release device and trapeze hook. The control lines are tied to the trailing (back) edge of the kite through some of the dome lines and are then attached to the sides of the control bar.

KITE CONTROL

It’s essential to understand that the kite can only fly when all four of its main lines are taut. If they are not, the kite will not generate lift and will fall. The faster the kite moves, the greater the thrust force it can create, as lift is proportional to the square of the speed. You can enhance thrust not only by increasing horizontal speed but also by giving the kite angular speed (the kite moving up and down relative to the rider) and by increasing the angle of attack relative to the wind (creating a larger projection of the kite onto the wind).

From this, we conclude that the kite always flies in the direction where its leading edge is pointed.