A force may be thought of as a push or pull in a specific direction. This slide shows the forces that act on the Wright 1900 aircraft when flown as a kite. You can compare these forces to the forces on the aircraft when flown as a piloted glider and you will note only a few differences. There are also a few differences from the forces on a powered aircraft, but the similarities are so great that the Wrights were able to use kite experiments to determine the aerodynamic performance of their unpowered aircraft from 1900 to 1902. They also used kite experiments to learn the basics of flight control. You can also learn the basics of aerodynamics by flying a kite. The forces on your kite are exactly the same as the forces on the Wright brothers’ kite.
Weight is a force that is always directed toward the center of the earth. The magnitude of the force depends on the mass of all the parts of the aircraft. The weight is distributed throughout the aircraft, but we can often think of it as collected and acting through a single point called the center of gravity. In flight, an airplane rotates about the center of gravity, but a kite rotates about the connection point of the control wires. A kite’s motion is confined or pinned like a door which rotates on its hinges. The kite’s weight is always directed toward the center of the earth.
To make a kite fly, we must generate a force to overcome the weight. This force is called the lift and is generated by the motion of the air over the kite. Lift is an aerodynamic force (“aero” stands for the air, and “dynamic” denotes motion). Lift is directed perpendicular (at right angle) to the wind direction. As with weight, each part of the kite contributes to a single lift force. Most of the lift of the Wright 1900 kite was generated by the wings. The lift acts through a single point called the center of pressure. The center of pressure is defined just like the center of gravity, but using the pressure distribution around the body instead of the weight distribution.
As the air moves past the kite, the kite resists the motion of the air. This resistance force is called the drag of the kite. The direction of the drag force is always in the direction of the wind. Drag acts through the center of pressure in the same way that lift acts through the center of pressure. (In reality, there is only one aerodynamic force on the kite. Engineers break this force into lift and drag to more easily explain the motion of an object along its flight path.)
To keep the kite at a fixed location, a pair of control lines are attached to the kite. The control lines generate a force called tension which is used to overcome the drag. Without the control lines, the kite would move in the direction of the wind and there would be no relative velocity between the wind and the kite. The lift would go to zero and the kite would fall to the ground because of gravity. For convenience, the tension force is often broken into two components, one vertical and one horizontal.
When the kite is in stable flight, the lift is equal to and opposes the combination of the weight and the vertical pull of tension. The drag is equal to and opposes the horizontal tension. Compared to the forces on an airplane, the horizontal pull on a kite plays the roll of the thrust. The vertical pull of the line tension is mainly the weight of the line; the kite must lift its own weight and the weight of the line.
The relative strength of the forces determines the motion of the kite as described by Newton’s laws of motion. If the wind velocity increases, the lift increases and exceeds the weight of the kite. The kite then moves vertically and the tension force increases because of increased drag. The vertical component of tension increases because of the change in angle that the tension force makes with the vertical. A new balance point is established and the kite achieves a different stable condition.