Gliders and kites

Gliders and kites both have wings to generate lift. A glider’s wings are aerofoils. Well made kites are also aerofoil wings, which gives them lift.

Gliders

Gliders are planes without a motor. They have four forces acting on them – lift, weight (gravity), drag and thrust. Although gliders do not have their own power to provide thrust, the weight of the glider produces the thrust to keep it moving through the air by flying downwards at a shallow angle of descent.

Gliders are as slim and light as possible. They (and other aircraft) are designed to have shapes and skins that are smooth to allow the plane to slip more easily through the air at higher speeds, reducing profile drag. Because there is no powered thrust, drag must be minimised as much as possible.

Lift is the all-important force for gliders. This is created by the wings counteracting the weight. The wings on a glider are longer and narrower than those on powered aircraft. The wingspan is very long compared to the width of the wing. This helps to overcome induced drag, which occurs at the wing tips at lower speeds.

Weight can be made to work for or against a glider. A lightweight glider can stay in the air longer. A heavier weight can fly faster and further. Some gliders carry ballast tanks of water to give them more speed, but the pilot can dump the water if they wish to lighten the plane when they want to land.

Gliders are usually launched into the air by powered planes. The planes tow the gliders using a long rope (aero-tow). The gliders take off before the powered plane because their wings generate more lift. At the desired altitude, the rope is released. The plane and the glider then turn in opposite directions, and the glider begins its unpowered flight.

To stay in the air for longer periods of time, glider pilots will often get help from Mother Nature. The glider slowly descends after being released from the tow, but it will rise if it gets into rising air:

• Thermals – in some places, the terrain absorbs heat from the Sun more rapidly than surrounding areas. As the air near the ground is heated, it expands and rises, creating a column of warm, rising air.
• Ridge lift – air hitting a mountain or ridge is redirected upwards, forming a band of lift along the windward side of the slope.
• Wave lift – this is similar to ridge lift but is created on the leeward side of the ridge by winds passing over the mountain and going downwards, after which it bounces upwards further from the mountain to make the glider rise up.

Kites

Kites have been around for over 3,000 years. They were the first known flying objects made by people.

To fly, a kite must have wind (moving air particles). The kite must be positioned in such a way (angle of attack) that the wind is split over the surface of the kite (wing). If the angle of the kite causes a difference in air pressure between the airflow over the top and the bottom of the kite so that there is greater pressure under the kite, a force called lift is created. Lift is also created as air is deflected downwards by the trailing edge of the wing.

Kites need to have a large wing surface that will be become convex (bulge out) in the wind so that the wind will slide over it. Kites need to be of equal weight on each side of their centre – one side must not be heavier than the other. They need to be light and sturdy, and they need to be tethered so that they have a fixed angle of attack to the wind. The tail is important because it creates added drag, causing the kite to point in the right direction – towards the wind.

The tethering string can help to get a kite into the air. If there’s not a lot of wind, pulling a light kite with the string against the still air has the same affect as the wind. The air gets pushed down, and the kite gets pushed up.