How Do Gliders Fly?

Gliders are planes without an engine. 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.

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.

Launching

Gliders are not powered planes and are unable to provide thrust for launching. Generally, they are towed into the air by a powered plane or they glider can be equipped with a small engine used only for takeoff and is then stowed during flight.

credit: 123rf.com

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.

Lift Forces

For a glider to stay in the air for longer, the pilot has to hunt for areas of lift or rising air to help the glider gain height. There are several ways of finding lift:

RIDGE LIFT
credit: airexperiences.co.uk

As wind blows against a hill or a mountain side, the air is pushed up and over the top. 

This creates a band of rising air in front of the hill that glider pilots use to help gain height. 

A glider pilot will go back and forth along the hill side in the band of rising air to climb higher and higher, this is called Ridge soaring.

THERMAL LIFT
credit: airexperiences.co.uk

Have you seen birds of prey circling without flapping their wings?
They have found a thermal.

A thermal is a column of warm rising air. If the air rises quicker than the glider or bird is going down, they will be pushed up. Birds will feel a push under their wings from the rising air and will start circling to stay in the column to gain height, same with gliders.
As the air rises it will cool, the moisture will then condense to form fluffy cumulus clouds (these tend to be handy markers for the pilots).
A glider pilot can then leap frog from thermal to thermal to stay in the skies for longer.

WAVE LIFT
Wave lift
credit: airexperiences.co.uk

As wind flows over a mountain, the air gets forced up and over similar to ridge lift. However wave lift occurs at higher altitudes with taller mountains. The airflow will then oscillate behind the mountain causing smaller waves. Often, the sign of wave lift is distinguished by a “wave lenticular” cloud formation.
By facing the glider in the strong upward draughts of the oscillation, the glider will be carried upwards many thousands of feet and can then glide for hundreds of miles.

Landing

Landing a glider is much like landing a conventional plane, except there is usually a single small wheel located directly under the pilot. The wings on gliders are very strong, and the tips are reinforced to prevent damage in case they scrape along the ground during a landing. Even so, pilots can usually manage to keep both wing tips off the ground until the plane has slowed sufficiently (kind of like riding a fast bike down the runway). Glider tails typically have a tiny wheel that prevents the tail from scraping while on the ground.

When landing the glider, the pilot needs to be able to control the glide path (the rate of descent relative to distance traveled) in order to bring the glider down in the right location. The pilot has to be able to reduce the amount of lift produced by the wings without changing the speed or attitude of the glider. He does this by deploying spoilers on each wing. The spoilers disrupt the airflow over the wing, drastically reducing the lift it produces and also increasing the drag.

Sources:

Cover Image: https://www.pipistrel-usa.com/taurus/

https://science.howstuffworks.com/transport/flight/modern/glider4.htm

https://www.sciencelearn.org.nz/resources/306-gliders-and-kites

How do gliders fly?

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