Gliders are so light that the lift created by their wings can overcome the opposing pull of gravity. However, without engines, gliders cannot take off. There are two widely used methods of launching gliders into the air. They can be catapulted upwards from the ground, or they can be towed up by an aeroplane. The cable between the plane and the glider is then released, and the glider can fly solo. A glider flight is an extraordinary experience, as it is almost silent except for the sound of the wind.
The wings on a glider have to produce enough lift to balance the weight of the glider. The faster the glider goes the more lift the wings make. If the glider flies fast enough the wings will produce enough lift to keep it in the air. But, the wings and the body of the glider also produce drag, and they produce more drag the faster the glider flies. Since there’s no engine on a glider to produce thrust, the glider has to generate speed in some other way. Angling the glider downward, trading altitude for speed, allows the glider to fly fast enough to generate the lift needed to support its weight.
The way you measure the performance of a glider is by its glide ratio. This ratio tells you how much horizontal distance a glider can travel compared to the altitude it has to drop. Modern gliders can have glide ratios better than 60:1. This means they can glide for 60 miles if they start at an altitude of one mile. For comparison, a commercial jetliner might have glide ratios somewhere around 17:1.
If the glide ratio were the only factor involved, gliders would not be able to stay in the air nearly as long as they do. So how do they do it?
The key to staying in the air for longer periods of time is to get some help from Mother Nature whenever possible. While a glider will slowly descend with respect to the air around it, what if the air around it was moving upward faster than the glider was descending? It’s kind of like trying to paddle a kayak upstream; even though you may be cutting through the water at a respectable pace, you’re not really making any progress with respect to the riverbank. The same thing works with gliders. If you are descending at one meter per second, but the air around the plane is rising at two meters per second, you’re actually gaining altitude.
