Why gliders glide

As you can see in the diagram above, the Schleicher ASH 31 glider has an aspect of High aspect ratio wings produced less induced drag, which is what makes them so efficient on gliders. So why don't all aircraft have high aspect ratio wings? There are several different factors. First, high aspect ratio wings bend more than shorter wings, which means they need to be designed with stronger design specs.

Since gliders are light, the bending isn't as much of a problem. But with heavier aircraft, like airliners, a high-aspect ratio wing would be impractical. Next, high aspect ratio wings are more susceptible to wing warping when ailerons are used. Since gliders fly a relatively slow speeds, wing warping isn't as pronounced, but it would be a real problem in a fast aircraft. Maneuverability is another major factor. High aspect ratio wings decrease maneuverability, because they have a higher moment of inertia.

Think of it like a tightrope walker: they carry a long rod to balance themselves, preventing them from quickly falling left or right. It's great if you want to stay in one place, but not so great if you want to quickly move or roll left or right.

Finally, airport size limits the aspect ratio an aircraft can have. Take the Boeing for example. The has an aspect ratio of approximately 9. Obviously, that wouldn't be practical. Like most airplanes, gliders use ailerons, a rudder, and an elevator to fly. Flaps are fitted on gliders to control descent rates by producing drag and increasing lift. Many modern gliders also use airbrakes or spoilers which, when used, drastically disrupt airflow over the wing, increasing drag and reducing lift.

Another significant difference between powered airplanes and gliders is that gliders normally have only one landing gear, situated directly below the pilot. Having only one gear save a lot of weight, but what happens to the wings on takeoff and landing when you've only got one gear? The wingtips are protected by skids or small wheels, and when the glider lands, it comes to rest on the main gear and one of the wingtips.

Inside the cockpit, the glider pilot uses a quick-release mechanism to release the tow rope. Once the glider is at a desired altitude, the rope is released and the glider and tow plane turn in opposite directions.

The cable is then attached to the bottom of the glider. Once the winch is activated, the glider is pulled along the ground at high speed toward the winch and takes off. In a short amount of time, the glider gains substantial altitude during this process and releases the winch line before continuing flight.

Glide ratio measures the performance of an aircraft's glide; many modern gliders have a glide ratio better than This means that if you start at an altitude of 1 mile, you can glide for 60 miles. In comparison, a Boeing has a glide ratio.

But if glide ratio was the only thing keeping gliders in the air, they wouldn't flying for very long. 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 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 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. Thermals are columns of rising air created by the heating of the Earth's surface. As the air near the ground is heated by the sun, it expands and rises. Pilots keep an eye out for terrain that absorbs the morning sun more rapidly than surrounding areas.

These areas, such as asphalt parking lots, dark plowed fields and rocky terrain, are a great way to find thermal columns.

Pilots also keep a look out for newly forming cumulus clouds, or even large birds soaring without flapping their wings, which can also be signs of thermal activity. Once a thermal is located, pilots will turn back and circle within the column until they reach their desired altitude at which time they will exit and resume their flight.

To prevent confusion, gliders all circle in the same direction within thermals. The first glider in the thermal gets to decide the direction -- all the other gliders that join the thermal must circle in that direction. Ridge lift is created by winds blowing against mountains, hills or other ridges. In a powered aircraft, the thrust from the engine opposes drag, but a glider has no engine to generate thrust.

With the drag unopposed , a glider quickly slows down until it can no longer generate enough lift to oppose the weight, and it then falls to earth. For paper airplanes and balsa gliders, the aircraft is given an initial velocity by throwing the aircraft. Some larger balsa gliders employ a catapult made from rubber bands and a tow line to provide velocity and some initial altitude. Hang-glider pilots often run and jump off the side of a hill or cliff to get going.

Some hang-gliders and most sailplanes are towed aloft by a powered aircraft and then cut loose to begin the glide. The powered aircraft that pulls the glider aloft gives the glider a certain amount of potential energy.

The glider can trade the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

The answer is that they are designed to be very efficient , to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy.

Pockets of rising air are called updrafts.