How Does a Glider Work?

Launch

Since gliders don't have an engine to provide thrust, they must be launched. Whether it's a balsa wood model or a full-scale acrobatic piloted glider, something has to put it in the air. Piloted gliders are typically taken up to a predetermined altitude by a tow plane. The propeller-driven tow plane is connected to the glider with a long section of rope. The glider pilot flies the glider directly behind the tow plane until a predetermined altitude is reached, where the glider pilot releases the tow rope and the tow plane returns to the airfield. The tow pilot also has a release mechanism for emergency purposes.

When the tow rope is released, the only thing holding the glider in the air is Bernoulli's Principle. This basic principle of aerodynamics states that fast-flowing air has lower pressure than slow-flowing air. The wings of a glider, like most aircraft wings, have a curvature through the cross-section called the camber. This curvature forces the air to move faster over the top of the wing than the bottom, which creates higher pressure on the bottom than the top. This difference in pressure produces the lift that keeps the glider aloft. A glider's wings also have a very high aspect ratio, which means they are much longer than they are wide. This increases the surface area that the air pressures are acting on, which means that a very slight difference in pressure produces a big difference in lift. These high-aspect ratio wings are what make gliders drop in altitude so slowly.

Once aloft, a pilot usually likes to stay aloft for as long as possible. One way to do that is to find thermals. Thermals are pockets of hot air that are rising due to their decreased density. If the pilot can stay in this pocket of hot air, she can "ride the thermal" to a higher altitude. The glider pilot will also want to do banks, climbs, stalls and other maneuvers while in the air. She will use the stick to move the control surfaces on the wings and the tail. When these surfaces are moved, the pressure near that surface changes due to Bernoulli's Principle, causing the glider to move. Ailerons near the wingtips make the glider roll, elevators on the horizontal tail section make the glider pitch up or down, and the rudder on the vertical tail section makes the glider turn left or right.

The glider flies too fast to land easily and come to a stop in a reasonable distance. The pilot, therefore, has some work to do on landing. First, she has to control the glide slope, which is how steep or shallow she is approaching the airfield. Second, the pilot has to control the flaps and speed brakes. When the flaps are extended, they change the camber of the wing, creating higher lift at lower speeds. If there is a cross wind, the pilot must turn sideways a little to keep from getting blown off her approach or even stalling the glider. This is called "crabbing" into the wind. The pilot must also turn the glider back to align the landing gear with the landing strip before touchdown. Finally, right before touchdown, the pilot must pull up the nose of the glider a little to coast into a smooth landing.