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A sailplane is a type of aircraft without a powerplant, which utilizes its lightweight design and highly efficient airfoils for the purpose of flight. The sailplane has no means to generate thrust, therefore it requires an engine-powered aircraft to tow it to a sufficient altitude, where it is released for flight. Sailplanes feature an aerodynamic design with a narrow passenger compartment, long, thin wings and minimal surface area exposed against the relative wind. Often used for recreation, sailplanes offer a less expensive alternative to conventional aircraft, as they require little maintenance and burn no fuel. The principal cost incurred with sailplanes is the tow fee administered by the pilot of the towing aircraft.
Aircraft generate lift with their wings, also known as airfoils. When the airfoil moves through the air, airflow is accelerated over the top surface of the wing, resulting in a loss of pressure. This low pressure present over the wing results in a vacuum effect, which forces the aircraft upward. Sailplanes are designed with highly efficient airfoils with a massive wingspan in comparison to engine-driven aircraft. This allows for a larger amount of airflow to be converted to lift, enhancing the aircraft's lift-to-drag ratio and overall ability to remain airborne.
A sailplane is controlled by the pilot in a very similar fashion to a conventional, engine-driven aircraft. Pilots use a control stick or yoke to control elevators and ailerons, while foot pedals are used to control the rudder's surface. The elevator controls the aircraft's pitch, the ailerons control roll and the rudder moves the aircraft side to side around its vertical axis. Many sailplanes also feature spoilers, which allow the pilot to reduce airspeed by increasing drag. More expensive sailplanes also feature wing flaps, which allow the pilot to adjust the wing's camber, increasing its ability to produce lift at low airspeed.
The sailplane requires a tow from a powered aircraft in order to become airborne. It is also possible to launch a sailplane through the use of a ground-based winch system. Once airborne, the sailplane pilot releases the connecting cable and then flies unassisted. Pilots make use of upward-moving pockets of air warmer than their surroundings, known as thermals, to propel the sailplane vertically. Sailplane pilots flying in mountainous terrain often seek lee waves, upward-moving sections of air found on the leeward base of mountain ridges, to sustain flight.