and are in general slower, smaller, and less expensive than sailplanes. Hang gliders most often have flexible wings given shape by a frame, though some have rigid wings. Paragliders and paper airplanes have no frames in their wings. Gliders and sailplanes can share a number of features in common with powered aircraft, including many of the same types of fuselage and wing structures. For example, the Horten H.IV was a tailless flying wing glider, and the delta wing-shaped Space Shuttle orbiter flew much like a conventional glider in the lower atmosphere. Many gliders also use similar controls and instruments as powered craft. Types of glider The main application today of glider aircraft is sport and recreation. Sailplane Gliders were developed from the 1920s for recreational purposes. As pilots began to understand how to use rising air, sailplane gliders were developed with a high lift-to-drag ratio. These allowed longer glides to the next source of "lift", and so increase their chances of flying long distances. This gave rise to the popular sport of gliding. Early gliders were mainly built of wood and metal but the majority of sailplanes now use composite materials incorporating glass, carbon or aramid fibers. To minimize drag, these types have a streamlined fuselage and long narrow wings having a high aspect ratio. Both single-seat and two-seat gliders are available. Initially training was done by short "hops" in primary gliders which are very basic aircraft with no cockpit and minimal instruments. Since shortly after World War II training has always been done in two-seat dual control gliders, but high performance two-seaters are also used to share the workload and the enjoyment of long flights. Originally skids were used for landing, but the majority now land on wheels, often retractable. Some gliders, known as motor gliders, are designed for unpowered flight, but can deploy piston, rotary, jet or electric engines. Gliders are classified by the FAI for competitions into glider competition classes mainly on the basis of span and flaps. A class of ultralight sailplanes, including some known as microlift gliders and some known as "airchairs", has been defined by the FAI based on a maximum weight. They are light enough to be transported easily, and can be flown without licensing in some countries. Ultralight gliders have performance similar to hang gliders, but offer some additional crash safety as the pilot can be strapped in an upright seat within