result. By the late 1930s the widespread adoption of all-metal stressed skin construction of aircraft meant, at least in theory, that the aerodynamic penalties that had limited the performance of pushers (and indeed any unconventional layout), were reduced; however any improvement that boosts pusher performance also boosts the performance of conventional aircraft and they remained a rarity in operational service—so the gap was narrowed but was not closed entirely. During World War II, experiments were conducted with pusher fighters by most of the major powers. Difficulties remained, particularly that a pilot having to bail out of a pusher was liable to pass through the propeller arc. This meant that of all the types concerned, only the relatively conventional Swedish SAAB 21 of 1943 went into series production. Other problems related to the aerodynamics of canard layouts, which had been used on most of the pushers, proved more difficult to resolve. One of the world's first ejection seats was (per force) designed for this aircraft, which later re-emerged with a jet engine. The largest pusher aircraft to fly was the Convair B-36 "Peacemaker" of 1946, which was also the largest bomber ever operated by the United States. It had six Pratt & Whitney Wasp Major radial engines mounted in the wing, each driving a pusher propeller located behind the trailing edge of the wing. Although the vast majority of propeller-driven aircraft continue to use a tractor configuration, there has been in recent years something of a revival of interest in pusher designs: in light homebuilt aircraft such as Burt Rutan's canard designs since 1975, ultralights such as the Quad City Challenger (1983), flexwings, paramotors, powered parachutes, and autogyros. The configuration is also often used for unmanned aerial vehicles, due to requirements for a forward fuselage free of any engine interference. The Aero Dynamics Sparrow Hawk was another homebuilt aircraft constructed chiefly in the 1990s. Configurations Airships are the oldest type of pusher aircraft, going back to Frenchman Henri Giffard's pioneering airship of 1852. Pusher aircraft have been built in many different configurations. In the vast majority of fixed-wing aircraft the propeller or propellers are still located just behind the trailing edge of the "main lifting surface", or below the wing (paramotors) with the engine being located behind the crew position. Conventional aircraft layout have a rear tail (empennage) for stabilization and control. The propeller may be close to the engine, as