This was later known as the Handley Page H.P.20 Several years later, having subsequently taken employment at the Handley-Page aircraft company, Lachmann was responsible for a number of aircraft designs, including the Handley Page Hampden. Licensing the design became one of the company's major sources of income in the 1920s. The original designs were in the form of a fixed slot near the leading edge of the wing, a design that was used on a number of STOL aircraft. During World War II, German aircraft commonly fitted a more advanced version of the slat that reduced drag by being pushed back flush against the leading edge of the wing by air pressure, popping out when the angle of attack increased to a critical angle. Notable slats of that time belonged to the German Fieseler Fi 156 "Storch". These were similar in design to retractable slats, but were fixed and non-retractable. This design feature allowed the aircraft to take-off into a light wind in less than 45 m (150 ft), and land in 18 m (60 ft). Aircraft designed by the Messerschmitt company employed automatic, spring-loaded leading-edge slats as a general rule, except for the Alexander Lippisch-designed Messerschmitt Me 163B "Komet" rocket fighter, which instead used fixed slots built integrally with, and just behind, the wing panel's outer leading edges. Post-World War II, slats have also been used on larger aircraft and generally operated by hydraulics or electricity. Several technology research and development efforts exist to integrate the functions of flight control systems such as ailerons, elevators, elevons, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag, inertia (for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), and radar cross-section for stealth. These may be used in many unmanned aerial vehicles (UAVs) and 6th generation fighter aircraft. One promising approach that could rival slats are flexible wings. In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. The X-53 Active Aeroelastic Wing is a NASA effort. The adaptive compliant wing is a military and commercial effort.