tighten into turns and may result in a spiral dive from which recovery is not possible. In the worst case, the wing structure can be stressed to the point of failure. At large angles of sweep and high speeds, in order to build a structure stiff enough to resist deforming yet light enough to be practicable, advanced materials such as carbon fiber composites are required. Composites also allow aeroelastic tailoring by aligning fibers to influence the nature of deformation to a more favorable shape, impacting stall and other characteristics. Stall characteristics Any swept wing tends to be unstable in the stall, since the wing tips stalls first causing a pitch-up force worsening the stall and making recovery difficult. This effect is less significant with forward sweep because the rearward end carries greater lift and provides stability. However, if the aeroelastic bending is sufficient, it can counteract this tendency by increasing the angle of attack at the wing tips to such an extent that the tips stall first and one of the main characteristics of the design is lost, on a conventional wing the tips always stall first. Such a tip stall can be unpredictable, especially where one tip stalls before the other. Composite materials allow aeroelastic tailoring, so that as the wing approaches the stall it twists as it bends, so as to reduce the angle of attack at the tips. This ensures that the stall occurs at the wing root, making it more predictable and allowing the ailerons to retain full control. History Prewar studies Belyaev, the author of the below mentioned DB-LK project, tested forward-swept wing gliders BP-2 and BP-3 in 1934 and 1935. Other prewar design studies included the Polish PWS Z-17, Z-18 and Z-47 "Sęp" series. World War II and aftermath Forward-swept wings designs, some whose design had begun during the prewar period, were developed during the Second World War, independently in Germany, Russia, Japan, and the US. An early example to fly, in 1940, was the Belyayev DB-LK, a twin-boom design with forward-swept outer wing sections and backwards-swept tips. It reportedly flew well. Belyayev's proposed Babochka research aircraft was cancelled following the German invasion. The American Cornelius Mallard flew on 18 August 1943. The Mallard was powered by a single engine, but it was followed by the Cornelius XFG-1 prototypes, which were flying fuel tanks, unpowered and designed for towing by larger aircraft. These Cornelius