1939 conducted the first wind tunnel tests to investigate Busemann's theory. Two wings, one with no sweep, and one with 45 degrees of sweep were tested at Mach numbers of 0.7 and 0.9 in the 11 x 13 cm wind tunnel. The results of these tests confirmed the drag reduction offered by swept wings at transonic speeds. The results of the tests were communicated to Albert Betz who then passed them on to Willy Messerschmitt in December 1939. The tests were expanded in 1940 to include wings with 15, 30 and -45 degrees of sweep and Mach numbers as high as 1.21. With the introduction of jets in the later half of the Second World War, the swept wing became increasingly applicable to optimally satisfying aerodynamic needs. The German jet-powered Messerschmitt Me 262 and rocket-powered Messerschmitt Me 163 suffered from compressibility effects that made both aircraft very difficult to control at high speeds. In addition, the speeds put them into the wave drag regime, and anything that could reduce this drag would increase the performance of their aircraft, notably the notoriously short flight times measured in minutes. This resulted in a crash program to introduce new swept wing designs, both for fighters as well as bombers. The Blohm & Voss P 215 was designed to take full advantage of the swept wing's aerodynamic properties; however, an order for three prototypes was received only weeks before the war ended and no examples were ever built. The Focke-Wulf Ta 183 was another swept wing fighter design, but was also not produced before the war's end. In the post-war era, Kurt Tank developed the Ta 183 into the IAe Pulqui II, but this proved unsuccessful. A prototype test aircraft, the Messerschmitt Me P.1101, was built to research the tradeoffs of the design and develop general rules about what angle of sweep to use. When it was 80% complete, the P.1101 was captured by US forces and returned to the United States, where two additional copies with US-built engines carried on the research as the Bell X-5. Germany's wartime experience with the swept wings and its high value for supersonic flight stood in strong contrast to the prevailing views of Allied experts of the era, who commonly espoused their belief in the impossibility of manned vehicles travelling at such speeds. Postwar advancements During the immediate post-war era, several nations were conducting research into high