Piasecki YH-16 Transporter - World largest helicopter in 1950s (plus bonus Kaman Huskie, Doak VZ4 and Douglas XV1)

In 1946, the US Army Air Corp issued an requirement for a wide-ranging helicopter capable of rescuing downed strategic bomber crews. The aircraft also had military application as a large troop and cargo transport. Piasecki submitted the Model PV-15, and was awarded a contract for the development of two prototypes, designated YH-16 Transporter. The first prototype was powered by two Pratt & Whitney piston engines and made its first flight in 1953. During construction the second prototype was modified to Model PV-45 standard through the replacement of its piston engines with two Allison turboshafts. The change in power-plants prompted a redesignation to XH-27, though this was changed to YH-16A prior to the aircraft's first flight in 1955. Both transporter variants were at times fitted with varying types of experimental horizontal tail surfaces, but none of these designs were adopted for permanent use. The YH-16A became the world¡¦s first twin turbine helicopter and established an unofficial speed record of 166 mph. In 1956, the crash of the second prototype led to the cancellation of the YH-16 program.