Boeing Vertol BV-347

The Boeing Vertol BV-347, also known as the Model 347, is an experimental helicopter that Boeing Vertol built from a converted CH-47A Chinook and used to test potential Chinook upgrades, several of which were later built into subsequent CH-47 versions. Development began in January 1969 when the US Army contracted Boeing Vertol to develop an improved Chinook; the Army loaned a single CH-47A, serial number 65-7992, in exchange for exclusive rights to the research data, and Boeing funded two phases of modification. Phase I brought redesigned four-bladed rotors of increased diameter, a fuselage stretched by 110 inches that cut rotor overlap from 35 to 22 percent, a raised rear pylon, uprated T55-L-11 engines and a quieter cockpit. Phase II added small variable-incidence wings with full-span flaps that gave extra lift and allowed a 60 degree banked turn. It first flew in Phase I configuration on 27 May 1970.