used for support on missile bases included a specific requirement to mandate the use of the General Electric T58 turboshaft as a powerplant. The Air Force had a large inventory of these engines on hand for its fleet of HH-3 Jolly Green Giant rescue helicopters and using the same engine for both helicopters would save costs. In response, Bell proposed an upgraded version of the 204B with the T58 engine. Because the T58 output shaft is at the rear, and was thus mounted in front of the transmission on the HH-3, it had to have a separate offset gearbox (SDG or speed decreaser gearbox) at the rear, and shafting to couple to the UH-1 transmission. Twin-engine variants The single-engine UH-1 variants were followed by the twin-engine UH-1N Twin Huey and years later the UH-1Y Venom. Bell began development of the UH-1N for Canada in 1968. It changed to the more powerful Pratt & Whitney Canada PT6T twin-engine set. The US also ordered the helicopter with the USAF receiving it in 1970. Canada's military, the U.S. Marine Corps, and the U.S. Navy first received the model in 1971. In 1996, the USMC launched the H-1 upgrade program via the award of a contract to Bell Helicopter for development of the improved UH-1Y and AH-1Zs variants. The UH-1Y includes a lengthened cabin, four-blade rotor, and two more powerful GE T700 engines. The UH-1Y entered service with the USMC in 2008. Design The Bell UH-1 Iroquois is a utility helicopter designed for military use. It has a metal fuselage of semi-monocoque construction with tubular landing skids and two rotor blades on the main rotor. Early UH-1 models featured a single Lycoming T53 turboshaft engine in versions with power ratings from to . Later UH-1 and related models often featured twin engines and four-blade rotors. All members of the UH-1 family have similar construction. The UH-1H is the most-produced version, and is representative of all types. The main structure consists of two longitudinal main beams that run under the passenger cabin to the nose and back to the tail boom attachment point. The main beams are separated by transverse bulkheads and provide the supporting structure for the cabin, landing gear, under-floor fuel tanks, transmission, engine and tail boom. The main beams are joined at the lift beam, a short aluminum girder structure that is attached to the transmission via a lift link on the