Bristol Type 173

The Bristol Type 173 was a British twin-engined tandem-rotor transport helicopter built by the Bristol Aeroplane Company and designed by Raoul Hafner as a civil passenger helicopter that also drew military interest. It developed the single-rotor Type 171 Sycamore by pairing two Sycamore Leonides engine and rotor installations under a new fuselage, with the rotor gearboxes joined by a shaft so that one engine could drive both rotors. The first Mk 1 prototype hovered on 3 January 1952 and was later evaluated by the Royal Air Force and trialled at sea on HMS Eagle; a second prototype was leased to British European Airways and lost in a 1956 air show accident. The type never entered production but was developed into the Bristol Belvedere, which flew with the Royal Air Force from 1961 to 1969.