1,024,969. Helicopter power plant. ROLLSROYCE Ltd. Sept. 14, 1964, No. 37542/64. Headings B7G and B7W. [Also in Division F1] A helicopter power plant comprises a jet propulsion engine and means for directing gases of the engine through a free turbine mounted for rotation about an axis which is normal to the longitudinal axis of said engine, the free turbine being coaxial with and mechanically coupled to the helicopter rotor. A helicopter comprises a fuselage 11 and fixed wings 12, 13 which carry gas turbine engines 14, 15 and rotors 16. Each engine is a by-pass engine in which air from the L.P. compressor which does not pass to the combustion equipment flows through ducts 25 and 27 to be diverted to nozzle 22 or to duct 30 leading to circulation control slits formed in the rotor blades. The combustion gas may flow to the nozzle 22 or may be diverted to drive a free turbine 19 connected by a shaft 18 and epicyclic gearing to a drive shaft 17 for the rotor. The drive mechanisms of the two engines are cross-connected by bevel gears and a shaft 52 to ensure that at least one engine will drive the rotor and the ducts 27, 30 are cross-connected at 40 to ensure that a flow of air will be maintained at the rotor if one engine fails. During take-off and landing valve mechanisms are operated to divert combustion gases to turbine 19 and by-pass air to the rotor blades and in forward flight the valves are operated to divert combustion gases and by-pass air to nozzle 22, lift being provided by wings 12, 13 with the rotor blades positioned fore-and-aft of the fuselage. An alternative engine form is described, mounted on a relatively high wing to reduce the length of shaft 18, in which the by-pass air duct terminates in a nozzle separate from the nozzle for the combustion gases.