Free-turbine turboshaft

A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by a separate turbine, downstream of the gas turbine and is not connected to the gas turbine. This is opposed to the power being extracted from the power spool via a gear box.

The advantage of the free turbine is that the two turbines can operate at different speeds, and that these speeds can vary relative to each other. This is particularly advantageous for varying loads, such as turboprop engines.

Most turboshaft and turboprop engines now use free turbines. This includes those for static power generation, as marine propulsion and particularly for helicopters.

A major market for turboshaft engines is that for helicopters. When turboshaft engines became available in the 1950s, they were rapidly adopted for both new designs and as replacements for piston engines. They offered more power and far better power to weight ratios. Helicopters of this period had barely adequate performance; the switch to a turbine engine could both reduce several hundred pounds of engine weight, 600 lb (270 kg) for the Napier Gazelle of the Westland Wessex, and also allow considerably more payload weight. For the Westland Whirlwind, this converted the inadequate piston-engined HAS.7 to the de Havilland Gnome turbine-powered HAR.9. As one of the first anti-submarine helicopters, the HAS.7 had been so restricted for weight that it could carry either a search sonar or an attack torpedo, but not both.

The free-turbine engine was particularly favoured. It did not require a clutch, as the gas generator could be spun up to operating speed without requiring the output shaft to rotate. For the Wessex this was used to give a particularly fast take-off from a cold start. By locking the main rotor (and the power turbine) with the rotor brake, the engine could be spun up to operating speed, then lit, and when the engine core is at the operating speed of 10,500 rpm the brake is released and drive to the rotor smoothly increased as the power turbine gains speed. This was used to bring the rotor to speed from stationary in just 15 seconds and a time from engine start to take-off of only 30 seconds.

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Wikipedia