Free-turbine turboshaft

, sectioned A free-turbine turboshaft ingests air through an intake. The air passes through a compressor and into a combustor where fuel is mixed with the compressed air and ignited. The combustion gases are expanded through a compressor-driving turbine, and then through a "free" power turbine before being exhausted to the atmosphere. The compressor and its turbine are connected by a common shaft which, together with the combustor, is known as a gas generator, which is modelled using the Brayton Cycle. The (free) power turbine is on a separate shaft. Turboshaft engines are sometimes characterized by the number of spools. This refers to the number of compressor-and-turbine assemblies in the gas generator stage and does not include the free power turbine assembly. As an example, the General Electric T64 is a single-spool design that uses a 14-stage axial compressor; the independent power shaft is coaxial with the gas generator shaft. Risk of overspeed One particular failure scenario, a gearbox failure, showed a free-turbine arrangement to be more at risk than a single-shaft turboprop. It could suffer a turbine overspeed to destruction after losing its connection to the propeller load. (In a single-shaft arrangement with a similar gearbox failure the turbine would still have most of its load from the compressor). Such a failure resulted in the 1954 accident of the second prototype Bristol Britannia, G-ALRX, which was forced to land in the Severn Estuary. A failure in the Bristol Proteus propeller reduction gearbox led to an overspeed and release of the power turbine of Nº3 engine. It cut through the oil tank and started a fire that threatened the integrity of the wing spar. The pilot, Bill Pegg, made a forced landing on the estuary mud. The Proteus gears were redesigned and an emergency fuel shut-off device was fitted to prevent a similar reoccurrence. Writing in 1994, Gunston found it remarkable that protection was not common on free-turbine engines. However, certification regulations allow other methods for preventing excessive overspeed such as disc rubbing and blade interference. == Applications ==

Most turboshaft and turboprop engines now use free turbines. This includes those for static power generation, as marine propulsion and particularly for helicopters. Helicopters showing the circumferential air intake of the Gazelle and the two exhausts (red covers) per side Helicopters are a major market for turboshaft engines. 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. Piston helicopters of this period had barely adequate performance; the switch to a turbine engine could both reduce several hundred pounds of engine weight, for the Napier Gazelle of the Westland Wessex, This was the first Bristol gas turbine and its broad design had been produced by Frank Owner at Tockington Manor. It first ran in July 1945 and in December 1946 was the first turboprop to pass a 100 hour type test. The advantage of the pusher propfan with a free power turbine is its simplicity. The prop blades are attached directly to the outside of the rotating turbine disc. No gearboxes or drive shafts are required. The short length of the rotating components also reduces vibration. The static structure of the engine over this length is a large diameter tube within the turbine. In most designs, two contra-rotating rings of turbine and propeller are used. Intermeshed contra-rotating turbines can act as the guide vanes for each other, removing the need for static vanes. and a flight-rated version, the PLT27, was also developed but lost a major contract to the GE T700 turboshaft. Turboshaft engines were used to power several gas turbine locomotives, most notably using the Turbomeca Turmo in Turbotrain (France) and Turboliner (United States) service. == See also ==