In the type of turbine engine with compressor, the turboshaft engine appeared late, but has been widely used in helicopters and vertical/short-range landing aircraft.
The turboshaft engine began to be mounted on a helicopter in December 1951 for the first flight. At the time it belonged to a turboprop engine and did not have a self-contained system. Later, as helicopters became more common in military and national economies, turboshaft engines gained independence.
In operation and construction, the turboshaft engine is similar to the turboprop engine root. They all evolved from the principle of a turbofan engine, except that the latter turned the fan into a propeller, while the former turned the fan into a helicopter rotor. In addition, the turboshaft engine has its own characteristics: it is usually equipped with a free turbine (ie, a turbine that does not drive the compressor, which is designed for output power), and is mainly used in helicopters and vertical/short-range landing and landing aircraft.
Structurally, the turboshaft engine also has a basic structure of a gas generator such as an intake port, a compressor, a combustion chamber, and a tail pipe, but it is generally equipped with a free turbine, as shown in the figure, the front is a two-stage ordinary turbine. It drives the compressor to keep the engine working. The second stage is the free turbine. The gas works in it. The drive shaft is specially used to drive the helicopter's rotor to rotate and make it fly off. In addition, the gas flowing out of the turbine is ejected through the tail nozzle, and a certain thrust can be generated. Since the jet velocity is not large, the thrust is small, such as the conversion to power, which is only about one tenth of the total power. . Sometimes the spray speed is too small, and even does not produce any thrust. In order to reasonably arrange the structure of the helicopter, the nozzle of the turboshaft engine can be up, down or to the sides, unlike the turbojet engine, which is not backwards. This is conducive to the overall arrangement of the helicopter design.
The turboshaft engine is used in helicopters and it cooperates with the rotor to form the power unit of the helicopter. According to the theory of the turbofan engine, in theory, the larger the diameter of the rotor, the better. The same core engine produces the same cycle power, and the larger the rotor diameter is, the greater the lift generated on the rotor. In fact, due to losses in the energy conversion process, the rotor is not made infinite, so the diameter of the rotor is limited. In general, the air flow through the rotor is 500-1000 times the air flow through the turboshaft engine.
Turboshaft engines have a much higher power-to-weight ratio than the turboshaft engine and another type of powerplant commonly used in helicopters, the piston engine. And in terms of the power generated by the engine, the turboshaft engine is much larger. The power of the turboshaft engine currently in use can reach up to 6,000 horsepower or even 10,000 horsepower, and the pistons are far apart. Economically, the turboshaft engine has a slightly higher fuel consumption than the best piston engine, but the aviation kerosene used is cheaper than the gasoline used by the former, which is partly compensated. Of course, turboshaft engines also have their shortcomings. It is more difficult to manufacture and has higher manufacturing costs. In particular, due to the lower rotational speed of the rotor, it requires a heavier reduction gear system that is heavier than a turboprop, and sometimes it weighs more than half of the total weight of the engine.
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