In a turbine powered helicopter, the turbine is typically a Turbofan engine. However, unlike Turbofan engines on a jet aircraft, instead of the exhaust gases providing thrust, they are used to spin an auxiliary power shaft, which is what causes the rotor to rotate and provides power to the helicopter. But how, exactly, does a turbine engine work in a modern helicopter?
The majority of turbine engines are high-bypass turbines. This means that immediately past the engine’s intake, the fan assembly (which are effectively a low-pressure compressor) pushes part of the air flow into the main compressor. The rest of the air is pressed into a bypass duct, a separate channel in the engine housing. In high-bypass turbofans, about 40-50% of the air is sent down the bypass duct.
On the surface, this doesn’t make a lot of sense. Don’t you need more air, not less, to make a turbine engine more powerful? In the case of turbofans, however, this is not so. More air is definitely NOT better. Since pressure ratio is the key performance characteristic of a jet engine, and in our case a turbine, designers put a lot of work into increasing this pressure ratio.
If an engine has to compress a lot of air, then the pressure increase is distributed over a large volume. However, by reducing the amount of air that flows into the compressor, more work can be done on a smaller volume, meaning a greater pressure increase. This is very good. Then, the designers increased the rotational speed of the compressor itself. With the compressor stages spinning around quickly, more work is done on the air that IS sent to the compressor, and this again means a greater pressure increase.
And this was where the original turbine designers ground to a halt, because at speed the temperatures that the nickel turbine blades were exposed to were in excess of their melting point. SO, a new process created Single Crystal turbine blades, which had a much higher melting point. On top of this, because they were cast with holes during the forming process, cold air is spread through tiny holes in each blase from the compressor during operation. The bleed holes produce a protective film of air, which keeps the turbine blades from coming into direct contact with the hot exhaust gasses.
Since helicopters don’t use rearward thrust to fly, the gasses are then passed through a second spinning turbine assembly, attached to the auxiliary output shaft, which is itself attached to the gearbox for the main and auxiliary rotors. Even with this added step, these engines produce exponentially-higher power levels than their piston engine counterparts, and are often much smaller and lighter, resulting in high performance aircraft that are light and agile to fly.