Jet (Gas-Turbine) engines are usually thought of as thrust producing engines and not engines that drive rotors or
propellers. Jet engines are used for both thrust production and for drive power. If you have ever heard of
"turboshaft" or "turboprop" engines, they are basically jet engines that do not produce thrust. They take the
power from the production of thrust and turn it into mechanical power to drive rotors or propellers. The way
the turbine blades are shaped determines whether they change thrust into great amounts of power to drive a
shaft, or if they allow most of the thrust to be used as propulsion and take little drive power away to
just drive the compressor section of the engine.
Helicopters (In almost all cases where jets are used) use turboshaft jet engines to power their rotors. The
UH-1 Huey, the Bell Jet Ranger, and the AH-64 Apache all derive their power from gas-turbine engines. Some older
or smaller helicopters use "reciprocating" (Piston) engines for their power source, but most of the helicopters
in use today use gas-turbine engines. They are light, very powerful and economical. The best part is that they are
very reliable as well. Failure rates for gas-turbine engines are very low because there are not as many internal
moving parts as there are in a reciprocating engine.
Lets look at how a jet engine works. A jet engine works on four very simple principles: "Suck, Squeeze, Burn and Blow".
(Stop laughing now...this is serious stuff) The picture posted here shows a simple gas-turbine engine cross section.
In the front of the engine is the compressor section which "Sucks" in air and "Squeezes" it to make it more dense and better for combustion. Air is brought into the compressor by the turning compressor blades that are shaped like little airfoils. It works like a big fan to move air into the engine. In between the moving rows of compressor blades are stationary blade sets called "stators". The stators change the direction of the airflow and help in the compression process. As you can see by the picture, the area that the air can occupy gets smaller as the air travels through the compressor. The air then goes through the diffuser section which transports the air neatly into the combustion chambers, which are in the combustion section. There the air is mixed with fuel and is ignited to create a powerful reaction. (The "Burn" part) The explosive burned fuel and air mixture then travels into the turbine section where the force is turned into a combination of drive power and thrust (or exhaust). If the force is converted mainly into drive power to drive a
transmission, as in most helicopters, then it is referred to as a turboshaft engine. If the force is only converted to enough power to drive the compressor, and the rest is used as thrust, then it is considered to be a turbojet (or thrust producing) engine. (That is the "Blow" part). It is a really basic design, which can get a lot more complicated than this example, but for theory sake, this is an adequate representation of what happens.
An example of a helicopter turboshaft engine.
How does the power get from the engine to the rotors?
The power is transferred from the engine using a main gearbox which changes the power from the engine and sends it to the transmission. In the transmission RPM is reduced from thousands of RPM to hundreds of RPM. By doing this the torque is increased and the rotation is slowed to an acceptable level for the rotor system. The transmission drives the mast which gives direct rotation to the rotors. Often another shaft will come out of the transmission to directly drive the tailrotor as well.
An accessory gearbox mounted on the engine draws little engine power to drive things like the oil pump, the generator and the fuel control for the engine itself.
Runways; they are not just for breakfast anymore.