eli5: how do turbines on Helicopter do spin the rotors?


I understand the basic concept of a turbine and also of Helicopter blades / Rotor.

I just can’t figure out how to the turbine transfers the force to rotor, (different rpms, how the transmission works and so on..

In: Engineering

Helicopter rotors are shaft driven. The spinning shaft of the jet turbine feeds into a gearbox that rotates the shaft the rotors are mounted on.

This is probably much easier to understand [visually](https://i.ytimg.com/vi/SvyQUsuhZD8/maxresdefault.jpg). Any internal combustion engine (including jet engines) needs to compress the air coming in to make the ensuing combustion more efficient. Jet engines compress the air with a turbine on the front, which is powered by the exhaust.

As the fuel burns and expands and the air heats up and expands, it fills up the combustion chamber and exits at high velocity. A small amount of that energy is siphoned off by a turbine in the back, which turns a shaft connected to the turbine at the front, which compresses the air. If you’re familiar with a turbocharger in a car, it’s the same idea. A turbo**jet** engine allows the rest of the hot gas mixture to leave the engine and Newton’s 3rd Law (equal and opposite reaction) propels the craft forward. With a turbo**shaft** engine, the turbine in the rear of the engine is much larger, with more fan blades, to capture a lot more of the energy. The exhaust that leaves the engine is only going about as fast as normal car exhaust. All of the other energy turns the shaft, which is connected through a transmission to the main rotor shaft.

Helicopters generally do *not* adjust the rotor speed. Once they are spun up, they continue to go roughly that speed for the duration of the flight. To control the helicopter, the pitch of the blades is changed. Increasing the pitch generates more lift, so the helicopter goes up. Increasing the pitch on the back* tilts the helicopter forward and makes it travel forward. Reduce the pitch of the blades to decrease lift and maintain level flight, or reduce it enough for the helicopter to drop as necessary.

* Due to gyroscopic precession, forces applied to a spinning object (like a helicopter rotor blade) have the most effect about 90° in the direction of rotation from where the force is applied. Since most American helicopters use a counter-clockwise spinning blade, to lift the back of the helicopter the pitch of the blade is increased on the left side (from the perspective of the pilot).