How exactly is the free shaft part of a gas turbines controlled?

Question

As you can see in the picture, the free turbine still gets the same exhaust gas that the main turbine gets.

• How can it exactly spin freely from the main turbine?
• How can the free shaft even stay stationary while the main turbine is spinning at high rpm even though it is still exposed to the same exact exhaust gas flow which makes the main turbine rotate?
• Why does the exhaust gas also not rotate the free shaft?
• Does the free shaft have a clutch attached to it or something that disconnects it from the driven wheel or the propeller so that wheel or propeller can stay stationary even if engine itself is spinning at high rpm?

I also read that gas turbine engines can act as a clutch or a fluid coupling exactly because of the fact that free shaft can operate freely from the main turbine. Chrysler, for example, in their turbine car removed the torque converter from the TorqueFlite automatic transmission that was attached to the gas turbine because gas turbine itself could act as the torque converter and therefore didn't need a separate torque converter to disconnect the engine from the output shaft.

How exactly is this "free spinning shaft" action accomplished?

Answer 1

The free turbine is literally just a windmill that generates torque when the wind blows on it. You could actually just stick it outside the engine itself behind the tailpipe and it would do the job, although way less efficiently.

The wind is provided by the "gas generator" on a turboprop, or the "core" engine on a turbofan. The torque is created from aerodynamic lift on the blades by the air blowing by, so stalling unstalling has a huge effect on the torque that can be produced.

When the blades are stationary and air starts to flow past the blades, the blades are mostly stalled and produce little lift and therefore little torque.

You can hold the propeller blade of a PT-6 turboprop with your hand to prevent the free turbine from spinning, and start the engine and let it run at ground idle while you hold the blade still (this is done all the time for maintenance purposes, normally using a restraint device like a special strap).

As long as you don't let the blades move, the torque force you have to resist from the mostly stalled turbine blades is very light.

Let the propeller move however, and the blades start to unstall and turbine starts to generate high torque, and you were holding it, you would have to let go and get out of the way.

So as far as controlling it, it's simply a matter of controlling how hard the wind is blowing, which means controlling the flow through the gas generator/core with the normal jet engine controls (regulating fuel flow).

Answer 2

To answer the questions:

1. “How can it exactly spin freely from the main turbine?”

The free shaft i.e. the power turbine is not connected mechanically to the gas generator spool and can turn at independent speeds or even independent directions from the gas generator spool. Typically the power turbine shaft connects directly into a gearbox, which either drives a propeller, in the case of a turboprop, or a rotor in the case of a turboshaft engine.

2. “Does the free shaft have a clutch attached to it or something that disconnects it from the driven wheel or the propeller so that wheel or propeller can stay stationary even if engine itself is spinning at high rpm.”

In aviation applications, there is no clutch between the propeller or rotor transmission gearbox. The fact that the shaft is not linked to the gas generator shaft is what allows it to operate as a fluid clutch.

The power turbine and drive shaft does not need to be mechanically linked to the gas generator spool in order to generate mechanical power, provided the exhaust gases moving over the power turbine undergo a sufficient drop in enthalpy, transferring their energy into mechanical work. In short, the power turbine spins for the same reason as the gas generator turbine section does.

3. “How can the free shaft even stay stationary while the main turbine is spinning at high rpm even though it is still exposed to the same exact exhaust gas flow which makes the main turbine rotate?”

Consequently the power turbine could be mechanically locked stationary while the gas generator continues to operate (provided this does not exceed either the thermal or torque limits of the driveshaft and reduction gearboxes).

4. “Why does the exhaust gas also not rotate the free shaft.”

The exhaust gases DO rotate the free shaft. See my answer above.

I can’t comment on the drive system for the experimental Chrysler car, but one of the few land applications of turbine engines, the power pack for the M1 Abrams main battle tank, I believe uses a clutch between the power turbine driveshaft and the tank’s transmission.