Whether it be a Free Power Turbine or Fixed Turbine, all helicopters require a clutch mechanism. The fixed turbine just needs a somewhat more complex arrangement than the simple sprag clutch on a free turbine. Engineering wise the free turbine has the advantage that the N1 and N2 shaft turbines can be optimised in design to perform the individual task required, in the same way that some RR turbo fans have three shafts.