Originally Posted by RatherBeFlying
Bringing in Newton's any torque measured about a propeller (or jet core for that matter) is matched by another one.
In the piston case, it's applied by the crankshaft and resolved through the crankcase and engine mounts into the airframe.
With turbine engines, the propeller/fan receives torque from the turbine and there is no transmission of torque forces to the airframe (excepting bearing friction).
In both cases, thrust bearings carry that force to the airframe.
So yes, turboprop engines do not transmit torque to the airframe; however, slipstream effects as noted by many are quite significant and generally have a linear relationship to torque.
(For LOMCEVAK too...)
I'm afraid you don't understand the aerodynamics of a turbine very well; you seem to be neglecting the function of turbine stator a.k.a. nozzle guide vanes or turbine diaphragm.
Gases exiting the combustor are traveling straight aft. The turbine nozzle turns this flow in the desired direction of rotation, so that it is caught by the turbine rotor blades to create (you guessed it) torque! But the nozzle vanes, in turning that gasflow, create an equal and opposite (you guessed it) torque!
This latter torque is indeed reacted in the engine cases, but it is a small fraction of the output torque of the gearbox, and besides the turbine case is bolted right to the gearbox, so the turbine torque doesn't get transmitted directly to the mounts.
Let me give you some sample numbers:
Power Turbine rpm = 20000; PT torque=500 lb-ft
Shaft hp = rpm x torque/5252 = 1904 shp (see note)
Prop reduction ratio = 15:1
Prop rpm = 20000/15 = 1333 rpm
Prop torque = 500 x 15 = 7500 lb-ft
By the same formula, the prop is absorbing 1904 shp.
Note the constant 5252 applies to any rotating machine where the units are rpm and lb-ft.
