When the turboprop engine is active, the propeller shaft rotates and causes the propeller to rotate as well, which in turn generates thrust. The propeller shaft rotates because of the torque (or twisting moment) created by the turbines of the engine and then which is conveyed to the propeller shaft. My question is that if I am trying to conduct a FEA analysis for the mount at which this engine is attached, should I also apply this torque on it or not? I was thinking that I shouldn't apply any kind of torque at this engine mount since I don't think that engine itself is rotating at all because I don't think there is a physical connection between the turbines (or propeller shaft) and the engine casing (to which the mount is attached). But it is recommended that the torque should always be taken into account while conducting such analysis. Moreover, I don't know the torque (if) experienced by the engine casing and mount would be equal to what the propeller shaft or turbine is experiencing.
The torque applied to the propeller originates in the turbine section, from the lift forces being applied to the turbine blades, being little wings going round and round, by the accelerating gases leaving the burner can.
Therefore, it's the lift forces of the air mass passing the blades that are generating a Newtonian reaction force path from gas pressures acting in the opposite direction through the stator vanes and engine case. So the torque generated at the center of the turbine disc, where the through shaft connects, is seeing a reaction torque force in the opposite direction being applied to the engine casing, wanting to rotate the engine in the opposite direction along the turbine shaft axis.
Forget about the propeller for a minute and imagine you have a turboshaft engine in a helicopter, where there is just a drive shaft running to a completely separate transmission unit. The engine doesn't care that there is a rotor on top of the transmission, it just knows that it's spinning a shaft against resistance. The torque driving the rotor transmission is still originating in the engine's turbine section, and wanting to rotate the engine case in the opposite direction, restrained by the engine's mounts. Of course the torque forces are also generating a reaction force within the transmission wanting to rotate the helicopter opposite to the rotor's torque, necessitating a tail rotor.
With a turboprop engine with an integral propeller reduction gearbox, this reaction torque is being absorbed by the engine mounts, along with thrust loads and gyroscopic precession forces from the propeller, transferred to the propeller gearbox through the shaft bearings of the prop, to the engine case, to the engine mounts.
The result is that the load at any particular mount is the sum of the forces acting at any given time at that point from thrust, torque and precession forces acting on the case. On the other hand, a turboshaft-based turboprop like the General Electric T-64 has the propeller gearbox on a separate mount, so that engine mounts only see rotational torque from the turbine section.
It will depend on where the vanes are.
Turbines have vanes which redirect the flow, on the inlet side helping avoid compressor stall, and on the exhaust side capturing the rotational energy and converting it to thrust.
These vanes are rigidly attached and so the air hitting them will cause a rotational torque on whatever they are attached to.
This is a great question because it challenges generally accepted principles of prop torque generated by a piston engine. One clue in the question states "a twisting moment creating by the turbine", and there were thoughts of the slender pylons on jets: is there a torque stress on the motor mounts?.
Certainly a twisting stress on the turbine shift, whether it is run through a transmission or not. So let's load the prop while the jet is running. Notice that blades are symmetrically arranged around the shaft compared with pistons pushing one at a time away from the center of rotation.
The symmetrical push on all the turbine blades by the jet exhaust gasses does not produce a torque force on the mounts, only a torsional stress on the shaft. The symmetrical drag load of the prop blades does not produce a torque force on the mounts, only a torsional stress on the shaft.
One may surmise that if the turbine torque and the prop load torque are balanced around the center of rotation, there is no torque on the motor mount.
However, if one component comes out of balance (such as the prop), the motor mount may be easily torn off.
For more on turbo-prop designs, check out this question. Seems that Pratt and Whitney designers had some thoughts on the torque issue too.
And in this report the "ovalization" of the nacelle seemed to be of greater concern, solved by increasing the pylon attachment points from 1 to 2, spaced 120 degrees apart.
Interesting discussion. My first thought is that a pure turbojet engine applies zero torque to the mountings but with a turboprop the mountings feel all the propeller torque. Not to be overlooked are the 1P and gyroscopic loads from the propeller (and gyros from engine of course) which are significant. Stressing the mounting for a turboprop is rather more complex than a pure jet ! And I'll add my first stressing wisdom - draw a free bogy diagram!