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.
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1$\begingroup$ Before you run your FEA, draw a free body diagram. $\endgroup$– Zeiss IkonCommented Sep 3, 2021 at 18:31
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1$\begingroup$ I can't tell you for sure, but a helicopter's main rotor causes a torque on the entire aircraft, that is counteracted with the tail rotor. I imagine the propeller itself on a turboprop causes a torque on the mount. $\endgroup$– stevederekson555Commented Sep 3, 2021 at 18:54
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2$\begingroup$ Draw the free body diagram for the engine -- what's opposing the torque the engine applies to the propeller? $\endgroup$– Zeiss IkonCommented Sep 3, 2021 at 19:01
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1$\begingroup$ @ZeissIkon, since the turbine applies the torque on the propeller shaft, there must be something which should oppose this torque. The turbine doesn't have a direct physical connection with the engine casing. So I am thinking its the airflow around the turbines which is opposing this torque. $\endgroup$– Rameez Ul HaqCommented Sep 3, 2021 at 19:29
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2$\begingroup$ If the torque reaction isn’t applied through the engine mounts then where is it applied? $\endgroup$– FrogCommented Sep 3, 2021 at 20:29
5 Answers
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.
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$\begingroup$ I'm hearing you on the point a transmission or reduction gear can take the prop (or rotor) load away from the center of the drive shaft, producing torque. With a helicopter, it is geared to drive the rotor that is 90 degrees from the jet turbine shaft, no? $\endgroup$ Commented Sep 4, 2021 at 2:50
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$\begingroup$ John K, just to be clear here, so the torque that the propeller shaft is experiencing is exactly equal to what the engine casing is experiencing, and they both are equal to the torque produced at the center of the turbine disc? Or the torque produced at the center of the turbine disc is equally distributed to the propeller shaft and the engine casing? $\endgroup$ Commented Sep 4, 2021 at 9:18
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$\begingroup$ @RameezUlHaq: To a first approximation, the shaft and the casing both experience the full torque produced by the disc. It’s like if you have a vertical stack of cardboard boxes with a heavy weight on top: each box experiences the full weight, the weight doesn’t get distributed through the column. $\endgroup$ Commented Sep 4, 2021 at 10:05
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.
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1$\begingroup$ Good point. Torque Force on the stationary vanes will try to twist the nacelle. So twist a tube made of aluminum foil and one made of 1/4 inch steel. What happens? So again, is there torque on the pylon holding the nacelle?. $\endgroup$ Commented Sep 4, 2021 at 10:28
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1$\begingroup$ Sigh, always these downvoters who never explain. I'd love to know what part I have missed in the very simple Newtonian mechanics of air impinging on a stationary surface. $\endgroup$ Commented Sep 9, 2021 at 20:30
Yes, it does...
We can regard the turbine as a 'black box' whose output is the rotation of a shaft. If that output finds a resisting torque, of any sort, the entire 'black box' will react, on its mount, with a torque of equal magnitude and opposite sense.
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.
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$\begingroup$ The mounts may experience torque when one changes the direction the spinning works is traveling (gyroscopic precession). $\endgroup$ Commented Sep 4, 2021 at 2:33
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$\begingroup$ @Auberron thanks, yes, we must also consider torque on the mounts from thrust. Notice the pylon attachment points to the wing would also be stressed in that manner. $\endgroup$ Commented Sep 4, 2021 at 3:23
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2$\begingroup$ Your answer is correct up to the penultimate paragraph. However, you will find that turbine torque is not balanced with prop torque - by design(!) For the engine mounts not to experience torque, the propwash and exhaust angular momentum most be equal and opposite. That would mean a lot of angular momentum is wasted in the exhaust stream. $\endgroup$ Commented Sep 4, 2021 at 9:02
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1$\begingroup$ I'm saying that any engineer worth their salt will make sure that there is a torque load on the pylon because else they're wasting valuable kinetic energy on angular momentum in the exhaust. $\endgroup$ Commented Sep 4, 2021 at 14:52
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!
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1$\begingroup$ Why would the propeller torque be experienced by the mount if there is no 'one to one' physical connection between the propeller and the engine casing? How can one calculate the gyroscopic loads from the propeller? $\endgroup$ Commented Sep 4, 2021 at 13:43
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1$\begingroup$ There is physical contact between propeller & engine casing via the gearbox. $\endgroup$– MikeCommented Sep 4, 2021 at 14:03
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1$\begingroup$ Gyro loads are calculated using the rotational mass inertia, revs and pitch & yaw rates. $\endgroup$– MikeCommented Sep 4, 2021 at 14:05