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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1Before you run your FEA, draw a free body diagram. – Zeiss Ikon Sep 03 '21 at 18:31
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@ZeissIkon, the free body diagram can be drawn. That is what my question is exactly; I am concerned about the engine mount only and I don't know if I am supposed to input torque on it or not. As far as free body diagram for engine is concerned (although which is not my purpose of study) I cannot include the airflow near the turbines, which I think is experiencing the torque (but in opposite direction) in regards with Newton's third law of motion. – Rameez Ul Haq Sep 03 '21 at 18:36
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1I 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. – stevederekson555 Sep 03 '21 at 18:54
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2Draw the free body diagram for the engine -- what's opposing the torque the engine applies to the propeller? – Zeiss Ikon Sep 03 '21 at 19:01
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1@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. – Rameez Ul Haq Sep 03 '21 at 19:29
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@ZeissIkon, just for the record, I don't necessarily believe that there should be something to oppose the torque on the propeller shaft. I mean it can undergo Newton's second law instead of third. – Rameez Ul Haq Sep 03 '21 at 19:36
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@RameezUlHaq A torque on the shaft can't produce an acceleration, unless that means the aircraft spinning out of control. – stevederekson555 Sep 03 '21 at 19:40
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2If the torque reaction isn’t applied through the engine mounts then where is it applied? – Frog Sep 03 '21 at 20:29
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@stevederekson555, I was talking about rotational acceleration on the propeller shaft only and hence on the propeller, not on the engine or engine mount or the aircraft itself. – Rameez Ul Haq Sep 03 '21 at 20:43
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@RameezUlHaq I think that a propellor always transmits a torque to the shaft. – stevederekson555 Sep 03 '21 at 20:48
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@stevederekson555, everybody knows that here, dude. Actually, its the other way round. The shaft transmits the torque to the propeller. But this idea that torque results in an opposite torque on the engine casing and hence on the engine mount (since mount is connected to the engine casing) confuses me. – Rameez Ul Haq Sep 03 '21 at 20:53
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@RameezUlHaq You misunderstood my comment. In a stationary state the aerodynamic torque on the propellor that is transmitted to the shaft is greater than what is transmitted to the propellor, so the shaft responds with a reaction torque which is what causes stress on the structure. – stevederekson555 Sep 03 '21 at 21:54
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@stevederekson555, stress on what structure? Stresses on the turbine blades only, or the stresses on the engine casing itself (which is attached to the mount)? – Rameez Ul Haq Sep 04 '21 at 09:02
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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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? – Robert DiGiovanni Sep 04 '21 at 02:50
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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? – Rameez Ul Haq Sep 04 '21 at 09:18
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@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. – Peter LeFanu Lumsdaine Sep 04 '21 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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1Good 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?. – Robert DiGiovanni Sep 04 '21 at 10:28
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1Sigh, 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. – Kenn Sebesta Sep 09 '21 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.
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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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The mounts may experience torque when one changes the direction the spinning works is traveling (gyroscopic precession). – Robert DiGiovanni Sep 04 '21 at 02:33
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@Rameez Ul Haaq In this paper the torsional moment is drawn at the aft section of pylon. See page no. 52. Maybe this would help. – Auberron Sep 04 '21 at 03:13
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@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. – Robert DiGiovanni Sep 04 '21 at 03:23
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2Your 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. – Sanchises Sep 04 '21 at 09:02
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@Sanchises by definition a "bearing" holds a moving object in place. Ball bearings allow an object to rotate while it is under gravitation force. In a jet the thrust is backwards, so the bearing must prevent the turbine from being pushed out the back of the nacelle (as well as supporting it under gravity). Picture an astronaut in space trying to turn a screw (without support the astronaut turns). So one could have exhaust gasses from a rocket attached to a turbine to rotate the astronaut the other way (creating twisting or torsion). The thrust net moves everything forward! – Robert DiGiovanni Sep 04 '21 at 09:50
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So, if we want to turn a screw a larger exhaust turbine is needed, so more energy is extracted and less action/reaction thrust is produced (why turbo props jets get most thrust from the prop) (and I'm starting to wonder about fans too). – Robert DiGiovanni Sep 04 '21 at 10:11
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Net thrust is forward, of course. I'm saying that there's typically guide vanes in place to remove angular momentum from the exhaust (also, intermediary turbine stators) that transfer a net torque to the engine housing. Bearings play a negligible role in the transfer of angular momentum (unless friction is very high). – Sanchises Sep 04 '21 at 11:37
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@Sanchises yes, I agree, also with Kenn Sebesta on that point. And you are right with net thrust forward, the bearings must prevent the prop (or fan) shaft from being pulled out of the nacelle too. So the loads on those bearings are fore and aft. The twisting torque force (of the vanes) seems to be taken up by the rim of the nacelle. I remember steel beer cans (compared to thin aluminum). Still not sure if that twist puts any load on the pylon. Great discussion anyways, thanks. – Robert DiGiovanni Sep 04 '21 at 12:09
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@RobertDiGiovanni, the twisting on the fixed vanes (also known as stators) is counter acting the torque produced at the rotors of the turbine (which in turn rotates the propeller shaft)? If it does, then it makes sense that why the engine mount would be taking the same amount of torque produced by the turbine (or to be precise, produced by the rotors of the turbines). And also, if what I said is true, then would the engine casing start rotating in the opposite direction to the shaft rotation if not mounted? – Rameez Ul Haq Sep 04 '21 at 13:54
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1I'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. – Sanchises Sep 04 '21 at 14:52
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@RameezUlHaq the greater danger seems to be deformation of the nacelle. – Robert DiGiovanni Sep 04 '21 at 16:45
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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1Why 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? – Rameez Ul Haq Sep 04 '21 at 13:43
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1There is physical contact between propeller & engine casing via the gearbox. – Mike Sep 04 '21 at 14:03
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1Gyro loads are calculated using the rotational mass inertia, revs and pitch & yaw rates. – Mike Sep 04 '21 at 14:05