How is torque varied on an engine

[rascalsailor]

Hi - was looking at a vid of a TBM850 where the pilot was adjusting engine torque. My question is - how is the torque physically varied?
Is it by changing engine rpm or by varying prop pitch - or similar?
Thanks
Russell

[AerocatS2A]

Same thing really. Normally you’d increase torque by increasing fuel to the engine which increases engine rpm. In response the prop pitch automatically increases to maintain the same prop rpm.

[rascalsailor]

n response the prop pitch automatically increases to maintain the same prop rpm.
Thanks - what is "prop RPM" as opposed to engine RPM?

[AerocatS2A]

The turbine engine fitted to the TBM is a “free turbine” which means the power turbine and propellor are free to spin at a different RPM to the main engine core. The propellor is also geared down significantly so it’s RPM is a lot slower than any of the turbine components.

[rascalsailor]

many thanks

[wiedehopf]

This is also important to get an understanding: https://en.wikipedia.org/wiki/Constant-speed_propeller

So the constant-speed controller is keeping rpm constant, so if you increase power the constant-speed controller will increase pitch to keep the RPM constant.
Altogether increasing power increases torque because the pitch is automatically increased.

[DaveReidUK]

Bear in mind also that a PT6 installation on a transport aircraft will typically have a propeller lever that allows the pilot, within limits, to alter the propeller RPM (i.e. by altering the engine RPM, since there is a fixed relationship between the two). This allows maximum PRPM to be selected on approach so that in the event of a go-around increased thrust is obtained by simply altering the blade angle without the need to acclerate the prop.

I don't know if the PT6 on the TBM features the same facility.

[wiedehopf]

With a given torque limit higher RPM just means higher thrust from the propeller is available, at low speeds at least.

No fixed relationship between engine and prop RPM, it's a free power turbine driven by the gas generator.
Yes the TBM has a prop lever too.

[DaveReidUK]

No fixed relationship between engine and prop RPM, it's a free power turbine driven by the gas generator.

Well the prop is geared to the free turbine (typically around 15:1 on the PT6), that's what I meant. :O

[RVF750]

OK. Think about this from a very basic viewpoint. There are a pair of rpm sensors on the propellor shaft. The difference in the signal from each, indicates a twisting on the shaft. This is the torque applied to it from the engine. The speed of the prop is governed, but the load applied to it from the engine is measured by this twist. That is a torque gauge in essence.

Hope that helps. This is why Turboprops use Torque gauges and piston aircraft use Manifold pressure to determine the engine's load.

[Intruder]

Only slightly off topic...

On the aircraft carrier USS Enterprise, the torque or power provided to each propeller was limited such that the prop shaft twisted no more than 2 complete turns along its length.

[lomapaseo]

Only slightly off topic...

On the aircraft carrier USS Enterprise, the torque or power provided to each propeller was limited such that the prop shaft twisted no more than 2 complete turns along its length.

source please!

I can imagine for a shaft that short that a word or two is wrong

[nonsense]

OK. Think about this from a very basic viewpoint. There are a pair of rpm sensors on the propellor shaft. The difference in the signal from each, indicates a twisting on the shaft. This is the torque applied to it from the engine. The speed of the prop is governed, but the load applied to it from the engine is measured by this twist. That is a torque gauge in essence.

Hope that helps. This is why Turboprops use Torque gauges and piston aircraft use Manifold pressure to determine the engine's load.

If you've got a difference in RPM between the two ends of a shaft then you've got a problem. Soon the shaft will no longer be a shaft, it will become a (coil) spring. (Or a tangled mess, or two shorter shafts)

When there is a torque applied to a shaft, it will deflect; one end will become and remain twisted by a modest amount with respect to the other end (unless it is an aircraft carrier, apparently then the amount may not be modest!) But once the rate of change of torque with respect to time has returned to zero (the torque is now constant) the two ends of the shaft will be rotating at the same RPM.

A difference in RPM indicates a changing torque with respect to time.

[DaveReidUK]

If you've got a difference in RPM between the two ends of a shaft then you've got a problem. Soon the shaft will no longer be a shaft, it will become a (coil) spring. (Or a tangled mess, or two shorter shafts)

I wondered that too, but then I realised that the poster was talking about a phase difference from the RPM sensor rather than a frequency difference.

[Intruder]

source please!

I can imagine for a shaft that short that a word or two is wrong
It's been a long time, but a quick search yields:

https://oldnfo.org/2017/01/29/uss-enterprise/ sez 1 1/4 turns at flank, but that is not necessarily the limit.

https://www.quora.com/What-is-the-true-top-speed-of-a-USN-nuclear-aircraft-carrier sez 1 1/2 turns.

How long do you think "that short" is?!? I don't have the figures for Enterprise yet, but the longest shaft on USS Midway is 448' long, and the shortest is 236'...