Turboprop Torque as power indication????
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Turboprop Torque as power indication????
I have been reading up on a type rating course on a turboprop and I am getting confused on the torque indication vs turbine % as a measurement of "power". What is the torque actually measuring. (In a descent, zero thrust, flight idle, does N2 reduce to idle, Prop RPM remain at 1400 ish, and torque reduces to Zero.) I guess I am asking, is Torque your "primary" gauge of power / thrust??
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Can only help with generals then
Torque % is the percentage of your max rated power. It is measured in the gear box by a starin gauge bridge.
Don;t have a clue what the back bits do in a free turbine I am afriad
Torque % is the percentage of your max rated power. It is measured in the gear box by a starin gauge bridge.
Don;t have a clue what the back bits do in a free turbine I am afriad
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The torque gauges are a direct measurement of torque being transferred from the turbine to the propeller gearbox. It's the only thing you use to gauge power (as long as the temps are within range).
For practical purposes the compressor speed is irrelevant, it just does what it does. You rarely even look at it. If your curious though the compressor sits at about 95% during climb and cruise, and I think about high 70s, low 80s for flight idle.
Also there's no N1 or N2 for this engine, only Ng (compressor speed, all on the same spool) and Np (free turbine speed).
For practical purposes the compressor speed is irrelevant, it just does what it does. You rarely even look at it. If your curious though the compressor sits at about 95% during climb and cruise, and I think about high 70s, low 80s for flight idle.
Also there's no N1 or N2 for this engine, only Ng (compressor speed, all on the same spool) and Np (free turbine speed).
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Np = prop speed (and, via the gearbox, implies LP turbine speed)
Ng = compressor / gas generator speed
(Torque x Np) is the power delivered to the slipstream, and drives the aeroplane. Since it's a governed variable pitch prop, Np alone doesn't get the job done.
CT7 torque is measured by wrapup in the LP turbine internal shaft; it is a calibrated torsional spring (i.e. torsion in the shaft is directly proportional to delivered torque)
Ng = compressor / gas generator speed
(Torque x Np) is the power delivered to the slipstream, and drives the aeroplane. Since it's a governed variable pitch prop, Np alone doesn't get the job done.
CT7 torque is measured by wrapup in the LP turbine internal shaft; it is a calibrated torsional spring (i.e. torsion in the shaft is directly proportional to delivered torque)
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No N1 and N2 in turboprop engines. The correct terminology are NH being High pressure compressure and NL being Low pressure compressor. This is true for the PW120 series. I know Ng is found in the PT6 series engines.
As has been mentioned, torque, i. e. the force "twisting" the propeller shaft, is indeed generally the main indication for power practically - most power setting tables just refer to a torque value for a given FL/temperature combination.
But to understand these tables, one of course has to know that the power delivered by the engine is Torque * prop RPM (and if so desired * a constant to get this abstract number to match a unit scale, be it hp, ftlb, Nm or whatever). Consider also that 100% torque against a feathered propeller will be usually both way outside of the engines limitations and not produce any noticeable thrust.
This might lead to occasional confusion when comparing the climb power and max continuous power tables for example - on the DH8-300, the torque values for climb power were a lot higher than for MCP, but the MCL tables were for 900rpm while the MCP required 1200rpm. So while the torque looked higher, the power actually provided by the engine was lower on MCL than on MCP.
NH, NL and ITT were secondary indications as long as no value went out of limits, they were just the result of the torque/NP setting and did their own thing apart from that.
But to understand these tables, one of course has to know that the power delivered by the engine is Torque * prop RPM (and if so desired * a constant to get this abstract number to match a unit scale, be it hp, ftlb, Nm or whatever). Consider also that 100% torque against a feathered propeller will be usually both way outside of the engines limitations and not produce any noticeable thrust.
This might lead to occasional confusion when comparing the climb power and max continuous power tables for example - on the DH8-300, the torque values for climb power were a lot higher than for MCP, but the MCL tables were for 900rpm while the MCP required 1200rpm. So while the torque looked higher, the power actually provided by the engine was lower on MCL than on MCP.
NH, NL and ITT were secondary indications as long as no value went out of limits, they were just the result of the torque/NP setting and did their own thing apart from that.
Last edited by Tu.114; 3rd Aug 2012 at 06:40.
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A quick divergence:
In a fixed shaft turbine: when increasing power (with the power leavers)
Is the gas generator wanting to accelerate however the csu is coarsening pitch to absorb the generated power excess and therefore an increase ng is not noted.
In a fixed shaft turbine: when increasing power (with the power leavers)
Is the gas generator wanting to accelerate however the csu is coarsening pitch to absorb the generated power excess and therefore an increase ng is not noted.
jpilotj
Yes youv'e got it.
Although on a turboprop engine the CSU is referred to as Prop Governor or Prop control Unit.
On the L188 Electra (Allison 501 fixed shaft), as you advance the power levers the rpm increases a little until the prop has responded and a steady state is restored. Likewise as the power levers are retarded the rpm reduces a little until the prop has responded.
From memory the normal engine rpm is 13820 (1020rpm at the prop) and during power increase this would rise to about 14300 and fall to about 13500 during reduction. If small power changes are made the rpm change will be much less.
If you slam the power levers from Idle to Max power it takes about 6/10 of a second for the prop to respond and you might see a peak of 14500rpm. This near instant power increase gives excellent "Go Around" performance.
As an aside, the torque gauges are calibrated in Horse Power rather than Foot Pounds of torque.
Yes youv'e got it.
Although on a turboprop engine the CSU is referred to as Prop Governor or Prop control Unit.
On the L188 Electra (Allison 501 fixed shaft), as you advance the power levers the rpm increases a little until the prop has responded and a steady state is restored. Likewise as the power levers are retarded the rpm reduces a little until the prop has responded.
From memory the normal engine rpm is 13820 (1020rpm at the prop) and during power increase this would rise to about 14300 and fall to about 13500 during reduction. If small power changes are made the rpm change will be much less.
If you slam the power levers from Idle to Max power it takes about 6/10 of a second for the prop to respond and you might see a peak of 14500rpm. This near instant power increase gives excellent "Go Around" performance.
As an aside, the torque gauges are calibrated in Horse Power rather than Foot Pounds of torque.
Last edited by dixi188; 3rd Aug 2012 at 14:55.
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A TP has some limitations, SHP, Tq and N1 among others. As the only TP i have ever flown had PT6, ill use that as an example.
To find SHP delivered to the prop use this formula:
SHP = (Tq x N1) / 5252. A PT6-42 would give (2230*2000)/5252=849SHP
5252 is a fixed value that as far as i remember fits all turboprops.
To find SHP delivered to the prop use this formula:
SHP = (Tq x N1) / 5252. A PT6-42 would give (2230*2000)/5252=849SHP
5252 is a fixed value that as far as i remember fits all turboprops.
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5252 is a value that as far as i remember fits all turboprops.
An aside - 
- I dont know about the PT6, but the CT-7 has torque and Ng gauges calibrated in %, and only the prop RPM in actual RPM, so that formula wont work if the original poster is looking at the same installation of the engine/aircraft type as I'm used to. (85% x 1230 ) / 5252 = 19.9 SHP!!! 
- I dont know about the PT6, but the CT-7 has torque and Ng gauges calibrated in %, and only the prop RPM in actual RPM, so that formula wont work if the original poster is looking at the same installation of the engine/aircraft type as I'm used to. (85% x 1230 ) / 5252 = 19.9 SHP!!!
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Remember, different aircraft can use different terminology for the same thing. What was Ng and Np in the Twin Otter was N1 and N2 in the King Air 100 and Beech 99.
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What is the torque actually measuring?
It's the twist or Torque on the power shaft.
There are two tubes. One is connected at one end to the power turbine, and the other to the prop gearbox - this is the shaft that spins the prop, of course. Another tube inside that has one end connected to the first tube and one end is free. The outer tube twists because of the load. The other doesn't twist (no load), and is the reference for the twisting or "Torque" of the outer tube.
A transducer measures the difference in twist et voila, you get %TQ.
Last edited by ImbracableCrunk; 7th Aug 2012 at 16:31.