Turboprop Torque as power indication????

[wangus]

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??

[mad_jock]

Your going to have to say which engine type you are looking at.

Free turbine is different to fixed shaft.

[wangus]

GE CT-7 Free Turbine
Baffled!

[mad_jock]

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

[wangus]

So you rely on Torque indication more than N2% to control your speed I guess, say on approach, or straight and level for example?

[mad_jock]

I would have thought so, I fly fixed shaft so as such we dont have a N2 it stays the same as the prop rpm is constant.

[Rednobius]

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).

[barit1]

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)

[LimaFoxTango]

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.

[Tu.114]

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.

[jpilotj]

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.

[dixi188]

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.

[wangus]

Many thanks all for your explanations. Appreciate the inputs.

[Flap Sup]

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.

[barit1]

5252 is a value that as far as i remember fits all turboprops.

Correct, so long as torque is measured in pound-feet, Np in rpm, and BOTH refer to the same shaft - gearbox input OR gearbox output.

[De_flieger]

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!!! :8

[ImbracableCrunk]

I think the -5A2 and -9B have a 16.9:1 reduction ratio from the power turbine to the prop.

That still doesn't make that 5252 formula work.

[JammedStab]

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.

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.

[barit1]

If torque is displayed as a %, then you need to know what 100% is equivalent to - AT THE PROP SHAFT.

[ImbracableCrunk]

What is the torque actually measuring?The simple version:

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.

[barit1]

Go to en.wikipedia.org/wiki/torque - then scroll down to "Relationship between torque, power and energy" - for the essential writeup.

[wangus]

Been reading and re-reading all the helpful and interesting contributions. I now find myself struggling on another aspect:

When you complete the climb and level off in the cruise, I assume you don't reduce the "power" levers, rather you pitch to level, accelerate, and ONLY reduce your propeller condition?? Compressor sits around 96% most of time as pointed out.
(And) If you are more or less always at 96% (ish) the free/power turbine is always being subjected to a lot of energy isn't it? Surely 96% + power turbine + 1390 RPM means you are always going to be a "max speed and power."

Hope my puzzlement makes sense! Am I looking too deep?

[barit1]

Basically the right idea, pull back prop rpm for cabin noise, and at cruise altitude, the power turbine can absorb all the available gas energy from the gas generator. Just watch Ng & ITT limits.

[Tu.114]

Not untrue, but mind the engine limitations in this process. Looking at a free turbine engine here.

Consider what the power and condition levers do to the engine - the power lever basically governs the gas generator speeds (type dependent, it normally has direct control on propeller pitch in beta mode as well), while the condition lever sets the RPM the propeller is attempting to maintain (again, type dependent, it usually also governs the fuel shutoff valve for starting and shutting down the engine).

And these two are very much interdependent. Remember the formula "Power = Torque * RPM" - this time let me change it to Torque = Power / RPM to hopefully make it clearer. Now let the gas generator run at constant 96% Nh and pull back the condition lever - as the power provided from the gas generator is constant but the propeller is spinning at a lower RPM, You will see the torque value skyrocket, likely exceeding the engines torque limitation. Also, You will find the exhaust gas temperature (whatever it may be named in Your engine) to have risen - when the power turbine is spinning slower, it will exert a greater drag on the gas flow, causing the temperatures to rise. One might simplify it and say that the ability of the prop to digest the engines power is dependent on its RPM - the higher the RPM, the more power it can accept from the gas generator.

So normally, reducing the propeller RPM will entail also reducing the gas generator speed in order not to overtorque the engine. On the DH8-300, setting climb power was normally done using both hands, one on the power and one on the condition levers, as just pulling back the condition lever with the engines delivering normal takeoff power at 1200rpm and 95% torque would immediately have overtorqued them.

[barit1]

Of course, type dependent!

[De_flieger]

Hey there wangus, hopefully what we do with the CT-7 may be of help. What aircraft are you studying? Our procedures have the condition levers/prop RPM at max only for takeoff, landing, goaround and abnormal situations where maximum power is required. Obviously this is all type and SOP-dependant, but our procedures are takeoff, prop RPM at max (1396) and power levers advanced to set slightly below the required torque with the CTOT system (constant torque on takeoff) adding extra fuel to maintain the constant commanded power level for the takeoff run. At 1000 ft agl we set climb power which involves switching off the CTOT system, reduce power if necessary to a safe level and then move the condition levers to MIN (1230 PRPM) - this is to avoid the type of situation that Tu.114 describes where simply pulling the condition levers to MIN from takeoff power would over-torque or over-temp the engines. After that then manually advance power levers to the required torque. From there to configuring the aircraft for landing we wont touch the condition levers in normal operations.

When I level out and determine the required power from the power chart, typically I dont need to change the power by more than a few percent from the climb setting, we're just going from power taking us up and across to power taking us across a bit faster. Another aside - as the aircraft speeds up from maybe 150 KIAS to ~220 KIAS (or whatever you get on the day) there is a ram effect as, due to airspeed, more air enters the intake, and you might need to reduce the power levers by a couple of percent to maintain the specified torque level. At 1230 PRPM you wont be at max power there. An example might be cruising at 76% torque, 1230 PRPM, 96% Ng and 800 degrees ITT, but the condition levers have been at MIN since 1000 ft AGL. Your procedures may vary of course!

[wangus]

Thanks again for all the information. It is all with regards to SAAB 340B.