Pistons, Turbines, and Torque

[puntosaurus]

Why do piston engined helicopters not have torque measurement gauges ? Is Mainfold Pressure a better proxy for torque than any of the turbine operating parameters ? Are turbines just more able to explore torque limits ?

[riff_raff]

puntosaurus,

I can't answer your specific question regarding engine torque measurement provided for different engine types, but here's some basic issues with each type of powerplant regarding their output torque characteristics:

Turboshaft engines tend to have a much smoother and more uniform torque output due to their continuous type of combustion. Aircraft turboshaft engines normally employ some sort of torquemeter on their output shaft by necessity, since the driveshaft connecting the power turbine assy is designed to be as lightweight as possible, and thus has very little strength margin for excessive torque loads. The torquemeter on the output shaft allows the engine torques to be closely monitored.

Recip piston engines tend to have a torque output that varies widely during a single rotation of the crankshaft, due to its intermittent type of combustion. Depending upon the number of engine cylinders and firing order, the instantaneous output torque can be up to twice as great as the mean brake torque value. Since the engine and rotor drivetrain must be designed to cope with these higher instantaneous torque loads, this type of drivetrain would naturally have more robust torque margins than a turboshaft drivetrain.

Regarding monitoring of manifold pressure on a gasoline recip engine, manifold pressure is a fairly good indicator of how hard the engine is being asked to work. But as for the actual instantaneous torques that the engine might be producing on the drivetrain for a particular MAP value, that relationship might be a little tenuous over time and varying operating environments. Direct torque measurement, such as strain measurement in turbine engine torquemeter shafts, would be much more appropriate.

Good luck.
riff raff

[John R81]

Like a car coming to a hill, more throttle needed to maintain same speed - and same gear, same speed = same RPM. More power needed to overcome the climb at same speed and weight.

[[email protected]]

In answer to the original question it is probably to do with power - a recip runs out of power available before it can get near to causing torque problems for a gearbox but a turbine, being far more powerful, is more likely to reach a gearbox/drivetrain limit before running out of puff.

[mini]

Torque is the only measurable output from a rotary engine, be it turbine or IC.

Power is calculated from torque and time, i.e. work done.

A typical piston engine will have a Torque Curve, a graphical representation of turning effort (Torque) vs RPM. A mathematical calculation applied to the torque and the RPM will result in the power (BHP) at that instant, the same calculation carried out over the entire torque/rpm curve will result in a power curve.

Thus, manifold depression is not a straightforward indication of output torque, unless you compute the RPM into the equation - and assuming its carburettors...

I suspect the reason piston powered helo's don't have torque indicators is more cost driven TBH. The tech application is the same regardless of propulsion source.

[VeeAny]

Puntosaurus

This came up at the last Redhill safety evening.

Manifold pressure is at best a predictor of engine power output, torque is a measurement of it.

MP is measured on the input side of the engine the best we can do is use the relationship between MP and HP for a fully servicable engine to predict what the engine power output will be, luckily for all the piston engines whose charts I've looked at, with constant RPM (like we usually operate in helicopters) the MP vs HP curve is a straight line.

Torque on the other hand is measured on the output side of the engine and if the engine is off spec the Tq measured is still what you are getting, the N1 or TOT you may be seeing to achieve it may not be where they should be for a spec engine but the Tq output is still what the gauge says.

Take the example of running on 1 Mag for instance engine power output is not what it would be when running on 2 Mags, to achieve the same power (at constant RPM) the MAP must increase to maintain RPM often by 2 inches, there is no increase in Tq or power output for this MAP increase, we just have no way of measuring the power or Tq output.

Now consider a turbine engine with a dirty compressor the N1 may be 5 or 10% higher (illustrative figure only) for the conditions of the day to achieve the same Tq, we can see the input side of the engine is not right and take maintenance action accordingly (if we notice) but still respect the Tq limits of the engine (not normally limiting until OEI) and drivetrain.

Consider that if you climb at constant Tq in a turbine engined helicopter power output stays constant, if you climb at constant MP in a Lycoming engined piston (and probably other brands, but I'll stick to what is provable) power output increases. Both of these conditions would be done at constant RPM to be true, but thats how we usually fly.

The MP indicator is a very useful tool if you appreciate its limitations, the Tq gauge is much more useful, particualrly when combined with its supporting gauges showing N1 and TOT, or NG and TIT or T4 or whatever anyone wants to call them.

Using manifold pressure as a proxy for torque I suppose is like using TOT alone to predict engine power output, you could do it, but why would you if you have a choice.

I guess mini is right its a cost issue rather than anything else.

The obvious weakness to whichever way you predict or measure power output is the indicating system itself if it is giving false readings, whilst the physics won't change what you think you are doing with your engine clearly won't be.

[ShyTorque]

If a piston engined helicopter pilot used a Tq gauge rather than MAP, the possibilty of overboosting would also be more of an issue.

[Quick Release]

Cheaper and easy... why build an intricate torque mechanism for your piston engine and possibly an acc gearbox to accommodate it when you have a MP gauge to work with....

[lelebebbel]

the possiblity of overstressing the machine is the same, no matter what instrument you use. It's just a matter of

a) knowing about it (i.e. having an instrument accurate enough to tell you that you have positively exceeded a limitation)

and

b) the consequences (inspection or maintenance required by the maintenance manual after said indication)

Apparently, the transmission systems on todays piston helicopters are overbuilt enough that the manufacturers don't require any maintenance after an overboost. They could, if they wanted to, very well say that you have to pull the transmission apart after a MAP exceedance.

There are some turbine helicopters out there without a torque gage or limit.


Back to the original question:
Measuring MAP is a very easy way to get a fairly accurate idea of the engine output on a piston engine, and it is incredibly easy to measure, thus cheap.
You can't do this on a turbine, so you need the more complicated torque gage system - which also happens to be more accurate.

[ShyTorque]

I was referring to overboosting being a potential cause of detonation damage to the engine, not transmission output.

[puntosaurus]

Some good intel here, and lots of possible answers.

It's possible it's a cost thing, but I find that hard to fathom. If Tq is the best measure then why isn't it used on pistons. I can't believe an oil pressure based Tq measurement system is that hard or expensive to install. Pistons have gearboxes too, although maybe not within the engine itself.

I'm inclined to believe that there is something inherent in turbines which allows them to deliver so much torque (less mass to accelerate within the engine ?) that you need to monitor it. I'm thinking also that maybe the hostility of the environment in which the free turbine and drive shaft operate, and the fragility of the components make it important to measure the torque parameter.

Open to further suggestions; where are the turbine design people when you need them ?

[birrddog]

Helicopters with piston engines typically(?) have a belt to attach the drive shaft to the transmission, where as turbines are connected by gears to the free turbine.

I would guess the impact of torque on a gear system in the turbine would be more critical than a somewhat elastic belt in the piston configuration.

[riff_raff]

puntosaurus,

With any rotorcraft drivetrain, the thing that matters most are the instantaneous torques that pass through the drivetrain's parts (gears, bearings, shafts, sprag clutches, flex couplings, splines, etc.). And it's important to note that it's instantaneous torques, and not mean torques, that matter. The stresses that a rotorcraft's drivetrain components experience are mostly a function of torque, not power (although some drivetrain parts, such as spiral bevel gear sets, are power limited due to scoring).

As I noted in my previous post, the output torque characteristics of a recip engine are more harsh than a turbine engine due to the intermittent nature of the recip engine's combustion. But any rotorcraft drivetrain must be able to handle instantaneous torque spikes far above the mean, due to things like torsional vibration responses in the system from blade harmonics or gear tooth dynamic loads. That's why torque must be closely monitored with any engine, recip or turbine.

As for measuring torque from a recip engine, in the old days when helos were powered by big air-cooled radial piston engines, the piston engines had a device in the output gearbox called a torquemeter that measured the torque reactions passing through the gearbox. The flight engineer throttled the engines to run at a MAP setting based on the torquemeter reading. The torquemeter was a purely mechanical (hydrostatic) device, so measuring engine torque can be done without complex electronic devices.

[Pilot DAR]

Torque on either a turbine or piston engine can be easily measured by oil pressure from a reduction gear driven reaction measuring system, within the engine's reduction gearbox. All turbine, and the big radial piston engines, had a reduction gearbox as a required element of their design. Today's (mostly Lycoming) piston engines are direct drive to the belt drive, and therefore do not have a reduction gearbox at which the torque could be measured. The big radials in some airplane propeller drive applications also hade a torque measuring/indicating system, oftem presented to the pilot as Brake Mean Effective Pressure (BMEP). This was useful for the pilot to determine power setting, but much more importantly used to sense the loss of torque instantly, so as to actuate a propeller autofeather system much faster than the pilot ever could. Direct drive propeller piston engines cannot and thus do not have this capability.

Measuring the engine torque at the piston engine driven internal oil pump would work in theory, but in practice would be much too variable, as normal oil pump wear, and pressure variability (which is okay for the engine itself to safely operate) would not be precise enough to take any meaningful measure of engine power being produced.

The torque impluse of the piston engine is much greater than the average torque of the very smooth turbine. Taken to extreme, diesel engines have a much greater power (torque) impulse into the [propeller] absorbing the power. This is why metal propellers and diesel engines do not work well together. The power impulse is so great that propeller blade tend to break.

For a piston engine equipped helicopter, the manifold pressure gauge is adequate to indicate the power being transmitted through the rotor drive train. The transmissions have the extra strength to tolerate any variability in power input which the MP gauge cannot indicate. MP guages have an adjustable orifice at the inlet line to the instrument, whose purpose is to damp out the induction system impulses, so the indication to the pilot is smooth. If this is adjusted too open, you will see the pointer flicker, as it tries to indicate every power stroke. If it is adjusted too closed, the instrument will lag, and be slow to indicate changes in power - easy to overboost accidentally in this case!

Pilot DAR

[puntosaurus]

Thanks for that. The radial story clinches it for me. I'm pretty sure torque is measured when torque is important, rather than when it's easy to measure.

So on a big radial, several of pistons are attached to a single crank of the shaft and the loads are more extreme. On a turbine, the little free turbine blades are taking the full twisting force required to turn the rotor, supported only at one end, and doing it whilst exposed to very high temperatures. Your lycosaurus on the other hand has a (relatively) huge crankshaft, each heavy well supported piston taking the load and applying it to a single well supported crank. So torque just isn't as important, at least to the engine, unless you're looking at an overspeed.

I'm sure that's not the full story, and there are gearbox issues as well, but it certainly seems to get to the heart of the issue of when torque needs to be measured. Maybe some maintenance folk would like to comment on what gets inspected in an overtorque, engine, gearbox, or both. Where is the damage mostly found ?