Torque Gauges
 

My experience of torque gauges has all been on turbine helicopters, from the Westland Scout to the Chinook, where the gauges always measure in percentage of maximum.

My only experience of fixed-wing gauges has been on home flight simulators where there seems to be a bewildering array of figures measuring I know not what?

Is there an industry standard other than percentages? If so, what is the unit of measurement?

Thank you in advance

The unit of measurement is always mechanical torque i.e. a known force applied at a known radius from the axis of the coupling shaft. Typically the unit is a pound-foot (*) or perhaps in EU nations, a newton-meter.

That said, the cockpit may be simplified by calling the engine (or gearbox) spec value 100%, and scaling the instrument appropriately. Makes for easier type-to-type transition.

The same scaling occurs in RPM instruments on turbine equipment. Rather than specifying max power as 17,876 rpm, that value gets labelled 100% and the gage calibrated from there.

* The unit of physical work is a foot-pound per Watt's definition. (Note the labels are swapped)

You should go fly a SAAB 340. It has helicopter engines (GE CT-7) and it's in a percent format, as well.

About the same speeds, too. ;)

Many thanks for the comments.

Do many aircraft indicate torque in ft/lbs or do most use percentages?

You'll find torque gauges in the C-130K/H models, which present in inch-pounds...

DH8-300 and -400 show torque in %. On the -300, the absolute maximum (maximum takeoff power, only used in single engine operation) was 105% though as opposed to normal takeoff power at 95%, so 100% must have been a somewhat random number. If I remember correctly, the Piper Cheyenne indicated ftlb.

On some newer, smaller turboprops (PAC750XL) the torque gauge is measured in psi. I also belive the PC-9 is the same.


Weird as psi is not a torque unit. I don't remember exactly how it is measured, but I have a vague recollection that it is something to do with measure the force of oil being thrown off the main shaft as it spins, this being related to the torque that the engine is producing.....or something like that.

The DHC6 Twin Otter used ft/lbs. I think the same goes for most of the PT6 series.

The Alison engine in the CV580 and L 188 indicate torque in horsepower, which to me seems the most sensible way.

Older helicopters such as the Whirlwind and Wessex have gauges marked in Ft/lbs.

But then as long as there is a clearly marked limit and you don't go over it, what do units matter to the pilot?

The Puma HC1 didn't have torque gauges at all, only a main rotor collective pitch gauge. The CP limits changed with the flight regime and that meant you had to know the graph!

Big Pistons Forever:Yes, it's a nicety made possible by the fact it's a single-shaft engine running a single constant RPM. But if RPM varies, the HP varies proportionately. The equation for SHP goes like this, if torque Q is expressed in lb-ft:

SHP = Q x RPM / 5252

Also please note the torque is NOT ft/lb nor in/lb - torque is the PRODUCT of force and radius, not a fraction of the two, regardless of how the gage is marked. :*

But some torque sensors are hydraulic devices (particularly on large recips, and some helo gearboxes), and the torque is calibrated as some fluid pressure. In that case the gage will read psi or kPa or whatever, and the conversion to true torque will have some constant factor in the equation.

I wouldn't say the 501 is indicating torque in HP. HP is measure of work. It's using a torque measuring system to show how hard the engine is working. Same as a BMEP gauge. It's a number telling you what's going on in the engine derived from a torque measurement.

True but a distinction of zero practical use. Pilots are interested in the power the engine is putting out. The most intuitive and obvious manifestation of that is to tell the pilot what the horsepower the engine is producing.

You are of course correct. I should have been been clearer in my post as a hp reading is easy to provide in a constant speed engine.

I find it interesting that a Cessna 210 I occasionally fly has a GEM engine monitor which among a bunch of other things gives a % of total horsepower value. This makes setting power easy as all the performance charts are presented in % horsepower.

A similar system would be easy to provide for a PT6 type engine but I guess there is not the interest.

HP, N1, EPR, RPM, MAP, BMEP and torque are just numbers to pilots. While it is cool to look down and see 16,000 HP rolling down the runway, I don't think most pilots are doing the math, "If I can raise 8,800,000 pounds one foot in one second and we weigh 120,000 pounds I'll climb at 73 feet/sec." And since the engines aren't lifting the plane that's not what's going to happen.

Wasn't 243 BMEP in an R2800 = 2400 HP? All just numbers.

I have flown aircraft where the engine power was measured and set by: Engine/Prop RPM, (fixed pitch piston props),
"Inches of mercury" Manifold Pressure (constant speed piston props),
Percent engine RPM (simple jets),
Torque pressure in Foot-Pounds. (turbo-props)
Torque pressure in PSI, (turbo-props)

It doesn't really seem to matter what the units are, or even what they measure. Ultimately they are just the numbers you expect to see or expect to set, under various conditions of flight.

I don't really see how I would be better off with the instrument indicating horsepower, (or kilowatts), or what I've currently got, which is psi. It is just a number, like they all are.

Helicopter pilots need to know torque rather than horsepower because the transmission is limited to the amount of torque it can safely handle, rather than how much the engine/s can produce. The engine/s driveshafts may be limited differently to to the main gearbox and there are usually different limits for takeoff, cruise, single engine situations.

Most Beech products are ft/lbs, I believe the DHC-6, and the PC12 are "/#'s torque. No Idea about other PT6 airplanes. Please correct me if I'm wrong

My original point was that I had only seen torque meters in helicopters where all those I have flown have used percentages. I was interested to see what, if any, the alternatives were. There clearly are several and, as has been mentioned, if you know the figures it makes little difference.

The manufacturers must have reasons for using different methods. Anyhow, my thanks to all for taking the trouble to reply.

Boslandew

One must always make due allowance for drift..... ;)