Pilatus PC-12 power setting rationale.

[walnlar]

The Pilatus PC-12, using the P&W PT-6, has the engine torque levels (power) displayed in PSI units (pressure). Being an engineer I cannot relate PSI (pounds per square inch) to levels of torque. Torque has always been in units i.e. - ft./lbs., in./ounces or N/M. What does the PSI unit represent?

[sky's the limit]

We use PSI to measure TQ on certain helicopter types as well. It's simply taking the oil pressure in the transmission and translating that into a TQ value, no different than using a percentage value.

stl

[Boreas]

Being an engineer I cannot relate PSI (pounds per square inch) to levels of torque.

Are you a civil engineer?

Jokes aside, torque can definitely be related to pressure - not expresses, but
related to units of pressure. As STL mentioned, it has to do with the indirect
way in which its measured.

There's a helical spline on the outside of the second reduction gear that displaces
as torque increases. This displacement creates an oil pressure that can be translated
into a torque value.

And so... Fnet = 0

[photofly]

Hopefully an engineer will remember that torques are distances multiplied by forces, and so there should be no oblique stroke in the abbreviation, the stroke indicating a division. And that the correct abbreviation for a metre is a lowercase m. Thus "inch ounce", "ft.lb" and "Nm".

[DonutHole]

lol, awesome.

[GyvAir]

I agree with the gist of the original posters question. Using PSI to indicate torque is rather non-intuitive. Knowing how the system works, you can understand it, but it seems to me that by leaving the indicator marked in PSI instead of ft.lbs or percent torque, they've really left the job unfinished. It's as though after spending all the time engineering and experimenting to get the system to work, they rushed it out the door with an off-the-shelf lab instrument jury-rigged into the panel, rather than taking the last step of ordering up an indicator face marked for the specific installation. To me, it makes no more sense than if it had been an electrical or strain gauge type torque indication system with the indicator marked in milliamps or volts.

[ahramin]

What difference could it possibly make? How does labeling the guage in ft.lbs tell you anything more than labeling it in psi? Does 2000 ft.lbs tell you something useful that 60 psi doesn't?

[Boreas]

What difference could it possibly make? How does labeling the guage in ft.lbs tell you anything more than labeling it in psi? Does 2000 ft.lbs tell you something useful that 60 psi doesn't?

I think it does make a difference. If most of the primary engine parameters were
expressed in 'foreign' units, things would definitely get confusing. Especially in
abnormal situations. I'm sure we can all agree that there's more to it than just
keeping needles inside their respective green ranges...

Is the PSI torque on the steam gauge version or on the EFIS PC12?

[ahramin]

What do you mean by foreign units? Other language or different system or what?

What abnormal situations involve needing to know the torque of the engine in ft.lbs?

[Dagwood]

I fly a constant speed piston engine. Moving the throttle changes the "Manifold Pressure" in inches of mercury. Confusing. I would prefer "Torque", or "Horsepower", or "% Power", but no, the aircraft manufacturers are stuck on the super-confusing manifold pressure as a measurement of power....

[ahramin]

Just flew an RV-7 back from KOSH and the engine instruments gave you a % of horsepower reading, that was pretty cool.

But really, manifold pressure is a reading of the air pressure in the intake manifold, how much simpler could it get? Well, I guess you could hook it up to a metric hectopascals gauge instead of an inhugs gauge but we like to use different systems in North America anyway.

[sky's the limit]

What difference could it possibly make? How does labeling the guage in ft.lbs tell you anything more than labeling it in psi? Does 2000 ft.lbs tell you something useful that 60 psi doesn't?

I think it does make a difference. If most of the primary engine parameters were
expressed in 'foreign' units, things would definitely get confusing. Especially in
abnormal situations. I'm sure we can all agree that there's more to it than just
keeping needles inside their respective green ranges...

Is the PSI torque on the steam gauge version or on the EFIS PC12?

Remove all the units completely and leave a few lines of different colours... the units make zero difference to anything. You're over thinking this one a touch.

stl

[photofly]

I fly a constant speed piston engine. Moving the throttle changes the "Manifold Pressure" in inches of mercury. Confusing. I would prefer "Torque", or "Horsepower", or "% Power", but no, the aircraft manufacturers are stuck on the super-confusing manifold pressure as a measurement of power....

It's appropriate to measure and display manifold pressure in in inches of mercury because inches of mercury is a unit of pressure.

Torque and power are not the same thing. Torque multiplied by RPM will give you a quantity that at least has the correct physical units of power.

Manifold pressure is neither power nor torque, nor a very good indicator of either. For instance, with the engine stopped, the manifold pressure rises to about 30 inches. But it's certainly not producing any power, or developing any torque for that matter.

At a fixed density altitude, manifold pressure multiplied by RPM is a better indicator of power, even though it still has the wrong physical units. For as long as the engine maintains a constant RPM, manifold pressure will give you a proxy idea of power (changes), which is probably how you use it. It's certainly how I use it. But 20" of MP at 1700 rpm is a very different amount of power from 20" of MP at 2300rpm.

If you want a proper power meter on your constant speed piston engine, JP Instruments will sell you one for about $1k. it takes input from the RPM, manifold pressure, static line, fuel flow and OAT, and maybe some other stuff too.



BTW - if you find it super-confusing, that's because you're expecting it to be easy to find a simple quantity to measure that tells you "power" on a piston engine. It isn't simple, and manifold pressure is not it.

[Dagwood]

^thanks photofly. My post was mostly in jest/sarcasm as measuring engine power by manifold pressure doesn't make sense until you understand how it works and what it is actually measuring. Likewise a turboprob power gauge in PSI doesn't make sense until you understand how it works and what it is actually measuring.

Someone flying a PC12 has probably flown a const speed without thinking twice about manifold pressure it's lack of intuitive-ness.

[photofly]

Haha - my bad. Still, hopefully someone found it useful.

[GyvAir]

What difference could it possibly make? How does labeling the guage in ft.lbs tell you anything more than labeling it in psi? Does 2000 ft.lbs tell you something useful that 60 psi doesn't?

By that logic, your truck's speedometer might as well read in driveshaft rotations per minute. Or, as this is an aviation forum, airspeed could just read in PSI.
For a torque indicator, my vote would be for it to be marked in percent maximum torque. I agree that having the actual torque indicated in units isn't of any particular use.

[ahramin]

Actually, by your logic knowing the width of your car in centimeters is as important as knowing your speed. Some numbers are more important or useful than others. Knowing the speed of your aircraft in specific units is quite important for navigation. That's why we changed from mph to knots. A gauge that read your airspeed in psi would be fine for takeoff and landing but would be a pain for figuring out your ETA.

For an engine power gauge I still haven't heard any examples of what useful information is conveyed by knowing the torque instead of the psi.

[Boreas]

Torque [lb ft] x RPM / 5252 = Horsepower
I use that all the time

Seriously though, having the appropriate units on a gauge goes hand-in-hand
with having it properly labeled. It provides a greater level of clarity!

Its not dissimilar to having control levers that are color coded and shaped
differently. Can you fly a plane with all black, spherical-knobbed levers?
Absolutely! Does it help to distinguish them from each other? I definitely
think so.

But I wouldn't be surprised if some of you disagree with that also...

[Colonel Sanders]

airspeed in psi

I wonder if I could get an ASI calibrated in kilograms per square foot? It would go well with a VSI reading in inches per month and an altimeter that displays furlongs above sea level.

[photofly]

airspeed in psi

I wonder if I could get an ASI calibrated in kilograms per square foot?

Nope, because kg isn't a unit of force. But you could get one calibrated in kilograms per squared second per foot

[Colonel Sanders]

Well, how about we assume 1G of gravity on planet Earth?

Admittedly it would not read correctly on Venus.

[photofly]

Well, how about we assume 1G of gravity on planet Earth?

Admittedly it would not read correctly on Venus.

You could put kgf (a horrible kludge meaning the force generated by a mass of 1kg at sea level) but just kg won't do, because the units are wrong.

[Colonel Sanders]

a horrible kludge

remember, you're talking to a software guy

[photofly]

a horrible kludge

remember, you're talking to a software guy

Right. Hardware guys get their units right ... mostly.

[ahramin]

I think I'm starting to get it. An engine gauge with torque as the unit obviously measures engine torque, but to someone who has never flown a PT-6 before the purpose of a gauge marked in psi might not be immediately obvious. Don't see how it would matter as these aren't planes that you just jump into and fly without training on first but it reminds me of sometimes when an inexperienced pilot transitions to a constant speed prop and has trouble wrapping their head around the manifold pressure and rpm gauges.