The N2 turbine drives the rotor via direct mechanical links - reduction gearbox to freewheel unit to short shaft to main gear box to mast to rotor.
Torque is a measure of how much twisting force is required at any particular time to keep the rotor turning at the desired rpm (as set by the N2 governor).
Imagine you had to turn the mast by hand - the faster you had to turn it, the more twist force you'd need to put in, and likewise more drag on the rotor (ie more collective pitch), more twist force. In autorotation, airflow turns the rotor, so no torque required from the engine.

As you may already know, torque is how we describe a force acting at some distance from a pivot point - eg tightening a wheel nut when changing a tyre - the same force applied further out on the wheel brace gives you more torque. As a function of both force and distance, torque is measured in units like foot-pounds, or newton-metres.

The connection with oil pressure is that the device used to measure torque on helicopters, called the torquemeter (funnily enough), on many machines uses small axial movements of two meshing gears in the drive train to allow a greater or lesser amount of pressure oil through a valve to a gauge, giving a more or less direct readout of the torque acting in the system.
Therefore instead of giving us a readout in foot-pounds, we get it in pounds per square inch of oil pressure (psi, as per the Bell 205), or just percent, as in the 206.

Percent of what? you may ask - percent of the maximum rated allowable torque that the engineers say we can put through the gearbox. It's just a figure that the designers work out, so you can often exceed 100 percent if the flight manual says you can - eg 105% for 5 seconds, or whatever.

Anything over the flight manual limits is an overtorque, and means an engineering inspection.

Ok, thanks. The turning the tire nuts example let me wrap my brain around it better, and now I better understand the connection to N2 and Nr.

Thanks.

Starting engine without introducing fuel only cools down the thermocouples, not the whole engine, so we try to keep that method for really urgent situations only. Or am i wrong? Opening the engine and letting it cool on a light breeze helps reaching those 150° though.

Opening the engine cawlings it will help to cool down the engine also be aware of readings after cranking the engine. The temp.reading goes down fast while cranking it but after a few seconds it raises again.Its like when you burn a finger. If you blow on it it stops hurting but when you stop it hurts again.:)

Sí, eso es una buena comparación, helipedro. :D

Wasted as a pilot! Professor of Physics perhaps?
 

Arm out the window, Great explanation. Good to see that Bachelor of Education wasn't in vain! Bit nippy up there in the hills I imagine! Cheerio.

Torque? surely in the US this is called Tork? (cheque = Check, etc...)

Thanks, Sir HC - haven't got to the long white socks and pocket full of pens yet, but.
Not cluey enough to pick who you are either!
Yes, it has been a smidge on the brass monkey side for the last month or so...

Ng Question
 

What is the small needle (not depicted in the gauge pic) indicating? I drew it, now I have to program it, haha.

http://lh3.google.com/benchmark.avio...s.Producer.jpg

I believe you are talking about the 206 N1 gage as mentioned in other threads, if so:

The little needle represents 1% increments of the big needle. Each one of the marks on the big needle is 10% and one turn of the small needle is 10% and thus each mark represnets 1%.

BTW, the picture does not show on my screen...

Regards,


Chuck

Many years ago a senior RAAF MTP was discussing N1 obtained during a MTF on a Huey - got "about" 95% - when challenged "surely you could be more accurate - what about the little needle? "What little needle?" came the reply. Taken to Huey by "junior" MTP and little needle pointed out.
GAGS
E86

Chuck,

You probably need to upgrade something to view the latest .jpg format, not sure what though.

Patrick

A generic 206 panel.....

http://img.photobucket.com/albums/v1...ry/intrum1.jpg

A generic 206 panel.....
 

SASless. Any idea how much the manufacturer charges to fit the needles that are missing on your generic panel?
I guess it's a bit like buying a car, unless you check what items will be fitted to the one you are buying you'll get the bog standard one with a wheel at each corner and an engine somewhere under the bodywork......

BTW - chuckolamofola's answer was correct, even thought he was flying blind......

Patrick_Waugh's .jpg doesn't show on my screen either, and I've got all the latest gizmos for seeing wotsits an all........

Patrick Waugh

Chuck doesn't need to upgrade anything. Your picture link isn't working.

Is this what you were trying to post?

http://img.photobucket.com/albums/v1...asproducer.jpg


Two possible reasons:

1. There's a limit to the length of URLs the PPRuNe software will accept as a links.
   I can't remember what it is - it's not reached very often - but you may have found it.
   "lh3.google.com/benchmark.avionics/RMPhH3eYABI/AAAAAAAAAyE/12VvVfCzFiM/Gas.Producer"
   is a very long link for a little gauge. ;)
2. I'm not sure Google's picture hosting allows direct links. Many hosting services don't.
   Lots of us use Photobucket which does - and it's free.

Heliport

Motoring the engine does more than just cool the thermocouples. Air is sent from the compressor through the entire engine, and out the exhaust. It won't cool the engine completely, but it does cool it down as the air goes through. It can take awhile for the metal to cool completely, and a good rule of thumb is 30 minutes, but you can motor the engine until it reads less than 150 and then start it without much danger of overtemping. I've done it probably hundreds of times, and never had a hot start.

I've been taught to always pop the IGNITOR fuse when you crank the engine for cooling down to below 150 before starting... Is this neccessary or can you simply with-hold the fuel till the TOT drops, and then continue as per normal start proceedures introducing the fuel?

Maybe for safety if there is a chance of fuel still being available to burn in the engine...?

This is SOP according to the Bell manual.

Patrick

The little gauge has a specific name!!

They used a name for it at Flight Safety DFW for both the 212 & 222 ratings.

Could it have been the "Vernier" gauge or something similiar?

Yes, it is a vernier gage. He was a mathmetician who invented a slightly misaligned scale that allowed you to read the last digit to within 10% for any linear scale, so techically the little dial is not a vernier, but it serves that purpose and is so called.

The N1 is a very sensitive parameter. Since the power of the engine is driven by the amount of air the gas producer sends to the burner, and the amount of air is proportional to the square of the N1/Ng, the last few percent do a lot of work for the pilot, and must be shown by a gage that also must show the big swing from idle to max power.

Here is a site that explains verniers.
http://www.upscale.utoronto.ca/PVB/H...r/Vernier.html