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I need help clarifying terminology between helicopter manufactures.

 

Torque = Torque

TOT= T4. Temp of gas producing stage

N1 = NG. % rotation speed of the 1st (compression) stage

N2 = NF. % rotation speed of power turbine stage

NR = % rotation speed or the rotor

 

I'm aware that you can NG out in an A-star, but N1 is never a factor in the MD unless there is an engine problem? Why would that be? Am I missing something or confused?

Also, I'm a little embarrassed to ask, but how is touque measured? I know it measures by revolutions, but how would that measurment actually give you tourqe on a system. Is torque normally a main rotor gearbox limitation?

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Focusing in on your torque questions:

 

Torque = r x f. http://en.wikipedia.org/wiki/Torque

 

What that means is that torque is a force exerted over a distance. It's not measured in revolutions as you thought but units such as foot-pounds in the US or Deca Newton Meters in Europe for Airbus helicopters. In this regard it's not different than the torque measured as the output of your car.

 

Torque limitations are commonly linked to the transmission or gearboxes, as they're usually not built to transmit the full power that can be created by the engines.

 

Hope that helps. I'll let someone else talk to your other Q's.

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N1 or NG refers to the gas producer. Since the gas producer and the compressor share a drive shaft, the point is moot. But when you say N1 or NG, most people think gas producer.

 

Torque is not measured in revolutions but it senses oil pressure at the engine. That's why they always tell you if you have abnormal or fluctuating torque indications, you should next check engine oil pressure, as they are related. Torque is usually limited by the transmission, mast, head and drive shaft.

 

I have never exceeded NG but it can happen at higher altitudes and power settings since the engine has to work that much harder to maintain NR.

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To simplify it, think of the way a turbine engine looks in the most simplistic way with the intake, compression, ignition and exhaust stages. The Ng is the gas producer, which means it's the first set of turbine blades, the ones that suck the air into the engine. When starting a turbine engine Ng is your go-to instrument to watch for how well the start up is going. Not only does it supply air for the fuel/air mixture, it also supplies cooling air for the rest of the engine since they can easily run at 700-800 degrees centigrade. So NG is an indication of how fast the turbine is spinning to pull in enough air to mix with fuel as well as enough air to cool the engine. NG increases with power produced and an increase in NG increases fuel consumption. The limit of the NG is the centrifugal stresses imposed on the turbine blades. So when it's cold out, and you have lots of power and the helicopter can handle the loads, you'll reach an NG limit before anything else. Now on a hot day (assuming torque still isn't a limiting factor) the T4 or TOT will be a limiting factor due to the NG's inability to cool the engine.

 

Torque is a lot like the manifold pressure on the piston helicopters. It's a measurement of the pressure inside the engine and it uses engine oil as a measurement. As you require more power out of the helicopter, it increases the pressure inside the engine. Very simply, in the engine stage where the power produced by the engine is transferred to the drive train through a series of gears, one of the gears in placed on a pinion and allowed to move forward and backward. As the pressure inside the engine increases or decreases and the gear moves, it allows more or less engine oil to leak into a chamber. That chamber measures the flow of oil and measures the torque output of the engine based on the amount of oil that drips through. Torque will be the limiting factor on hot days in the summer. Over-torqing an engine creates so much pressure inside the engine that it can warp and damage it.

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I guess I'm an "old guy", I remember when it was gas generator/compressor (N1/NG) and power turbine (N2/NF).

 

"Zippiesdrainage" has it pretty much right, except when the air is dense your compressor (N1/NG) won't have to turn as fast to produce power, which means lower readings, and you'll torque limit first. Vice versa high DA, your first limit will be NG and/or temp.

 

T4 is some kind of engineer gibberish related to exactly where the temperature probe(s) is(are) in the engine. I guess there's T1, T2, and T3 as well as T4 postition. You'll also hear the temp reading referred to as TOT on some engines. If the the gauge has a redline, I don't really care if it's a TOT or T4, or whatever.

 

Torque is generally measured at the engine, kinda like posted. The 222 used some kind of mast sensor for torque, so there's other ways of measuring that.

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I guess I'm an "old guy", I remember when it was gas generator/compressor (N1/NG) and power turbine (N2/NF).

 

"Zippiesdrainage" has it pretty much right, except when the air is dense your compressor (N1/NG) won't have to turn as fast to produce power, which means lower readings, and you'll torque limit first. Vice versa high DA, your first limit will be NG and/or temp.

 

T4 is some kind of engineer gibberish related to exactly where the temperature probe(s) is(are) in the engine. I guess there's T1, T2, and T3 as well as T4 postition. You'll also hear the temp reading referred to as TOT on some engines. If the the gauge has a redline, I don't really care if it's a TOT or T4, or whatever.

 

Torque is generally measured at the engine, kinda like posted. The 222 used some kind of mast sensor for torque, so there's other ways of measuring that.

 

And the UH-60 uses a sensor on the engine output shaft for torque. So lots of different ways to measure it. Torque is also sometimes referred to as power, which is not technically correct, but since the NP/NR stays relatively constant at 100% it's usually not incorrect in practice.

 

As the various T*T terms imply, turbine temperature can be measured at different places as well. Turbine inlet, turbine outlet, or somewhere in between the various turbine stages. Wally is right though, wherever it's measured it will have a max limit and that's all you really need to know. It's a useful measurement though, since it will often be the limiting factor of the power your engines can produce, and the limit won't change based on the atmospheric conditions, so it can be used as a more reliable indicator than torque or MP.

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The Delta Ng is the easier way of reading the Ng. Old steam gauge Ng's are a simple gauge but might get a bit hairy to read limitations when numbers aren't simple rounded numbers and have decimal points in them. The Delta Ng gives you a digital display for more precise use of power and limitations as well as an additional gauge which uses a computer to determine limitations based on OAT and DA which it uses to calculate onto a very simple gauge that ranges from -5 through +5. The number 0, right in the middle of the gauge is your takeoff max power limit, (easier than trying to see 101.9 on the old steam gauge) and a nice yellow arc from -3.5 to 0 to let you know how much your pulling past your MCP toward your Max takeoff power.

 

So think of a bit like your torque gauge in the essence that it's there to make your life easier with big easy to read limitations and numbers. The numbers on it represent the remaining limit of the stress you can put on the Ng (gas generator turbine) if it's well below 0 (your limit) your fine you're using less than the max, but as it gets close to that 0 start taking notice. It's like a percentage in reverse, not telling you how much of 100% your using it's telling you what percentage is left to be used.

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I'm aware that you can NG out in an A-star, but N1 is never a factor in the MD unless there is an engine problem? Why would that be? Am I missing something or confused?

 

Kind of like the old saying, comparing apples to oranges, both are fruits, but different flavors. It’s like trying to compare two different engine designs, the MD500E’s 250-C20B engine vs. the AS350B’s Arriel 1B engine. It’s not an issue of NG vs. N1. It’s more about different design philosophies and different compressor efficiencies.

 

The ability of the compressor to supply air in sufficient quantity to satisfy the needs of combustion and cooling is effected by atmospheric changes due to temperature and altitude. In order to overcome these atmospheric changes and deliver the maximum mass of air at the correct pressure and velocity, the compressor must operate at very high rotational speeds.

 

Due to the vibratory stresses to turbine components being subject to high centrifugal loads at high temperatures, the limiting of exhaust gas temperatures is also reliant on the compressor’s ability to move large airflows.

 

However, there is a definite RPM limit in the form of tip speed on both the axial flow and centrifugal compressor. The speed at the tips of the rotating airfoils (axial and centrifugal stages) are governed to keep the airflow over any part of the airfoils from creating shock waves when the tips reach the speed of sound. These shock waves are due to compressibility that result in loss of compression and compressor efficiency.

 

Older compressor designs permitted airflow only in subsonic ranges. Newer designs have allowed for Mach 1.2 or above. The compressibility effects are controlled by innovations made in compressor and diffuser design.

 

As an example, the Arriel 1B compressor is made up of a single stage transonic axial compressor feeding a single stage centrifugal compressor. The Arriel 1B compressor is able to cover a wider range of atmospheric changes due to increased temperature and altitude; therefore, T4 topping is normally not an issue. The 250-C20B has a narrower range and compressor efficiency decreases with increased temperature and altitude at the higher RPM (N1) range, resulting in the engine topping at its TOT limit.

 

The older AS350-B had the Max Ng placard that was used to find the Max T/O Ng. The delta-Ng gage was a welcome improvement. We covered this subject in a May 2014 post: Link Post "Delta Ng", May 2014.

 

The delta-Ng gage is an easier way to account for changes in temperature and altitude and determine the correct Max T/O Ng. It was done manually with the following placard:

 

Scan-1_zpslyn21leg.jpg

 

Ratings 250-C20B Airrel 1B

N1/Ng @ 100%: 50,970 RPM 51,800 RPM

N2/Nf @ 100%: 33,290 RPM 39,794 RPM

MAX TOT/T4 793ºC 810ºC

MAX SHP 420 HP 641HP

Wt.: 158 Lbs. 253 Lbs.

Stages of compression

6 Axial; 1 centrifugal (250-C20B)

1 Axial; 1 centrifugal (Airrel 1B)

 

To see the effects the compressor’s diameter has on tip speed and airflow, take a 6” diameter compressor at the 51,800 RPM @ sea level. That would yield a Mach 1.2 tip speed. However, there’s a trade-off between tip speed and diameter.

 

An increase in compressor diameter increases the airflow by the square of the change; doubling the diameter quadruples the airflow for a given rotational speed.

 

As an example, the larger engines like the PT6 moves more air; however, their N1/Ng speeds are in the 30,000 - 33,000 range.

 

Tip speed (ft./sec) = π x Diameter in ft. x (RPM/60)

Speed of sound at sea level =1116.9 ft./sec

Edited by iChris
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