N1....N2, Bingo!

[Barannfin]

I have no turbine time so I am just looking for some simple clarification and explanation. I would like to know what exactly N1 and N2 are.

My understanding of the turbine engines as installed in helicopters is that first the air is compressed and ignited in the first stage of the engine (N1?) and then goes through more turbine wheels (N2?) that are not directly connected to the first stage. And it is these that are connected to the tranny and transfer the energy to the rotors.

Is this anywhere near correct?

[Nick Lappos]

Yep!

The turbines that we use are free turbines, actually two turbines that are parked close together. The Gas Turbine is the one that makes the stream of hot gas that will eventualy make all the output power. The gas producer has a compressor that packs the air, a combustion section that fuels it and burns it, and a hot section that extracts some of the power so that the compressor can be run. Also, the accessories are run by the gas producer, so the oil pump, fuel pump and other housekeeping stuff draws its power. This gas producer spins independantly, and we call its speed the N1 or Ng. About 2/3 of all the energy the engine burns is consumed by the gas producer (in effect, this 2/3 is the overhead, none of it actually does anything else but keep the engine running.)

If the engine is a pure turbojet, it ends with the gas producer. the stream of hot gas that pours out of the gas producer section is guided through a nozzel and forms the "jet" in jet engine. This thrust pushes the airplane forward.

If the engine is a turbo shaft, there is a power section that backs a new wheel into the gas stream, and connects this wheel to the transmission of the helicopter, or the prop of the turboprop. This wheel has the job of extracting almost all the power from the gas stream, leaving it just enough energy to fall out of the engine by itself. The speed of this powersection is labeled N2, or Np. Often, for ease in packaging, the power turbine shaft is run up the engine inside the gas producer shaft and out the front end of the engine. This makes the engine more compact, and lighter, since there need not be a big stiff frame to hold everything in alignment. As a bit of trivia, the N2 runs in the opposite direction as the N1, so the torques of the two counteract and cancel. This releaves some of the stress on the engine mounts.

On most modern jet airliners, there is a big propellor at the front hidden inside the engine shroud, that's why the engine pods are so big these days. Nobody wants to call them propellors, because that would look like a step backwards, but they are big shrouded props. The ratio of amount of cold air the jet engine pushes via the prop, as compared to the air that goes all the way through the combustor and leaves as hot jet is called the bypass ratio. In a pure turbojet, like the old 707's engine, the bypass was 0%, everything went through the insides. In a modern engine, the bypass ratio can be 87%, so that 7/8 or the air that the engine move is never taken inside, it is just pushed by the prop. This is a big efficiency improver, done for better gas milage. The big fan is exactly analogous to a big helicopter rotor, where less power is needed to make the thrust if the disk loading is lower.

[Vfrpilotpb]

Hi Nick,

Your last few lines have created another question, if the disc loading is reduced it needs less power, would the disc loading be reduced by adding additional rotor blades to an already existing system, or is it not that simple?
Vfr

[Nick Lappos]

vfrpilotpb,

The disk loading is calculated by dividing the thrust by the disk area, and has units of pounds per square foot. If we add more blades, or widen the existing ones, we don't change the disk loading, we would reduce the "blade loading" of the rotor system. Believe it or not, that can cost more power, because with more blade area, we can operate at lower angle of attack for each blade, and that almost always reduces the efficiency (increases the power for a given thrust). With more blade area, the disk can produce more thrust before it stalls, but at a lower thrust, too much blade area takes more power.


The reason why more disk area takes less power is explained by that downwash formula we used when discussing VRS. A bigger disk handles more air, so the speed change on the bigger package of air can be less for a given thrust. Less speed change means less power needed.

Nick

[Barannfin]

Thanks alot Nick, your response was exactly what I was looking for.:)

[Irlandés]

Barannfin,

if you want to do some more reading, "The Helicopter Pilot's Manual Vol 2 - Powerplants, Instruments and Hydraulics" by Norman Bailey has a good introduction to gas turbines. It takes the Allison 250 series turboshaft engine (pretty standard helicopter fare, I believe) and goes through the processes and components at an easy to understand level. It's backed-up by plenty of schematics and gives an introduction to concepts such as compressor stall and surging.

The book is a little pricey for it's contents at 33 Euro (subjective ;) ) but you might be able to borrow it off someone. I'd loan you mine but I suspect we're in different hemispheres! :rolleyes:

Seeya!

Irlandés :D