3 spool

[Wheezy Child]

OK, so after a harrowing self teach on 3 spool engines I think I have made a breakthrough.

Can someone confirm 2 things for me please?

1. Fan, IP compressor and HP compressor all powered by there own turbine. Each is connected by its own shaft (therefore 3) and these shafts sit inside one another?

2. An advantage of a 3 spool engine is that only one spool needs to be spun by the starter. Question is which spool? I think it would be the HP spool as it appears the smallest with probably the least inertia.

Who would have thought I have an interview coming up!!?

Cheers all

WC

[exeng]

Wheezy,

Correct on both one and 2. However the fact that the starter drives the lower inertia H.P. spool is an additional benefit, but the main benefit is that each compressor spool is better matched to the airflow so in theory there are increased stall/surge margins over a twin spool engine. Surge margins at different power settings are however maintained by the use of engine bleeds and variable intake guide vanes on both twin and triple spool engines.

A couple of disadvantages of triple spool engines (in theory at least) are increase in weight, increased complexity and therefore higher lifetime maintenance costs.


Regards
Exeng

[Wheezy Child]

PPRUNE rocks!

Thanks ExEng. That was exactly what I was looking for!
I hope now they ask me the question!!!

Cheers

WC

[extreme P]

What can you tell me about the RB211-524?
Triple spooled engine
Physically smaller than other similar engines [3 spools]
Less shaft-flex and distortion [longer life]
3 sections mean each section works closer to its optimum
N1 100% = 3900rpm
N2 100% = 7000rpm
N3 100% = 10611rpm
Easier to start as only 1 shaft needs to be turned
Better thrust to weight ratio than competitors
Better propulsive efficiency
Lower fuel consumption
Reduced noise
Slightly heavier due to the extra spool
86.3" fan diameter [75% of thrust]
Modular construction [easier to build and maintain]
Seven-stage IP compressor
Six-stage HP compressor
Single annular combustor with 24 fuel burners on the G/H-T
Single-stage HP turbine, single-stage IP turbine and a three-stage LP turbine

I believe the starter rotates either the N2 or N3 section. Depends on the version of RB211 you are discussing.

[Macgyver]

Pardon my ignorance, but what does "HP" and "IP" stand for? High pressure compressor, Low pressure compressor??

[BelArgUSA]

Intermediate Pressure for IP

[SMOC]

I'm fairly sure all RB211s have the HP/N3 shaft connected to the gearbox therfore the pneumatic starter, the new Trent 1000 for the 787 has the IP/N2 spool connected to the gearbox, but uses an electric starter/generator.

[16down2togo]

It sounds so good but it is such a pain to operate. Takes forever to start them, hot and high you have trouble with the autostart and the performance, the complicated reversers are deactivated quite frequently, maintanance has to crawl in.
And last not least: It climbs like a snail above FL 250!
Compare it to the CF6, that's a pilots engine, I can easily live with the imperfect 2 spools on this one.

[Flap Track 6]

A couple of disadvantages of triple spool engines (in theory at least) are increase in weight,

Well, theoretically the triple spool is lighter than a twin spool, but for some reason RR couldn't seem to make the RB211-524 lighter than the competition on the B747. However, the triple spool Trent is the lightest engine on the B777 and the Trent1000 is lighter than the GenX.

[BelArgUSA]

As a pilot of old 747s, I have flown the 3 different engines. A pilot point of view -
xxx
The JT9s have the reputation of being very easy to maintain, and are known to be able to sustain bad treatment (from pilots like me) for a long time before requiring overhaul. The CF6 is, like our friend 16down says, a pilot's engine, it is my favorite when it comes to handling the thrust levers, although I personally prefer EPR to N1 as means to set power. I liked the RB211 as well, but it is often a maintenance nightmare.
xxx
I remember my airline having leased a 747-238B (ex Qantas) for a few years, the engines were (supposedly) JT9D-7J, but actually, one of the engines was a JT9D-7A and this was not known by the crews (including myself). We often operated that -7A at 7J power (EPR) settings, and it survived the bad treatment for well over a year or two, when it was discovered it was actually a -7A and was soon replaced by a proper -7J.
xxx
In short, JT9s can suffer gravel and birds with little worry for FOD, which is not the reputation of the CF6s and certainly not for the the RB211s. Our director of maintenance jokes in saying that with a RB211, to expect damage if ingesting little butterflies...
xxx
Nice peace of technology, that RB211, and a pain to study in the classroom.
xxx
:)
Happy contrails

[spannersatcx]

The JT9s have the reputation of being very easy to maintain:eek: These things are the worst ever to work on. You try getting the reverser fwd after it's got stuck in full reverse, operate the 3 way valve, can't do that as you need to open the cowl, which you can't do as the reverser is out:{ If you do manage to get the cowls open try closing them again!:(
Having worked on (rather still working on) all 3 I'd say the RR, then the CF6 then the Pratt for ease of maintenance.

[Nepotisim]

I would have to agree with spannersatcx also having worked on all three. Each has their own foibles in certain areas during maintenance.

I would also be more worried about a butterfly if I was in a CF6 rather than a Roller. Rollers are pretty tough but not as robust as the Pratt.

[rockytan]

any idea what is the meaning of N in N1, N2, N3? cheers

[rmm]

On the weight issue, the following is from the 744 maintenance manual.
RB211 - 13,882 lbs (6302 kg)
CF6-80C - 12,725 lbs (5785 kg)
The figures are engine dry weight with inlet cowl, fan cowls and thrust reversers fitted.

[NutLoose]

N1, N2 and N3 are the spools....... LP, IP and HP stages

If you look at the other post above

3 sections mean each section works closer to its optimum
N1 100% = 3900rpm
N2 100% = 7000rpm
N3 100% = 10611rpm

The RPM's are to do with speeds of the stages, the optimum speed where a blade is most efficent is near the speed of sound tip wise, so if you have a big blade such as on a RB211 it will be running near the speed of sound and if it just had one compressor, the smaller blades rearwards at the compressor will not be running as near efficent, having 3 separate stages the RPM's can vary and allow each spool to run more efficently..... in an ideal world you would run every stage independant, but the weight gain would negate this.....

Incidentally on the RB199, the 2nd spool If memory serves me correctly runs in the opposite direction to reduce the forces of having all three compressors rotating in the same direction.

[Flap Track 6]

any idea what is the meaning of N in N1, N2, N3?

Further to NutLoose's answer, N is the scientific/engineering designation for rotational speed, in revolutions per second.

The Pegasus has its spools rotating in opposite directions to cancel out gyroscopic forces on the airframe (especially useful whilst hovering). There is some debate over whether this clever idea originated from Hawker or the Bristol Engine Company ...

[barit1]

Not sure where the counter-rotation idea originated, but the very earliest Lycoming T53 helo engine had it. Later T53's did not.

Actually, counter-rotation offers a couple of advantages - One turbine nozzle (stator) can be eliminated, with weight and efficiency benefits. As pointed out earlier, the gyro loads from the rotors are self-cancelling, as are engine seizure loads for the guys who design the mounts.

One very real disadvantage is if differential (intershaft) bearings are needed - they turn at VERY high speed and are a real engineering challenge. But despite this, several modern engines are counter-rotating. :ok:

[TURIN]

I think the question regarding 'N' was trying to ask "What does the letter N mean?" Not what does it refer to.
I tried an internet search and gave up.

[Macgyver]

This might seem like a dumb question.... the 211's have a 7 stage IP compressor and a 6 stage HP compressor, but what about the LP compressor? There's 3 spools, so why does the technical data (such as given on the rolls royce website) only mention the number of stages for the IP and HP compressors?

[Mad (Flt) Scientist]

Because on many engines the LP stage is JUST the fan i.e. there's a single stage only. That's what I'd assume in the absence of anything else.

(Much like if an aircraft has a quoted wingspan today, I assume it's a monoplane, or if it has a quoted wing sweep one would assume aftwards sweep rather than forward sweep)

[Intruder]

Because on many engines the LP stage is JUST the fan i.e. there's a single stage only.
I'm not familiar with any 1- or 2-spool engines that have only the fan on N1. Which engines are so designed?

[spannersatcx]

RB211-524 manual says:

C. The engine is a high by-pass ratio, three-shaft turbofan consisting,
basically, of a gas generator system and a mechanically independent L.P.
system.
D. The gas generator system comprises a seven-stage intermediate pressure
(I.P.) compressor, a six-stage high pressure (H.P.) compressor, an
annular combustion section and single-stage I.P. and H.P. turbines.
E. The L.P. system consists of a single-stage compressor (fan) driven by a
three-stage turbine.

The CF6-50 engine is a dual-rotor, axial-flow, gas turbine turbofan.
Basic design is based on a high compression, high bypass ratio concept.

B. An integrated, 4-stage, front fan and low pressure compressor is
driven by a 4-stage low pressure turbine which combines to form the N1
rotor.

The 14-stage high pressure compressor is driven by a 2-stage
high pressure turbine which combines to form the N2 rotor. Each rotor
is mechanically independent of the other.

[Mad (Flt) Scientist]

@Intruder. I was thinking 3 spool. I can't think of any where the LP has anything except the fan.

Obviously a single spool engine would be unlikely to have just a fan stage (unless it was a centrifugal single stage, perhaps, and we hardly can call that a "fan"

[barit1]

I'm not familiar with any 1- or 2-spool engines that have only the fan on N1. Which engines are so designed?


The military TF34 is one, and the early CF34's too: Single-stage fan driven by a 4-stage LP turbine, and a 14-stage HP compressor and 2-stage HP turbine core. Said core is a scaled-up version of the T64 helo & turboprop engine.

Note the CF34-8 & -10 are altogether different in layout from these original engines.

I'm not aware of any single-spool engines with a fan, which would amount to in effect a "leaky" stage 1 of the compressor.

[BelArgUSA]

Single spool engines, with a fan... (aft fan in these cases) -
GE's CF-700 (aft fan version of the CJ-610/J-85) and
CJ-805-23 (on the CV 990) -
xxx
Strange, that in the case of CJ-805-23, GE kept the "CJ" designation.
xxx
Was the RR Conway R.Co 12 a single or twin spool...?
Had a fan, for its bypass but... cannot recall.
xxx
:)
Happy contrails

[411A]

The RR Conway was 2-spool.

[Intruder]

The military TF34 is one, and the early CF34's too: Single-stage fan driven by a 4-stage LP turbine, and a 14-stage HP compressor and 2-stage HP turbine core. Said core is a scaled-up version of the T64 helo & turboprop engine.
That makes sense -- evolving a free-turbine turboshaft or turboprop to a fan on the LP stage. Thanks.

[rockytan]

Thanks guys. Just curious if anyone know the history of the word N?Mathematical terms & percentage!!

[BelArgUSA]

Maybe...
xxx
I remember my classes of math and physics, which were in French language, we used the value "n" as a designated or arbitrary amount. While in the Anglo-Saxon world, people will state "I owe you "x" number of dollars", a French language mathematician or physicist would say "I owe you "n" number of dollars"... maybe in an English language math of physics classroom lecture, the "n" is used as well as the French do, to designate a "number"...
xxx
With jet engines, the 100% "N" value is rather arbitrary. Some engine manufacturers might decide to call the maximum rated RPM as 100%, while others will decide the maximum RPM as 98.5% or 101.2%... then like the examples for N1, N2 and N3 (of the RB211) given in previous postings, but might not be the maximum or continuous RPM values...
xxx
We are all familiar with engine stations designations, such as "Pt-2" or "Tt-7" as pressure or temperature values for various "stations" from inlet to exhaust of engines, "N" is another conventional engineering value, expressing RPM...
xxx
This is my interpretation of what "N" means. On the J-85, the RPM indicator was simply called RPM, not "N1", as there was only one shaft. Since I made a major clean-up of my old manuals, for "paper recycling" some years ago, no idea as to where or what publication I can refer to.
xxx
:)
Happy contrails

[HotDog]

On the CJ 805-3B (or the Mil. J79) CV 880 engine, max N1 was 103% rpm and 1% eng variation equaled 5% thrust variation.

[lomapaseo]

That brings to mind what is the relationship of "redline" N value to the 100% value quoted by the manufacturer?

PS just a brain teaser :}

[MrBernoulli]

Another advantage for the 3-spool that I don't think I have seen here yet:

3 spools allows the engine to be smaller in cross-section because there is an extra spool developing some of the power. This reduces some of the overall drag and could also be a positive where, for example, an aircraft design requires more clearance from the ground when 'hanging an engine under the wing. On the BA 777s a RR Trent 800 has 2 feet less diameter on the engine casing than a GE 90.

[rockytan]

Thanks guys

[washoutt]

In my university, N was short for Number of rotations of the spools, N therefore.

[DaveReidUK]

Thanks guys

Some of them may even be still alive. :O

[sorvad]

Haha! rockytan, that's gotta be a record.... 15 years from response to acknowledgement..... You don't work for the CAA by any chance do you? :}