Does anyone know why the the triple spool Rollers have 8 IPC stages and only 7 HPC and what the advantage of this could be? Isn't it better to load the HPC more heavily than the IPC, or having an even amount of blisks/blings in each compressor stage to make it more equally loaded?

Hi:
Effectivity.
In teoria, each compressor stage should be turning at different and specific rpm from each other.
In the practice, stages are grupped to make them less complex.
With two spools, for instance, on 14 stage compressor, You will have 4 low pressure stages turning at one rpm (N1), and 10 high pressure stages, turning at other same rpm (N2).
On 3 stage compressor, for 14 stages compressor, Yo may have the fan stage turning at N1, 3 Stages turning at (N2) and 10 stages turning at another, N3 speed.
This allow for some what more effectivity rpm for each stage, allowing less fuel consumption and lower compressor stall possibility

Rgds.

(2)

At the same time, 3 spools make engine heavier. The relationship between dimension/ Fuel consumption will be lower when engine power is bigger.

Rgds

Thanks

RRUltrafan wrote:

Does anyone know why the the triple spool Rollers have 8 IPC stages and only 7 HPC and what the advantage of this could be? Isn't it better to load the HPC more heavily than the IPC, or having an even amount of blisks/blings in each compressor stage to make it more equally loaded?

The Trent 700, 900, 1000, XWB have 8 IPC and 6 HPC, not 8/7, the RB211-524 has 7 IPC and 6 HPC.

The main reason for the 3 spool design is to optimize the rotational speed of each spool for the role it performs.

Apprentice wrote:

(2)

At the same time, 3 spools make engine heavier. The relationship between dimension/ Fuel consumption will be lower when engine power is bigger.

Rgds

I dont think that is a valid across the board statement, the A380 engines the Trent 900 has a dry weight of 6160 kg, and the GP7200 6718 kg. On the 787 the the Trent 1000 dry weight is 6033 kg a,d the GEnx‐1B/P2 has a dry weight of 6147 kg.

K. Sorry for the misstated no. of HPC blisks/blings... So in the end, the number of stages for each spool is just for optimal efficiency of the different spools to achieve a higher and better compression ratio as well as a more balanced engine?

Zeme, Hi:
You are Right, it is a not true for all engines,
I was taught that 50 000 lbs thrust engines like CF6- it is more efficient that a similar 3 spool engine. .
It is a strong exercise to probe that, but it had been done .
Rgds

RRUltrafan, Hi: as stated, for optimal performance, each compressor stage should be turning at it’s (calculated) optimal speed.
But, this is no practical. You should group several stages in a rotot and calculate optimal (medium?) speed for the rotor.

Rgds

RRUltrafan wrote:

K. Sorry for the misstated no. of HPC blisks/blings... So in the end, the number of stages for each spool is just for optimal efficiency of the different spools to achieve a higher and better compression ratio as well as a more balanced engine?

I dont think it optimum for all phases of flight or for all flight profiles, it would be designed around cruise optimisation which is better for longer flights. I have read that twin spool is better for short haul as the aircraft spends a higher proportion of a flight in climb and descent.

The intermediate compressor on a 2 spool is turning very slowly as it's on the same shaft as the fan. They're not very efficient, which is why I guess they're called 'booster' stages. Much better on a 2 spool to add stages to the HPC.

Interestingly for the 'Advance' engine RR is moving more stages to the HPC and adding a second HPT stage. I don't know what the reasoning for this is.

I guess to increase the pressure ratio even higher

zeke wrote:

I dont think it optimum for all phases of flight or for all flight profiles, it would be designed around cruise optimisation which is better for longer flights. I have read that twin spool is better for short haul as the aircraft spends a higher proportion of a flight in climb and descent.

Hmm, I'm not sure that's true. RR engines on widebodies are know for being better at shorter ranges as they have an advantage at climb and descent. I think it's particularly at descent where a 3 spool can be throttled back further than a 2 spool. Also, RR's proposal for the A320NEO was a 3 spool.

I don't know where the cross over is for 2 spool vs 3 spool in either thrust rating or stage lengths. I know the Williams were working on a very small 3 spool for a while, but RR's smaller engines are mostly 2 spool - e.g. their business jet engines.

Apprentice wrote:

(2)

At the same time, 3 spools make engine heavier. The relationship between dimension/ Fuel consumption will be lower when engine power is bigger.

design objective for RR was among other things a lighter engine. via shorter, stiffer spools.

2/3 spool weight differences are overlayed with plastic fan or not ( with the plastic fan allowing a lighter containment.)
With RR 3 spool and GE plastic fan judgement on only 3 vs 2 spool is difficult.

The plastic fan weight advantage may be just due an FAA waiver for incomplete blade out testing

Regarding the weight, a triple spool will have more weight for bearings and other attachments, and tend to have fewer total stages because the IPC is more efficient. The bigger the engine gets, the more stages it tends to have, so the stage count effect dominates the bigger the engine gets. This is why the Trent is the lightest 777 engine, while the RB211-524H on the 744 was heavier than the PW and GE options.

As far as the range tradeoffs, the triple-spool is a better climber, because the IPC is uncoupled from the heavily-loaded fan, so it doesn't get bogged down to a suboptimal RPM. What counts against it is interstage pressure losses, which gives twin-spools an edge at longer range. Combine the climb and weight advantages, and you tend to see triple-spool engines being relatively better at shorter ranges. I've read that on the 787, the RR is best below 4000 nm, and the GE is better above that. Now, as they get PIPed, that crossover slides back and forth, but their relative strengths stay the same.

I'm sure lightsaber could chime in with a better articulation for all of this.

Hi: I have received another explanation, 3 spools will allow to accomodate stages’ speed to a more optimal schedulle.
At time it was introduced, Plastics’ Blade were not even a dream, so I do not take them in account.
As 3 spools make engines heavier, there was not a choise for small turbines (< 20 000 lbf). It’s use make sense in big engines.

Rgds

Apprentice wrote:

Plastics’ Blade were not even a dream, so I do not take them in account.

The RB211 was massively delayed from ( beyond other things ) the new plastic blades that RR wanted to introduce.
( IMU) they could not meet the birdstrike requirements at the time and continued with hollow Titanium blades.
in between RR rotated through a belly up orientation, reanimated by government money.

kurtverbose wrote:

Hmm, I'm not sure that's true. RR engines on widebodies are know for being better at shorter ranges as they have an advantage at climb and descent. I think it's particularly at descent where a 3 spool can be throttled back further than a 2 spool. Also, RR's proposal for the A320NEO was a 3 spool.

I think the premise that the RR engine was heavier and thus resulted is more fuel burn was stopped after the 744.

I think the Trent 700 is the lightest and most selected on the A330, the Trent 800 lighter than the
GE on the 777-200/300, Trent 900 lighter than the GP7200 on the A380, and Trent 1000 lighter than the GEnx in the 787.

zeke wrote:

kurtverbose wrote:

Hmm, I'm not sure that's true. RR engines on widebodies are know for being better at shorter ranges as they have an advantage at climb and descent. I think it's particularly at descent where a 3 spool can be throttled back further than a 2 spool. Also, RR's proposal for the A320NEO was a 3 spool.

I think the premise that the RR engine was heavier and thus resulted is more fuel burn was stopped after the 744.

I think the Trent 700 is the lightest and most selected on the A330, the Trent 800 lighter than the
GE on the 777-200/300, Trent 900 lighter than the GP7200 on the A380, and Trent 1000 lighter than the GEnx in the 787.

Hi: Agree.Just IMHO, problem is not about when it was made, but about how big is it.
In smaller engines, 3- spool engine loose its advantage.

Rgds

Apprentice wrote:

Hi: Agree.Just IMHO, problem is not about when it was made, but about how big is it.
In smaller engines, 3- spool engine loose its advantage.

That is why you go GTF there for similar gains.

zeke wrote:

kurtverbose wrote:

Hmm, I'm not sure that's true. RR engines on widebodies are know for being better at shorter ranges as they have an advantage at climb and descent. I think it's particularly at descent where a 3 spool can be throttled back further than a 2 spool. Also, RR's proposal for the A320NEO was a 3 spool.

I think the premise that the RR engine was heavier and thus resulted is more fuel burn was stopped after the 744.

I think the Trent 700 is the lightest and most selected on the A330, the Trent 800 lighter than the
GE on the 777-200/300, Trent 900 lighter than the GP7200 on the A380, and Trent 1000 lighter than the GEnx in the 787.

I know all that. I think you clicked the wrong 'reply to' button.

So if the triple spools boast efficiency on the large long range aircrafts during climb, why does it lose its appeal and efficiency with smaller aircrafts? Shouldn't it be optimal for the smaller aircrafts (A320, 737, CS100 etc) with shorter range?

RRUltrafan wrote:

So if the triple spools boast efficiency on the large long range aircrafts during climb, why does it lose its appeal and efficiency with smaller aircrafts? Shouldn't it be optimal for the smaller aircrafts (A320, 737, CS100 etc) with shorter range?

No, it's too heavy at the smaller sizes.

They also take longer to start.

zeke wrote:

They also take longer to start.

Start as "get it running on idle from stopped", or start as "get it to full thrust from idle"?

Trent XWB is a beast I know, but would not call it slow to full thrust, I actually thought it gets plane running awfully quick, more like small planes, not widebody with 11+hrs fuel.

Cheers,
Adam

Normal starts seem to take longer.

zeke wrote:

Normal starts seem to take longer.

Starter works on the HP spool only?

WIederling wrote:

zeke wrote:

Normal starts seem to take longer.

Starter works on the HP spool only?

Hi: Yes, Starter make turn HP Spool only, Intermediate and LP Spools, are turn by air pasing through (similar to 2spools, in which starter turns Hp and air LP),. Having 3 spools to turn, make air pressure increase slowly during start, and full start process slowly.

Aside: When a 3- Spools powered a/c come to You with APU or APU’s air bleed inop, You better search the best TWO Peumatic Air Starters in the airport, to connect them both to aircraft Pneumatic Syatems’ connectors and use them during engine start or You risk to ground your plane. For a two spools engines’ start using a ground cart, one should be enough.

Rgds

gloom wrote:

zeke wrote:

They also take longer to start.

Start as "get it running on idle from stopped", or start as "get it to full thrust from idle"?

Trent XWB is a beast I know, but would not call it slow to full thrust, I actually thought it gets plane running awfully quick, more like small planes, not widebody with 11+hrs fuel.

Cheers,
Adam

Hi: Start is defined as process when N1 spool go from full stop position (no 0 rpm, but Full Stop, verified by Ground Crew on the interphone), to the moment N2 & N1 reach stable idle speed.

Aceleration is, from Idle speed to certain thrust (or rpm) below TO. This maximum value (EPR o RPM) for check acceleration varies with differents engines.
(Reason You don’t go to full TO power when CHECKING engine’s acceleration is to avoid engine’s OVERBOOSTING)

Rgds

Apprentice wrote:

Aside: When a 3- Spools powered a/c come to You with APU or APU’s air bleed inop, You better search the best TWO Peumatic Air Starters in the airport, to connect them both to aircraft Pneumatic Syatems’ connectors and use them during engine start or You risk to ground your plane. For a two spools engines’ start using a ground cart, one should be enough.

Not sure I can agree with you on that all of the A330/340s use two 3” HP hoses for a start, regardless of engine type. I have only ever used one cart easily able to maintain 28-30 psi during the start. The 777 needs 3x3” HP hoses connected for an external start.

I suppose if you have units only designed with one outlet like for narrow bodies you would need two of them.

Hi: You are very locky.
B777. Engine are heavier and requieres much more power to reach “Fuel On” Speed
On 330/340 if powered by RR, theoretical You may use only 1 Cart, but it will have ro be strong!
This is a fact, 3 spools engine requiere more power to be started. Also, heavely engines (3-Spool), need more power. As bigger ones (B777)
Rhds

For bigs, heavy engines, is better 2ea Ground Starter Unit than 1ea w/ 2 houses.
, What we need is. Air Power, and....
The available units, also elsewhere are similars, with two hoses and Power’s deficiency.
Air Power, for this case may be calculated as Gb x Pg, Gb. Is air flow mass and Pg is Gas’s pressure. If You use 2 hoses, to keep Gb at similar levels, Pg will be down and so air power, If You decide to keep Pb stable, Gb will be down....
As a mechanic, I had been involved in so many Assisted Starts, with Crew demanding more pressure and Ground Crew throtle up cart to the maximun. And in so many palces.... For me, only difference is ceason: summer or winter....

Slds

And last but not least, for B777, if we need to start one, powered by GE-90, 2 carts, with 1 hose each is enough. For more heavy RR / PW, 3 hoses are needed
Rgds

Apprentice wrote:

Hi: You are very locky.
B777. Engine are heavier and requieres much more power to reach “Fuel On” Speed
On 330/340 if powered by RR, theoretical You may use only 1 Cart, but it will have ro be strong!
This is a fact, 3 spools engine requiere more power to be started. Also, heavely engines (3-Spool), need more power. As bigger ones (B777)
Rhds

Always have used only one cart on the A330/A340 with Trent’s never an issue.

Apprentice wrote:

And last but not least, for B777, if we need to start one, powered by GE-90, 2 carts, with 1 hose each is enough. For more heavy RR / PW, 3 hoses are needed
Rgds

You need 270 ppm for a 777 to start (most airports have 300 ppm units for widebodies 767/747/A330/A340/A350/A380 only need 250 ppm), for the GE90-110/115 they have special 400 ppm units. You have actually got it around backwards, the Trent engines have less demand to turn a spool as each spool is smaller and lighter, it just takes longer for them to accelerate the other two spools.

Zeke, hello, as I said You are a lucky man.
My experience differs from this. Back to B777-A, with al three engines, GE were the lighther. R R the heavier and need 3 Hoses, to avoid a hung start.. Its my experience. Sorry.
Airports providers dont use to expend a lot o money on ground equipment. And, on each airport where I had been, the parameters into account is Pressure, that, by the way, is the parameter to read in the cockpit during engine start. But, at the end we need both Gb and Pb, because we need to do a work (turn air starter), and for that, a powerful equipment is needed.
Rgds.

Airports setup for mainly short haul of narrow bodies will only have 180 ppm units, you will need two of those on a wide body. While the cockpit we only see the pressure, without the ability to provide the mass flow the pressure will drop right off.

I tend to only fly into larger international airports where 250 ppm+ units are normal. Even use those used to drive the packs for passenger boarding in some ports. Most of the ports I operate into the ground equipment is not owned by the airport, it is owned by the ground handling agent, where that equipment is not up to our specification (normally the GPU) we will have our own.

Hi Zeke, congratulations for working in such kind of company.
I had found weak air starters in Europe, America, South America.
Beside at main base, rest of places, airports own the ground equipment.

In the places I had been the Ground Air Start’s Cart and the Air Conditioning Cart are different, and even are connected in different places.
You may pump Air Through Pressure Connector and it will reach the ACM and lower the temp, but hear it is not allowed because Carts use to Overtemp.

Rgds

Apprentice: the GE-90's (especially the higher thrust versions i.e. -110b - 115b) are much heavier on the 777's than the Rollers/PW engines. The Trent 800 Rollers were favored by airlines who wanted slightly less range with a higher passenger and cargo capacity, while the GE-90's were favored for range at a lower capacity.

RRUltrafan, , Good Morning:
Thanks for info. While on training for B777A, a long time ago,We were told that RR was the heavier engine of the three (for -A variant). I Did never check the data.

Rgds

LH707330 wrote:

What counts against it is interstage pressure losses, which gives twin-spools an edge at longer range.

Do three spool engines have more stages than two stage engines? I would have thought the opposite, since in a three spool engine each stage is operating at a closer-to-optimum speed. Or do I misunderstand something?

kitplane01 wrote:

LH707330 wrote:

What counts against it is interstage pressure losses, which gives twin-spools an edge at longer range.

Do three spool engines have more stages than two stage engines? I would have thought the opposite, since in a three spool engine each stage is operating at a closer-to-optimum speed. Or do I misunderstand something?

Generally, the triple-spools have fewer total stages, especially in the IPC. I could be wrong, but what I've read is that it's the gaps in the turbine section though that create the issue. Most RR engines have 1 HPT, 1 IPT, then a bunch of LPT, whereas big twin-spools use 2 HPT and many LPT. The gaps (and consequent pressure losses) between the HPT/IPT/LPT are larger as a result of the architecture, so you lose a bit of thermodynamic efficiency before you get to the LPT. So while the architecture is more compact and thus lighter (for big engines), the twin-spool has a thermodynamic advantage at those lower thrust settings in cruise. The TXWB has a two-stage IPT, so this effect may be less prevalent there, but I've not seen any comparisons between it and other engines of similar vintage to know for sure.

zeke wrote:

kurtverbose wrote:

Hmm, I'm not sure that's true. RR engines on widebodies are know for being better at shorter ranges as they have an advantage at climb and descent. I think it's particularly at descent where a 3 spool can be throttled back further than a 2 spool. Also, RR's proposal for the A320NEO was a 3 spool.

I think the premise that the RR engine was heavier and thus resulted is more fuel burn was stopped after the 744.

I think the Trent 700 is the lightest and most selected on the A330, the Trent 800 lighter than the
GE on the 777-200/300, Trent 900 lighter than the GP7200 on the A380, and Trent 1000 lighter than the GEnx in the 787.

Trent 700 is heaviest on A330 (6160 kg 13552 lb vs 9500 lb GE CF6-80E and PW4000 about 10000 lb)
https://www.easa.europa.eu/sites/defaul ... e%2003.pdf
page 4 sec III - 5

but A330 with Trent 700 flies about 500 nm farther than the other two. So most airlines select Trents for their A332 and A333.

777 - RR says Trent is lighter

The lightest engine on the 777 is 8000lb lighter than with the GE90, 5,400lb lighter than with the PW4090 and 6,500lb lighter than with the PW4098. This translates into more payload. - https://www.rolls-royce.com/products-an ... technology

Trent 800 dry weight (Dry engine weight (kg) 6078 (13400 lbs)) is actually a bit less than Trent 700 (surprise!)
https://www.easa.europa.eu/sites/defaul ... 102013.pdf
p. 5

GE90 dry weights GE90-76B, GE90-77B, GE90-85B, GE90-90B, GE90-94B 7892 kg (17400 lbs)
GE90-110B1, GE90-113B, GE90-115B 8761 kg (19316 lbs)
https://www.easa.europa.eu/sites/defaul ... 112017.pdf

Pratt 4090 and 4098 EASA only shows 9420 lb (4273 kg) dry weight for 94" Pratts (e.g. PW4060). The Smithsonian shows a PW4098 weighing 7,264 kg (16,000 lb), but the condition (dry or fully equipped) is not specified.

The RR flyer says the Trent 800 is 8000 lb lighter than the GE90 (assume GE90-94). EASA reports Trent 800 to be 13400 lb, and GE90 to be 17400 lb, or 3000 lb more. I must assume the 8000 lb less figure given by RR is for fully equipped engines, and for 2 engines, as the EASA specs are dry weight for one engine.. So Pratt 4090 would be about 15000 lb dry, and the RR figures about match the 16000 lb weight for PW4098 reported by the Smithsonian

In any case, it is clear that the Trent 800 weighs less. So after the Trent 700 the RR engines weigh less. More due to weight growth in competitive engines (Pratt 4098 is over 6500 lb more than Pratt 4060) while RR held weights nearly constant while significantly increasing thrust.

Your numbers are misleading as you do not understand that some engines the reverser and accessories are part of the engine whilst other engines it is not.

I think the weight for the T700 includes the full mixed exhaust assembly, which is why it's quoted as being heavier. Incidentally, the mixed exhaust accounts for a roughly half-point reduction in drag.

LH707330 wrote:

I think the weight for the T700 includes the full mixed exhaust assembly, which is why it's quoted as being heavier. Incidentally, the mixed exhaust accounts for a roughly half-point reduction in drag.

Versus the chevroned 787 nacelle?

WIederling wrote:

LH707330 wrote:

I think the weight for the T700 includes the full mixed exhaust assembly, which is why it's quoted as being heavier. Incidentally, the mixed exhaust accounts for a roughly half-point reduction in drag.

Versus the chevroned 787 nacelle?

Versus an unmixed bypass duct in general, chevron or not.

LH707330 wrote:

Versus an unmixed bypass duct in general, chevron or not.

noise reduction via chevrons takes about .5 ..1% more fuel. so it is not a wash imu.

WIederling wrote:

LH707330 wrote:

Versus an unmixed bypass duct in general, chevron or not.

noise reduction via chevrons takes about .5 ..1% more fuel. so it is not a wash imu.

I was referring to the weight of the nacelle with mixed vs. unmixed, not the overall chevron versus no chevron.

LH707330 wrote:

I was referring to the weight of the nacelle with mixed vs. unmixed, not the overall chevron versus no chevron.

hehe.
I was looking more to sfc effects.

So the higher loaded 8 stage IPC is for higher efficiency and a better balance between the spools.