Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Two or Three Spools?

Tech Log The very best in practical technical discussion on the web

Two or Three Spools?

Old 7th Apr 2021, 20:17
  #1 (permalink)  
Thread Starter
 
Join Date: Aug 2013
Location: Not lost, but slightly uncertain of position.
Posts: 13
Two or Three Spools?

A question here for those in the know after a debate on another forum. Is Rolls-Royce new UltraFan design considered a two spool or a three spool engine? On their homepage RR describes that the engine uses Advance 3 core design, but as I see it the UltraFan only have two turbine sections and only two concentric axels, with the inner axel driving the LP compressor as well as the fan section through a reduction gearbox, kind of the same way as on Pratt & Whitneys GTF design.

Any thoughts?
F-16GUY is offline  
Old 7th Apr 2021, 20:38
  #2 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 673
Looking at the mobile page (so who knows how the hell it relates to the normal page) as well as the Wiki, it seems that the Advance is a 3 spool engine like the rest of the 211/Trent lineage, and the Ultrafan is based on that but goes to 2 spool/geared fan
Vessbot is offline  
Old 7th Apr 2021, 21:18
  #3 (permalink)  
 
Join Date: Mar 2002
Location: Florida
Posts: 0
well if you took away the fan and just ran it as a pure jet, the question becomes how many stages of compressor and small dia turbines do you need to achiever an optimum pressure/fuel burn.

This sounds like a Tdtracer analysis is needed (keep the nacelle drag low and the weight low and at the same time improve the fuel burn enough to fill a niche)
lomapaseo is offline  
Old 7th Apr 2021, 22:10
  #4 (permalink)  
 
Join Date: Feb 2002
Location: UK
Age: 56
Posts: 3,200
When PW announced the GTF concept I remember a spokesperson from Rolls being a bit sniffy saying its like having a "two and half spool engine".

Looks like PW are on to something after all... as long as they get the bowed rotor problem sorted. 😁
TURIN is offline  
Old 8th Apr 2021, 11:16
  #5 (permalink)  
 
Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 682
F-16GUY

I agree.
at time 30 secs says "we have removed the low speed turbine" and the diagram only shows 2 spools.
The IP turbine drives the fan through the new clever gearbox - but it is still marketed as having 3 shafts.

Last edited by Goldenrivett; 8th Apr 2021 at 11:22. Reason: extra info
Goldenrivett is offline  
Old 23rd Apr 2021, 05:23
  #6 (permalink)  
 
Join Date: Nov 2017
Location: Under the radar
Posts: 35
The reduction gearbox allows the fan to be driven directly from the IPT with both at optimal RPM. So LPT (running at a lower, fan optimised RPM) is no longer required. It's still essentially a three shaft engine it's just recovering the fan drive energy mechanically, directly from the core, instead of via LPT

As I understand it, by running each stage closer to optimal RPM, the gearbox allows high OPR over fewer, smaller diameter stages, so the core is more efficient, has less parts and is lighter. However the forces on each compressor blade are much greater because the pressure ratio of each individual stage is then higher.

This puts RR in a very good position to develop an efficient large GTF because triple spool core architecture shares similar characteristics (i.e. less stages, high pressure ratio per stage.) so their previous experience with blade materials/cooling/geometry etc. will be applicable.

I think P&W did well to develop their GTF and it's certainly been a commercial success. But they will need to work on the core to scale it up. RR have the advantage with core tech but they are still working on the fan and gearbox. So essentially the two manufacturers are pursuing the same goal from opposite directions.

Last edited by Sonikt; 24th Apr 2021 at 00:52.
Sonikt is offline  
Old 23rd Apr 2021, 12:42
  #7 (permalink)  
 
Join Date: Mar 2005
Location: N/A
Posts: 4,034
As the chief engineer says at 1:33 in the video it's a two spool engine, but it's a three shaft, the gearbox/fan counting as a separate shaft is my take away, spool and shaft having different meanings.
megan is online now  
Old 24th Apr 2021, 00:25
  #8 (permalink)  
 
Join Date: Nov 2017
Location: Under the radar
Posts: 35
Yes I agree. The "shaft" driving the fan, not being in any way part of the core, is not a "spool" in the classical definition. But the average observer won't spot the subtle difference here.

​​​​​​Manufacturers often use different terminology to highlight their own products. I guess in the case of RR we could say it's just marketing bluff to call their GTF "three shaft", due to the similarly of the expression with "three spool", for which RR decades of experience is renowned.

Maybe it's a bit misleading but as per my previous, the comparison does still have some basic technical merit, so IMO it's "honestly misleading", if such is possible! Unfortunately for RR most people are somewhat cynical about manufacturer claims these days, hence this thread.

Perhaps more amusing, is that RR persist with the expression "intermediate pressure spool" when there is seemingly no longer any low pressure section. No doubt will become something for the old hands to explain to the young uns' why this anomaly exists!

Last edited by Sonikt; 24th Apr 2021 at 00:40.
Sonikt is offline  
Old 24th Apr 2021, 12:18
  #9 (permalink)  
 
Join Date: Oct 2019
Location: north of Harlow and south of Cambridge
Posts: 6
So, finally!

The two spool concept wins!!!! 😉

so many years of the British teaching this yank that the 3 spool was so much more “refined” than 2 spool.

70 Mustang is offline  
Old 24th Apr 2021, 22:51
  #10 (permalink)  
 
Join Date: Nov 2017
Location: Under the radar
Posts: 35
LOL you can have that one if you like 🙂

But seriously, "refinement" is not really the issue IMO, flexibility is. A key characteristic of triple spool, is that fan RPM is fully independent of core RPM, because it's on a separate shaft. This has long been commercially advantageous to RR, due to much lower development costs for simple derivatives.

But as you rightly pointed out, RR don't say to customers "triple spool is best because it's good for RR" they have to find another way to say it's better 🙂

With triple spool, a common high/intermediate pressure core (i.e. the difficult/expensive bit) can be easily (and therefore cheaply) scaled up or down, without significant modification, just by throttle push. Only the (separate, lower cost) LPT need be tweaked to achieve optimal fan RPM at a range of throttle settings/BPR.

GTF can achieve the same characteristic, but via gearbox setting, instead of via LPT setting. It's no coincidence, that RR are designing their gearbox, to work at a range of different ratios.

Last edited by Sonikt; 25th Apr 2021 at 00:15.
Sonikt is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.