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"two spool"?

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Is there a source for the claim that the Proteus is a "two spool" engine? It's a free turbine engine, so there are two separate shafts, but these aren't described today as "two spools" as there is only one compressor unit (both axial and centrifugal sections form one unit). I can't see this as an early use of the term, against contemporary use, as the Proteus is so early that it pre-dates the two spool engines and even by 1949 [1] it's described as a "free turbine". Andy Dingley (talk) 21:50, 14 January 2018 (UTC)[reply]

The Bristol Proteus is not a "two spool" engine as defined in modern terms. Although it dose have a tubular shaft that goes through the axial compressor rotor it is, I believe a free turbine. - Samf4u (talk) 04:23, 15 January 2018 (UTC)[reply]
Agreed. Was there ever a contemporary use of the term though? (I can't find it). I would be against using it here, but it might be a historical footnote in any coverage of two spool engines. Andy Dingley (talk) 11:45, 15 January 2018 (UTC)[reply]
In August 2005 an IP user added "two spool" to the article giving no reference. I have found several sources that give detailed descriptions of the Proteus written in the 1950's and none of them call it a two spool engine. Some exceptions include The Gatwick Aviation Museum and All-Aero.com. Both of their descriptions are very similar in calling it a two spool engine but I think they used Wikipedia as a source. - Samf4u (talk) 21:14, 16 January 2018 (UTC)[reply]
The Proteus is a single-spool 'free turbine' engine with an additional separate turbine for driving the propeller that is not mechanically connected to the power section, hence 'free'. One of the advantages of this design is that the starting system does not have to turn the heavy mass of the propeller which was important at the time for commercial use, as cartridge starters were not suitable for civil use on aircraft with such large propellers as the SARO Princess, not to mention those such as were used on the SR-N4 hovercraft.
Interesting point. Do you happen to know what the drivetrain on the SR-N4 was? Was there any sort of clutch in there, or was there just a brake and it relied on the turbine being free to stop the whole set of shafts and gearboxes windmilling on startup?
We do have an article at free-turbine turboshaft, BTW. Needs plenty of work, and a lot of sourcing, if you're interested. Andy Dingley (talk) 11:01, 10 April 2019 (UTC)[reply]
I think that one of the plus points of the free turbine is that there is no need for a clutch as the gas output of the engine at low engine speeds (low rpm) does not produce sufficient torque via the power turbine to turn the propeller, which I suspect is why the design is/was also used in small helicopters. 1967 Flight article on the SR.N4 power section here: [2] — Preceding unsigned comment added by 95.148.8.216 (talk) 11:54, 10 April 2019 (UTC)[reply]
Reading your comments, and knowing that people "in-the-know" know what a spool is, I still wondered "What is a SPOOL and where did the term originate?" Looking through Flightglobal archives it seems to have originated at Bristol with the invention of the Olympus insofar as it appeared for the first time in Flight, as far as I can see, in 1952 and was supposedly used colloquially at Bristol for the Olympus split/dual/divided/compound/you-name-it compressor, ie a "two-spool arrangement".[3] However, the NACA was already using the term in its then-confidential research memoranda by 1952, the earliest I can find, and also give a definition in a 1956 publication[4]: For a single-spool gas generator the spool comprises compressor, combustor and turbine. For a two-spool gas generator the LP spool comprises the first compressor and its turbine mounted on a common shaft, and the HP spool comprises the second compressor, combustor and turbine. The reason for including the combustor in the spool definition is because it affects how the compressor and turbine work together. The inclusion of the combustor in the HP spool is also shown in Figure 12.2 of this book[5]Pieter1963 (talk) 23:11, 18 April 2019 (UTC)[reply]
I see in Turbofan talk that it originated in Barnoldswick, ref cotton spinning spools.Pieter1963 (talk) 15:14, 19 April 2019 (UTC)[reply]
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