Advantages of a Two-Spool engine?
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Joined: Jan 2000
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From: nowhereinparticular
Advantages of a Two-Spool engine?
Seems to be plenty of info floating around about the "advantages of the 3 spool engine" eg. the RB211 & Trents - but what, if any, are the advantages of a two-spool engine such as the CFM56-5C4? I've searched hi & lo but can't find the answer. Also, which design is better and for what? ie reliability, SFC, Pwr/Wt ratio yadda yadda...?
Any wisdom appreciated
Any wisdom appreciated
Joined: Mar 2000
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From: Arizona USA
Two spool engines are generally lighter in weight (or, for you JAR types
, mass).
But, generally, the three spool design is more fuel efficient, as each rotor turns at its optimum RPM.
Sudden thought...I have spent 25 years in RR powered aeroplanes, and not one engine failure, only two precautionary shutdowns.
The Brits may be a bit (tin hat firmly in place) pigheaded, but they (RR) sure do build GOOD engines.
Hats off to them! None better IMHO.
Come to think of it, the Smiths autopilots were first rate as well.
The Trident could land....when others "went away, somewhere else".
Boeing should have learned a lesson. Lockheed certainly did.
, mass).But, generally, the three spool design is more fuel efficient, as each rotor turns at its optimum RPM.
Sudden thought...I have spent 25 years in RR powered aeroplanes, and not one engine failure, only two precautionary shutdowns.
The Brits may be a bit (tin hat firmly in place) pigheaded, but they (RR) sure do build GOOD engines.
Hats off to them! None better IMHO.
Come to think of it, the Smiths autopilots were first rate as well.
The Trident could land....when others "went away, somewhere else".
Boeing should have learned a lesson. Lockheed certainly did.
Last edited by 411A; 14th June 2002 at 20:53.
Joined: Jul 2001
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From: Gold Coast
Have to disagree with a two-spool being cheaper to make, as the extra parts & machining goes up by more than a factor of two.
True enough it makes planning the airflow a lot easier with bigger engines, but packaging the entire device is a lot harder, as is making sure that the shafts are stiff enough.
True enough it makes planning the airflow a lot easier with bigger engines, but packaging the entire device is a lot harder, as is making sure that the shafts are stiff enough.
Joined: Dec 2000
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From: United Kingdom
As the engine gets larger the weight disadvantage of the 3 spool design actually decreases. In fact the RR Trent engines on the B777 are actually lighter then the GE & P&W engines, and also have a higher thrust rating (at least they did on inital certifiaction). 
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"In other words, 2-spool more parts, so more expensive."
Interesting. By "parts", I gather you don't just mean spools, but blades/stages, support bearings, airflow management components, fuel management components, gearbox accessories, etc, for specific engines from specific manufacturers?
Was this information something you were taught in the classroom, or did you take it upon yourself to count the parts?
Regards.
Q.
Interesting. By "parts", I gather you don't just mean spools, but blades/stages, support bearings, airflow management components, fuel management components, gearbox accessories, etc, for specific engines from specific manufacturers?
Was this information something you were taught in the classroom, or did you take it upon yourself to count the parts?
Regards.
Q.
Joined: Mar 2002
Aviation Qualifications: CPL
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From: near an aeroplane
The main advantage of twin spools over a single spool is the fact that the rotor speed better matches the airflow. On a large single spool engine the rotor speed is more or less optimal about halfway down the compressor section. So in effect this means it will be turning too fast for the first stages, and too slow at the later compressor stages. This can be partially resolved through blade design, variable inlet vanes, variable stator vanes and such but it will never be optimal. By creating more spools the N1 spool can use a lower speed (6 to 7000 rpm) compared to the N2 spool (over 13000 rpm) which better matches the local airflow through the engine. A triple spool engine is one step further of course and takes this principle to its next logical step. The drawbacks? More bearings, extra shafts, more complexity, and therefore better used on larger engines where the weight penalties will be least.
(N1 and N2 speeds as quoted are not based on any specific engine type!)
(N1 and N2 speeds as quoted are not based on any specific engine type!)
Joined: May 2000
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From: New York
I think the 3 spool engines are a bit more simple in that they don't have as many bypasses/bleeds, etc. but probably forfit that savings in the extra spool. I'm now flying the 757 with the Rolls 211s and 76's with GE's. Both are very fine engines, supremely reliable, fuel efficient, and responsive. 211's do start a bit slower, but only if you're looking for it. It's an irrelevant point.
Both are superb engines.
The finest engine ever made was the Rolls Merlin. It turned the P-51 from a good airplane into a great one.
Both are superb engines.
The finest engine ever made was the Rolls Merlin. It turned the P-51 from a good airplane into a great one.
Joined: Jul 2001
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From: Gold Coast
Okay , I just realised why my answer seemed so odd.
For some reason I completely missed the [b]3[/i] spool bit, instead reading it as single spool.
Imagine that I've said something profound about the extra stiffness and weight of a triple spool compared to a two spool engine, and skillfully compared then both as the sizes of the engines grow from about 50,000lbs thrust to 100,000lbs+.
I'll just shut-up right about now ..... Grrrr ....
For some reason I completely missed the [b]3[/i] spool bit, instead reading it as single spool.
Imagine that I've said something profound about the extra stiffness and weight of a triple spool compared to a two spool engine, and skillfully compared then both as the sizes of the engines grow from about 50,000lbs thrust to 100,000lbs+.
I'll just shut-up right about now ..... Grrrr ....
Joined: Dec 2001
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From: what U.S. calls Žold EuropeŽ
One advantage of 2 spools are lower gyroscopic moments. With the two spools turning in opposite direction they nearly erase each other moments resulting in smaller pylon loads. The lighter inner spool turning at higher rpm has about the same gyroscopic moment as the outer and heavier one turning slower.
For 3 spools inner moment compensation is not working as well.
For 3 spools inner moment compensation is not working as well.
