Advantages of a Two-Spool engine?

[Monty Python]

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

[411A]

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.

[Intruder]

I suspect the 2-spool engine has lower maunfacturing costs due to its relative simplicity.

[18-Wheeler]

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.

[OzPax1]

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).

[twistedenginestarter]

18-wheeler

I've read your post several times and I cannot guess what you were trying to say before you appear to have typed something completely wrong. Can you check it again?

[18-Wheeler]

Nope, that's how I wanted to write it.
In other words, 2-spool more parts, so more expensive.
2-spool more flexible with regard to airflow.

[QAVION]

"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.

[Ford Airlane]

The GEs sure light off quicker than the RRs. The difference there is amazing!

The only thing I find annoying with the RRs though is the droning noise associated with un-synched N1 which is not apparent on the GEs.

[cirrus01]

Empty Weight difference on the 777 quite dramatic with different engines..........

RR Trent powered 200 series about Four and a half Tonnes lighter than a similar equiped GE90 powered A/C.

[gas path]

.......And a darn sight better

[Jhieminga]

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!)

[Intruder]

18-Wheeler:

The original question was in reference to 2-spool (GE and P&W) vs 3-spool (RR). Single-spool engines are not in the equation. Do you still disagree that the 2-spool is cheaper to manufacture?

[Roadtrip]

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.

[18-Wheeler]

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 ....

[Volume]

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.

[OnTheStep]

double spools also exhibit more stable operation, ie- less prone to surging

[GlueBall]

And last but not least: Two spoolers feature more panel space; No N3 guages to scan.

[C Montgomery Burns]

Is 'pod-nod' a feature peculiar to triple spools (we had it a fair bit on our L10s with their RB211-22Bs and -524B02s) or does it affect twin spools as well?