|Quoting Air2gxs (Reply 2):|
All shafts travel in the same direction. Clock-wise for PW & GE, counter-clockwise for RR.
Correct. Prior engines are co-rotating. Note that the F119 and newer military Pratt are all counter rotating.
|Quoting B747FE (Reply 5):|
The Rolls Royce Trent 900 was designed with a contra-rotating HPC/HPT module.
The future is contra-rotating. There was a bearing "breakthrough" in the 90's that made it easier to take the higher effective RPM's a coutra-rotating design imposes while at the same time allowing a smaller package (needed to make a contra-rotating possible, parts have to fit...). (e.g., low spool at 1300, high spool at 12,500 creates an effective RPM of 1300+12,500 or 13,800 RPM.) A double spool gets a free 2% drop in TSFC. A tripple spool (RR) gets a free 3% drop in TSFC.
Since I worked for Pratt for over 4 years, I hope you all don't mind me going into a LOT more detail on contra-rotating engines. I really believe, now that the bearings aren't a big issue, that all future engines that aren't derivatives of exiting designs will be contra rotating.
If you know how a compressor (or turbine) works, there are alternating rotating airfoils (blades) and stationary (as in non-rotating) airfoils (stators). The stators need to take out the rotational inertia that the spool imparts on the gas path. (Note, there is diffusion going on, but I'm going to simplify.) There is a loss associated with this. Also, there is that rotational velocity that has energy. By not taking direct advantage of that energy to compress the gas (or extract work in a turbine), there are unavoidable losses. In a contra rotating design, you can match the airfoils so that at the switch between the contra-rotating spools there is no need for a stator at certain design points. This dramatically improves efficiency.
Now Pratt will always put in a stator between the contra-rotating spools. Why? Better efficiency off of the design point. However, this lengthens the engine hitting the design with a weight penalties. In the F119 vs. F120 competition for the F-22, the Pratt engine won mostly due to improved fuel economy off of full power. Why? GE
likes to take out the stator between contra rotating stages (in military designs, I don't know about GenX.); this shortens the engine (less weight) and simplifies the design (fewer parts). But there is a noticible hit in TSFC in reduced power operation (4 to 6%)! In the case of the F-22 this cut the subsonic ferry range too much. Theoretically, you could optimize the airfoils so that no stator is needed, but no one has done it right yet.
|Quoting Caboclo (Reply 16):|
EMBQA, what's the difference between shafts and spools?
Depends on the engine company. At Pratt, a Spool includes the following parts: Compressor rotor and blades, shaft (tube that bearings ride on and connect compressor rotor to turbine rotor press fitted into rotors), bearings, turbine rotor and blades. However, that's Pratt. I cannot speak for other corporate cultures.
|Quoting Finkenwerder (Reply 9):|
Rolls Royce did it 30 years ago with the Pegasus ............Counter rotating shafts !!
An amazing design well respected as being ahead of its time by the rest of the industry.
I was turned on to this thread a bit late. Hope the info is useful.