IFIXCF6 From United States of America, joined Sep 2004, 108 posts, RR: 0
Reply 7, posted (9 years 11 months 3 weeks 1 day 20 hours ago) and read 9473 times:
Quote: I can not answer for all turbines but as an engineer I would say that they would be rotating the same direction due to the bearing speeds.
If one shaft was turning 5,000rpm and the other 2,000rpm then the bearing would be at 3,000rpm if rotating the same direction and 7,000 if turning the opposite direction.
This is not the way bearings are used in engines I have maintained. Specifically, I know more about CF6's than other engines. The -80C2 has 7 total bearings, with the 1B, 2R, and 6R supporting the Fan/LPT (N1). N2 (core/HPT) uses bearings 3R, 4R, 4B, and 5R. The "R" and "B" indicate roller or ball bearings. The balls are thrust bearings, hence 1 on each rotor. Why 2 #4's. No idea. But you can see the rotors (yes, one is on a spool, one is on a shaft), are supported by seperate bearings.
The case for or against counter-rotation is probably an aerodynamic efficiency question. Since stators (non-moving vanes between each set of rotating blades) are used to straighten the airflow, I think it has possibilties. However, turning the Fan/LPT in one direction, and the core/HPT in the other does not seem to be that great an improvement. What if you could alternate rotation from one stage to the next, throughout the engine? A mechanical nightmare, but what...in terms of efficiency? Could you eliminate stators, with their drag? Or just reduce their profile? I don't know. I'm just a mechanic. But these thoughts come to mind.
Lemurs From United States of America, joined Mar 2005, 1439 posts, RR: 4
Reply 8, posted (9 years 11 months 3 weeks 1 day 19 hours ago) and read 9466 times:
Quoting Gigneil (Reply 6): If I'm not mistaken, the GEnx will also feature contra-rotation
You're right on. If you go to GE's press site for the GEnx and look at the flash demonstrator applet, they discuss it there. They state this allows for some of the efficiency gains in this new core, unsurprisingly.
There are 10 kinds of people in the world; those who understand binary, and those that don't.
Jetlagged From United Kingdom, joined Jan 2005, 2602 posts, RR: 25
Reply 10, posted (9 years 11 months 3 weeks 1 day 9 hours ago) and read 9373 times:
Quoting IFIXCF6 (Reply 7): Since stators (non-moving vanes between each set of rotating blades) are used to straighten the airflow,
Stators do more than simply straighten the airflow. They increase the air pressure by acting as a diffuser.
Stators aren't always stationary either. The GE CF6 and Pratt & Whitney JT9D engines use variable incidence stators for compressor stability. This is one complication the RR three spool design can avoid.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
PW100Testpilot From Netherlands, joined Mar 2005, 26 posts, RR: 0
Reply 13, posted (9 years 11 months 3 weeks 1 day 4 hours ago) and read 9328 times:
A PW-100 (that's a turboprop, maybe not answering your question, but maybe interesting to know) has 3 shafts rotating into each other, an HP-shaft (HP impeller - HP turbine), a LP-shaft (LP impeller - LP turbine) and a PT-shaft (Power Turbine - Reduction Gear Box).
These shafts rotate in different directions, the LP-shaft CW with appr. 27000 RPM, the HP-shaft CCW with appr. 33000 RPM and the PT-shaft CW with 20.000 RPM
Efohdee From United States of America, joined Oct 2004, 214 posts, RR: 0
Reply 15, posted (9 years 11 months 3 weeks 23 hours ago) and read 9278 times:
The Textron Lycoming T55 712/714 turboshaft has contra-rotating turbines. Four total turbine stages. Two gas generator turbines drive the compressor. Two power turbines turn in the opposite direction to drive the output shaft. This is said to offset the effects of torque.
I think the Textron Lycoming T53 UH-1 Huey engine is the same, but I'm not sure. I've never torn into one.
Lightsaber From United States of America, joined Jan 2005, 14023 posts, RR: 100
Reply 18, posted (9 years 11 months 2 weeks 6 days 16 hours ago) and read 9154 times:
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.