PW 125B Turboprop engines.

[low n' slow]

Hi all.

Just started a new type course, the F50. In the powerplant chapter, I learnt that the Powershaft and HP spool rotate clockwise and that the LP spool rotates counterclockwise (as seen from behind looking in the direction of flight).

It's not that I'm not accepting this, I am, but I'm just curious, are there any other engines operating in this way? That is, having the different spools rotating in different directions? I've never heard of it and can think of a lot of reasons for why this configuration is a bad thing (different directions of precession forces etc... Then again, someone with a much bigger salary has thought that one out for me...). Just curious.

/LnS

[Escape Path]

While I cannot answer the technical bit from your question, I can say that the PT6 also works that way; both turbines (power and gas generator) rotate in different directions.

[barit1]

Not at all unusual.

For one thing, if they rotate the same direction, a stator stage (nozzle guide vanes) is necessary between the two rotors to insure the gas flow has the correct vector upon entering the downstream (low pressure) rotor. If they contra-rotate, the extra stator can be eliminated, for less weight and lower pressure loss.

And the counteracting precession moments can be a good thing, for reduced mounting loads in the airframe. Also, if the engineer is designing for simultaneous seizure of all rotors, the mounting torque loads are reduced.

Several modern turbofans (Trent, GE90, GP7200...) have this feature.

[howiehowie93]

The Pegasus has contra-rotating spools too so they cancel out the gyroscopic moment when the Harrier was in the hovver.

Lots of other engines have them too as the previous poster said: RB211, RB199 for some that I worked on.

regards
H

[low n' slow]

Wow, I'm learning new stuff here.

I guess at some point in my training, someone told me that the spools never contra rotate and that's been the truth for me ever since and there's never been anything to challenge this thought until now. Thanks for clearing this up for me.

The precession forces are reduced in total, and I guess that's one of the main benefits of having this configuration. But lets say there are three spools or shafts running in the engine as in the PW125B. If the engine is rotated, as in lift off, two spools will want to precess one way and the other will precess the other way. I'm assuming that the fixation of the spools is very rigid, but are there no rubbing problems or similar? As far as I've understood, there might be some play in the assembly and if this is so, then while the precession is in total neutralized, the opposing forces within the engine must cause some extra stress?

Thanks all!
/LnS

[barit1]

The engine cases are of necessity very strong and stiff, because they are pressure vessels after all, and further must maintain shape to prevent tip rubs on the blades. Thus reacting the precession forces is a relatively minor task.

There is an additional challenge to the bearing designers, though, if differential (intershaft) bearings are employed. If the inner ring is spinning 20,000 rpm clockwise, and the outer ring 30,000 rpm counter-clockwise, then the bearing sees a 50,000 rpm spin. Not an easy job. :eek:

[low n' slow]

Ok! Sounds interesting. What RPM are most bearings rated at or limited to?
The PW125B HP spool spins at 33300 RPM and the LP spool spins at 27700 RPM when att 100%. Thats 61000 RPM, quite a lot!

/LnS

[barit1]

I'm not sure of the PW125B architecture - whether it employs differential bearings, or whether the outer rings (outer race in some dialects) are all fixed.

The bearings people seem to have the technology to handle whatever design challenges are thrown their way, but there must be a limit somewhere. :ooh:

[dyn]

In old service bulletin PW125B is indicated by BS647, what does this code mean?

[tdracer]

There is an additional challenge to the bearing designers, though, if differential (intershaft) bearings are employed. If the inner ring is spinning 20,000 rpm clockwise, and the outer ring 30,000 rpm counter-clockwise, then the bearing sees a 50,000 rpm spin. Not an easy job. :eek:

Two spool engines rarely have bearings that are not mounted to structure - there is no need for it.
Three spool engines are a different story - one shaft is supported off another shaft. I doubt it is a coincidence that Rolls RB-524 and Trent engines are somewhat notorious for having bearing issues.