1. High RPM high spool. 22,300RPM maximum. What that does is a very efficient high compressor and even better for the high turbine.
I am sure I don't need to tell you, but for others: RPM in of itself is relatively meaningless. What dictates the pressure rise per stage (really work per stage) is the tip Mach number, which is a function of RPM, blade tip radius, as well as the temperatures coming out of the LPC. As the fan pressure ratios have dropped and bypass ratios consequently increased, the physical size of the cores has gotten smaller and smaller, pulling in the HPC tip radius - forcing HPCs to spin faster to maintain the same tip Mach numbers. Furthermore, as the LPC is more heavily loaded (as noted below) the temperature at HPC inlet is increased, further demanding greater physical speed to maintain tip mach number.
2. Contra rotating. This is the root cause of the EIS issues. For a very high speed high spool going the opposite direction of the low spool creates a very fast bearing speed. The LEAP is corotating to avoid the risk. About a 3% fuel burn advantage going counter rotating.
Your statement about bearing speeds is only relevant if there is an inter-shaft bearing, rather than all bearings being to the static structure. Historically P&W have not used inter-shaft bearings, e.g. on the JT9D and PW4000 (see Jack Kerrebrock, Aircraft Engines
). Do you have any source showing that the GTF uses intershaft bearings (I say this not hostilely but earnestly)
Most of the sources I am seeing attribute the GTF startup problems to thermal rotor bow rather than a bearing issue (e.g. http://aviationweek.com/commercial-aviation/new-pw-president-has-nothing-hide-gtf-starting-issue
). This makes sense to me as the low speed shafts especially are becoming smaller and less stiff as the overall core size shrinks. Rotordynamics is becoming an evermore present issue with these designs.
3. 3:1 gearbox on the 10,047 rpm low spool means the low turbine is much simpler and more efficient. Most of why the pw1100G is lighter than the LEAP is the reduction in low turbine stages.
Correct. The higher rotational speeds of the LPT due to the gearbox allow higher tip Mach numbers, thus more work extraction per stage and fewer overall stages. It also allows them to not dramatically increase the LPT radius to maintain tip Mach numbers. Compare the GTF cross-section (http://356007295890291112.weebly.com/up ... ig.jpg?250
) and LEAP-X (https://theflyingengineer.files.wordpre ... leap-x.jpg
) cross sections and it is immediately obvious that the number of stages and radii are very different.
4. I started a new number to talk the low compressor. By having it at a decent Mach number not only makes it more efficient, but much higher pressure ratio per stage. That helps engine efficiency.
Agreed. I wouldn't be surprised if the OPR of the LPC was on the order of 3-4 for the GTF.
5. Low RPM fan. That cuts shock wave losses. Oh, the engine would have done better with a 3.5:1 gearbox, but the gearbox is the longest lead item. Hence why the MRJ gets the C-series box instead of one built for half the horsepower as was needed.
The reduction in shock loss is one immediate benefit of the GTF architecture, but a longer term benefit is that it enables further decreases in fan pressure ratio. One can always decrease fan pressure ratio through blading design (decrease turning), but with the higher RPMs associated with direct shafts and the required compromise between the fan and LPC/LPT, you would still incur the shock losses for essentially no reason. The GTF architecture opens up a new part of the design space of very efficient, low pressure ratio fans.
One thing I would like to note is that increased tip Mach number does not directly imply increased efficiency. Too high of a tip Mach number and you can do a lot of *work* per stage, but the efficiency of the stage will drop greatly as shock losses grow. It is rare to see tip Mach numbers above 1.4-1.5 as that is roughly where the total pressure ratio across a normal shock really starts to fall off. Too low of a tip Mach number, however, and the work per stage is low and you will need more stages to achieve the desired pressure ratio, which costs weight and complexity.