|Quoting tortugamon (Reply 14):|
Looking forward to your updated analysis ferpe; its almost the end of the weekend (your time)
It is always nice to know your work is hotly awaited
. I was putting the finishing touches on the updated analysis yesterday when my new laptop with the analysis software on it decided it didn't want to find any hard-drive anymore (probably a serious virus after trying to find engine maintenance manuals on these "you want XXXX, we got it!" websites
) so here comes the analysis without the finer adjustments:
First some fundamentals (these are better described in the TF42 thread (http://www.airliners.net/aviation-forums/tech_ops/read.main/329330/):
1. Thrust is coming from the engine accelerating the mass of the airflow passing the engine faster then the airplane speed:
thrust = airflow mass * airflow overspeed
2. The funny thing is that the closer this overspeed is to the planes speed the more efficient it is, but to get any reasonable thrust out of it you then need to accelerate a lot of air = high BPR.
3. The core needs to drive a big fan to push this big mass of air out the back with this low overspeed. The power (hp) is directly related to the core airflow and how hot this is before it enters the turbines (T41). High temp = lots of shaft HP
generated by the turbines at a given core airflow, at a given max T41 you need to increase the core airflow if you wnat to drive a higher fan airflow.
had produced an engine concept for 100klbf which has a BPR of 12 i.e. the core needs 8% of the airflow to produce the shaft hp to drive the fan. Jointly the core and fan channel flow produce 100klbf with some 3430lb of air pushed out the back at 940ft/s (multiply the two and you get 100klbf. The overspeed is also called specific thrust and is a good measure on what type of engine one has. Engine for SR71 = very high specific thrust otherwise there is no overspeed at M3
, engine for C-series which flies at M0.78 needs less overspeed and benefits from lots of air = high BPR and low specific thrust which brings good propulsive efficiency).
So when Boeing asks for more thrust GE
can either increase the overspeed (higher fan pressure ratio, ie spin the fan faster) or increase the airflow and try keep the fan PR
low (good for low noise level which is directly dependent on low specific thrust). GE
increased the fan diameter with 2,5'' to 131,5'' which is 4% more fan area then before. The airflow in my analysis at TO
has therefore also increased with some 4% from 3430 to 3560 lb, see the tables (original engine first, stronger one below, click on the tables to see the figures better and ease the comparison as you get them in separate tabs):
For elegance sake I rated the new version at 105klbf (I have no info that this will indeed be the rating). A number of things can be seen from the tables:
I have introduced flatrating of TO
and ToC thrust i.e. the thrust needed comes from the airframe model and the Turbine entry temp TET (T41) and EGT (T45) is then dictated by this thrust and the +15°C ambient temp for TO
and +10°C for the climb thrust at ToC. I could not add these point to my original analysis as my computer went sic
. You see that this cost us some 78K on T41 and 66K on T45 (EGT). If we assume that the design has a redline T41 at 2000K and T45 at 1100°C we can see that the original design left no room for in service deterioration.
Jetlife2 helped me with a rule of thumb for deterioration: start with 20°C and then add 5° for every 1000 cycles you want the engine to stay on wing. So with the typical 5000 cycles for a longhauler engine we need an additional 45 °C for the EGT on top of the 66°C we need for flat rating. In all we need the engine to come out of the factory with an EGT of 1055°C max at TO
+15°C which is what we have with the revised analysis engine. The original run to hot, ie had a to small core, GE
probably had slightly larger core (or they have higher redlline temps then I assume). The core size can be seen at the airflow, we had 267 lb/s originally and now 320 lb/s, a 20% increase. I would guess GEs increase was around 15% or so.
So we now have a larger core in relation to total flow, this means we loose about 2% in cruise TSFC at average weight ( 0.49 goes to 0.50 ). As mentioned before my original engine was a bit tight on core airflow so all in all GE
has probably lost about 1% uninstalled, on an installed aiframe level it can be slightly higher as the engine dia has increased which increases engine and nacelle weight and nacelle wetted area (this is always the drawback of a larger fan dia).
With the above I have tried as best I can to describe how engine OEMs have to reason when the airframe OEM ask for more thrust. One has a certain technology level for e.g. the Turbines (I assumed 2000k T41 and 1100°C T45) and a certain max PR and component efficiency (these are coupled, see the TF42 thread). If you then need more TO thrust you have to increase airflows and a higher relative core airflow cost you TSFC.
This is just as far as I have understood it and I am still at a corse level with my understanding but as the saying goes "it is at least better then handwaving"
[Edited 2013-04-29 14:28:16]