I have not seen any indication through usage over the past 15 years that 1505 is anything other than spool rate in practical usage.
Whether this is some kind of C2/FF/bleed calculation may remain unclear and depending on how you interpret the numbers theoretically, you can get varied interpretations.
The bottom line is the left side Y reflects engine spool up rate at low RPM and the right side Y is spool (RPM rate change) at high RPM. This is most easily observed using AirEd.
In practical usage, low Y on the left side results in slower starter and ignition RPM and and high Y on the right side results in a more responsive (faster reaction time) engine RPM change
at high RPM.
When using AirEd, Setting low Y values on the left X is useful for reducing starter surge. When used this way, it's important to increase the Y rapidly in the table (at least by X = 0.32)
to prevent too-slugish response at lower approach air speed RPMs.
Fan jets (high inlet area engines) tend to have slower spool times at higher RPMs at cruise than turbojets.
1505 is a relative table, not absolute values which is part of it's deceiving nature so if the cfg file inlet is large, 1505 will output slower response than the reverse.
What is x and Y exactly? I have ceased to care years ago. I just use it to control low and high RPM spool and I get the results I want.
For instance, when configuring the SR-71 engine which has very slow startup spool time and near instant response at max power. I have the following values (x = y
0.0029 = 1.51
0.118 = 7.6
0.0207 = 13.6
0.024 = 23
...
0.167 = 100
...
0.6869 = 125
For my purposes, I simply interpret 0.6869 X as 70% engine power/(FF?) so the spool above 70% is interpolated based on the highest value.
By default, FSX ignition is ~22% RPM regardless. I have found it useful to interpret the table where x = 0.167 - x = 0.32 as close to ignition RPM.
If you HAVE to reason it all out, I think there is a case for those who have observed the table seems to be inverted from a typical interpretation.
For me, I do not try to figure it out and simply set low Y values for slow spool and high Y values for faster spool between zero and seventy percent Fuel flow.
The results of my settings above is during starter spool, the increase is quite slow and there is little surge at ignition as opposed to default 737 values.
At runway acceleration max power and Mach 3.2 cruise, the RPM responds to throttle changes in less than 2 seconds.
In the Concorde, I also adjust the low scale to prevent the startup RPM from surging to 100% or more (which is the default FS2002 behavior.)
Some engine have significant surge at startup so you need to know your engine's characteristics when playing with this table.
Some evidence Y is related to N2 is if the right Y is too low, N2 will be limited (reduced) regardless of the 1503/1504 values.
-Pv-
