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Captain Sim wrote on Jan 30^th, 2020 at 3:38pm:


OK, so I did a little observation with some typical cruising situation. data from FPPM 757-200 PW2037 FAA LB/F/FT REV02 -3 ENROUTE -All Engine -LRC Table(It is same on FCOM-PI-All Engine PW2037- LRC Control) . And I do found something interesting.

Color Code:
BLUE:CONTROLED
GREEN:MATCH
AMBER:NOT MATCH


Here we see, at FL290 with 220lb, I match the KIAS from LRC table thus the Mach number didn't match...
As my understanding, This is because FS/P3D's IAS is actually EAS (factoring compressibility when converted from IAS, or, NOT factoring it in when coverted from TAS as that's what FS do). While on real ADIRU it only fix installation error thus CAS, that's the difference you see here. the P3D one is more "accurate" but the real bird is not....

But what matters for my trip is the GroundSpeed, when level flight without wind, it's same as TAS, if I match Mach and TAS/GS, the FF goes high.

But in any case the EPR is ridiculously high, The EPR typically only gose like 0.95-1.45, so 0.1 more is like 20% of difference

Same thing with 180lb of weight


for FL320, same deal, if match the Mach number thus ground speed, the FF goes higher than data, that's why I feel I need fuel_flow=0.9 for most my flight



But up to FL340, 220lb start to match, but not with lower weight


Still, the EPR is bad...

and for FL380, things changs again... FF gose much lower than data...That's when I feel if I run with Fuel_flow=0.9, I'll get too much fuel on landing if I cruising high...



Actually, I also know this effect, the problem with FS/P3D is the shock wave drag factor dose not change with AOA, so with high alt things going worng like this, I see it on many mid-to-high end Add-ons as well.

But there is some thing can be done, When I tweak aerodynamic, I normally would add more induced drag than parasite drag to get better match on high alt, since the IAS will drop thus high AOA. also get lower cruise_lift_scalar can get even higher AOA, but be careful to not ruin TO/other performence...

Things I would suggest CS to look into:
More tweak on aerodynamic to fit over all fuel flow, I know there are many "FS Limitations", But it certainly could be better (I tried, and get better result on PSS757 back to FS9, so I know it CAN BE DONE)
Look into the EPR gauge, it just doesn't working well... It can't even go lower than 1.0 (guess what, at FL210 and low weight as 140lb, you cruise at 0.99 EPR)
It would be better to have a customized ADIRU data for airspeed reading, rather than just use FS's EAS,especially for those high performence Airliners.