w.i.p Fuel usage inconsitencies at varying speeds and altitudes since FM update

[KLaFaille]

There seems to be inconsistencies with fuel flow which worsen as altitude and speed increases. As speed increases fuel flow starts high, dips, peaks, then declines again. There is also a disparity with the amount of lbs/nm of fuel being used in FPAS vs lbs/hr that is shown on the IFEI. The actual fuel usage appears to align with the lb/nm displayed in FPAS.

This data was taken on the Syria map, 29.92 barometer, 20 degrees C, clean jet with no pylons, and 50% fuel load. Infinite fuel was used to negate any airspeed changes due to decreasing weight of the aircraft. The final mach number in each chart is the maximum speed attainable at that flight level. Y axis is lbs of fuel, X is speed in mach. IFEI lbs/hr usage is 100x times the value on the chart so an IFEI value of 142 on the chart = 14,200 lbs/hr and for FPAS a vaule of 42 = 42 lbs/nm.

In most tests throttles were advanced at .4 mach and I started collected data at .5 to allow for engine spool time. The 40,000 test data starts at where the throttles were put into AB, this is due to the aircraft struggling to maintain attitude control below that airspeed at that altitude. Acceleration in full MIL at 40,000 feet was not recorded due to the aircraft needing nearly full MIL just to maintain altitude. Unfortunately, I am unable to go back to the prior version and run these tests in that environment. There are no tracks because each test was 100% just the jet flying in a straight line with me pausing every additional .02 mach to record data.

I can't say I'm particularly knowledgeable of all the exact variables that go into calculating fuel flow vs speed and altitude, but something certainly seems off and there are usage shenanigans happening after the FM update. It doesn't seem logical that the lbs/nm burned should start to decline as if air resistance is falling off as the aircraft approaches the maximum speed for a given altitude, all while the IFEI fuel value keeps climbing. The transition point from where lbs/nm usage is increasing to it all of a sudden leveling and tapering down seems very suspect to me as well.

FWIW, playing on Gray Flag prior to the update I would typically take off from an airbase and do CAP in an orbit up at 48,000-49,000 feet around mach 1 +/- .1 and with a loadout consisting of 3 tanks, 6x120C, and 2x9X. I would have a loiter time of 45 minutes to an hour before coming down to tank. I can't go back and check prior to the FM patch now, but I seem to recall up around those altitudes fuel consumption was something around 7 or 8 lb/nm in full AB. Now a similar loadout barely has the ability to maintain an orbit up around 43,000, is not able to reach mach 1, and fuel consumption is in the 40ish lb/nm range.

 

Edited February 28, 2024 by KLaFaille
additional clarification

[KLaFaille]

I was able to roll back to the 12/19/23 version 2.9.2.49629 and ran the 1200, 12,000, and 30,000 foot tests again and got the same results; the traces matched each other almost exactly and well within any sort of margin of error. The only difference I found was that the jet could reach higher speeds in MIL on 2.9.2.49629 than it can now in 2.9.3.51704.

First value is full MIL in previous version 2.9.2.49629, the second is full MIL in current version 2.9.3.51704:

1200 FT: .97 vs .95

12,000 FT: 1.02 vs 1.0

30,000 FT: 1.13 vs 1.08

The results still don't make any logical sense to me, especially in the FPAS curves, where usage abruptly just levels out and starts to decline. Though, as I said, I don't pretend to understand the math or theory that governs fuel usage. I think the other changes in the FM were enough to have me take a closer look at fuel usage, as it seemed different now, which, as it turns out, is not the case.

Edited February 29, 2024 by KLaFaille
Added chart comparing version before FM with current one.

[BIGNEWY]

On 2/29/2024 at 6:36 PM, KLaFaille said:

Though, as I said, I don't pretend to understand the math or theory that governs fuel usage. I think the other changes in the FM were enough to have me take a closer look at fuel usage, as it seemed different now, which, as it turns out, is not the case.

 

Fuel consumption is currently undergoing a review internally. 

thank you