Ground refueling work around.

[Frederf]

It doesn't happen with the power off. I've tested it a lot of different ways. You can't use the fuel gauge without enough power applied to the airplane that also starts to move around fast. It only takes a few seconds.

 

Here is a track showing that the external fuel stays put until you have ground power or engine. Airplane starts with 20% internal and full 300g external. My total fuel according to rearm dialog is 48% of internal capacity before I turn on main pwr, 47% after I turn on main pwr and it's 47% after ground power is applied and all the fuel rushes from external to internal.

 

viper fuel change.trk

[Nealius]

Here's my track showing the 370gal tanks drained to 0% and the internal tank drained 50% when setting the total fuel slider to 50% in the rearm window.

 

The rearm window weights are also wrong. Payload is 4Mk82, 2AIM120C, 2AIM9M, LITENING, 370gal tanks. I should probably test a clean jet vs. bags only for easier math.

 

FUEL %	REARM WINDOW	MISSION PLANNER
50%	33,466lbs	35,036lbs
100%	37,049lbs	38,616lbs

 

Setting 81% fuel takes the totalizer down to 7,999lbs or so, with the needles showing 400 in the external tanks.

Viper Fuel.trk

Edited July 6, 2021 by Nealius

[Frederf]

Yeah, you're right and I'm wrong. I've been testing when the airplane starts with partial fuel not when reducing internal with full externals. By timing defueling an off airplane from 100% to 0% with no and three external tanks I get 163 and 321 seconds for 7163 and 14118 lbs. fuel respectively. That's a relationship of 1.97 ratio for each. Fuel is being removed and added at a rate of 20 kg/second. Just by time it's clear that it's moving more fuel.

 

Power on/off is determining if externals do/don't fill internal capacity when not being defueled or fueled. When being defueled or fueled the external tanks are being affected regardless of power.

Looking at the fuel system in the flight manual there's not really any particular bug in logic (rates are super fast though). Everything is working normally but the tools we have are confusing. The rearming window fuel slider initializes in an unexpected way (total not internal) and the setting is an end-state internal fuel goal. The ground service adds/removes fuel until it hits this goal regardless of what wacky hijinks happen during that process. The downside is that 100% internal goal can leave all sorts of unusual external fill states and there's no direct way to ensure a total fill. The only way to refuel external tanks completely is for the internal fueling process to go on long enough that the external tanks get filled during that process which depends on external tank which requires the most to fill and internal capacity minus the current internal amount.

[Nealius]

This is problematic online, where we do not have an accurate way to set fuel. The Viper in its current state has almost no bring-back capability. On-speed approach/landing speeds at GW above 28,000lbs or so are 10+ knots higher than real-world values, and the nose comes down prematurely during aerobraking at these weights as well, meaning you have to plan your mission such that you come back with enough fuel to stay under GW 28,000lbs or so. Else you'll run off the end of the runway.