Found an odd bug with fuel weight, how an aircraft behaves when empty (after burning all fuel) changes drastically depending on the amount of fuel added initially. It happens when engines are attached to rotators for thrust vectoring and should not be an issue with conventional fixed engine aircraft.
When I fill the plane up with 10% fuel and run it dry, it performs as intended.
When I fill the plane up with 100% fuel and run it dry, it becomes unstable, as if the empty weight of the aircraft and CoM changed somehow, like the fuselage blocks becoming lighter than their normal empty weight after burning all the fuel.
It makes it very difficult to build a balanced aircraft. The aircraft in question is completely original with no mods whatsoever.
EDIT: To help people understand the issue better, see examples below.
TestPlaneA with 40 gallons - Take off and fly around until fuel runs out. When the engines flame out, try to keep the aircraft in the air. When it reaches minimum speed, the nose will dip down but the aircraft remains perfectly stable and controllable.
TestPlaneB with 400 gallons - Very same aircraft but with more fuel. Take off and fly around until fuel runs out (may take a while longer to burn all fuel). When the engines flame out, try to keep the aircraft in the air. When it reaches minimum speed, the tail will dip down instead of the nose and the plane goes into a tailspin.
Same aircraft in same state under same conditions, yet the weight distribution is different. The more fuel it burns, the more dramatic the effect becomes.
@AndrewGarrison Seems CoM of aircraft is not calculated properly when the fuel is depleted and is a lot at take off time
Did some digging on the net and found the cause of the problem, the bug is caused by thrust vectoring engines, specifically engines attached to rotators (which this particular aircraft has). Removing the rotators fixed the problem. Best to use wing blocks for thrust vectoring instead.
EDIT
@randomusername All fuel tanks are arranged to be on the CoM already from the start, like any other plane. No need for XML edits either, 950 gallons is enough.
@Minecraftpoweer I will upload the thrust vectoring version as well eventually since many people play with unlimited fuel, if you remove all fuel from the plane, or don't burn any fuel in-flight, the bug does not occur. The standard version (which does not bug out) ended up being just as nimble anyway.
EDIT2: Added examples to the original post, please note that the aircraft is not the final product.
@Dragon77 can you upload it unlisted and tag me?
@Minecraftpoweer Doesn't matter, the plane is empty (no fuel, nothing in fuel tanks, bone dry, running on air etc.) when the bug occurs.
If I add 10% fuel ---->>and use up all the fuel in-flight<<---- the plane remains stable.
But if I add 100% fuel ---->>and use up all the fuel in-flight<<---- the plane becomes very unstable.
or
If I only add 10% fuel, there's no problem.
If I add 100% fuel and burn it down to 10% in-flight (meaning the aircraft's fuel load, weight and CoM is the same as above), it becomes unstable.
i.e. the --->>empty weight<--- of the aircraft changes/bugs out.
@Dragon77 or just if you have one of those scales where you put one thing on each side, both sides have full glasses and both weigh the same. if you remove all liquid from one of the glasses the one which still has liquid weighs more
@Minecraftpoweer Let me try a simpler explanation:
Take an empty glass, the empty glass weighs 100g with the CoM on the bottom end. Add a tiny bit of water to the glass, it weighs more and the CoM moved upwards. Drink the water in the glass and the empty glass weighs 100g again with the CoM back on the bottom end.
Now take the same 100g glass and fill it to the brim with water, it weighs more and the CoM moved upwards even more. Drink all the water in the glass but, instead of the empty glass weighing 100g again with the CoM at the bottom end, it now weighs only 50g with the CoM at the top.
That is the problem, doesn't matter how much water you added or how much the water weighs, if the glass is empty, it should weigh 100g with the CoM on the bottom end, not 50g with the CoM at the top.
@Dragon77 because if there is a lot of fuel it weighs more, when the fuel is depleted a lot of weight will dissapear thus changing the COM and if the COM ends up like a lot further back your plane might stall easier
@Minecraftpoweer I know, but that doesn't matter at all if there's no fuel left in the aircraft. The issue is with how differently the plane behaves after expending all fuel (or very little left). If I fill the plane with 10% fuel and use up all the fuel, it behaves differently than when I fill it up with 100% fuel and also use up all the fuel. If there's no fuel left in the plane, it should behave the same regardless of how much fuel I started with.
@Dragon77 because when the fuel decreases the weight also does
@Minecraftpoweer That's pretty obvious, yes. The issue is not with the weight of fuel but, the difference in how the plane behaves when empty, depending on how much fuel added before flying. See the bit below, by "run it dry", I mean using up all fuel.
"When I fill the plane up with 10% fuel and run it dry, it performs as intended.
When I fill the plane up with 100% fuel and run it dry, it becomes unstable..."
I may also use a mod in the meantime until the issue get fixed, if at all (it was first reported more than two years ago).
because fueal has mass and mass+gravity equals weight? idk im not a scientist but fuel weighs so i usualy only have modded fuel and i make the fuel block 0 mass so it doesn't matter if it runs empty
@CursedFlames It is indeed the exact same plane, only fuel load changed.
I usually test planes with as little fuel as possible in order to test flight characteristics when the plane is at it's lightest, and also to test empty weight unpowered glide capability. I fuel it up properly once it "enters service".
So it's the exact same plane, just with different fuel loads? That's really odd if so, though I can see why no one would notice it: you rarely spend all your fuel.