So I recently designed a semi-stable fighter, and its CoM and CoL are lined up at nearly the same position. When you start out flying, its mostly stable (it also has variable artificial stability). However, when the fuel level drops to around 50% or below, the aircraft becomes unstable and uncontrollable at lower speeds (and less stable at higher speeds as well), and even the stabilizer system can't override it. The only way to get out of this is to use the thrust-vectoring, but it becomes an issue when landing due to the fact that you need to use minimal power to land the aircraft.
So, on to the question: does the change in the fuel level have an effect on the mass or balance of the aircraft, or am I missing some other important detail? I previously thought that there wasn't simulation for balance or mass changes caused by fuel level changes while flying, but I could be mistaken.
Note: even adding increased control doesn't fix the issue. The issue also happens only whenever the fuel level gets to around 60/50% or below.
Link to the aircraft.
@PlaneFlightX
@HydroMoney22
I believe I overlooked a very important factor: stalling. The aircraft has a takeoff airspeed of about 175mph (about 282kph) IAS. I've been trying to land the aircraft at much lower speeds. The issue might be that the aircraft is just destabilizing due to a stall. Interestingly, the aircraft will stay in the air until it enters an uncontrolled pitch-up, most likely due to the CoM/CoL alignment. I'll do some more flight testing to see what needs to be done in order to land the aircraft properly, assuming that an aerodynamic stall is the issue.
@HydroMoney22 Thanks for the information!
@PlaneFlightX Thanks! I'll test it out to see what happens.
T
Fuel impacts COM, if you have fuel in a fuselage block then that fuselage block will have more mass and the mass will decrease as you use up your fuel. If you want to negate this you can either place a fuselage block with fuel at the centre of mass so the mass changes but the COM does not, or you can set the mass scale to 0 on a fuselage fuel piece and place it where ever to have fuel that does not effect mass at all.
Personally, I grab a fuselage block, fit it to the thickest section of the body, fill it with fuel, set mass scale to 0, and then go into the fuel percent and set it super high for a large amount of fuel. And none of the mass changes.
Fuel mass is simulated, I think. The best way to test this would be to make a contraption which can drive onto another contraption (the second contraption being a platform with springs), with the first contraption being mainly fuel with a engine to comsume it. You could then test how much the springs were compressed based on the level of fuel.