Gas Aircraft and Fuel Burn

I am going to be flying my first Ardupilot Gas Plane model soon which will have a 100 oz gas tank flown with a Fuel Injected Gas Engine. I was curious to know how well Ardupilot handles fuel burn as I tend to set my planes up for a best endurance configuration. How well will it handle the fuel burn after I burn off lets say 80oz of fuel? I was just going to tune it with full fuel to start off. Looking forward to flying my first gas model with ardupilot!

I design my planes best to keep the fuel tank in the CG. Even with that, in some of my designs the plane would end up nose heavy at the end of the fuel capacity. I never had trouble with the Arduplane adjusting to this. Just make sure u check for vibrations at full throttle on the ground.
Good luck.

The issue I have, is I tend to tune my planes on the tail heavy side and fly near min airspeed to get the most flight time. Will the autopilot be able to handle this as a large amount of fuel is being burned off. Lets say I take off with 60 oz of fuel and need to make an emergency auto landing. Will it land the same at 60 oz of fuel vs almost no fuel?

I’ve flown gas planes a lot and never had an issue with balance due to fuel load on landing, but the planes do tend to have the fuel tank close-ish to the CoG.

Balance shouldn’t be an issue, but what about airspeed? Approach speeds / stall speeds are different depending on the weight of the aircraft. I guess its not too bad on the smaller scale, but on the larger scale it could cause issues for planes that are not exactly over powered and are behind the power curve. It seems people are tuning on the higher fuel loading side so it doesn’t cause an issue.

We are building a hybrid multi-rotor which has battery and fuel engine.
the fuel tank is a bit large and heavy and the center of gravity moves while fuel consumption.
I wanna ask if the pixhawk can handle this CG movement during the flight.
if it can’t, what should I do?

The principle thing you need to worry about is whether the partially full tank can shift the CG of the aircraft to a state that the stability margin is significantly reduced below a safe limit. Stability margin is the distance between CG and the aerodynamic center divided by the mean chord. Positive margin (CG ahead of aerodynamic center) ensures natural pitch stability. Its value above 3-5% ensures the aircraft is controllable. You do not want to rely on arduPilot’s gyro stabilization to overcome a naturally unstable state.

If your fuel tank’s geometry is longitudinally dominated (ie like a horizontal coca cola bottle along the length of the fuselage) then you need to worry about CG shifts associated with the liquid motion and their impact on the above stability criteria.

Essentially you want to validate the sufficiency of the stability margin in extreme states of the tank when it is partially and fully drained.

In the case of tanks with principle dimensions in the vertical and span-wise orientations, it should be sufficient to check just the full and empty tank states for static margin variations.

In the case of longitudinally oriented elongated tanks, one needs to check fully sloshed states of partially full tank. These states are when all the remaining liquid in the tank has shifted to the back of the tank for 75,50,25 percent full conditions. You should be able to run back of the envelope calculations to figure out where the aircraft’s CG is in each case and whether pitch stability is maintained and static margin is above the minimum value recommended.

If there is a violation of the safety limits, then the tank either needs to be shortened or shifted to the front of the aircraft. If shifted, the aircraft will need to take off nose heavy and fly slightly below the ideal cruise condition. However, it is better that it flies slightly inefficiently for many flights than for it to fly efficiently for the first=last flight. ;).