I just did the exact same test, but a bit differently, and I don’t think the differences matter that much. Read on if interested:
Armchair flight test 1:
Shivering cold and dark, at some random gate at KSAN. Used the video-game fuel-n-balance window to load the “problem” plane (baron g58) with 100 percent of total fuel. Parking brake on. Weather: default ‘Clear skies’ preset, which sets wind to 2 knots from 272 which I never changed. Prop and mixture levers fully forward, throttle lever fully idle (read zero). Started both engines, waited 10 seconds. Now I see the default rpm is stuck at 930 rpm.
Released the parking brake.
Aircraft doesn’t move at all. I can make it move by pushing the throttle levers forward slightly to increase the rpm to 1000 or something. And then it starts to move and I can step on the brakes to bring it to a complete stop and wait for the rpm to go back to 930 rpm. I can release the brakes now and plane wouldn’t budge. I can again push the throttle levers a bit to make it move…and rinse and repeat.
Armchair flight test 2:
Repeat everything in 'Armchair flight test 1, but load the plane up with only 15 or 20 percent of total fuel (or the sim’s default, 50 percent of fuel) this time. Soon as you release the brakes after starting both engines and verifying that the rpm is at default 930, the plane starts to move on its own, and will reach a maximum 40 knots without ever touching the throttles if left unchallenged.
Physics? More fuel means more weight? Less fuel means less weight? 