RE: Belly tank for more fuel capacity on Yellow F-15
Most external tanks on full scale planes are pressurized for exactly that reason. Bleed air on jets, Vacuum pump exhaust air on the old warbirds. Larger tubes might help, but the column of fluid may be to high. Planes like the P-51 and T-28 run boost pumps 100% of the time just for their main tanks. You may need one between your external and main tanks or find a way to pressurize your external tank.