You can edit the aircraft.cfg file to add another fuel tank. In the example below, I have added fuel tank Center2. 0.6, 0.0, -0.42
is its location, which matches where the front seat passenger position is (Heidi's seat). 12.000, 0.000
is 12.000 gallons of fuel followed by the unusable fuel which in this case is 0. 12 gallons of fuel equals 72 pounds, which the cub can easily fly with. If you make it 16 gallons, you will still be under the minimum 100 pounds that Heidi can weigh if you add her.
Center1 = 2.500, 0.000, 0.000, 12.000, 0.000Center2 = 0.6, 0.0, -0.42, 12.000, 0.000
If you add Heidi using the Payload & Fuel Manager (SHIFT+4), both Heidi and her weight are added to the plane. So if you add the extra fuel tank, don't include Heidi. This, I believe, is what FAC257 did in his Piper Cub - Around The World Adventure
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