twin otter aft fuel tank
hello phu, the short answer is : pitching moments, gravity & fuel sloshing. the fuel tanks of the twin otter are mounted in tandem in the fuselage & have pitching moments around the center of gravity when the aircraft is climbing/descending. by doing so the aft fuel tank is either rotated down & tilted aft or is rotated up & tilted forward. now gravity kicks in & wants to equilize the fuel levels. this has an adverse effect on the piching moments. the answer is : flapper valves, which have an equilizing effect. depending on the tilt, this effect is an all/ nothing affair for valve 5( collector tank) & an all/calibrated, restricted affair for valves 6 & 8, due to the hole in these valves.
greetings,
bm