"The central fuselage is filled with helium, which makes it buoyant so it can ascend like a balloon." "And inside that there's another bag with compressors on

"The central fuselage is filled with helium, which makes it buoyant so it can ascend like a balloon." "And inside that there's another bag with compressors on it that brings air from outside, compresses the air, which makes the aeroplane heavier and then it descends like a glider."