As shown in Figure 1, the motion plane is placed below the
centre of buoyancy for improving the vehicle stability. By
loading/unloading the ballast and adjusting the position
of the sliding mass with respect to the centre of buoyancy,
the net weight and the position of the centre of gravity are
changed in order to achieve the propulsion: the so-called
buoyancy-driven engine. The wings provides drag and lift
forces that propel the vehicle, whereas the tail improves
stability.