Skyships are equipped with a pair of ballonets, one forward and one aft comprising a maximum of 27% of the envelope's gross volume. Ballonets are air-filled compartments within an airship that compensate for the expansion and contraction of the helium, thereby maintaining envelope pressure. They can be filled, or partially filled, with air. Emptying of air is done through four valves under the envelope. The valves open automatically as the ship climbs; air is released, the ballonets collapse and the helium expands. During descent, air from the propeller ducts, and/or electric fans is used to inflate the ballonets, and keep the envelope from collapsing. The ballonets can be independently filled by using shut-off dampers in the air supply trunking. Through differential inflation a measure of pitch trim can be obtained.