Airships get off the ground with lifting gas. Lifting gas includes any gas that is lighter than air such as hydrogen, helium or natural gas. When the dirigible pilot replaces the air in the ship's ballonet with the lifting gas, the air surrounding the ship has a greater specific weight and the airship rises off the ground. It takes about 1,000 cubic feet of hydrogen to lift 20 lbs., whereas 1,000 cubic feet of helium can lift 60 lbs.
Much like a ship in the water, an airship controls its direction with a keel and rudder system controlled by the people flying the ship. Propellers spin, so the dirigible goes forward rather than just hovering in one spot. Flight control units called "elevators" control the angle of the ascent and descent of the ship.
As the dirigible goes higher, the air pressure gets lower, allowing the lifting gas to expand. To keep the helium at a constant pressure, the pilot uses a system of valves to put air into the ballonet. The pilot strives to maintain a balance between the pressures of the helium and air within the ship using the valves. This balance keeps the ship at the desired altitude.
Going down is the reverse process of going up. The pilot increases the amount of air in the ballonet while releasing lifting gas. The air in the dirigible makes the ship heavier, and so the ship loses altitude. The pilot can increase the air within the dirigible until it lands or he can keep some lifting gas in the ship to fly at a lower altitude
