A blimp is a non-rigid airship that floats in the air solely due to the buoyant gas within the giant balloon-like bag. It typically carries over 5,000 cubic meters of helium at a density about seven times less than air. An airship floats in the air in the same manner as a ship floating in water. The weight of the airship must equal the buoyant force of the gas inside the bag. In order to increase the altitude of the blimp, the pilot increases its buoyancy by adding gas from pressurized tanks to the blimp, which expands the flotation bladders, displacing the heavier air and increasing the buoyant force until the reduced weight of the blimp equals the reduced weight of the air. To lower the airship, the buoyancy is decreased by releasing gas from the flotation bladders, which then decrease in size, displacing less of the heavier air.
Because helium is a safer gas than hydrogen, it is what is used in today’s blimps.