A blimp, which is a type of non-rigid airship, stays in the air through a combination of buoyancy and aerodynamic forces:

1. Buoyancy: The blimp’s envelope is filled with a lighter-than-air gas, typically helium. This provides the main lift that keeps the blimp afloat. The buoyant force of the helium offsets the weight of the blimp’s structure and payload.
2. Aerodynamics: The blimp’s shape and tail fins create aerodynamic lift as the blimp moves through the air. This additional lift force helps stabilize the blimp and allows it to be maneuvered.
3. Ballonets: Inside the blimp’s envelope are air-filled ballonets. The pilot can adjust the amount of air in the ballonets to control the blimp’s buoyancy and altitude. As the blimp ascends, the ballonets release air to allow the helium to expand. During descent, the ballonets take in more air to increase the blimp’s weight.
4. Superpressure: The blimp’s envelope is kept under a slight positive pressure relative to the outside air. This superpressure helps maintain the blimp’s shape and structural integrity, preventing it from collapsing.

In summary, a blimp stays airborne through the combined effects of buoyancy from the lighter-than-air gas, aerodynamic lift from its shape, and the pilot’s ability to adjust the blimp’s buoyancy using the ballonets. The slight internal superpressure also helps preserve the blimp’s structural integrity during flight.