The non-rigid airship in its simplest form is a streamlined envelope, containing the lifting gas, with a gondola suspended below it containing the crew accommodation, propulsion and fuel. Most gas airships today are non-rigids because of their comparative simplicity and sturdiness. The modern non-rigid or pressure airship maintains its shape solely by the pressure of the helium in the envelope, supplemented by an adjustable volume of air within internal ballonets. A non-rigid airship, sometimes called a blimp, consists of:
the envelope - the primary structure containing the helium gas and the ballonets.
the ballonets - air bags inside the envelope which regulate the internal pressure, shape and trim. Air is squeezed out of the ballonets as the gas expands with increasing altitude and forced back in again as the helium contracts when the airship descends.
the gondola is the airship's cabin containing the cockpit, engine compartment and facilities for crew, passengers and cargo. It is suspended from cables attached either to an internal load curtain or externally to the envelope sides.
propulsion is provided by airscrew engines and may include vectored thrust to assist take-off, landing and holding stationary flight.
the empennage - fins, rudders and elevators provide dynamic stability and steerage.
Source: 'An introduction to the Airship' - Edwin Mowforth