Cargo Airships Prospective - Faculty of Aerospace Engineering

shaped airships. Achieving smaller size of the airship is pretty much wanted, because it
affects directly the needed size of the infrastructure, namely the hangar.
Figure 4. Hybrid airship three sources of lift – Aerostatic, Vectored thrust and Aerodynamic lifts (Hybrid
Air Vehicles (HAV) website)
An important feature, found typically in non-rigid or semi-rigid airships, is the ballonets.
When the airship change its altitude, the most important factor influencing it, is the
differential between the lifting gas density, inside the airship, to the density of the ambient
atmosphere. This way, when the airship climbs, the lifting gas inside it expands, activating
pressure on its envelope. In order to avoid such condition (due to heavy penalty induced on
the weight of the envelope material capable of withstanding higher than atmospheric
pressures), ballonets are used. Ballonets are in fact non-permeable pockets inside the airship,
designed to be filled with outside air. When the airship rises, and the lifting gas expands,
occupying more volume inside and reducing its density, the air flows out of these pockets,
allowing to keep the airship's hull form and same internal pressure over all extent of flight.
When the airship reaches the height, in which the ballonets are fully deflated, it is called
airship's pressure height and it is usually the maximum designed height the airship allowed to
reach. In the emergency case that the airship rises beyond this limit, due to weather conditions
or direct sunlight, emergency procedures are usually implemented, by releasing some of the
lifting gas in order to relieve pressure and descend. Pressure height can be designed, by
allocating enough volume for the maximum expansion of the ballonets and filling the airship
with specific amount of lifting gas. For example, for airship designed for pressure height of
10,000 ft in normal ISA conditions (typical pressure height value for most of the modern
cargo airship designs), it is needed to allow about one quarter of its volume to the maximum
expansion of the ballonets at the sea level (SL) (where the atmosphere pressure is the highest).
This way, the higher the designed height pressure altitude would be chosen, the higher lifting
gas volume for given payload will need to be assigned (larger airship).