Airships depend on buoyancy from the atmosphere to float rather than thrust to maintain airspeed for aerodynamic lift to remain aloft, hence the designation “lighter-than-air (LTA)”. However, to simply float in equilibrium (neither sinking nor rising) and in still air without using power (no thrust), where the weight of LTA gas plus air contained in the aerostat is included as part of their all-up-weight (AUW), they instead should be as-light-as (ALA) the air they displace, where the surrounding air then pushes them up (the buoyancy experienced, an externally applied up-lifting force) countering their total weight. Thus: when LTA they would ascend (rise), when HTA they would descend (sink) like submarines in water, and when ALA they would just float like balloons – moving gently with the air currents they are in (not to be forgotten as a means for conveyance without use of power).
Large aerostats, which are the visible hallmark of non-rigid airships (Blimps), generally use an envelope that contains the LTA gas (usually helium under current rules) used to inflate them – making it possible for the aerostat to displace the atmosphere and thereby gain sufficient buoyancy for the airship (including other parts) to float. The aerostats also often have smaller internal flexible air cells called ballonets. At sea level, if one were to look at a cut-away view of a Blimp, the smaller ballonets would be located on the interior lower surface, perhaps one forward and one aft within the aerostat’s otherwise helium filled envelope.
Ballonets are used mainly to contain air taken in that compensates for LTA gas volume change due to temperature and/or pressure changes, which expands or contracts in accordance with the gas laws. If one were to fill an aerostat’s envelope full with LTA gas at ground level then, as it rose to higher altitudes and due to atmospheric pressure reduction with height gained, the gas would try to expand but would be constrained from doing so by the envelope. Super-pressure thus would develop and increase, trying to burst it, which would occur if too much – needing ways to prevent this risk. The trick is not to fill it full with LTA gas in the first place and then use compensating air taken in or vented to accommodate LTA gas volume changes and thus fill the aerostat full – perhaps over-filling with air a little to develop low super-pressure that stiffens its form. The flexible ballonet membrane thus prevents air/LTA gas mixing and, with means to take air in and vent it in controlled ways, enables the low super-pressure developed to be held within acceptable limits.
Ballonets thus may be compared to air bladders that can expand and contract. If there were no way to vent the rising internal pressure, the hull eventually would rupture as height increases. Instead of venting expensive helium to compensate for internal pressure rise, air in the ballonets thus is vented instead. However, at the altitude when the air has been fully vented, so with the ballonets essentially lying flat on the aerostat’s lower surface, further ascent then will cause super-pressure to quickly rise. This altitude therefore is called the ‘pressure height’, depending on atmospheric conditions at the time and the quantity (not volume) of LTA gas put into the aerostat (its gas fill). It should be noted that the quantity is the mass (thus weight) of helium put in to puff it up; where the objective is that this quantity remains as a constant amount without loss due to leakage or venting. Naturally, leakage thus must be minimised and venting avoided by not rising above the pressure height if possible; when LTA gas vent valves may automatically open for safety reasons (depending on preset super-pressure opening levels decided for safe operation).
What happens when the gas pressure relief valves open is that the quantity (so weight) of LTA gas contained in the aerostat then reduces (exacerbating the weight balance issue). However, this doesn’t cause an immediate problem, where the airship may continue to rise into thinner (less dense) air, but that still is of greater density compared to the LTA gas. The displaced air mass (so weight) therefore reduces by a greater extent (reducing buoyancy) when an equilibrium state may be reached again without further ascent – allowing the LTA gas vent valves to close. Nonetheless, when the airship then descends the weight (quantity) of contained LTA gas then is less, so a greater quantity of air (weighing somewhat more) eventually will be needed in the ballonet to keep the aerostat’s envelope puffed up and thus structurally stable.
As the airship descends and atmospheric pressure increases (squashing the contained LTA gas) fans pump air into the ballonets to compensate for diminishing LTA gas volume – thereby maintaining constant super-pressure (differential pressure between the internal and outer atmosphere’s pressure) to keep a stable aerostat form that functions structurally without problem. Now, the size of the ballonets is fixed during design based on the altitude (thus pressure height) needed for normal operations. The higher the airship is required to go, the larger the ballonet airspace that must be provided to compensate for gas expansion. This is why airships are best operated at low altitudes, normally not greater than 10,000 ft (3050 m). Altitudes of say 20,000 ft (6100 m) are possible, but then the ballonet capacity needed would be twice as much (about 50% of the aerostat’s total capacity), which then needs further consideration with regard to the way they behave dynamically (sloshing) under pitching action and the increased size of the aerostat necessary for its payload and operating purpose.
If LTA gas has been lost due to venting above the pressure altitude then, as the airship descends there may not be enough ballonet capacity (becoming full) for air taken in to maintain a stable form. Because of this a way (e.g. a valve) to enable air to be put directly into the LTA gas chamber (mixing with it) is provided. However, this additional air then increases aerostat weight above normal operating levels – causing the airship to be HTA and thus sink faster than normal (in exactly the same way that ships/submarines sink when they take water in). Nonetheless, ‘faster than normal’ for airships usually is still slow (compared with purely HTA aircraft) due to their aerostat’s vast size and thus aerodynamic drag; where descent rate may be compared to that of parachutes. The bigger problem is purification of the LTA gas (a long process) in order to put more in – restoring normal quantity for further operation.
When aerostat envelopes are filled with somewhat LTA gas their volume slowly increases (puffs up) without adding much weight, but causing displacement of a significantly greater weight of surrounding air, which eventually overcomes the aerostat’s weight, buoying it up, when it then floats. This is guaranteed due to the physics involved (Archimedes’ principle). Depending on size plus the type of aerostat and its restraint method, this can be dangerous if pockets of LTA gas form in various places then flow together; causing localised buoyancy that rapidly increases (affecting behaviour), a particular problem for unidirectional and oddly shaped aerostats. Nonetheless, after just floating and presuming the gas fill operation is substantially incomplete, further LTA gas fill then will puff the aerostat up more, causing increased buoyancy that must be restrained. Tension in the restraint lines thus increases.
The atmosphere’s displacement (thus buoyancy) therefore increases proportionally to the volume of LTA gas put in, not easy to determine; where one must know the quantity (mass) put in plus the temperature it attains, purity (so density) and the pressure it’s held under to calculate its volume.
Airship design thus involves compromises of payload weight and the behaviour of the contained LTA gas to reach altitudes desired, where the higher the altitude the bigger the ballonets needed and thus the greater the weight of air at low altitude carried in them to lift (reducing payload capacity), just like ships full of water. Naturally, ability to float is desired to fly without effort, where airships should simply be ALA. Their LTA state therefore should not be too much, which makes it difficult to control flight; where excess buoyancy then must be constantly countered with either negative aerodynamic lift (needing airspeed) and/or vertical thrust downward to prevent uncontrolled ascent. Conversely, if operating in a significantly HTA state, then they need sufficient airspeed for aerodynamic lift with a form that suits to become fully airborne, which also may be with vertical up thrust. However, this is less problematic for controlled flight due to being the way that HTA aircraft fly. Even so, unless substantially HTA, the benefit of buoyancy to augment flight may simply make behaviour at ground level difficult; due to not being fully ground borne (landed), so little weight on the ground. Since the aerostat still is big the airship (an aircraft) then may behave like tumble weed in turbulent windy weather, needing ways to hold them.
Airships thus are dirigible (steerable) aircraft that use an aerostat (as balloonists do) to enable aerostatic lift (buoyancy) from the atmosphere, which may enable flotation. Whether they do float then is dependent on enough atmospheric displacement by the aerostat for buoyancy to support ‘all up’ airship weight. However, just like any other body with airspeed and depending on shape, their aerostat also may act as an aerodyne to develop aerodynamic lift, which is a normal method used when flying traditional unidirectional airships with a cigar form. Such airships often use up to about 10% aerodynamic lift to fly with an HTA state. Types with a widened aerostat (in fashion at the moment) shaped to develop greater aerodynamic lift (labelled hybrids) just introduce more issues for developers to manage that likely will take considerable time to sort out and certify.
Previously, airships were classified under three construction methods, namely, non-rigid, semi-rigid and rigid. Rigid types used a large internal skeletal truss/framework assembly for their aerostat which contained individual LTA gas bags in compartments along their length protected within by exterior dope tensioned fabric skin covers. History didn’t fare well for this class in that only the Graff Zeppelin (LZ127) lived up to its operational expectations. The non-rigid class conversely enjoyed an unparalleled degree of success, where they contributed 55,000 USA led cross-Atlantic sorties in defence of convoy shipping with only one airship loss due to enemy action and no convoy losses in all weather, day/night operations. During the First World War smaller British non-rigid airships shepherding convoys achieved similar results in warding off submarine attacks, preventing ship losses.
Non-rigid airships achieve their rigidity from the very slight overpressure applied to the aerostat’s envelope. They don’t “pop” like party balloons when punctured; in fact the rate of gas leakage is very low as the internal pressure generally is no greater than that found in ordinary homes when the doors are closed with the air conditioner working. Semi-rigids are types somewhere between the other two.
