Airships, which are dirigible buoyant aircraft, mainly use buoyancy from the atmosphere to float from “lighter-than-air (LTA)” principles. This needs an aerostat (an air displacement body) suitable for that and other purposes. They also use thrust to maintain airspeed underway, needing efficient form minimising drag. Airspeed then enables +/- aerodynamic lift on their aerostat as well, working as an aerodyne to remain aloft against heaviness and downdrafts or to counter lightness and updrafts when necessary. Note: see Definitions for clarification of terminology.
It should be noted that aerodynes are bodies that get buoyancy applied on them in the same way as all bodies in the atmosphere (i.e., aerostatics, much the same as hydrostatics). However, while considered to be aerodynamic ‘lifting bodies’, aeronautical definitions often overlook ability to displace the atmosphere. This usually is because of insignificant buoyancy gained compared with true weight that, like sinking ships flooded with water, includes other things they contain.
If air in an aerodyne was evacuated (pumped out – so displaced) then atmospheric pressure externally applied would be a problem, tending to crush them (as for hydrostatic pressure on submarines), so could fail catastrophically from structural instability unless adequately reinforced. Besides, the air in them (providing oxygen) also is needed to support life.
On the other hand, aerostats are big aerodynes designed with lower structural weight and pressure stabilised methods, needing an LTA-gas instead for thin flexible bag or envelope, bladder or balloon displacement vessel support. The LTA-gas is not a ‘lifting’ substance, where natural ability, cause and effect relations need consideration. Flotation occurs and buoyancy increases as the LTA-gas is put into normally flexible vessels containing it. As the LTA-gas is introduced, it puffs the vessel out (previously lying flattened by atmospheric pressure) enabling it to then displace external air in greater amounts as size swells. However, without the flexible vessel LTA-gas wouldn’t lift anything (just dissipate into the atmosphere, pushed to high altitudes by it). Also, without the LTA-gas or simply filled with air, the vessel wouldn’t float from too much weight and little displacement.
It’s a conundrum needing both parts working together as a body with structural integrity against atmospheric pressure and other applied loads (e.g., component weights, dynamic forces, restraint forces…). The LTA-gas thus is an integral bulk component with minimal weight evenly supporting the thin flexible vessels of an aerostat displacing the atmosphere instead of thicker shelled vessels with integral stiffening members preventing collapse after their evacuation – usually a heavier combination.
If a rigid shell displacement vessel were used instead it would still need the LTA-gas to minimise weight, needing further methods to separate and replace the air otherwise filling it as the LTA-gas is introduced. The LTA-gas used therefore may be regarded as an essential structural component of an aerostat that, when lost, results in collapse without other ways to hold form.
To float in equilibrium (neither sinking nor rising) and in still air without using power (no thrust) aerostats must be as-light-as the air they displace, where the surrounding air then pushes them up (the buoyancy experienced, an externally applied upwards reactive pressure) countering total weight. When LTA, aerostats ascend (rise) and, when heavier-than-air (HTA), they descend (sink like submarines in water) but, when as-light-as the air displaced, they float like balloons moving gently with the air currents they are in (an important way for conveyance without power).
Flexible Hull Airships
Airship aerostats with a visible flexible envelope that also functions as their hull in the airstream subject to aerodynamic effects and loads from other parts they support, are used by non-rigid (Blimps) and semi-rigid types. Their envelope material normally has high tensile strength and modulus of elasticity that doesn’t stretch very much but can be folded and collapses easily without ways for pretension to stiffen it. Naturally, envelopes also must be able to hold gas without leakage.
Such airship aerostats normally also have smaller internal flexible air cells called ballonets. If one were to look at a cut-away view of a Blimp, the ballonets usually are located inside on the lower surface, perhaps one forward and another aft. Ballonets are used mainly to contain air (preventing mixture with the LTA-gas) taken in to compensate for LTA-gas volume change from temperature and/or pressure changes, which expands or contracts in accordance with the gas laws.
If one were to fill an aerostat’s envelope fully with LTA-gas at ground level (ballonets empty) then, as it rose to higher altitudes and from atmospheric pressure reduction with height gained, the gas would try to expand but be constrained from doing so by the envelope. Any super-pressure then would increase, trying to burst the envelope, needing ways to prevent that risk. The trick is not to fully fill the envelope with LTA-gas in the first place, instead using compensating air taken in or vented to accommodate LTA-gas volume changes – perhaps over-filling with air a little to develop low super-pressure that stiffens envelope form. Ballonets and their associated air intake systems, blowers and valves, thus are a way for steady super-pressure control within acceptable limits – functioning as air bladders that expand and contract as needed.
Instead of venting LTA-gas to compensate for internal pressure rise, air in the ballonets thus is vented. However, at the altitude when the air has been fully vented, so with the ballonets empty and lying on the aerostat’s lower surface, further ascent then will cause super-pressure to increase. This altitude 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 out; where the objective is that this quantity remains as a constant amount without loss from leakage or venting to maintain structural form. Naturally, leakage thus must be prevented and venting avoided by not rising above the pressure height – when LTA-gas vent valves automatically open for safety reasons (depending on pre-set super-pressure opening levels decided for safe operation).
When the gas pressure relief valves open the quantity of LTA-gas contained in the aerostat then reduces (losing weight). However, this doesn’t cause an immediate problem, where the airship may continue to rise into thinner air, but still of greater density than the LTA-gas. The displaced air mass 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 gas fill will be less, so a greater quantity of air (weighing somewhat more) then is 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) air must be pumped into the ballonets to compensate for decreasing LTA-gas volume – thereby maintaining constant super-pressure (differential pressure between the internal and outer atmosphere’s pressure) that holds envelope form.
Ballonet sizes are fixed during design based on the altitude (thus pressure height) needed for normal operations. The higher an airship is required to go, the larger the ballonet airspace needed to compensate for gas expansion. This is why airships are best operated at low altitudes, normally not over 3050 m (10,000 ft). Altitudes of say 6100 m (20,000 ft) are possible, but ballonet capacity needed then would be twice as much (about 50% of the envelope’s total capacity), needing further consideration of the way they then behave dynamically (sloshing) under pitching action and the increased size of the aerostat necessary for its payload and operating purpose. Higher heights are possible but the gas fill amount must reduce further unless aerostat size increases (weighing more) with diminishing returns and higher costs.
If LTA-gas is lost from 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 form. Because of this a way (e.g., a valve) to put air directly into the LTA-gas chamber (mixing with it) is needed. This additional air then increases envelope weight above normal operating levels – causing the airship to sink faster than normal (the same way that submarines sink from taking water in).
Nonetheless, ‘faster than normal’ for airships usually is still slow (compared with nonbuoyant aircraft) due to their greater size (thus drag) and remaining buoyancy; where descent rate may be compared to that of parachutes. The bigger problem is purification of the LTA-gas (a long process) to restore the gas fill amount for further operation.
When inflating large envelopes with LTA-gas, buoyancy increase proportional to external air displaced is guaranteed by the physics involved (Archimedes’ principle). However, depending on envelope size and its restraint method, inflation can be a dangerous process if pockets of LTA-gas form in various places and then flow together – quickly causing increased buoyancy at the place they combine (affecting behaviour).
This is a particular problem for oddly shaped and unidirectional (UD) envelopes, needing assured restraint and process control to centralise LTA-gas accumulation at the envelope’s apex. After flotation and before the gas fill operation is complete, further LTA-gas fill then puffs the envelope out further, flowing upwards to the apex and outwards to the extremities, gradually filling downwards to the envelope’s belly position (opposite to water filling a bucket) with the belly pushed up and probably inverted by atmospheric pressure until gas filling is complete and the ballonets have been sufficiently overinflated with air for low super-pressure. Buoyancy then will exceed aerostat weight substantially and must be restrained securely.
It also should be noted that purity of the LTA-gas needs to be near 100%, as air in it otherwise increases its weight. People working in it inadvertently breathing pure LTA-gas therefore would quickly pass out, suffocate and die, needing strict procedures for safety to be upheld.
Inflexible Hull Airships
Airship aerostats with an inflexible hull in the airstream fall into two categories:
- Monocoque stiff shell types able to hold form as aerodynes (like aeroplane fuselages) without pressurisation but that normally are pressurised to structurally stabilise them. They are filled with LTA-gas (replacing the air in them to reduce weight) and function in a similar way to flexible envelopes but without ability for folding.
- Stiff rigid framed types with pretensioned fabric covers providing a smooth profile, containing multiple individual thin membrane unpressurised LTA-gas cells (balloons) able to stretch and deform as bladders to adopt the compartment shape they are put into.
Their stiff materials normally have high tensile/compressive and bending strength with modulus of elasticity that doesn’t stretch or compress much but may break, buckle or deform permanently when overloaded or bent too much. They therefore are less forgiving than flexible types (that normally are able to recover from temporary overloading) and more difficult to repair.
The fabric covers of rigid framed airship aerostats provide protection against external effects for the multiple balloon gas cells contained within, enabling very lightweight material usage without super-pressure stabilisation methods (no ballonets).
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 gas fill’s quantity, temperature, 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 (reducing payload capacity), just like ships full of water. Naturally, ability to float is desired to fly without effort, where airships should simply be as-light-as the air they displace. Their LTA state therefore should not be too much, which makes it difficult for flight control; 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 from 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 aerostats are big, semi-buoyant airships 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. Semi-buoyant 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: 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, where 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 from enemy action and no convoy losses in all weather, day/night operations. In the First World War, smaller British non-rigid airships escorting convoys achieved similar results 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.