Definitions

Rev E, 11 Apr 2022

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This page provides definitions with explanation of words or terms used for lighter-than-air (LTA) technology and buoyant aircraft (balloons, aerostats, airships & derivatives using LTA-tech) primarily written for non-rigid types. It’s felt that this is needed to help people understand matters and redress false or inadequate definitions provided elsewhere.

As we learn about the technology improvements needed become apparent, needing appropriate corrections and additions. Following reasoned advice future revisions may be made. Note: This revision changes the definitions to make them relative for all buoyant aircraft (including new semi-buoyant types) instead of just LTA aircraft.

Glossary of LTA Technology and Buoyant Aircraft Terms

6DOF: This refers to the six degrees of freedom motions (i.e., u, v, w, p, q, r) of free bodies in three dimensional (3D) space relative to cartesian x, y, z axes where u, v, & w are linear velocities along respective x, y, z axes, while p, q, & r are respective rotational motions acting about the same axes with angular-velocities following the positive right-hand corkscrew rule.

Absolute pressure: The pressure of a gas or fluid – e.g., atmospheric pressure (p₀) is absolute.

Aerial-crane or Air-crane: An aircraft designed as a flying crane enabling pick-up, transfer, and accurate placing of heavy under-slung loads while remaining overhead in hovering or pseudo-hovering geostationary flight above the respective pick&put sites.

Aerodynamic drag: Resistance felt by a body in an air stream or from its movement through the air.

Aerodynamic head: Positive aerodynamic pressure during flight on an aircraft’s leading surfaces, such as its nose and due to airspeed.

Aerodynamic lift: The resulting force on aerodynes in an airflow acting upwards due to airspeed, perpendicular to the direction of flight or the air stream. The word ‘upwards’ is used in the same sense that a person’s head is above their feet.

Aerodyne: Any aerodynamically shaped body (not necessarily a vehicle) that is able to develop significant reliable aerodynamic lift due to airspeed for flight while minimising aerodynamic drag. Note: aerostats also are aerodynes that (depending on design) can develop aerodynamic lift, although not usually as efficiently as wings. Lifting bodies also are aerodynes; where the aerostats of new semi-buoyant aircraft (also functioning as a lifting body) augment aerostatic lift with appreciable aerodynamic lift to sustain flight against significant static heaviness.

Aerostat: A large LTA body or vessel that displaces the atmosphere under static conditions to develop buoyancy on it in accordance with Archimedes’ principle, functioning as a flotation aid for useful aerial carriage of gross aircraft weight normally based on:

• A closed envelope (usually flexible membrane), shell, bag or jacket containing an LTA substance (gas or hot air) puffing it out, which also may have systems for pressure stabilisation to maintain form against atmospheric pressure, or
• The combination of a set of thin flexible LTA-gas cells (bags) functioning in a similar way contained in a lightweight rigid structure with outer covers enclosing/protecting them.

Aerostatic equilibrium (EQ): Aerostatic EQ occurs when a buoyant aircraft’s gross weight* is exactly balanced/countered by buoyancy.

Aerostatic heaviness: The amount by which a buoyant aircraft’s gross weight* exceeds buoyancy.

Aerostatic lightness: The amount by which buoyancy exceeds a buoyant aircraft’s gross weight*.

Aerostatic lift: Buoyancy exerted by the atmosphere (an upward reactive force opposite in direction to weight) on a body that displaces it, tending to cause flotation or the body to rise. Note: all bodies in the atmosphere experience buoyancy, normally not noticed and ignored. However, aerostats are designed to utilise this benefit for flight with reduced need for power, enabling flotation or excess lift to ascend without installed power if designed for those purposes.

Air-ballast: Air from the atmosphere used as ballast, taken aboard and compressed (held in flasks at high pressure) or discharged back to atmosphere for buoyant aircraft weight change purposes. Note: this emulates similar practices used by submarines to rise or sink.

Aircraft: Flight vehicles supported by atmospheric air, such as: aeroplanes, airships, balloons, tethered aerostats, helicopters, gliders, autogyros, aerial drones, etc.

Airship: A free flying dirigible buoyant aircraft able to: 1) float in EQ without airspeed, power or upthrust, and 2) fly with powered propulsion plying directed courses. Notes: Aircraft only using HTA technology (e.g., aeroplanes and helicopters) are considered to be nonbuoyant types. However, aircraft using an air inflated wing able to fly without LTA-gas but that may be inflated with it for partial static lift may be semi-buoyant types. Ultimate responsibility lies with the aviation authorities to categorise and define the limits/requirements for each type.

Airspeed: The velocity of an aircraft relative to the air it is in.

All up weight: A nonbuoyant aircraft term for the gross or total weight of the flight vehicle (including disposable loads). Note: the weight of buoyant aircraft should include their aerostat’s inflation gases and weights should be ‘true’ rather than ‘effective’ values in the atmosphere. See separate effective and true weight definitions.

ARDD: Automatic Rapid Deflation Device. It is a means to rapidly release the LTA-gas from an aerostat under special conditions that may be operated remotely and/or automatically, depending on particular circumstances (such as taking off without control), either to keep the aerostat on the ground or to bring it down. It is needed to prevent unintended launch or runaway when other means of control are unavailable. Notes:

1. For most buoyant aircraft, particularly whilst moored, a ground anchored rip-line system is a secure way to ensure that, if an uncontrolled breakaway occurs, the gas is released, also used by the pilot and/or ground personnel when necessary.
2. ARDDs suit unmanned buoyant aircraft and may use a burn-wire system to make holes in the upper envelope that either operates automatically (due to a particular situation) and/or by a remote operator at the press of a button when safe to do so.

Ballast: Any material (usually sand or water) used aboard buoyant aircraft to:

• add weight for balance (such as lead at the nose of an airship, moving the cg forward to below the CB when tail heavy)
• counter static lightness
• release (cast overboard) if there is excess static heaviness or to reduce it and perhaps deliberately cause static lightness
• use during load exchange to maintain EQ, or to
• set a particular weigh-off state (static heaviness or lightness) desired for subsequent flight.

Note: Ballast may be of a fixed nature (to offset a permanent condition) or disposable to suit variable weight needs. Disposable ballast is used during load exchange (when buoyant aircraft gross weight may change substantially). It also is used for trim purposes if gross airship weight increases (e.g., from rain) or if buoyancy reduces (e.g., when super-heat reduces). Disposable ballast also must be of a form and/or releasable in a way that doesn’t harm. See also Air-ballast.

Ballonet: A chamber within a non-rigid or semi-rigid buoyant aircraft’s aerostat for air bounded by a flexible membrane preventing mixture with the LTA-gas in the aerostat that freely inflates or collapses when respectively filled or vented as part of a pressure management system to compensate for changes in LTA-gas volume as pressure and temperature change and to maintain aerostat super-pressure through controlled over inflation. Note: its function as part of the pressure management system ceases when empty of air (totally collapsed – usually at the pressure height) or fully inflated with air (usually when descending after significant LTA-gas loss).

Balloon: A simple bulbous/rotund aerostat as just an inflated envelope or LTA-gas cell – usually carrying a suspended basket or pod for people, payloads or ballast and buoyant aircraft systems, collectively providing weight acting as a pendant to keep the balloon upright and counter buoyancy. Note: See also: Barrage balloon, Gas balloon, and Hot-air balloon.

Barrage balloon: A wartime defensive tethered-aerostat method with a strong line (difficult to see) let up as an obstacle in the way of aircraft that may fly into them, protecting ground arrangements and people below from attack. They also could carry nets hanging between them.

Battens: Long shaped ribs used to support other parts (e.g., a nose cone) as well as to stiffen and spread load into a non-rigid aerostat’s envelope.

Bed down: The action to restrain an aerostat at ground level with a net or cover and/or lines to anchor positions, screw-pickets or heavy ballast around it. This may involve temporary mooring-out methods after removing protruding lower parts and then hauling it down – sometimes until squeezed against the ground (not necessarily).

Blimp: A colloquial term for a non-rigid airship often also used (perhaps falsely) to describe a tethered-aerostat.

Body of revolution: The 3 dimensional outer hull profile of a vessel (such as an aerostat) with varying circular section about a straight central axis.

Buoyancy: In air, buoyancy is the reactive externally applied aerostatic lift force from the atmosphere on the body causing its displacement (i.e., non-rigid/semi-rigid aerostat envelopes or rigid aerostat gas cells). Note: all bodies in the atmosphere experience it. Under ISA sea level conditions air has a density of 1.225 kg/m³. Hence, with standard gravity, g_n = 9.80665 m/s², a displaced 1³ m volume of air applies 12.013 N buoyancy on the body causing the displacement (regardless of what it contains).

Buoyant aircraft: Aircraft that have an aerostat for atmospheric displacement purposes to enable effective buoyancy, which includes semi-buoyant types that may not be able to achieve EQ without other lift augmentation methods (i.e., aerodynamic and/or vertical upthrust).

Buoyancy control: Perhaps a misnomer, as buoyancy from the atmosphere is practically impossible to control directly due to remoteness as well as the way it freely changes, needing responsive variable geometry displacement body methods. However, buoyancy and weight also are forces opposite in direction that both result from the effects of gravity (so are interrelated), where weight change may be used to counter (so control) buoyancy. Weight control and variable geometry methods thus are ways to adopt. Notes:

1. Because buoyancy results from displacement of the atmosphere, it may be controlled by the displacing body’s expansion/contraction.
2. Gross weight may be reduced by releasing ballast. It may be increased by collecting rainwater or (if used) by combustion engine exhaust water recovery, and by in-flight transfer (e.g., a bucket and line to collect water over-flown). The displaced volume (so buoyancy) also may be reduced by venting LTA-gas or by cooling it (although the latter isn’t easy) while imbibing air to compensate and maintain aerostat form – increasing weight. Alternatively, air may be pumped into flasks at high pressure and used as ballast.
3. Displacement may be increased by heating the LTA-gas or by introducing more from pressurised or liquefied storage (both depending on free gas expansion – facilitated by expelling previously imbibed air), where the latter is only practical from ground facilities.
4. Methods involving in-flight LTA-gas compression to reduce the displaced volume when desired are possible (increasing parasitic weight) but need further ways to imbibe/expel air that, in turn, increases/decreases aerostat weight.

Camp out: To remain in attendance with a buoyant aircraft at a temporary site away from its main base using available ground handling methods.

Capture: Buoyant aircraft activity for transition from flight to restrained from the ground.

Catenary curtain: An arched load curtain to evenly spread load from suspension system lines on a non-rigid aerostat’s envelope; where the shape of the arch takes the natural form of a slack line between two fixed points – normally used upside down.

Conventional airship: A UD airship built in the style of most previous types, which were classic rigid, non-rigid or semi-rigid types. Note: this could change in the future if other types overtake the number of classic style airships produced.

Classic airship: An airship in the traditional style adopted before 1900 with an elongated ellipsoid or cigar shaped UD aerostat or hull form.

Control reversal: This is when the intended effect of a tail surface elevator control movement causes the aircraft to do the opposite thing (e.g., descend when climb is desired). It may occur from a combination of strong pendulum stability due to low cg position and poor airspeed. If understood this can be of benefit to assist take-off with substantial heaviness, but otherwise could cause a crash.

Cycloidal propeller or Cyclorotor: A propeller (like paddle wheels) with straight parallel blades equally offset and positioned around a drive-shaft (connected to them by radial arms) and with a swash plate plus rod and hinged link mechanisms similar to helicopters for blade pitch control as they rotate around the drive-shaft axis. The arrangement is similar to Voith Schneider propellers used by some ships (e.g., tugboats) but for aircraft. Note: The benefit of such propellers is that they enable rapid vectored thrust at full power in any radial direction through 360°. However, they don’t exist yet as certified propellers for aircraft in regular production but may become available soon from current developments.

Dead weight: The weight of an item to be lifted – usually given to be its effective weight.

Disposable load: Crew, payload, fuel, oil, releasable ballast and other such things onboard that need replenishment; sometimes referred to as tare or useful weight.

Differential pressure: The difference of absolute pressure on each side of a surface, e.g. (p_i – p₀) where p_i is the pressure inside an aerostat and p₀ is atmospheric pressure.

Dirigible: Able to be steered on a specific course. Airships often are referred to as dirigibles due to the way this ability was established by them, although it is a term that may be used to describe the controlled motion of any vehicle or sailing vessel.

Displacement: For a buoyant aircraft, the weight (or amount) of atmospheric air displaced by its aerostat (the displacement vessel) in it. This may be determined knowing the air’s density and the volume of air displaced.

Displaced volume: This is tricky to define because the volume of LTA-gas used is variable (dependent on pressure and temperature) and aerostats usually have compensating air within them. For non-rigid and semi-rigid types, this air is used to fully inflate and pressure stabilise their envelope after a partial LTA-gas fill (necessary to allow for expansion) and, for rigid types, filling the space between their outer covers and the gas cells (neutralising atmospheric pressure on the outer covers) that is difficult to quantify. Further:

• During ascent the LTA-gas expands while the air vents until, at the pressure altitude, the LTA-gas either exactly fills the aerostat’s envelope with ballonets empty or, in rigid types, the gas cells exactly fit their bounding compartments, both of known volume from geometry.
• Ignoring the volume of the aerostat’s parts and other aircraft features (considered negligible by comparison) these geometric volumes then are equal to the displaced volume at that altitude. Using the gas laws and atmospheric data charting temperature, pressure and air density, the displaced volume then may be calculated for other altitudes – enabling the gas-fill for use and resulting buoyancy to be determined.
• Even so, rigid airships in the past often were just filled full of LTA-gas (hydrogen) under warm hangar conditions (causing super-heat before undocking) and loaded up with sufficient disposable ballast to release under way as necessary, buoyancy then reducing after undocking from loss of super-heat, but while fuel was consumed (losing weight) not worrying about it.

Docking: Movement of a buoyant aircraft into a large shed or hangar.

Ducted propeller (or fan): A conventional screw propeller concentrically and closely located within a short annular shroud aerodynamically designed to enhance propeller disc airflow, significantly improving efficiency compared with a similar propeller on its own. A ducted fan is similar but uses a larger number of rotor blades.

Dynastat: An interesting but maybe needless technical term probably introduced by people with an HTA background relatively new to LTA matters, otherwise used as a product name for a novel buoyant aircraft with flight characteristics said to be between aerodynes and aerostats. However, this doesn’t recognise that aerostats also are effective aerodynes (always were) but not vice versa. Other terms introduced for commercial purposes (such as HAV or hybrid airship) also have been coined, confusing matters. Notes:

• LTA technology evolved (including both aero-static and -dynamic sciences) over a period of more than 100 years before nonbuoyant powered aircraft began, continuing thereafter until c1940 when HTA technology became dominant and airships faded (losing knowledge).
• Mowforth¹ tried to be clear with his definition (supported by illustrations of example types) saying that, for hybrid airships, “When aerodynamic lift is used the vehicle combines the characteristics of an aerodyne and an aerostat and may then be termed a dynastat”, also saying, “This type of hybrid must generally take off and land with a ground run, as does a conventional aeroplane.” However, already mentioned, airship aerostats naturally have both aero-static and areo-dynamic ability, so what are hybrids?
• Wikipedia² (under the title ‘Hybrid airship’) now states, “a dynastat is a hybrid airship with fixed wings and/or a lifting body and is typically intended for long-endurance flights.” Maybe! However, wings and lifting bodies (both aerodynes) displace the atmosphere a little (normally causing negligible aerostatic lift – not zero) confusing matters between lifting bodies and aerostats.
• The issue then is what is the term needed for, apart from a product name, where all aerodynes and aerostats (the common technical terms used historically) otherwise would be dynastats?!
• As the intention probably was to provide a term for new UD airships being developed that function and look like aeroplanes, the following definition is offered: a UD semi-buoyant airship with wings and greater airspeed operating with significant static heaviness needing aerodynamic lift for take-off, climb, level flight and safe landing with a short ground run.
• Such types have yet to be certified and enter commercial service, perhaps needing special arrangements to manage behaviour at ground level in difficult weather.

Effective buoyancy: The buoyancy on an aerostat less the weight of LTA-gas inflating it.

Effective weight: The dead weight of an item as measured in the atmosphere or other surrounding fluid/gaseous substance buoying it at sea level under ISA standard conditions.

Elevator: An aerodynamic control surface hinged about a mainly horizontal axis usually at the rear of an aircraft to develop a downward or upward force in flight from airspeed that lowers or raises the aircraft’s tail, causing angular pitch to respectively ascend or descend from resulting wing and body aerodynamic lift (+ve or –ve). Whether it does or not is another matter, where tail surface elevators can cause the opposite effect – see Control reversal.

Empennage: A collective term for the tail surfaces of an aircraft.

Envelope: For non-rigid and semi-rigid aerostats, the outer skin, bag or jacket (usually a flexible membrane) containing LTA-gases and/or air used to puff it out and stabilise structural form.

Eta patch: A bonded fabric assembly normally with a single D-ring line attachment point held to the patch with splayed load spreading finger tapes or cords, affixed to a non-rigid envelope to carry applied tensile line loads.

Exhaust water recovery: The process to condense and extract water using a suitable system from the exhaust products of an internal combustion engine. This is possible because of the burning process which reforms the mixture of fuel carried and oxygen drawn from the air outside, making water (H₂O) and other products (depending on the fuel). Note: Due to the chemical relationships of the process, it’s possible to collect a greater weight of water (due to added oxygen from the air) than the mass of fuel used, enabling an airship’s overall weight to be maintained (even increased, depending on efficiency) while fuel is consumed.

Fender: A protective unit with shock absorbing ability to prevent vulnerable structure being damaged at possible ground contact or other impact positions.

Flight: For buoyant aircraft this is the motion in and through the atmosphere (the air). This may be with airspeed (due to propulsion), when induced aerodynamic forces arise and heading/course are controlled, or without airspeed, when it drifts as a floating body with ground speed and direction as for the airflow (wind).

Gas balloon: A balloon filled with an LTA-gas (often hydrogen instead of hot air or helium) for efficiency to minimise size/weight and maximise flight endurance. It is sustained in flight due to just buoyancy – otherwise free to drift with the airflow.

Gas cell: An ultralightweight closed flexible bag (like a balloon or bladder) to contain a partial LTA-gas fill, which normally swells without stretching significantly or collapses to suit gas volume expansion or contraction, used in rigid airships and simply retained inside a bounding compartment.

Gas fill: Quantity (mass) of LTA-gas put into an aerostat’s envelope or its gas cells. Note: except for leakage or deliberate venting and from topping up, the gas quantity remains constant during flight – so, although volume may vary considerably, the amount (thus weight) does not change.

Geostationary: Ability to hold a particular station and height without rotating in 3 dimensional space relative to a point on the earth’s surface.

Gondola: The nacelle or car of an airship normally arranged with a cabin and flight deck for passengers and crew.

Gross weight (or mass): The all up weight, including the weight of contained LTA-gas. Note: The LTA gas put into buoyant aircraft aerostats is an essential component supporting their flexible containers (envelopes, gas cells) as displacement bodies against atmospheric pressure. The LTA gas fill weight therefore should be included in the buoyant aircraft’s weights table.

Ground handling: Primarily, the operations/activities to manage buoyant aircraft at ground level, including: launch, capture, cross-field movement, mooring, unmooring, docking, undocking, haul down, bed down, ballasting and ballast management, load exchange, loading/unloading, weigh-off, and so forth. Note: Ground handling also includes operating site setup, equipment management, and ground systems, as well as operating site assistance concerning non-rigid aerostat test and inflation, buoyant aircraft assembly and rigging, aerostat pressure watch, weather monitoring, site security, LTA-gas plant management, buoyant aircraft breakdown, packing for shipping and similar tasks typical at airports for nonbuoyant aircraft. Road train duties to follow buoyant aircraft on tour to support them at temporary sites also needs ground handlers to load/unload vehicles and drive them.

Grounding: Descent of a buoyant aircraft to the ground with intent to remain there, usually followed by capture (so restrained) and mooring (securely held). Note: Buoyant aircraft at or near EQ remain primarily airborne at ground level, so do not actually land unless the buoyancy keeping them essentially afloat (so airborne) somehow is lost. Thermal types normally are landed when their hot air is vented, but this doesn’t usually occur for LTA-gas filled types.

Haul down: The action to pull an elevated aerostat restrained by ground lines against buoyancy down to the ground using manpower, tirfors, capstans or winches on the lines for the purpose. Note: if buoyancy first is countered by sufficient ballast attached to the aerostat, then haul down is eased and may be undertaken with just manpower – only sensible if the elevated aerostat is within reach.

HAV: The acronym of a company (Hybrid Air Vehicles Ltd) used colloquially recently by journalists to describe HAV’s and any similarly arranged designs in vogue at the moment from media coverage of new types proposed (none in certified service) based on (not evident from the acronym or the company’s name) a combination of buoyancy and dynamic lift to fly. Technically, it’s a mishmash of terms obfuscating the ‘nature of the beast’ – a flying car, Helistat or what!? See also ‘Hybrid Air Vehicle’ as a term below.

Heavy operation: The method used to deliberately fly buoyant aircraft with significant aerostatic heaviness, which needs augmentation of buoyancy experienced with enough induced aerodynamic lift from airflow over its aerostat and/or upthrust.

Helistat: Used as a product name, this was a novel prototype airship with a UD aerostat for buoyancy to support gross weight. It had a lower belly mounted module with 4 pylons radiating symmetrically (2 each side) supporting helicopters (with their tail stem & rotor systems removed) at the pylon ends for vertical lift (essentially operating clear of the aerostat). The arrangements also needed an empennage for stability and control of flight plus other typical airship parts/systems for management. Flight otherwise was to be controlled via the helicopter rotors’ tilt systems. It was to enable rotor lift with full effect to carry payloads without need for load exchange (so no compensating ballast). As a term, Helistat thus may be used to define other buoyant aircraft designed this way. However, see also ‘Hybrid airship’ and ‘Rotastat’.

Helium: An inert (so non-inflammable) gas used to partially inflate buoyant aircraft aerostats at ground level (allowing space for expansion) as a bulk component to puff out, support and stabilise their flexible containers (i.e., the aerostat’s envelope or gas cells) that, in turn, displace the atmosphere. Under ISA sea level conditions pure helium has a density of 0.169 kg/m³. Hence, with standard gravity, g_n = 9.80665 m/s², pure helium of 1³ m volume under the same conditions weighs 1.657 N, acting against buoyancy on the helium’s container.

Hot-air balloon: A bulbous naturally shaped non-rigid aerostat with an open lower aperture, inflated with air (puffing it out) that subsequently is heated by a burner supported on frame legs over a basket for payloads and/or people – suspended from the aerostat below its aperture. The hot-air-balloon is sustained in flight due to just buoyancy, gained when the weight of contained air reduces sufficiently from heating (causing expansion and thus cold air with greater density to be expelled) otherwise free to drift with the external air.

Hull: The exposed outer shell or membrane of a vessel such as an aerostat. For example, one may refer to the outer profile of a rigid airship’s aerostat as its hull.

HTA: Heavier-than-air; where the material or substance (e.g., gas) of a part or assembly concerned simply weighs more than air.

HTA aircraft: Aircraft unable to achieve significant aerostatic lift, needing lift by other means (e.g., aerodynamic lift and/or vertical thrust) to take off and fly.

Hybrid airship: An awkward term that confuses matters. Wikipedia² states, “This is an aircraft that combines characteristics of LTA technology with HTA technology, either fixed-wing or rotary-wing”. Mowforth¹ says that it is, “An airship in which a substantial proportion of the gross weight is sustained in flight by aerodynamic or rotor lift, the remainder being carried by aerostatic buoyancy”. Notes:

• The main problem with both definitions is that they are too broad (mixing things inappropriately and incorporating rotor technology that needs separate treatment). Also, the emphasis is on UD types (which classic airships are) based on HAV’s methods, but where airships also can be omni-directional (O-D).
• HTA & LTA technologies also are not independent pure sciences, both embracing the same aeronautical technology, where the science began with LTA methods as the basis for low airspeed aircraft and then was extended with higher and higher airspeed HTA types. There is nothing hybrid about that and airships are unlikely to fly at the high airspeeds of HTA types.
• An alternative definition is: A semi-buoyant aircraft using lifting body principles and/or winged to gain significant aerodynamic lift (say 40%) augmenting buoyancy to carry overall aircraft weight, maybe able to float (reach equilibrium) when fuel and/or payload is low.
• It’s mooted that they have the advantage of being able to operate without ballast. However, the term ‘semi-buoyant aircraft’ makes the ‘hybrid airship’ term superfluous and where semi-buoyant types also reduce need for ballast.
• Also, what is hybrid about the airship type, where uninformed people may think it is something to do with being diesel/electric powered?
• For types adopting rotor technology and methods, see Helistat and Rotastat.

Hybrid air vehicle (HAV): Another inappropriate term (as for hybrid airship) meant for new airships using a combination of aerodynamic lift, buoyancy and thrust to fly, but just a matter of fact for buoyant aircraft developed since 1783. The term therefore is considered to be redundant. Note: It is a newly coined term currently associated with UD types using a broadened or deltoid aerostat (perhaps also wings) to develop greater aerodynamic lift than possible for conventional airships before – falsely said to be a hybrid of HTA and LTA technology. This is because LTA technology has always included dynamic lift and thrust methods appropriate for airships. With the industry for aircraft today dominated by nonbuoyant types it is believed the term was introduced to lead people away from past airship tragedies and think about the new possibilities. However, this doesn’t make it a good technical term to adopt.

Hydrogen: An inflammable gas used to partially inflate some buoyant aircraft aerostats at ground level in a similar way to helium but needing ways to prevent ignition. Under ISA sea level conditions pure hydrogen has a density of 0.0853 kg/m³. Hence, with standard gravity, g_n = 9.80665 m/s², a 1³ m volume of pure hydrogen under the same conditions weighs 0.8365 N, acting against buoyancy experienced.

Landing: An HTA term that, for buoyant aircraft (usually remaining mainly airborne after touchdown), normally is the loss of buoyancy from collapse of their aerostat (then no longer displacing the air) due to venting the LTA inflation substance (gas or hot-air) so that its effective weight transfers to support from the ground (landed). Note: Because nonbuoyant aircraft usually land in a different way to buoyant aircraft, this term should be avoided to obviate confusion; where the terms ‘capture’ and ‘grounding’ should be used instead for the similar action unless heaviness is substantial – when the aircraft then may be classed as a semi-buoyant or nonbuoyant type.

Launch: For balloons and airships, activity for release and take-off into free flight. For tethered aerostats, the action to pay out restraint lines for aerostat ascension.

Lenticular: Of a similar outer profile to a lens or lentil seed – the shape of a discus.

Lifting body: An aerodyne (normally without wings), which an aerostat also is, shaped to develop significant aerodynamic lift from airspeed for flight while minimising aerodynamic drag.

Lifting gas: A misleading/false technical term widely used by many people to describe the LTA-gas put into aerostats, which has mass and so weight rather than lifting ability. Notes:

• Buoyancy experienced by a body in water is derived by the same principle (espoused by Archimedes over 2000 years ago), but without such incorrect referral to the air that usually fills their displacement vessel as a ‘lifting substance’; where it’s recognised that the vessel is buoyed externally by the water it is in – not the substance or things it contains!
• Buoyancy on an aerostat in the atmosphere is derived in the same way, i.e., as an external force from the air pushing it up, rather than magically being lifted by the LTA-gas inside.
• The term therefore should not be used any more.
• See the definition for ‘LTA-gas’, offered as the correct term to use.

Load or weight exchange: The process to exchange one load or weight with another of equal amount (mass) in order to maintain balance and/or EQ within manageable limits. It applies to any change in the combination of payload, ballast, fuel or other disposable load. It also applies to moored buoyant aircraft that remain afloat during maintenance to, for example, replace a heavy part such as an engine with ballast – re-exchanged when the part is reinstalled.

Load panel: A panel that acts like a catenary curtain affixed normally at its upper edge to spread single point tensile line loads applied on a fabric or membrane structure.

Load patch: Similar to an Eta patch for use to spread tensile line loads applied on a fabric or membrane structure.

Loiter: Prolonged flight at low to zero airspeed, as may be needed for patrol, observation, survey, photographic and so forth duties; or simply to wait until the ground crew are ready for capture & mooring, the weather is manageable to continue, and/or the circumstances are right (say for an event, such as showing up at the right time and then remaining generally overhead for a while). Note: buoyant aircraft suitably designed to have ability & facilities for the purpose also may deploy a sea anchor or grapnel to hold against the wind without power, waiting out on duty for longer periods (loitering in the patrol area with intent – ready for action).

LTA: Lighter-than-air; where the item or substance concerned simply weighs less than air.

LTA aircraft: Buoyant aircraft able to float in aerostatic EQ and ascend from lightness without lift augmentation by other means (e.g., aerodynamic lift and/or up-thrust).

LTA-gas: This is the correct term for the gaseous substances put into aerostat envelopes, shells or gas cells as a material or integral component with bulk to support their thin outer membranes, holding form and preventing collapse under atmospheric pressure. The contained gas thus enables their function structurally as displacement bodies to get buoyancy. The only reason these gases are used is to minimise weight, where the aerostat otherwise would be useless for purpose.

Metalclad: A type of rigid aerostat construction based on a thin metal (aluminium) shell (monocoque) with minimal supporting structure, just enough to sustain self-weight, requiring moderate pressure stabilisation to function reliably under load during flight and against atmospheric pressure. Rarely used, such vessels also are classified with other pressure airships that need super-pressure to stabilise their aerostat’s form (preventing collapse).

Module: Any unit or nacelle to contain people, systems, freight and so forth individually or together, that is readily mountable & removable or can be simply picked up & set down to enable: maintenance in a workshop, quick replacement with fresh pre-prepared arrangements, rapid role change (say from passenger transport to aerial surveillance) and load exchange – helping to minimise turn-round time.

Monocoque: Literally ‘single-shell’. Monocoque hulls comprise a continuous hard shell that, for a buoyant aircraft’s aerostat, may be an assembly of moulded sandwich panels able to carry in-plane shear and compression, differing from the thin single ply shell of former metalclad types. Note: Aerostats based on monocoque construction with sandwich panels are ‘rigid’ and shouldn’t need internal super-pressure to stabilise the shell due to greater bending stiffness of the panels. Airships with such monocoque aerostats have frequently been proposed, but none have so far been built. They are suitable for very large airships that naturally have higher airspeed, enabling form to be maintained.

Moor or Mooring: For buoyant aircraft, the action for connection to a restraint system. Classic airships typically moor to a mast. Mooring also may be by a ground anchor system with lines that fix the buoyant aircraft’s aerostat.

Mooring mast: A vertical stanchion or tower, which may be braced, with a coupling facility at its head for UD buoyant aircraft (typically conventional airships) to connect to. Connection normally is at the buoyant aircraft’s nose, but sometimes through an alternative point (e.g., a chin position). An important aspect of the mast and coupling arrangements is that it must freely permit the buoyant aircraft’s weathervane, pitch and roll movements under any weather conditions – allowing it to align with the wind. Notes:

• Masts for conventional airships may be fixed permanent/temporary installations (i.e., tower, stick or expeditionary types) or mobile (allowing relocation by road and/or airfield movement with an airship attached). They also may be tall (a high mast) with the airship held aloft, or short (a stub mast) with the airship held next to the ground (facilitating access, load exchange and servicing).
• Masts for tethered UD aerostats are taller to accommodate lower rigging lines and held on an automated turntable system with sensors to determine wind direction keeping the aerostat nose to wind.

Multi-hull airship: Airships with two or more aerostats of conventional form connected by cross-members as a single unit, usually held horizontally analogous with marine “catamarans” or “trimarans”.

Multi-lobe hull: An aerostat of “lobed” (e.g., cloverleaf or wide/body) cross-section. Usually adopted for non-rigid airships and formed by internal lacing or webs of the aerostat’s envelope. Notes: cloverleaf types are said to improve aerostat bending stiffness and be easier for attachment of suspended structures. New wide-body and deltoid types (like mattresses) also have adopted such multi-lobe aerostat methods.

Non-rigid: Buoyant aircraft with a flexible membrane aerostat envelope mainly pressure stabilised to function structurally without frameworks as the main carriage system for principal parts, systems, equipment and the payload.

Nonbuoyant aircraft: Aircraft that do not have or use an aerostat for atmospheric displacement purposes to enable effective buoyancy – so are HTA types.

O-D: Omni-directional

Parachute balloon: A barrage balloon with a basket for people that, when elevated, may be used as a lookout point for distant observation, from which occupants also may alight with a parachute for training purposes.

Parasitic weight: Useless or excess aircraft parts and systems weight (without needed purpose), reducing payload and/or other disposable load (e.g., fuel) that may otherwise be carried.

Payload: The items (people, cargo, systems and equipment) normally carried for revenue earning or particular customer purposes.

Pendulum stability: The effect from low mass (acting at the cg) where gravity causes a centring moment towards a resting position directly below the centre of buoyancy (CB). If disturbed from the rest position the buoyant aircraft rocks (pitches and/or rolls) as a pendulum until the motion stops from air resistance or control, when it regains a steady upright attitude.

Pick&put: The action to pick up an item and then put it down, usually at a different location.

Platform: A term often used for a buoyant aircraft holding a geostationary position with long endurance at altitude as a stable means to support installed user systems or a payload aloft.

Pressure airship: A dirigible type with an aerostat structure of non-rigid, semi-rigid or monocoque form, using super-pressure to structurally stabilise that form against atmospheric pressure and the effects of flight.

Pressure altitude or height: The altitude (1) when a non-rigid’s LTA-gas expands to completely fill its containment vessel (the aerostat) without increasing super-pressure and when any ballonets first become empty of air. In rigid types with frame structure and outer covers it is when the gas cells swell (due to gas expansion) to fill their bounding compartments. Notes:

• If the buoyant aircraft ascends further super-pressure then increases (begins to develop in rigid types) until gas vent valves open, when LTA-gas is lost. Depending on rate of ascent (if it continues) and gas loss rate, super-pressure rises further with danger that the aerostat then may fail from excess structural stress with catastrophic effect.
• Aerostat weight reduces from gas loss, exacerbating the situation. However, atmospheric displacement reduces faster from ascent into thinner air (reducing buoyancy) causing heaviness that helps to slow, stop ascent and initiate descent.
• Assuming the aerostat doesn’t fail (unlikely from design factors of safety) when the buoyant aircraft descends to levels where the valves close and super-pressure subsequently returns to its previous value with the aerostat’s envelope or bounding compartments just full of gas again, the pressure height (2) will be at a higher altitude than before (from reduced gas-fill).
• Further descent causes gas volume to reduce, needing air to be blown into the ballonets (for non-rigids) to maintain super-pressure or imbibed into the space between gas cells and outer covers (for rigids) to counter increasing atmospheric pressure. When reaching the previous pressure height (1), assuming temperature/pressure conditions are the same, buoyancy experienced then will be less, needing ballast release from heaviness to regain EQ.
• Depending on continuing descent rate, if too fast non-rigids’ gas volume may reduce at a greater rate than compensating air filling ballonets (reducing super-pressure). Non-rigid aerostat stiffness then reduces with potential loss of form (not good but survivable).
• Assuming this doesn’t occur and, depending on the gas loss at altitude, as descent continues a non-rigids’ ballonets may become full (so ineffective) resulting in further super-pressure loss, when another way to force air into the envelope is needed to maintain form (degrading LTA-gas purity).
• Loss of LTA-gas from ascent above the pressure height (1) thus causes increased aerostatic heaviness. It therefore is customary to operate below this height, as possible. Pressure height only increases from reduced LTA-gas fill, hence the available buoyancy. Buoyant aircraft in general thus should operate below the pressure height, unless using it deliberately to control altitude, and fly at low altitudes in denser air to maximise static lift for payload carried!

Pressure management: A system of ducts, fans (blowers), valves and air filled ballonets for non-rigid aerostat super-pressure control.

Pressure stabilised: This refers to the way super-pressure contained by a non-rigid or semi-rigid aerostat’s envelope of flexible membrane or thin monocoque shell form is used to stiffen it; where positive differential pressure induces bi-axial tension in the membrane or shell, enabling it to function as a rigid structure under compressive load effects without buckling.

Pressure watch: Ground activity to monitor & control a non-rigid or semi-rigid aerostat’s super-pressure and other things affecting their condition, plus monitoring the weather when they are moored (parked).

Prismatic coefficient: The ratio of an aerostat’s volume and the volume of a cylinder with the same diameter and principal dimension (length or height).

Release:

1. The act of launch when a buoyant aircraft is fully ‘let go’ from ground restraint to thus take up free flight. Cast off.
2. Let go from mooring restraints – a mast or ground pickets.
3. Drop ballast or cast it overboard.

Rigging: The various lines (cables, wires, cords, ropes) and their associated fittings (buckles, links, turnbuckles, shackles, etc) used in a suspension system and for: bracing, handling, mooring, climbing atop, safety and so forth.

Rigid: A buoyant aircraft that either (1) has an aerostat structure of light wire-braced frames, girders and connecting members with separate internal compartments, each bounding a flexible LTA-gas cell, with light outer covers for smooth aerodynamic form (typical of former Zeppelins) or (2) uses a stiff outer monocoque shell, which also may be compartmentalised, to contain LTA-gas; both using the stiff structures to support their principal parts, aircraft systems, equipment and the payload. Notes: Rigid buoyant aircraft enable very large types to be constructed and flown. When produced a way to undertake gas-fill practically and safely is needed; where the amount of LTA-gas is vast (making supply difficult, needing to be undertaken in stages) and the buoyancy gained as a result is huge (needing ways for restraint). Rigid construction helps to overcome these issues.

Rip system: This is a simple method using a strong line attached to rip panels on the upper surface of a non-rigid aerostat’s envelope that, when used, causes a large hole to be made by tearing or cutting action, releasing the LTA-gas – necessary to rapidly destroy buoyancy. When the line is connected at its lower end to a fixed ground point it causes deflation if the aerostat were to break free and as it drifts down wind. Note: The line’s end must be located and affixed in such a way that it either is available for ground personnel or for the pilot to deliberately use under emergency conditions to land a buoyant aircraft (enabling escape). Tragically³, this was not accessible for one pilot of a helium filled non-rigid consumed by fire that started from fuel leakage.

Rotadyne: An aircraft sustained in flight entirely by powered rotors (e.g., a helicopter). Also, a powered rotating body or ring with radiating blades that pitch collectively to develop positive or negative aerodynamic lift.

Rotastat: Wikipedia² (under the title ‘Hybrid airship’) now states, “A rotastat is a hybrid airship with rotary wings and is typically intended for heavy lift applications” Maybe! Mowforth¹ says, “With rotor lift the vehicle’s characteristics become those of a rotadyne and an aerostat, and it becomes a rotastat.” Also, “This type of machine is capable of VTOL.” In addition, it’s mooted that such buoyant aircraft have the advantage of being able to operate without ballast. Perhaps! Even so, from Mowforth’s illustrations, it’s clear that there are at least three possible versions.

1. Those with a fixed UD aerostat similar to that of conventional airships but carrying a lower structural arrangement of arms or pylons extending each side to support helicopter type rotor systems for vertical lift at their ends. These arrangements also may have separate propellers for thrust and control in other directions. Note: See Helistat
2. Contraptions called Cyclorotors based on aerostats that rotate either about a longitudinal or vertical axis, carrying a set of hinged blades (wide rotors) radiating equi-spaced about the rotation axis, each supporting hinged T-planes at their ends (functioning like cycloidal propellers), all held in place with bracing cables. They also have propellers to cause rotation of the whole buoyant aircraft, working as a giant floating/swirling system configured to direct aerodynamic forces generated by the blades and T-planes for vertical lift to carry under-slung payloads and to thrust in any other direction for motivation plus control. Note: They also develop Magnus effect lift, which may not be desired but is a consequence from rotating their aerostat.
3. A buoyant aircraft with a rotating spherical aerostat on a horizontal axis that deliberately gets aerodynamic lift derived from the Magnus effect (as for a swerving ball) needing airspeed. Airspeed is enabled by thrusters at each end of the rotation axis supported on a hanging yoke.

Apart from confusion created by saying it is a hybrid airship, the technical term Rotastat is difficult to understand from the different ways it may be considered, where it leads one to think it applies to a rotating aerostat (items 2 & 3) instead of an aerostat with rotors (item 1), so a rotorcraft. However, what it needs for the term Rotorcraft to be used instead is for aeronautical people to be openminded about aircraft, as a Rotorcraft could be, “Any aircraft with rotary wings or rotor blades to develop lift from rotation about a vertical axis”, which appears to be the intent of the term.

Semi-rigid: A buoyant aircraft using an aerostat with a flexible membrane envelope (usually pressure stabilised) together with a rigid framework (usually a keel with bow members) providing stiffness to spread load as the main carriage system for principal parts, systems, equipment and payloads. The keel enables bigger types with low envelope super-pressure; where it spreads the weight of aircraft parts and payloads to be supported by the aerostat, where buoyancy experienced increases in proportion with L³ (i.e., displaced volume).

Semi-buoyant aircraft: Aircraft that have an aerostat for atmospheric displacement purposes to gain buoyancy but needing other lift augmentation methods (i.e., aerodynamic) to fly and remain airborne.

Slenderness ratio: For conventional UD types, the ratio of the axis of revolution’s distance between an aerostat’s ends and its maximum diameter.

Slosh: For aerostats, a sort of wave movement of gas cells or the air in a ballonet and the ballonet membrane (similar to liquid slosh).

Super-pressure: The differential pressure of a non-rigid aerostat or closed balloon at its lowest position.

Super-heat: For an aerostat, the result of a temperature difference between internal LTA-gas and external air; where positive super-heat is when the LTA-gas temperature is greater than the air’s temperature.

Suspension system: For buoyant aircraft with a non-rigid aerostat, this is the arrangement of internal and/or external rigging lines supporting lower units (baskets, modules, nacelles, pods, gondolas, cars and so forth).

Take-off: An HTA term that, for near EQ buoyant aircraft (practically airborne beforehand due to buoyancy), is the act of launch into free flight. Because nonbuoyant aircraft take off in a different way to buoyant aircraft, this term should be avoided to obviate confusion; where the terms ‘release’ and ‘launch’ should be used instead for the similar action of a buoyant aircraft’s movement from the ground into free flight.

Tethered aerostat: This is a buoyant aircraft continuously held by a single line (the tether) from a ground winch system – used to let it up or to haul it down against excess lift (where buoyancy experienced is greater than gross weight plus the weight of the tether).

Thermal airship: An airship that uses heating methods to reduce the density of the contained LTA substance (normally hot-air instead of gas) and usually non-rigid.

True Weight: The weight of an item as measured in a vacuum at sea level.

Undercarriage: A ground fender beneath a buoyant aircraft’s lower features to protect them from impact damage and to minimise the shock felt by occupants, sensitive systems & structure when the descent rate is arrested. Notes: a buoyant aircraft’s undercarriage may be a bumper bag, skid or wheeled leg arrangement (similar to nonbuoyant aircraft). However, wheels usually must be allowed to rotate freely about their leg and don’t normally need brakes, being fenders to absorb and accommodate the direction of impact rather than landing units along a runway that support aircraft weight. The fender’s shock absorber stroke also should be longer due to the different dynamic spring/mass systems of buoyant aircraft and with damping to prevent spring-back or rebound that causes relaunch. Ways to spread the impact load also are needed for soft surface grounding sites and the fenders need high fatigue tolerance from many ground impacts while mast moored (unless fixed or kept indoors).

Undocking: Movement of a buoyant aircraft out of its hangar.

UD: Unidirectional.

Unmooring: Buoyant aircraft release from a mooring restraint system – usually a mast.

Upend: Rotation of a horizontally aligned UD buoyant aircraft such that the end points of its longitudinal axis move respectively to the upper and lower positions with the longitudinal axis then vertically inclined. The rotation may be either way. This can occur at a mast when a conventional airship appears to undertake a nose stand with its tail in the air above the mast or in free flight with low gas-fill (such as for stratospheric types at low altitude) if the gas moves to one end or the other, when it may pitch nose or tail up from pendulum action and the way slosh occurs.

Valving off: The release of LTA-gas (venting) from an aerostat either deliberately (e.g., in order to increase weight of air contained in the ballonets without increasing super-pressure) or automatically should super-pressure rise above set limits via pressure relief valves for safety; such as when ascending above the pressure height.

Vectored thrust: Directed force from an adjustable propeller system at various angles as required for manoeuvring or control, lift, propulsion and air braking.

Vertical launch and capture (VLC): For buoyant aircraft, the respective actions of release into flight with vertical ascent and grounding from flight with vertical descent.

Vertical take-off and landing (VTOL): This is a nonbuoyant aircraft term that is a natural ability for most buoyant aircraft with regard to vertical launch, ascent/descent and return to the ground, where the acronym often is used to convey the ability. However, because buoyant aircraft normally remain airborne at ground level from buoyancy experienced without airspeed (so don’t land), a better term and acronym is: vertical launch and capture (VLC).

Virtual inertia: An ‘added mass’ effect from surrounding air movement entrained by the buoyant aircraft’s own movement

Weather-vane: For UD buoyant aircraft, the rotating (yaw) motion about a nose attachment point to a mooring mast, where they move like a weather cock around the mast (depending on the wind direction) to align with the wind (nose into wind), minimising aerodynamic loads. Note: Buoyant aircraft also may pitch and roll that needs consideration to manage.

Weigh-off: The action to determine a buoyant aircraft’s EQ state prior to launch or during flight (normally prior to capture) by:

• Prior to launch, raising it a little, holding it steady, then letting go and observing whether it ascends or descends. Then adding or removing ballast and repeating the process until it neither ascends nor descends, when EQ is established. Following this a particular amount of ballast may be added or removed to suit the flight plan.
• Prior to capture and in level flight, reducing airspeed to practically zero with attitude that minimises aerodynamic lift and zeroing vertical thrust, then observing the descent or ascent rate and using judgment to assess EQ state. Following this a particular amount of ballast or LTA-gas may be released to suit capture. New types with air-ballast facilities instead may take more air aboard or discharge it.

Weight exchange: See load exchange.

Zeppelin: A rigid airship with an internal cable braced framed structure of the type produced by Luftschiffbau Zeppelin GmbH up to 1938 and now any airship produced by ZLT Zeppelin Luftschifftechnik GmbH & Co KG. The term is sometimes applied indiscriminately to any type of airship (normally rigid types).

* Total, gross or all-up weight, including payload, ballast, etc.

Notes

• The rev B edition was updated and broadened following review of the glossary published by Mowforth¹ in, “An introduction to the Airship”, which is an excellent and comprehensive treaties of the subject matter originally from a time when the re-emergent industry was setting up. Some definitions here were added (although revised) for completeness and some were revised for compatibility with his glossary. Terms not added were considered either to be not in common use today, although this may change in the future (when they may be added), or to suit other purposes – so not generally applicable to all types. Some definitions here also are different, written to avoid common misconceptions of buoyant aircraft physics and practices.
• The rev C edition was mainly to revise the layout (no longer a table).
• Rev D amends the document’s header and footer and is to correct some definitions mainly associated with the physics of buoyancy in the atmosphere.
• Rev E makes general corrections, additions/deletions and changes the definitions to make them relative for buoyant aircraft today (including new semi-buoyant types) instead of just LTA aircraft.
• For further definitions of buoyant aircraft words or terms see:
  BS 185-7:1969⁴
  The Rigid Airship⁵
  Appendix E “Glossary of Airship Ground Handling Terms” of Giles Camplin’s thesis⁶
• For more information or interaction to enable further revisions, please contact us.

References

1. Book, “An introduction to the Airship”, by E Mowforth, 1985 (third edition, 2007).
2. Wikipedia, Hybrid Airship
3. Aviation Safety Network, summary report, American Blimp A-60+, G-TLEL, 12 June 2011, Reichelsheim, Hessen, Germany
4. ANSI, Glossary of aeronautical and astronautical terms. Lighter-than-air aircraft (aerostats) (British Standard)
5. Book by E. H. Lewitt, 1925, “The Rigid Airship: A Treatise on the Design and Performance“
6. Thesis, “Rediscovering the Arcane Science of Ground Handling Large Airships”, by Giles Camplin, City University, London, February 2007.