Definitions

Rev G, 30 Nov 2025
Author: Charles Luffman

© 2025 Luffships Limited. All Rights Reserved.

This listing provides definitions with explanation of words or terms used for buoyant aircraft (balloons, aerostats, airships & derivatives), primarily written for non-rigid types but not exclusively. It’s felt that the definitions are needed to help people understand lighter-than-air (LTA) matters and redress false or inadequate terms provided elsewhere.

As we learn about the technology, improvements needed become apparent – leading to appropriate revisions. Following reasoned advice to the author (as above) future revisions may be made. Note: This revision is a general update to correct and ease the terms for buoyant aircraft (BA).

Glossary of Terms

6DOF: This refers to the six degrees of freedom (i.e., u, v, w, p, q, r) that free bodies may move along or about within three dimensional (3D) space relative to cartesian x, y, z axes, where u, v, & w are linear velocities along respective x, y, z axes, and p, q, r are respective rotational motions 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, putting heavy under-slung loads accurately and safely down 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 shaped body (not necessarily a vehicle), including sails, able to hold general form against aerodynamic and aerostatic effects of external pressure causing lift and drag from airflow over it. 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-BA (also functioning as a lifting body) augment aerostatic lift with appreciable aerodynamic lift to sustain flight against significant aerostatic heaviness.

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

Aerostatic equilibrium (EQ): This occurs when a BA’s gross weight* is balanced by buoyancy.

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

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

* Total, gross or all-up weight (AUW), including payload, ballast, LTA-gas and air contained, etc.

Aerostatic lift: Buoyancy exerted by the atmosphere (an upward reactive force opposite in direction to weight) on a body that substantially 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), which may be discharged back into the atmosphere without harm for aircraft weight change purposes. Note: this emulates similar practices used by submarines to rise or sink.

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

Airship: A free flying dirigible BA using an aerostat to: 1) perhaps float in EQ without airspeed, power or upthrust, and 2) fly with powered propulsion plying directed courses.
Notes: Non-BA (e.g., aeroplanes and helicopters) don’t use an aerostat. However, aircraft using air inflated wings or an aerodyne lifting body able to fly without LTA-gas but that may be inflated with it to reduce weight for aerostatic lightness may be semi-buoyant types. Ultimate responsibility lies with the aviation authorities to recognise, categorise and define the limits/requirements for safe flight of each type.

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

All up weight (AUW): A non-BA term for the gross or total weight of the flight vehicle (including disposable loads). Note: the AUW of BA 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.

Automatic Rapid Deflation Device (ARDD): This is a contrivance 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 inadvertently without control), either to keep the aerostat on the ground or to bring it down. It’s needed to prevent unintended launch or runaway when other means of control are unavailable. Notes:

  1. For most BA, 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. It also is needed for use by the pilot and/or ground personnel when necessary. However, rip lines are a destructive way that may lead to total loss of the aircraft and its LTA-gas that’s unsuitable for UAS flight. It should be noted that the authorities only require safe situation containment.
  2. ARDDs suit unmanned BA 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.
  3. Secondary ways to limit subsequent downwind drifting and LTA-gas loss also are needed, easier to enable with OD types.

Ballast: Heavy stuff (usually sand or water) used aboard BA to:

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 BA 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 bladder within a non-rigid or semi-rigid BA’s aerostat (normally for air) bounded by a flexible membrane preventing mixture with the LTA-gas in the aerostat. It’s meant to freely inflate or collapse 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. Also 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 BA 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, this 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, gn = 9.80665 m/s2, a displaced 13 m volume of air applies 12.013 N buoyancy pushing up on the body causing the displacement (regardless of what it contains).

Buoyant Aircraft: These are aircraft using an aerostat for atmospheric displacement purposes to enable significant buoyancy. It also 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 (an externally applied force) is not possible to control directly due to remoteness as-well-as the way it freely changes. It results from external air displacement by the body in it, so needs responsive weight control and reactive variable geometry displacement-body methods instead. Buoyancy and weight 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 therefore are the 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 quickly 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 – maybe increasing weight. Otherwise, air may be pumped into flasks at high pressure and used as ballast (easy to later vent).
  3. Displacement may be increased by heating the LTA-gas or by introducing more from pressurised or liquefied storage. Both depend on free gas expansion – enabled by expelling previously imbibed air and where the latter normally is undertaken 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 BA at a temporary site away from its main base using available ground handling methods.

Capture: This is BA 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 catenary form of a slack line between two fixed points – normally used upside down.

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

Classic airship: An airship in the traditional style adopted before 1900 with an elongated ellipsoidal 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 expected (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 hardly exist at the moment in regular production as certified propellers for aircraft 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. (pi – p0) where pi is the pressure inside an aerostat and p0 is atmospheric pressure.

Dirigible: This means 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 BA, 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 to fully fill out their envelope. For non-rigid and semi-rigid types, this air is used to 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:

Docking: This is movement of a BA 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 a non-BA background relatively new to LTA matters, otherwise used as a product name for a novel BA with flight characteristics said to be between aerodynes and aerostats. However, this doesn’t recognise that aerostats always were effective aerodynes but not vice versa. Other terms introduced for commercial purposes (e.g., HAV or hybrid airship) also have been coined, confusing matters. Notes:

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 help 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 IC 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 BA, 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 by buoyancy from the atmosphere – 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, normally 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) doesn’t normally change.

Geostationary: Ability to hold a particular station and height without rotating in 3 dimensional space above 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 (AUW), including the weight of contained LTA-gas (constant) and air (variable). Note: The LTA-gas and air put into BA aerostats are essential components, together supporting their flexible containers (envelopes, gas cells) as displacement bodies against atmospheric pressure. These gaseous elements therefore should be included in the aircraft’s weights table because their aerostat otherwise would collapse or not fly.

Ground handling: Primarily, the operations/activities to manage BA 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, BA assembly and rigging, aerostat pressure watch, weather monitoring, site security, LTA-gas plant management, BA breakdown, packing for shipping and similar tasks typical at airports for non-BA. Road train duties to follow BA on tour to support them at temporary sites also needs ground handlers to load/unload vehicles and drive them.

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

Hangar: A large shed or building for aircraft. Hanger: An implement to hang clothes on or anything hung from a line.

Haul down: The action to pull an elevated aerostat restrained by its tether or ground lines against buoyancy down to the ground using manpower, tirfors, capstans, vehicles 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. Even so, this is only sensible if the elevated aerostat is within reach and with skilled crew aware of the dangers involved able to act or let go promptly.

Heavier-than-air (HTA): where the aircraft, its material, parts and substances used weigh more than the air they displace.

Heavy operation: The methods used to deliberately fly airships with significant aerostatic heaviness, needing augmentation of buoyancy with enough induced aerodynamic lift from airflow over its aerostat and/or upthrust as-well-as perhaps control-reversal techniques to take off.

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 BA designed this way. However, see also ‘Hybrid airship’ and ‘Rotastat’.

Helium: An inert (so non-flammable) gas used to largely inflate aerostats at ground level (allowing space for expansion) as a bulk component to puff out 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, gn = 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 then is heated by a burner supported on frame legs over a basket or nacelle 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 shell or membrane of a vessel such as an aerostat forming the outer profile. For example, one may refer to the outer skinned profile of a rigid airship’s aerostat as its hull.

Hybrid Airship: An awkward term that confuses the related natural effects of aeronautical science experienced by all aircraft to a greater or lesser extent. Aeroplanes experience both aerodynamic and aerostatic loads. So do airships! Does that make them somehow hybrid? Wikipedia2 stated, “This is an aircraft that combines characteristics of LTA technology with HTA technology, either fixed-wing or rotary-wing”. Mowforth1 said that it’s, “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”. It needs to be understood that LTA and HTA are just terms for the conditions aircraft may be designed to fly under, which is a choice for aircraft developers to exploit or not but needs an aerostat for significant buoyancy from the atmosphere. Notes:

Hybrid Air Vehicles (HAV): The acronym of a company (Hybrid Air Vehicles Ltd) used colloquially by people to describe their alternative semi-BA and any similarly arranged designs in vogue through media coverage (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, hybrid is a mishmash of natural aircraft abilities obfuscating the ‘nature of the beast’.

Hybrid Air Cruiser (HAC): Just another way (as for hybrid airship) made up to market aircraft using a combination of aero-dynamic and -static lift plus say ‘battery assisted pedal power’ to fly. With the industry for aircraft today dominated by nonbuoyant types it’s believed the hybrid term was introduced to lead people away from past airship tragedies and think about new possibilities. However, this doesn’t make it a good technical term to adopt.

Hydrogen (H2): An inflammable gas when mixed with oxygen in air, used to partially inflate some aerostats instead of helium. Its use needs acceptable ways to prevent ignition. Maintaining purity ensures that! H2 is a green fuel that many other vehicle and power system developers have turned to, where the safety issues largely have been solved. Some aircraft authorities (as for EASA) have introduced similar ‘level of safety’ rules for other aircraft fuels and combustible substances to allow H2’s use in aircraft instead of banning it. Under ISA sea level conditions pure H2 has a density of 0.0853 kg/m3. Hence, with standard gravity, gn = 9.80665 m/s2, a 13 m volume of pure H2 under the same conditions weighs 0.8365 N, acting against buoyancy experienced – so about half the weight of helium.

Landing: A non-BA term that, for BA remaining mainly airborne after touchdown, normally requires the loss of buoyancy from venting the LTA-gas or hot-air (causing collapse of their aerostat (then not displacing the air) so that its effective weight transfers to support from the ground (landed). Note: Because non-BA land in a different way to buoyant types, the landing term should be avoided to obviate confusion. The terms ‘capture’ and ‘grounding’ should be used instead for the similar action. However, if heaviness is substantial then the aircraft may be classed as a near non-buoyant type, when the term becomes relevant.

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:

Lighter-than-air (LTA): This concerns an item or substance that simply weighs less than air.

Load or weight exchange: The process to exchange loads or weight with others of equal weight (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 BA that remain afloat during maintenance to, for example, replace a heavy part such as an engine with ballast – removed 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. Load patches also can be designed to carry loads in any direction and be used to affix parts.

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

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.

LTA-Vacuum-Ball: A spherical vessel similar to a bathysphere but of lightweight construction designed to resist atmospheric pressure without collapse when partially evacuated, reducing overall weight, to then be buoyed-up from becoming LTA. It’s a possibility to use in aerostats instead of or in combination with LTA-gas filled methods, providing a way using air-pumps and valves to control overall aerostat weight – allowing controlled ascent or descent in the atmosphere.

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: Readily mountable & removable lightweight aircraft units or nacelles to contain people, systems, freight and so forth, used individually or in groups that can be simply picked up & safely set down. They thus 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 BA’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 certified. They are suitable for very large airships that naturally have higher airspeed, enabling form to be maintained.

Moor or Mooring: For BA, the action for connection to a restraint system. Classic airships typically moor to a mast. Mooring also may be by a ground anchor system that fully fixes the BA’s aerostat.

Mooring mast: A vertical stanchion or tower, which may be braced, with a coupling facility at its head for UD BA (typically conventional airships) to connect to. Connection normally is at the BA’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 BA’s weathervane, pitch and roll movements under any weather conditions – allowing it to align with the wind. Notes:

Multi-hull airship: Airships with two or more aerostats of conventional form connected by cross-members as a single unit, usually held horizontally are 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-Buoyant Aircraft: Non-BA types don’t have or use an aerostat, so are unable to get useful buoyancy, needing lift by other means (e.g., aerodynamic and/or vertical thrust) to fly that is unnaturally power intensive.

Non-rigid: BA 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.

Omni-directional (OD): Aircraft (like helicopters and rotund balloons) able to routinely take off and return to ground vertically, hover, hold station and altitude while rotating, and fly in any direction while facing any other direction without consequence, regardless of wind direction.

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, 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 BA 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 BA 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 thin shell 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 a frame structure and outer covers it is when the gas cells swell (due to gas expansion) to fill their bounding compartments. Notes:

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 a UD aerostat’s volume and the volume of a cylinder with the same diameter and principal dimension (length or height).

Release: This involves:

  1. The act of launch when a BA is fully ‘let go’ from ground restraint to thus take up free flight, e., cast off.
  2. Letting go from mooring restraints – either a mast or ground pickets.
  3. Dropping ballast or to 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 BA 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 BA 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 downwind. 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 BA (enabling escape). Tragically3, this was not accessible for one pilot of a helium filled non-rigid airship consumed by fire that started from fuel leakage.

Rotodyne: 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’) stated, “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 BA 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 classic 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 overall like cycloidal propellers), all held in place with bracing cables. They also have propellers for rotation of the whole BA, 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 BA 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 also supporting 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 BA 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: BA that have an aerostat for air displacement purposes to gain buoyancy but needing other lift augmentation methods (i.e., aerodynamic) to fly and remain airborne, needing power.

Slenderness ratio: For conventional UD types, the ratio of the aerostat’s length between each end and its maximum cross-section diameter. For OD types, the ratio of the aerostat’s plan width and its maximum height.

Slosh: For UD aerostats, a sort of wave movement of gas cells or the air in a ballonet and the ballonet membrane (similar to liquid slosh) as it pitches, where air moves to the lowest end while LTA-gas is pushed to the highest end, causing the CB and the cg to move apart in those directions upsetting balance. This generally isn’t noticeable for OD types but can occur to a lesser extent.

Super-pressure: The differential pressure of a non-rigid aerostat envelope 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 external air’s temperature.

Suspension system: For BA with a non-rigid aerostat envelope, this is the arrangement of internal and/or external rigging lines supporting lower units (baskets, modules, nacelles, pods, gondolas, cars and so forth). Lines also may be used to suspend things, such as a deployable boat, observation car or cargo items/units.

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

Tethered aerostat: This is a BA 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 non-BA term for a ground fender beneath a BA’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 BA’s ground fenders may be a bumper bag, skid or wheeled leg arrangement (similar to non-BA). 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 needs to be longer due to the different dynamic spring/mass systems of BA 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). Wheels more likely would be used for semi-buoyant types needing runways to launch when heavy, although skids also may be suitable.

Undocking: Movement of a BA out of its hangar.

Unidirectional (UD): moving or operating in a single longitudinal direction (forward or backward).

Unmooring: BA release from a mooring restraint system – usually a mast for UD types but surrounding ground anchors for OD types.

Upend: Rotation of a horizontally aligned UD BA 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 classic airship appears to perform 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 or air (venting) from an aerostat either deliberately or automatically to maintain or reduce super-pressure (e.g., during rapid ascent when the LTA-gas expands, which it also does when temperature increases.

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 BA, the respective actions of release into flight with vertical ascent and grounding from flight with vertical descent.

Vertical take-off and landing (VTOL): A non-BA term that is a natural ability for most BA with regard to vertical launch, ascent/descent and return to the ground, where the acronym often is used to convey the ability. However, because BA normally remain airborne at ground level from buoyancy experienced without airspeed, a better term and acronym is: vertical launch and capture (VLC), aligning with marine terminology for ships.

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

Weathervane motion: For UD BA, the rotating (normally 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, minimising aerodynamic loads. Note: BA also may pitch and roll, needing ways to manage. However, OD types instead are fixed at ground level, preventing weathervane motion – much easier to manage.

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

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).

Notes

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.