LTA Aircraft Definitions
Section 2: L to Z
Section 1: A to K
Rev D 1 Dec 2017
Author: Charles Luffman
© 2017-2018 Charles Luffman. All Rights Reserved.
This page provides LTA aircraft definitions with explanation of words or terms used for lighter-than-air (LTA) aircraft (i.e. balloons, aerostats, airships and so forth) primarily written for non-rigid types. It’s felt that this is necessary to help people understand matters and to redress false or inadequate definitions provided on websites such as Wikipedia.
Corrections and additions (if needed) may be made in future revisions following reasoned advice to the author, as above.
Note: specific with regard to use for LTA aircraft.
Landing: An HTA term that, for LTA aircraft (usually remaining airborne after capture), is the loss of buoyancy from collapse of their aerostat due to venting the LTA inflation substance (gas or hot-air) so that the aircraft’s full weight transfers to support from the ground (landed).
Because HTA aircraft land in a quite different way to LTA aircraft, this term should be avoided to obviate confusion; where the terms ‘capture’ and ‘grounding’ should be used instead for the similar action of an LTA aircraft’s descent to the ground.
Launch: For free balloon systems and airships, activity for release and takeoff 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 due to airspeed for flight while minimising aerodynamic drag.
Lifting gas: A misleading false term widely used by many people to describe the LTA gas put into aerostats, which has weight rather than lifting ability. 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 it’s buoyed externally by the water it’s in.
Buoyancy on an aerostat in the atmosphere is derived in the same way as an external force from the air that pushes it up, rather than magically being lifted by the LTA gas inside. The term therefore should not be used any more.
Note: See the definition for ‘LTA gas’, offered as the correct term to use.
Lighter-than-air (LTA): Literally that; where the item concerned simply weighs less than air.
Load exchange: The process to exchange one load for another 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 LTA aircraft 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: An Eta patch for use to spread tensile line loads applied on a fabric or membrane structure.
Loitering: 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: LTA aircraft with 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: Aircraft able to float in aerostatic EQ without lift augmentation by other means (e.g. aerodynamic lift and/or vertical thrust).
LTA gas: This is the correct term to use for the gaseous substance put into an aerostat’s envelope or gas cells as a component with bulk to support their thin flexible membranes, so that they don’t loose form against atmospheric pressure; enabling the air’s displacement without mixing and thus buoyancy on the aerostat. The reason it’s used is to minimise weight of the displacement vessel (the aerostat), of which it is an integral part that otherwise would be useless for the purpose.
Metalclad: A type of rigid aerostat construction based on a thin metal (aluminium) shell (monocoque) with minimal supporting structure, just enough to sustain its own 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 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’: a monocoque hull comprises a continuous hard shell, which for an LTA aircraft’s aerostat may be an assembly of moulded sandwich panels able to carry in-plane shear and compression, so differing from the thin single ply shell of former metalclad types.
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 LTA 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 that fixes the LTA aircraft’s aerostat.
Mooring Mast: A vertical stanchion or tower, which may be braced, with a coupling facility at its head for unidirectional LTA aircraft (typically conventional airships) to connect to; where connection normally is at the LTA aircraft’s nose, but sometimes through an alternative point (e.g. a chin position). An important aspect of the mast and coupling arrangements (mast and LTA aircraft) is that it must freely permit the LTA aircraft’s weather vane, pitch and roll movements under any weather conditions – allowing it to align with the wind.
Masts may be a fixed installation (permanent), temporary installation (i.e. stick or expeditionary types) or mobile (allowing relocation by road and/or airfield movement with an LTA aircraft attached). They also may be tall (a high mast) with the LTA aircraft held aloft, or short (a stub mast) with the LTA held next to the ground (facilitating access, load exchange and servicing).
Multi-hull airship: An airship with two or more aerostats of conventional form joined together as a single unit, usually a horizontal group analogous with a marine “catamaran” or “trimaran”.
Multi-lobe hull: An aerostat of “lobed” (e.g. cloverleaf) cross-section. Usually found in non-rigid airships and formed by internal lacing of the aerostat’s envelope, mooted to improve bending stiffness and be easier for attachment of suspended structures. New HAV types have adopted such multi-lobe aerostat methods.
Non-rigid airship: An airship that uses an aerostat with a flexible membrane envelope (usually pressure stabilised) without any framework as the main carriage system for the airship’s principal parts, aircraft systems, equipment and the payload.
Parachute balloon: A barrage balloon with a basket for people that, when elevated, may be used as a lookout point for distant observation and from which occupants may alight with a parachute – also used for parachute training purposes.
Parasitic weight: Useless or excess parts and systems weight (without needed purpose) unnecessary for an aircraft to bear, 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 LTA 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.
Platform: A term often used for an aircraft holding a geostationary position at altitude as a stable means to support installed user systems or a payload aloft.
Pressure airship: An airship with an aerostat structure of non-rigid, semi-rigid or monocoque form, which uses super-pressure to stabilise that form against atmospheric pressure and the effects of flight.
Pressure altitude or Pressure height: The altitude (1) when a non-rigid’s LTA gas expands to completely fill its containment vessel (the aerostat’s envelope) without increasing super-pressure and when the ballonets just become empty of air. In a rigid type it is when the gas cells swell (due to gas expansion) to just fill their bounding compartments.
If the LTA aircraft ascends further super-pressure then increases (begins to develop in rigid types) until gas vent valves open (2), when LTA gas is lost. Depending on rate of ascent (if it continues) and gas loss rate, super-pressure may continue to rise with danger that the envelope may burst or rigid structure may fail due to excess super-pressure with catastrophic effect.
Aerostat weight thus reduces due to gas loss, exacerbating the situation. However, atmospheric displacement reduces at a faster rate due to ascent into thinner air (reducing buoyancy), which helps to slow, stop ascent and (3) cause descent.
Assuming that the envelope doesn’t burst (unlikely due to factors of safety used in design) when the LTA aircraft descends to a level where the valves close (no more gas loss) and super-pressure returns to its previous value with the envelope or bounding compartments just full of gas again, the pressure height (4) will be at a higher altitude than before (from reduced gas-fill).
When the LTA aircraft further descends towards the ground, gas volume will reduce – needing air to be blown into the ballonets (for non-rigids) to maintain super-pressure (required to stabilise the envelope). When it reaches the previous pressure height (1), assuming temperature/pressure conditions are the same, buoyancy experienced will be less – causing aerostatic heaviness and needing ballast release to regain the previous EQ state.
Then, depending on descent rate (if too fast) gas volume may reduce at a greater rate than the blowers can manage to fill the ballonets of non-rigids with compensating air, causing reduced super-pressure, reducing aerostat stiffness with potential loss of form (not good but not necessarily catastrophic).
Assuming this doesn’t happen and depending on the gas loss at altitude, when nearing the ground the ballonet of non-rigids may become full (so ineffective) again resulting in loss of super-pressure, when another way to force air into the envelope is needed to maintain it (degrading LTA gas purity from mixing).
The loss of LTA gas due to ascent above the pressure height is wasteful and causes increased aerostatic heaviness; it is therefore customary to operate below this height wherever possible. The pressure height may be increased only by reducing the gas-fill and hence the available buoyancy. LTA aircraft thus should in general operate at low altitudes to maximise payload weight carried.
Pressure management: A system of ducts, fans (blowers), valves and 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 parked (moored).
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).
- The act of launch when an LTA aircraft is fully ‘let go’ from ground restraint to thus take up free flight. Cast off.
- Let go from mooring restraints – a mast or ground pickets.
- 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, safety and so forth.
Rigid airship: An airship that either has an aerostat structure of light wire-braced frames, girders and connecting members with separate internal compartments, each bounding a flexible gas cell, with light outer covering for smooth aerodynamic form (typical of former Zeppelins) or 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. Such airships enable very large types to be constructed and flown.
Note: when the aerostats of very large airships are 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 where the buoyancy experienced as a result is huge (needing a way to be restrained). Rigid airship 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 an 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.
Otherwise, the line’s end is located and affixed in such a way that it either is available for ground personnel or the pilot to deliberately use under emergency conditions when the LTA aircraft must be prevented from leaving the ground (to enable escape). Tragically, this was not available for one pilot when the airship he was flying was consumed by fire that started from leaking fuel lines.
Rotadyne: An aircraft sustained in flight entirely by powered lifting rotors (e.g. a helicopter). Also, a powered rotating body with radiating blades that pitch collectively to develop positive or negative aerodynamic lift.
Rotastat: Wikipedia3 (under the title ‘Hybrid airship’) states, “A rotastat is a rotorcraft/airship hybrid typically intended for heavy lift applications.” Maybe! Mowforth2 says, “With rotor lift the vehicle’s characteristics become those of a rotadyne and an aerostat, and it becomes a rotastat.” Also that, “This type of machine is capable of VTOL.” In addition, it’s mooted that such aircraft have the advantage of being able to operate without ballast. Perhaps!
Even so, as shown by Mowforth’s illustrations of typical types, it’s clear that there are at least two distinctly different LTA aircraft versions.
- Those with a fixed unidirectional 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.
- Contraptions based on an aerostat that rotates either about a longitudinal or vertical axis, carrying a set of hinged blades (wide rotors) radiating equi-spaced about the rotation axis and each supporting hinged T-planes at their ends (functioning like cycloidal propellers), all supported in place by bracing cables.The arrangements also have propellers to cause rotation of the whole vehicle, which work like giant floating and swirling propellers configured to direct aerodynamic forces generated by the blades and T-planes to enable vertical lift for under-slung payload carriage and thrust in any other direction for motivation plus control.
In addition, Mowforth illustrates an LTA aircraft with a rotating spherical aerostat on a horizontal axis that deliberately gets aerodynamic lift from airspeed (enabled by thrust units at each end of the rotation axis that are supported from a hanging yoke) derived from the Magnus effect (as for a swerving ball).
With regard to the first group, the arrangements clearly are for a type that has an aerostat to develop buoyancy to counter gross weight so that the aerodynamic lift of its helicopter type rotors can be used with full effect to carry a payload; otherwise functioning as conventional airships. In this respect they are helistats, as their individual type names suggest.
On the other hand, the second group clearly are rotating aerostatic propeller systems, where they appear to be correctly defined as rotastats. These rotastats also develop aerodynamic lift derived from the Magnus effect, which may not be desired but is a consequence from rotating the aerostat. The spherical type, which is omni-directional, thus also is a rotatstat. The explanation below thus is offered to better define the term.
A rotating aerostat that develops Magnus effect aerodynamic lift from airspeed that may also carry hinged blades and or planes to develop lift in other directions.
Semi-rigid airship: An airship that uses an aerostat with a flexible membrane envelope (usually pressure stabilised) together with a rigid framework (usually a keel with bow members) that provides additional stiffness to spread load, as the main carriage system for the airship’s principal parts, aircraft systems, equipment and the payload. This enables bigger types with low envelope super-pressure; where the weight of aircraft parts plus payload to be supported by the aerostat and buoyancy experienced increases in proportion with L3 (i.e. displaced volume).
Slenderness ratio: 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 or semi-rigid aerostat or closed balloon at its lowest position.
Super-heat: For an aerostat, the result of a temperature difference between internal gas and external air; where positive super-heat is when the gas temperature is greater than the air’s temperature.
Suspension system: For an LTA aircraft with a non-rigid aerostat, this is the arrangement of internal and/or external rigging by which external lower aircraft units (baskets, modules, nacelles, pods, gondolas, cars and so forth) are supported from it.
Takeoff: An HTA term, which for LTA aircraft (practically airborne beforehand due to buoyancy) is the act of launch into free flight. Because HTA aircraft takeoff in a quite different way to LTA aircraft, this term should be avoided to obviate confusion; where the terms ‘release’ and ‘launch’ should be used instead for the similar action of an LTA aircraft’s movement from the ground into free flight.
Tethered aerostat: This normally refers to an LTA aircraft as a non-rigid unidirectional aerostat with stabilising fins (that may be pressure stabilised fabric structures), which is continuously held by a single tether line from a ground winch system – used to let the aerostat ascend or to haul it down against excess lift (where buoyancy experienced is greater than gross weight plus the weight of the tether line). Rightly or wrongly, such arrangements often also are called Blimps. Other aerostat forms (e.g. simple balloons) without fins and restrained in a similar way also are tethered aerpstats.
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.
Undercarriage: A ground fender beneath an LTA aircraft’s lower features to protect them from ground impact damage and to minimise the shock felt by occupants when the descent rate is arrested.
Note: an LTA aircraft’s undercarriage may be a bumper bag, skid or wheeled leg arrangement (similar to HTA aircraft). However, wheels usually must be allowed to rotate freely about their leg and shouldn’t have brakes, since they are fenders that must accommodate the direction of impact rather than landing units along a runway that support aircraft weight.
Undocking: Movement of an airship out of its hangar.
Unmooring: Release from a mooring restraint system – usually a mast.
Upend: Rotation of a horizontally aligned body (e.g. an aerostat) of unidirectional form 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 may occur at a mast when a classic style 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 pitches 90° due to pendulum action.
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 LTA aircraft, the respective actions of release into flight and grounding from flight with vertical ascent and descent.
Vertical takeoff and landing (VTOL): This is an HTA aircraft term that is a natural ability for most LTA 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 LTA aircraft normally remain airborne at ground level from buoyancy experienced without airspeed, a better term and acronym is: vertical launch and capture (VLC).
Virtual inertia: An apparent added mass effect due to surrounding air movement entrained by the aerostat’s own movement.
Weather-vane: For classic airships, 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. The airship also may pitch in a similar way depending on wind strength, wind direction and EQ state.
Weigh-off: The action to determine an LTA 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.
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).
* Total weight, including payload, ballast, etc.
- The rev B edition was updated and broadened following review of the glossary published by Mowforth2 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 his own purposes – so not generally applicable to all types. Some definitions here also are different, written to avoid common misconceptions of the LTA aircraft physics and practices involved.
- 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.
- For further definitions of LTA aircraft words or terms see:
The Rigid Airship6
Appendix E “Glossary of Airship Ground Handling Terms” of Giles Camplin’s thesis7
- For more information or interaction to enable further revisions contact the author.
- Chapter XI, Common Airship Fallacies, item (2) Compressing gas or air for ballast, page 286 of book, “Airship Design”, by Charles P Burgess, 1927.
- Book, “An introduction to the Airship”, by E Mowforth, 1985 (third edition, 2007).
- Wikipedia, Hybrid Airship
- Aviation Safety Network, summary report, American Blimp A-60+, 2011
- ANSI, Glossary of aeronautical and astronautical terms. Lighter-than-air aircraft (aerostats) (British Standard)
- Book by E. H. Lewitt, 1925, “The Rigid Airship: A Treatise On The Design And Performance“
- Thesis, “Rediscovering the Arcane Science of Ground Handling Large Airships”, by Giles Camplin, City University, London, February 2007.