LTA Aircraft Definitions
Section 1: A to K
Section 2: L to Z
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.
6DOF: This refers to the motion of any body in three dimensional (3D) space that can move and/or be controlled in any of the six directions or ways (x, y, z, p, q, r) possible – i.e. linearly longitudinally along the ‘x’ axis, laterally along the ‘y’ axis and vertically along the ‘z’ axis, and in rotational pitch ‘p’ about the ‘y’ axis, yaw ‘q’ about the ‘z’ axis and roll ‘r’ about the ‘x’ axis.
Absolute pressure: The pressure of a gas or fluid – e.g. atmospheric pressure (p0) is absolute.
Aerial crane: An aircraft designed as a flying crane to enable pick-up, transfer (normally over short distances) and accurate placing, usually in hovering or pseudo-hovering geostationary position holding flight of heavy under-slung loads. LTA aircraft can be designed for this purpose with greater range, endurance and load carrying capability (size and weight) than helicopters.
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 force on an aerodynamically shaped body acting upwards due to airspeed, perpendicular to the direction of flight or of the undisturbed 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. Aeroplanes are aerodynes.
Note: most aerostats also are aerodynes that can develop aerodynamic lift, although not usually as efficiently as wings. Lifting bodies also are aerodynes; where the aerostats of some HAV types (configured as lifting bodies) augment aerostatic lift with appreciable aerodynamic lift to sustain flight when significantly aerostatically heavy.
Aerostat: The body or vessel of an aircraft that is LTA and causes significant displacement of the atmosphere under static conditions in order to develop sufficient buoyancy from it for useful support of overall aircraft weight normally based on:
- An envelope (usually flexible membrane) shell, bag or jacket containing an LTA substance (gas or hot air), which also may have systems for pressure stabilisation to maintain form, or
- The combination of a set of flexible LTA-gas cells contained in a lightweight rigid structure with outer covers enclosing them.
Note: it’s possible to develop HAV aerostats with ratios of aerodynamic to aerostatic lift that vary between 0 and 100% – i.e. from a fairly pure HTA aerodyne (an aeroplane) to a simple spherical LTA balloon (pure aerostat).
Aerostatic equilibrium (EQ): Aerostatic EQ occurs when an LTA aircraft’s gross weight* is exactly balanced by buoyancy experienced.
Aerostatic heaviness: The amount by which an LTA aircraft’s gross weight* exceeds buoyancy.
Aerostatic lightness: The amount by which buoyancy exceeds an LTA aircraft’s gross weight*.
Aerostatic lift: Buoyancy experienced from the atmosphere (an externally applied upward force opposite in direction to weight) on a body tending to cause flotation or the body to rise.
Note: all bodies in the atmosphere experience this force.
Aircraft: Any flying vehicle supported by atmospheric air, such as an: aeroplane, airship, balloon, helicopter, glider, autogyro, drone, etc.
Airship: A dirigible powered LTA aircraft with propulsion units and an aerostat for flotation in the atmosphere that flies in a near aerostatic EQ state, able to achieve aerostatic EQ during normal flight operations.
Airspeed: The velocity of an aircraft relative to the air it is in.
All up weight: An HTA aircraft term for the gross weight of the vehicle (including disposable loads and payload). For LTA aircraft it also should include the aerostat’s inflation gas.
ARDD: Automatic Rapid Deflation Device. It is a means to rapidly release the LTA gas from an aerostat under emergency conditions that may be operated remotely and/or automatically, depending on particular conditions (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.
For most LTA aircraft, particularly whilst moored, a rip system is the most secure way to ensure that, if an uncontrolled breakaway occurs, the gas is released, or to be used by the pilot and/or ground personnel when necessary.
ARDDs suit unmanned LTA 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 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 aerostatic lightness
- release (cast overboard) if there is excess aerostatic heaviness or to reduce it and perhaps deliberately cause aerostatic lightness
- use during load exchange to maintain EQ, or to
- set a particular weigh-off state (aerostatic heaviness or lightness) desired for subsequent flight.
Ballast thus 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 airship gross weight may change substantially). It also is used for trim purposes if gross airship weight increases (e.g. due to rain) or buoyancy reduces (e.g. when super-heat reduces).
Note: disposable ballast must be of a form and/or releasable in a way that doesn’t harm.
Ballonet: A chamber within a non-rigid or semi-rigid airship’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 gas volume as atmospheric 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 aerostat as just an inflated envelope.
Barrage balloon: A simple 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), 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 or screw-pickets 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 axis.
Buoyancy: In air, the aerostatic lift force of the atmosphere on the body causing its displacement.
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 applies 12.013 N buoyancy on the body causing the displacement (regardless of what it contains).
Buoyant aircraft: LTA aircraft that have an aerostat for atmospheric displacement purposes to enable effective buoyancy to be experienced, which includes types that may not be able to achieve EQ without other lift augmentation methods (i.e. aerodynamic and/or vertical thrust) such as some HAVs.
Buoyancy control: Perhaps a misnomer, as buoyancy from the atmosphere is practically impossible to control directly due to remoteness and the way it freely changes. However, 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.
Also, because buoyancy results from displacement of the atmosphere, it may instead be controlled through the displacing body’s volume change. Practically, it thus involves indirect methods that are manageable.
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) may be reduced by venting LTA gas or by cooling it, although the latter isn’t practical yet.
It may be increased by heating the LTA gas (provided it can freely expand) or by introducing more from pressurised or liquefied storage (also provided it can then freely expand), where the latter is only practical from ground facilities. Purported methods involving in-flight compression to reduce the displaced volume or increase weight when desired are fallacious due to resulting excess parasitic system weight.
Camp out: To remain in attendance with an LTA aircraft or aerostat at a temporary site away from its main base for ground handling purposes.
Capture: Activity of transition from free 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.
Conventional airship: A unidirectional 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 aerostat or hull form that is unidirectional.
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 due to a combination of strong pendulum stability from low cg position and poor airspeed. When understood this can be of benefit to assist takeoff if aerostatically heavy, but otherwise could cause a crash.
Cycloidal propeller: A propeller (like paddle wheels) with straight blades parallel, but equally offset and positioned, around a drive-shaft (connected to it 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. tug boats) but for aircraft.
The benefit of such propellers is that they may enable almost instant vectored thrust at full power in any radial direction through 360°. However, they don’t exist yet as a certified propeller for aircraft in regular production.
Dead weight: The weight of an item to be lifted – usually its effective weight.
Disposable load: Crew, payload, fuel, oil, disposable 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 pressure inside an aerostat and p0 is atmospheric pressure.
Dirigible: Vehicle ability to be controlled (steered) on a particular course (heading). Airships often are referred to as dirigibles due to the way this ability was established, although it is a term that may be used to describe the controlled motion of any vehicle.
Displacement: For an LTA aircraft, the weight (or amount) of atmospheric air displaced by its immersed body (primarily its aerostat – the displacement vessel). This may be determined knowing the air’s density and the volume of it displaced.
Displaced volume: This is tricky to define because aerostats usually have air within them; for non-rigid and semi-rigid types, 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 that is difficult to quantify.
However, 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 their geometry).
Ignoring the volume of the aerostat’s parts and other aircraft features (considered negligible by comparison) these geometric volumes 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 – so the resulting buoyancy and gas-fill to use.
Even so, rigid airships in the past often were just filled full of gas (hydrogen) under warm hangar conditions (causing super-heat when undocking) and loaded up with sufficient disposable ballast to release under way if gas vented when expanding, buoyancy reducing due to loss of super-heat while fuel was consumed (loosing weight), not worrying too much about it.
Docking: Movement of an airship into a large shed or aircraft 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: Perhaps an unnecessary term due to others (such as HAV, hybrid airship or buoyant aircraft that have been taken up) said to be a body something between an aerodyne and an aerostat.
Mowforth2 was 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.”
Wikipedia3 (under the title ‘Hybrid airship’) states, “a dynastat is a dynamic lift airship typically intended for long-range cruising.” Maybe! However, all aerodynes displace the atmosphere (causing some aerostatic lift) and aerostats develop aerodynamic lift/drag from airspeed.
The issue then is what exactly is the term needed for, apart from a product name, and where otherwise all aerodynes and aerostats (the common terms used) would be dynastats?!
Since the intention probably was to provide a term for new unidirectional airship types arranged (i.e. formed or configured) in such a way as to enable greater aerodynamic lift than possible for conventional airships, the following is offered:
A buoyant aircraft body arranged to develop greater aerodynamic lift than possible for classic style airships that needs airspeed for takeoff, climb, level flight and safe landing with a short ground run when aerostatic heaviness is significant.
This recognises that, as fuel is consumed, aerostatic heaviness reduces without means to regain weight lost, but that may still be significant when returning to the ground – particularly after an aborted take off. Such types have yet to be certified and enter commercial service, and may need special arrangements to manage unstable behaviour on the ground in difficult weather.
Effective buoyancy: The buoyancy experienced by 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, which buoys the item.
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 container, bag or jacket (usually a flexible membrane) to hold internal LTA substances (gases and/or air).
Eta patch: A fabric assembly with a single D-ring line attachment point reinforced with splayed load spreading finger tapes or cords, entirely affixed to a non-rigid membrane to carry tensile line loads applied.
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 the fuel’s hydrocarbons and oxygen from the air (drawn from the atmosphere outside) to make water (H2O) plus carbon dioxide (CO2).
Due to the chemical relationships of the process, it’s possible to collect a greater weight of water (due to added oxygen from the atmosphere) than the weight 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 capability to prevent vulnerable structure being damaged at possible contact or impact positions.
Flight: For an LTA aircraft this is its free motion in and through the atmosphere (the air). This may be with airspeed (due to propulsion), when induced aerodynamic forces arise and heading is controlled, or without airspeed, when it drifts as a floating vessel with ground speed and direction as for the air (wind).
Gas balloon: A balloon filled with an LTA gas (often hydrogen) instead of hot air, for efficiency to minimise size/weight and maximise flight endurance, which supports a suspended basket for payloads and/or people. It is sustained in flight due to just the buoyancy it experiences – otherwise free to drift with the air.
Gas cell: A lightweight flexible bag or vessel (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, this remains reasonably constant during flight – so, although volume may vary considerably, the amount does not.
Geostationary: For aircraft, ability to hold a particular station and height in 3 dimensional space relative to a point on the earth’s surface.
Gondola: The nacelle or car of an LTA aircraft normally arranged with a cabin and flight deck for passengers and crew.
Gross weight (mass): The all up weight, including the weight of contained LTA gas. The gas mass is needed for LTA aircraft dynamic analysis, which also needs virtual mass (inertia) of entrained air.
Ground handling: Primarily, the operations and activities to manage LTA 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 and unloading, weigh-off, and so forth.
Also, operating site setup, ground systems and equipment management, plus operating site assistance concerning non-rigid aerostat test and inflation, LTA aircraft assembly and rigging, aerostat pressure watch, weather monitoring, site security, LTA gas plant management, LTA aircraft breakdown, packing for shipping and similar tasks typical at airports for HTA aircraft. Road train duties to follow an LTA aircraft on tour to support them at temporary sites also involves ground handling to load/unload vehicles and drive them.
Grounding: Descent of an airship to the ground with intent to remain there, usually followed by capture (so grounded).
Note: airships remain primarily airborne at ground level, so do not actually land unless the buoyancy keeping them afloat (so airborne) somehow is lost. Thermal types normally are landed when their hot air is vented, but this doesn’t usually occur with 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.
Heavy operation: The method used to deliberately fly a dirigible LTA aircraft with significant aerostatic heaviness, which needs augmentation of buoyancy experienced with aerodynamic lift (needing enough airspeed) and/or vertical thrust upwards.
Helistat: This is an airship with an aerostat to develop buoyancy that supports gross weight, carrying a lower nacelle with arms or pylons extending symmetrically each side to support helicopter type rotor systems at their ends for vertical lift (essentially operating clear of the aerostat). The arrangements also may have separate propellers for thrust and control in other directions and, if designed as a unidirectional type, an empennage for stability and control of flight.
The arrangement enables aerodynamic lift of its helicopter type rotors to be used with full effect to carry a payload perhaps without need for load exchange (i.e. without needing compensating ballast). See also Hybrid airship and Rotastat.
Helium: An inert (so non-inflammable) gas used to partially inflate LTA aircraft aerostats at ground level (allowing space for expansion) as a bulk component to fill out, support and stabilise its flexible container (i.e. the aerostat’s envelope), which in turn displaces 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, a 13 m volume of pure helium under the same conditions weighs 1.657 N, acting against buoyancy experienced.
Hot-air-balloon: A bulbous naturally shaped non-rigid aerostat with an open lower aperture, inflated with air 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 experienced 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 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 as its hull.
HTA aircraft: Aircraft unable to achieve aerostatic EQ, needing lift by other means (e.g. aerodynamic lift and/or vertical thrust) to take off and fly without descending.
Hybrid airship: An awkward term that perhaps confuses matters. Wikipedia3 states, “This is an aircraft that combines characteristics of LTA technology with HTA technology, either fixed-wing or rotary-wing.” Mowforth2 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.”
The main problem with both definitions is that they are too broad (incorporating rotor technology that needs separate treatment) and where today the emphasis is on unidirectional types (which classic airships are) based on HAV methods. An alternative definition is: A buoyant aircraft using lifting body principles and/or winged to develop significant aerodynamic lift (say 40%) that supplements 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 terms ‘Buoyant aircraft’ and Hybrid air vehicle’ tend to make this term superfluous.
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): An aircraft that uses a combination of aerodynamic and aerostatic lift plus vertical thrust to remain airborne. It is a newly coined term currently associated with dirigible unidirectional LTA aircraft types (classic airships) with a broadened aerostat and/or wings to develop significantly greater aerodynamic lift than conventional airships today and where they are considered to be a hybrid of HTA and LTA technology.
Hydrogen: An inflammable gas used to partially inflate some LTA 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, gn = 9.80665 m/s2, a 13 m volume of pure hydrogen under the same conditions weighs 0.8365 N, acting against buoyancy experienced.