Delta

Delta (pronounced del-tuh)

(1) The fourth letter of the Modern Greek alphabet (Δ, δ).

(2) The consonant sound represented by this letter.

(3) The fourth in a series of items.

(4) Anything triangular, an allusion to the Greek capital delta (Δ).

(5) In mathematics, an incremental change in a variable, as Δ or δ.

(6) A nearly flat plain of alluvial deposit between diverging branches of the mouth of a river, often, though not necessarily, triangular.

(7) A word used in communications to represent the letter D (usually initial capital letter); Used in the ICAO (International Civil Aviation Organization) radiotelephony spelling alphabet (usually known as the NATO phonetic alphabet).

(8) In astronomy, a star that is usually the fourth brightest of a constellation (initial capital letter).

(9) In computing, a small but noticeable effect or the set of differences between two versions of a file; in informal use, a small but noticeable effect.

(10) In surveying, the angle subtended at the centre of a circular arc.

(11) A type of cargo bike that has one wheel in front and two in back.

(12) In electrical engineering (often attributive), the closed figure produced by connecting three coils or circuits successively, end for end, especially in a three-phase system.

(13) In finance, the rate of change in an option value with respect to the underlying asset's price.

(14) In chemistry, a value in delta notation indicating the relative abundances of isotopes or relating to or characterizing a polypeptide chain that is one of five types of heavy chains present in immunoglobins.

(15) In aerodynamics, a type of wing.

(16) The NATO code name for a class of Soviet nuclear-powered ballistic missile submarine armed with sixteen multi-warhead missiles.

(17) In physics, of or characterizing the atom or radical group that is fourth in position from the functional group of atoms in an organic molecule.

(18) In US Space Force use, a unit, nominally headed by a colonel, equivalent to a USAF operations wing, or an army regiment.

(19)   In electrical engineering, the closed figure produced by connecting three coils or circuits successively, end for end, especially in a three-phase system.

Circa 1200: From the Middle English deltha from the Latin delta from the Ancient Greek δέλτα (délta), from the Phoenician dalet & daleth (tent door).  It was akin to the Hebrew dāleth.  It was the fourth letter of the Greek alphabet (equivalent to the Modern English D) and was shaped like a triangle (Δ).  The sense of a delta being a "triangular island or alluvial tract between the diverging branches of the mouth of a great river" is because the Ancient Greek writer Herodotus (circa 484–circa 425 BC) used it to describe the mouth of the Nile River (now known as the Nile Delta).  That was picked up in English during the 1550s and applied to other river mouths (often of quite different shapes) by 1790.  The related forms are deltaic and deltification.  In pre-modern medicine, the deltoid muscle (the large muscle of the shoulder; triangular, resembling the Greek letter delta) was described in 1758, the name from the Ancient Greek deltoeides (triangular, literally "shaped like the letter delta).  In modern use, the "deltoid muscle" gained the short-form “delts” in 1977.  The related form is deltoidal.

The delta-wing

Gloster Javelin (1951-1967).  The first twin-engined delta-wing in service, the Javelin was the UK’s long-term solution to its need for an all-weather interceptor, the post-war technology gap until then filled by the stop-gap Gloster Meteor and that most improbable cold-war fighter, the de Havilland Mosquito.  The Javelin was the last aircraft to bear the Gloster name, the company absorbed into the morass created by the mergers, acquisitions and nationalization that befell British industry between Attlee and Thatcher. 

In aviation, the delta shape pre-dated aircraft by centuries, triangular stabilizing fins for rockets described in the sixteenth and seventeenth centuries and the first lifting wing in delta form patented in 1867 for use by a dart-shaped, rocket-propelled airplane.  However, although there were prototypes and much theoretical work was done during the first half of the twentieth century, it wasn’t until the late-1940s when jet propulsion made possible high-speed subsonic and supersonic flight that it was possible to build airframes which could take advantages of the delta-wing’s unique properties.

Lockheed SR-71 Blackbird (1963-1999).  A product of Lockheed’s Skunk Works, although in development as a high-altitude interceptor since 1959, the US government didn’t admit the SR-71 existed until 1964 and decided ultimately to use it for reconnaissance and research into supersonic flight.  The SR-71 in 1976 set the world record for the fastest flight by an air-breathing manned aircraft and the mark still stands.

The delta form offers structural advantages and aerodynamic characteristics suited to the fluid dynamics of airflow in supersonic conditions.  To suit different applications, design variations have evolved, with and without additional stabilizing surfaces.  The long root chord of the delta wing and minimal structure outboard is inherently structurally efficient, able to be built stronger and stiffer yet lighter than a swept wing of equivalent lifting capability.  It’s thus simple and relatively inexpensive to build.

Avro Vulcan (1956-1984).

Until the UK adopted the submarine-launched Polaris missile system, the Vulcan was the platform for the UK’s independent nuclear deterrent and it remained in service long enough to be deployed for its only combat mission, a flight of five aircraft used for (conventional bombing) missions during the 1982 Falklands War.  Each aircraft flew almost 4000 miles  (6,500 km) from Ascension Island, Victor tankers used for the air-to-air refueling, almost 1.1 million gallons (5 million litres ) of fuel burned each mission.

The long root chord also allows a deeper structure for a given aerofoil section, providing more internal volume for fuel and other storage without a significant increase in drag although, on supersonic designs, designers often take the opportunity to use a thinner aerofoil instead, thereby further reducing drag.  Usefully, the large root chord also provides a large surface area which assists in reducing the minimum speed; a low landing speed being a design objective in most military and civil applications.  Delta wings can be designed to induce vortex lift, so flow separation can be turned into a means of increasing lift and the whole structure is naturally stable in pitch, therefore not requiring a separate tail surface.

Aérospatiale/BAC Concorde (1969-2003).

The Anglo-French supersonic passenger jet operated commercially between 1976-2003.  Flying at twice the speed of sound and carrying up to 128 passengers, it was the longest lasting of its time, the Soviet Tupolev Tu-144 in passenger service only for a few years during the 1970s after which it operated both as a freighter and a research platform and the American SST never progressed beyond engineering mock-ups.  The Americans worked out the economics were never going to make sense; the Soviets had a different relationship to money and the Concorde lasted as long as it did only because the French & British governments wrote-off the development costs.

There are drawbacks.  The large wing area creates more viscous drag for the same amount of lift compared to a high aspect ratio wing; swept wings have a better lift-to-drag ratio than deltas.  Also, high-lift devices like fowler flaps are hard to integrate into delta wings with the rearward location of the trailing edge producing intolerable pitching moments when such flaps would be deflected.  In short, delta wings are superior only in supersonic flight; their best known design probably the Anglo-French Concorde.

Lindsay Lohan in delta skirts.

A delta skirt is characterized by its triangular shape, an allusion to the Greek letter delta (Δ); almost always, a delta skirt is fitted at the waist, flaring out towards the hem, creating the triangular silhouette.  The concept is adaptable and can be made from just about any fabric, the most popular including cotton, taffeta, silk and a variety of other synthetics.  Delta skirts tend to be shorter because to produce the triangular effect as length increases either some sort of internal structure is required or additional material needs to be used, increasing bulk and weight.  Although most associated with younger women, they have been adopted by a number sub-sets from the anime aesthetic, Goths and bohemians to hippies.