Cruciform

Cruciform (pronounced kroo-suh-fawrm)

(1) In geometry, a geometric curve, shaped like a cross, which has four similar branches asymptotic to two mutually perpendicular pairs of lines (equation: x ² y ² – a ² x ² – a ² y ² = 0, where x = y = ± a are the four lines).

(2) In engineering or design, being in the shape of a cross; cross-shaped.

(3) An emblem or escutcheon in the shape of a cross.

(4) In aeronautical engineering, a type of tail structure.

(5) In genetics, a cross shape in DNA (known also as the Holliday junction).

1814: From the Modern Latin cruciformis (1655-1665), the construct being from crux (genitive crucis) (stake, cross) + forma (form, shape), both Latin words of unknown origin; the he English form was first documented in 1814.  Early etymologists suggested crucis might have links with the Irish cruach (heap, hill), the Gaulish krouka (summit), the Old Norse hryggr (backbone) or the Old English hrycg (back) but modern scholars, although offering the odd speculation, concede only it may have been borrowed from “somewhere” an observation which, while probably true, isn't a great deal of help.  Form first entered Old English around the turn of the thirteenth century as forme & fourme (semblance, image, likeness), a direct borrowing from the Old French forme & fourme (physical form, appearance; pleasing looks; shape, image; way, manner), from the Latin forma (form, contour, figure, shape; appearance, looks; a fine form, beauty; an outline, a model, pattern, design; sort, kind condition), again a word of unknown origin, the most accepted theory being it may be from or cognate with the Ancient Greek morphe (form, beauty, outward appearance).  Cruciform is a noun & adjective, cruciformity is a noun and cruciformly an adverb; the noun plural is cruciforms.

Empennages: the cruciform & the T-Tail

On airframes, a cruciform tail is an empennage (the tail assembly, almost always at the extreme rear of an aircraft, provides directional stability while in flight, as the feathers on an arrow generate, the word derived from the French empenner (to feather an arrow)), a configuration which, when viewed in direct frontal or rearward aspect, assume a cruciform (lower case †) shape.  The accepted practice is for the horizontal stabilizer to intersect the vertical tail close to the middle, well above the fuselage.

Cruciform tail: Rockwell B-1 Lancer

The cruciform tail is a compromise.  While not offering all the aerodynamic advantages of the T-Tail (where the horizontal stabilizer is mounted atop the tail (upper case T) and thus almost completely removed from the wake of the engines), it doesn’t demand the additional structural strengthening (and thus weight) or suffer the same vulnerability to metal fatigue.

T-Tails: Lockheed C-141 Starlifter (produced 1963-1968), Wright-Patterson USAF (US Air Force) Base, Dayton Ohio (left), Vickers VC10 (produced 1962-1970) in BOAC livery, BOAC promotional photograph, 1964 (centre) and Lockheed C-5M Galaxys (produced (all versions) 1968-1989) at USAF Base, Dover, Delaware (right).

The T-Tail is used in aviation for a variety of reasons.  On freighters, the advantage is the horizontal stabilizers are moved away from the rear of the fuselage, meaning the space-consuming mechanical and hydraulic assemblies don't intrude on the rear-loading area, permitting both a larger door aperture and an uninterrupted load-path.  One unusual use was the Vickers VC10, a design distinguished by the four engines being mounted in the tail section, something which precluded the conventional placement of horizontal stabilizers.  The VC10 was a design cul-de-sac but as the K.3 aerial refueling tanker, the airframe remained in service with the Royal Air Force (RAF) until 2013.

The cruciform (left) and the T-Tail (right) can be borrowed from aeronautical design and applied to hairstyles: Lindsay Lohan demonstrates.