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Aviation History
1956
1956 - 1038.PDF
184 FLIGHT, 3 August 1956 An artist's impression of the four broad types of trail discussed below. (Below) B-47 trail and (lower pic- ture) typical decay into curved eddies or closed loops. IDENTIFICATION BY CONTRAILS . . . stretch from horizon to horizon; or it may be quickly re-absorbed, giving the effect simply of a short plume. This may prevent the contrail from developing sufficiently to show its characteristic form. Variations in the distance at which the contrail first appears behind the engine can also take place. Where the streams of a multi-engine contrail are divergent, and the aircraft is obscured by haze, delay in forming will lead to the conclusion that the engines are more widely spaced than they actually are. Classification of Contrail Types. When it is considered that various combinations of the above factors are not only possible, but indeed probable, it is apparent that the question of recognition by contrails is very complex, and unlikely to have a simple solution. Nevertheless, careful analysis reveals certain classifiable dissimi- larities which seem to suggest grouping of contrail types into four easily recognizable categories as a basis for further study: — (A) Single-engined aircraft—Hunter, Seahawk, etc.(B) Twin-engined aircraft with the engines well spaced—Meteor, Tornado, etc. (the four-engined Tornado has the trail-effect of a twin).(C) Multi-engined aircraft with the engines disposed across the span —B-47, Shackleton, etc. (D) Aircraft with four or more engines dose to the fuselage—Comet,Victor, etc. (Note that in these groupings engines paired in pods are considered asone engine, e.g., the inboard pods of the B-47.) A fifth group would appear possible, comprising those aircraft which have two engines mounted very close to the fuselage. Coming within this group would presumably be such types as the CF-100 and the Badger. However, it has not so far been possible to make sufficient observations to report with certainty on these aircraft. Within each of the above classifications many small variations have been noted, giving rise to the hope that with further patient research much progress might be made towards the identification of individual types. In the remainder of this report each category is separately considered and some pointers are given to the direc- tion in which such investigations might profitably proceed. Group A—Single-engined Aircraft. Thin, single-stream contrail-head. The speed of the aircraft appears to influence the shape of the head. The greater the speed, the more pointed the head. This isparticularly noticeable with the Hunter, the head being drawn out to a very fine point. A further feature of the Hunter contrail is a narrow,dark band, which forms along the centre of the trail, starting where the taper of the head reaches its final expansion. The dark band continuesto run along the tail of the contrail and is sometimes apparent when the contrail starts to decay. It is remarkable that a single-enginedaircraft should produce such an effect. No other variations have been noted in this group. Group B—Twin-engined Aircraft with Engines Spaced Well Apart.A twin-stream contrail head. The two streams when first leaving the aircraft are slightly divergent. The angle of divergence and the distancefor which they continue to diverge varies for different aircraft. The %twin stream, after diverging, runs parallel for some distance, and thenusually merges into a single stream. The distance behind the aircraft at which this occurs varies for different aircraft. Three types have beennoted: Canberra, Tornado and Meteor. Taking divergence of stream first, the aircraft with the greatest angleis the Canberra, followed by the Meteor and Tornado, in that order. The distance for which the streams continue to diverge comes in thefollowing order: Tornado, Meteor, Canberra. Nevertheless the two streams of the Meteor tend to unite quite a short distance behind theaircraft, whereas those of the Tornado remain separated for a longer distance, and those of the Canberra sometimes do not join up at all butfrequently form a narrowly spaced twin contrail which often breaks up into eddies similar to those formed by the contrail of the B-47 whenit is decaying (see below). Group C—Multi-engined Aircraft with Engines Disposed along theWing. A four- six- or eight-stream contrail head. Separate streams come from each engine, indicating the number of engines. The patternformed by the beginning of the streams conforms to the wing sweep as a general rule, and reveals if the wing is well swept, or straight.Some streams c*me out straight, some converge in pairs, and the tail can form into various shapes. Three types have been observed: Lincoln, B-36 and B-47 Stratojet:— Lincoln: Four parallel streams starting in a straight line. The streamsare almost equally spaced except for a slightly wider space between the . two centre ones. At some distance behind the head the four streamsmerge into a rough, broad band. B-36: Two groups of three parallel streams, with a space between thegroups. The streams in each group come together well behind the air- craft, to form two parallel, widely spaced streams, which eventuallymerge into a broad strip. If the wing-tip jet engines are in use, the groups consist of four streams each, instead of three. The streams ofthe jet engines diverge slightly from the outer piston engine streams, but eventually join the streams in each group to form the two parallel,widely spaced streams. The sight is very spectacular, giving the impres- sion that the aircraft is throwing out a tremendous amount of cloud. B-47: The head has two pairs of streams, the inner streams of eachpair starting to form slightly ahead of the outer ones, i.e., conforming to the sweep-back of the wings. The streams in each pair quickly joinup, to form two parallel streams. Sometimes these break up into curved eddies. The way these eddies form is interesting. The twincontrail breaks up into short sections and the ends of the opposite streams of the sections curve inwards and join together, to form shapes similar ;to an hour-glass. They sometimes break through at the neck, to form rough circles. The eddies become rapidly reabsorbed, and disappear. .-;The B-47 contrail does not always decay in this manner. It may become " persistent, and spread out by the action of the wind into a rough cloud "strip, in which case the twin contrail becomes obliterated. -; Group D—Aircraft with Four or More Engines Close to Fuselage. :;.The width of the single stream indicates the over-all distance across i the engines. If examined closely with binoculars the first beginning "of the stream may be seen to have a small vee space, with the point of the vee facing backward. This gives the effect of the streams converg-ing very rapidly to form one broad stream. In the case of the Valiant, after the short, broad head the contrail divides into two parallel streams,which stay apart for quite a long distance but finally merge together as the stream becomes diffused. In a turn, the two streams are thrownapart and do not join up. The stream on the inner side of the turn looks much denser than the one on the outside. It will be seen from the foregoing brief remarks that thorough and patient analysis can bring to light a great many useful pointers out of which a really practical system of aircraft recognition by r contrail may develop. It is hoped, with the help of observers all f over the country, to build up a body of information.
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