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Aviation History
1956
1956 - 1037.PDF
FLIGHT, 3 August 1956 183 • Hunter contrails, showing dark centre in developed stream. IDENTIFICATION BY CONTRAILS Possibilities Investigated by the Aircraft :'':'^-/ :'k. Recognition Society IN a broadcast interview following the All-England AircraftRecognition Contest in January 1954, Mr. Nigel Price, amember of the winning team, referred to the value of contrail study as an aid to the correct identification of high-flying aircraft.Among aircraft recognition enthusiasts his remarks aroused a good deal of interest and some controversy, and in the 2\ years thathave elapsed since the broadcast much useful investigation has been undertaken by members of the Aircraft Recognition Societywith the object of establishing the limits within which information gathered from contrails could be considered seriously as an aidto recognition, and, if possible, some accurate system of classifica- tion which could act as a guide to further study of the subject.For a variety of reasons, of which the most obvious and the most important is the absolute necessity of establishing beyond allpossibility of doubt by visual or other means the identity of the aircraft making the trail, reports admissible as evidence are hardto come by, and research by the Society will continue over a long period. However, a stage has been reached where an interimreport is possible, and Mr. L. Andenon, a past chairman of the Society, recently published some conclusions which are of con-siderable interest. His report begins with a note on the way in which contrails areusually formed: the exhaust gases from an aero engine contain a considerable amount of water vapour. The amount varies, butmay be taken to average about 1 \ lb for every 1 lb of fuel consumed. Free air also contains water vapour, but at the very low tempera- tures at high altitudes the amount which can be held as invisiblevapour is extremely small. Any in excess of this amount condenses out as cloud. The addition of water vapour from the exhaust gasesmay result in the critical point being reached and passed, the result being the formation of a contrail. [The density of the trailis a function of the weight of fuel burned per mile, and the use of afterburning greatly increases this—a point that was readily appar-ent during the record runs of the Fairey F.D.2—Ed.] The amount of water the air can hold depends upon the tem-perature: the lower the temperature, the smaller the quantity. The heights at which contrails can form will therefore vary inaccordance with the prevailing weather. In cold weather they will form at lower altitudes than when conditions are warm. Fig. 1shows the approximate variation of heights under various weather conditions. As an aircraft nears the height at which a persistent contrail willbe made, it is usual for a short or faint one to appear first, and this gradually develops into the persistent type as the aircraft climbsinto colder air. The process continues until the tropopause is reached, i.e., the point where the temperature ceases to decreasewith height. At this level, where the aircraft enters the stratosphere, the contrail usually begins to decrease in length, until eventuallyit is of the short, re-absorbed type once again.* Factors Influencing Apparent Shape of Contrails. It shouldbe remembered that the cross-section of the contrail of a single- engined aircraft is symmetrical, whereas that of a multi-enginedtype is not. Because of this, the attitude which a multi-engined aircraft presents to the observer modifies the shape of the contrail-head. For instance, a Canberra, which makes a contrail with a twin-stream head, would appear to be producing a single stream ifit were some distance away and flying at right angles to the observer's line of sight. The possibility of recognizing an aircraft 50.000 tt 40.000 ft 30.000 U 20,000 ft 10,000 ft COLD WEATHER HOT WEATHER - Fig. 1. Variation of trail height with weather. " by its contrail is limited to some extent by this factor, and themachine must be in a suitable position, relatively to the observer, before a valid opinion can be formed. Besides the attitude, there are a number of other factors whichmay mislead. Because of the variation of trail-forming height with temperature it is possible for trails to be as low as 12,000ft, or ashigh as 40,000ft, or even more. When by reason of haze the aircraft itself cannot be seen no judgment of scale is possible; and in thesecircumstances the distance apart of the head-streams in a multi- engined contrail may easily mislead. For example, the Tornado,when hidden by haze, and making a contrail at 14,000ft, is quite likely to be mistaken for an altogether larger aircraft. Anotherconsideration is the length of the contrail: it may persist, and *Fuller details are given in Air Ministry Publication M.O. 480 (A.P.3315) "Condensation Trails," price 9d, H.M. Stationery Office. (Left) Hunter—the more pointed—and Canberra trails. The latter is at a greater distance and presenting a side view, hence the illusion of a single-engined aircraft. (Right) Valiant trail, illustrating the characteristic division of the streams.
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