The Aeroplane Spotter, April 9th 1942

THE A E R O P LA N E SPOTTER 88 APRIL 9. 1942 r ^ : » - * r r . . 0 - . ; Hi v : > ¦ * *: ? S . i 5- I BShf ¦ < : I Q « Jl FOW C O M P A R ISO N ,— If the page is viewed from a distance of one foot the aeroplanes are shown as they would appear to an observer situated at the following distances. Left to right: Messerschmitt Me 11 Oca, 395 f t .; Junkers 7u 87b, 295 f t . ; Vickers-Armstrongs Supermarine Spitfire VB, 290 f t . ; Lockheed I 2a . 380 f t . ; Vickers-Armstrongs Wellington //, 460 f t . ; Martin Maryland / . 460 ft. HEIGHT ESTIMATION— I By j . s . b l a i r , b . s c., a m ./ .m * /,.£ . The author of this article offers suggestions whereby observers on the ground can check* within loose limits, their estimates of the height at which aeroplanes are flying. His methods will doubtless he of more interest to Spotters than to members of the Royal Observer Corps, who have their own means of ascertaining heights or checking their estimates. hut both Spotters and Observers may like to put the ideas to practical test and, having done so. comment on them. This article is the first seriout attempt in “ THE A E RO PLA N E SP O TT E R " to solve a problem that has long troubled those denied the use of special height-finding instruments in their Spotting. normally, they do not last any length of time at such a low level. (5) Timing of Sounds Occasionally, an accurate method of determining the distance (or height) of an aeroplane may be available, namely, by noting the time it takes for a sound from it to reach the observer. The difficulty is to know the precise time that the sound leaves the machine, but in certain circumstances a back-fire, or the stopping or starting of the airscrew may be visible from the ground, and if at the moment this occurs seconds are coanted, the distance may be measured (approximately 1,100 It. per second). (6) S p e e d a n d T im e Another fairly accurate method of assessing the height of an aeroplane demands a knowledge of its speed. If the time in seconds taken by the aeroplane to travel from vertically over­ head to an elevation of 45 degrees is multiplied by 1J times the speed, the height in feet will be found. This is useful only when the aeroplane is passing, or about to pass, directly over­ head. The same principle, however, may be used for finding the distance of an aeroplane, and hence, as previously men­ tioned, its height, by noting the time in seconds that it takes an aeroplane, when travelling approximately broadside on to the observer, to pass through an angle of 8i degrees. This time, multiplied by 10 times the speed, will be the distance away. Eight and a half degrees is the angle subtended by ins. at 24 ins. from the eye, or approximately the width across the knuckles of the clenched fist at arm’s length. The distance corresponding to an angle of 8J degrees is also shown in Fig. 1 (opposite). This method may also be used for an aero­ plane passing directly overhead. _ - ’ Accurate counting of seconds is essential. It should be remembered that the six pips of the B.B.C.'s time signal are spaced one second apart. Counting deliberately “ one hundred and one, one hundred and two, one hundred and three," and so on also gives reasonably accurate timing of second intervals. As regards the approximate speeds of aeroplanes the follow­ ing may be taken as a rough guide for those Hying normally at cruising speeds and not in any particular hurry:— Fighters .................. • :*... .. ... .250 m.p.h. Bombers ... ... . . . . ... 200 m.p.h. Two-motor trainers .............................. 150 m.p.h. Elementary trainers ... ... ... 100 m.p.h. The Observer must remember that the speeds required are those relative to the ground and in consequence allowance may have to be made for the speed of the wind. (7) Size of Aeroplane and Angle Subtended This is quite a reliable method, but necessitates:— (a) Accurate identification of the aeroplane. (b) A knowledge of the span or length of the aeroplane, or at any rate roughly the group to which it belongs. (c) Some form of scale to be held at a definite distance from the eye against which the apparent size of the aero­ plane can be measured. This is not so formidable as it appears at first sight: (a) should present no difficulties to a competent Spotter nor should (b) present difficulties; particularly as aeroplanes can be classed into some six main categories for size. As regards (c) the simplest arrangement is to have a scale which can be he!d at arm's length (this is, on the average, 24 ins. from the eye). The scale should be marked so that if the aeroplane is seen with one wing tip touching the left-hand side the other wing .tip will touch the scale at a point marked with the appropriate distance the aeroplane is away. Thus six scales are needed to cover the six main sizes. Often the length will be seen more easily than the span and scales both for length and span should be available. (To b: conclu ded I . A CCURATE ESTIMATION of the height of an aeroplane above the ground is one of the most difficult of the tasks of the Spotter or Observer. The Observer generally has some other means of cross-checking or verifying the correctness of his estimate, but such a check is seldom available to the Spotter and he is usually without any form of instrument which will enable him to measure the height of aeroplanes. Thus, height estimation becomes a matter of applying experience gained by long practicc, and in the early stages of training it will be little more than guesswork. Essentially, all height-judging depends on a comparison— even though sub-conscious— of the apparent size of the aero­ plane with its actual size. Except when an aeroplane is directly Overhead or comparatively low down near the horizon, one does not judge its height directly, but rather its distance. The height is subsequently found from the angle of elevation of the aeroplane. c In these circumstances the estimation of height can be made by two principal methods, namely:— (a) Where aeroplanes are at 20 degrees or more above the horizon, an estimate of the distance from the observer is made either consciously or stib-consciously and from this an estimate of the height in relation to the angle at which the aeroplane is seen. (b) When an aeroplane is less than 20 degrees or so above the horizon, errors in the estimation of the angle will make such large differences to the height, even if the estimated distance is correct,, that method (a) is not applicable. In these instances the method used should be to estimate the height above the horizon in multiples of the aeroplane’s length. The following methods of est:mation of height or distance may be found useful. All but the last are applicable to eleva­ tions greater than 20 degrees above the horizon. (1) Appearance of Small Details There are certain details common to all aircraft, such as the pilot’s head (particularly in open cockpit machines), the pitot tubes and the aerial mast. These small details will not be noticeable at distances of more than 1,000 ft. and the pilot's features are not likely to be distinguishable at more than 500 ft. (2) Comparison with Local Landmarks There may be local chimneys, church spires, etc., the heights of which are known, arid these will serve as a guide to the height of aircraft, particularly if, for example, in imagination three or four chimneys are piled one on top of the other. This method is somewhat uncertain as it involves not only the actual height, but an appreciation of angle as well. Those who have had any exj>erience of mountaineering will probably find that estimates of height based on the height of mountains or on the apparent size of well-known objects as seen from mountains may help them. <3) Cloud Heights This method forms only a very rough guide as the height of clouds varies widely. First of all, the type of cloud must be identified. The following table gives approximately the heights at which the different types of clouds are likely to found:— be Cirrus Cirro-cumulus Cirro-stratus Alto-cumulus r a t u s Cumulus (base) I * (top) • • t 25.000 to 40.000 ft. 20.000 to 30,000 ft. 7.000 to 20.000 ft. 2,000 to 4,000 ft. up tKlO.OOO ft. {or 15,000 ft. in thecase of thundtr clouds) (4) Vapour Trails Vapour trails are generally formed at and above 15,000 ft. Some have been known to form at less than 1,000 ft., but, a.4
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