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Aviation History
1919
1919 - 0602.PDF
air navigator's aid. If he uses a horizontal bearing plate having a transparent centre and lays across it a rod pivoted at one end, as shown in the diagram in Fig. 4, so that its length AB is parallel to the direction of the drift of the ground, AC, the line of symmetry being parallel to the fore and aft lines of the machine, and if he then draws a pencil line, AB, Fig, 6. on the bearing plate (previously oriented correctly) and then repeats this operation on a number of courses, he will obtain a wind star, and the intersection point, known as the wind point, will give him the velocity and direction of the wind. Thus, in Fig. 5 three such courses are shown, and all, it will be observed, intersect at the wind point; RC is then the velocity of the wind to scale, and the direction of RC relative to the bearing plate scale gives the direction from which the wind is blowing. The point R can, of course, be got by a cut of two lines only, but for accuracy such change of course should not be less than say 50 deg. ;. this can conveniently be done by turning first 25 deg. one way. and then the other and for equal times, since this does not interfere appreciably with the course desired to be made good. Of course, aircraft on closed patrols, for war purposes, change course several times on each patrol and a wind determination or check can be made at each " corner." On a long straight flight the position of the wind point along the line, AB, in Fig. 4 can, if desired, be obtained by timing over some object on the ground and so determining the length of PA, and hence the position of the wind point. Equally, once the wind point is obtained, the length PA will give the ground speed and the bar, AB, can be graduated directly in m.p.h. The procedure for setting AB to the drift angle depends on the height. At low heights it suffices to view the ground vertically below, but at considerable heights it is much more accurate to observe the angle which an object passed over Fig. 7. some minutes previously bears to the fore and aft line of the machine (a tail bearing). When over the sea a flare may be dropped to afford the necessary fixed point when nothing else is available. So much for the actual use of the method ; the reason why the drift lines in Fig. 5 all pass through the wind point in this convenient manner may seem to need explanation to those who are not prepared to take such construction for granted. Thus, in Fig. 6, if an aeroplane head due north along CA, and the observed drift line be AB, we know the closing side of the vector diagram will represent the wind, and that it must start somewhere along the line, AB, and must end at C. In short, AB is what is called a " position line " for the wind point. Now head the craft on a new bearing, say 75 deg. E. of N., along CD, and let DE be the observed drift line. Then we get a new vector diagram, and a new position line, DE, for the wind point; these two intersect at P, and this m ust, therefore, be the wind point, and PC the velocity and direction of the wind to scale (the scale being the same one as that by which CA and CD represent the air speed). This is for two courses only, but obviously the drift line for any third, fourth, or additional course must equally pass through the point P. Probably the obtaining of a wind star in this way by flying on several courses in succession affords as accurate a way as can be devised for the ascertaining of the wind elements at any desired altitude and time. More over, in the nature of the case, meteorological records must always be received some little time—not infrequently in practice, some hours—after the moment at which they were measured, whereas by this method the instantaneous values can be obtained and can be checked as often as desired. It should also be noted that AP and DP, in Fig. 6, will measure the ground speeds along each of these courses. Hence, once the wind point is found and the instrument set accordingly figures can be given at once for the ground speed along any contemplated course and the probable duration of flight to cover any desired mileage over the ground. Sometimes—especially in an ocean flight—a suitable object for drift observations will be seen on the earth's surface, but it may not be so placed as to be in the aircraft's path, or it may not be noticed until it is too late to alter course. For such cases the relative path—see Fig. 7—instead of being AB may prove to be CD. How is one then to determine the course being made good ? Perhaps the best method is to take times and bearings of the object as it passes through the points E. F and* G, such that the time from E to F is equal to the time from F to G. Then if the angles a and $ are small, i.e., not more than 15 deg., it is easy to find CD, the course-made-good, by marking off AE proportional to the angle /8 and AG pro portional to a and then to join FG. Thus, if a be 15J degs. Fig. 8. and 0 say 13 degs., we mark off AE at a length of 1.3 in. and AG at a length of 1.55 in. ; this gives us the points E and G. Draw a line through these points and we have the .bearing of the course-made-good ; its inclination to the course being steered will give the drift angle. With airships the ability to hover can be made use of. If an airship heads up wind and reduces speed until'some object below appears stationary, the speed of the airship relative to the air must be equal and opposite to the wind. Hence, the velocity of the wind can be read on the air speed indicator and its direction noted by reading the course being steered. Whether this method was actually used by the raiding Zeppelin airships I do not know, but it appears that use was made by them of an alternative and less attractive method. This alternative plan was to steer up wind and suddenly to put the ship on a wide circular uniform turn ; on com pleting 360 degs. change of course the ship would, if the wind be zero, be back over the starting point of the turn—in the other event it would be in the rear of this point by a distance equal to the space covered by the wind in the time of the turn. The additional time taken to recover this lost ground at a known air speed, is related to the total of the two times in the same proportion as the wind velocity is to the air speed. Thus, if T! and Tc be the two times and V the air speed, then wind velocity = TiV/(Tj + T2). The Prediction of Wind Air navigation requires two things of the meteorologists, first that they shall provide accurate information as to the average weather conditions along any specified air route, and secondly that they shall provide a forecast of the probable winds in the upper air during the ensuing two or three days. 6*2
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