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Aviation History
1917
1917 - 1383.PDF
DECEMBER 27, 1917. METEOROLOGY IN RELATION TO AERONAUTICS. By W. H. DINES, D.Sc, F.A6.S., F.R.S. Concluded from page 1346.) The Wind.—It will be well to separate the effect of thewind upon an aeroplane into two separate parts, and then to discuss the present possibilities of foretelling What wind islikely to be found at any given height. The wind is decidedly the most important element with which an airman has todeal. The first point to be considered is the gustiness, but thisis a matter on Which the meteorologist cannot give much information ; he looks rather to receive information fromthose engaged in the practical business of flying. He knows, indeed, that winds at the surface of the earth possess verydifferent characters in this respect, but he does not know to what extent the local ^situation is responsible for variationin the steadiness of the wind. On the coast winds off the sea are much steadier than winds off the shore, as Well asbeing stronger. So much is this the case that the anemo- grams from a coast station are as a rule distinguishable ata glance from anemograms from an inland station like Kew, but the better the exposure of the anemometer at an inlandstation the steadier is the wind recorded. A better exposure means in general a greater height above the ground, andsteadier winds are met with as the altitude is increased. The steadier winds from the sea and at greater heights is due tothe absence of obstacles like trees, houses, hills, &c, which break up the steady flow of air and cause eddies, and theseeddies will travel a long way down the wind, as is proved by the following curious fact observed at Southport. At South-port the anemometers are erected about 1 mile from the town, at Marchside, on the bank of the Ribble, and theexposure is an excellent one as the country is flat for miles round. The station is in charge of Mr. Baxendell, and henoticed that the trace showing the direction of the wind always changed its character as the wind shifted through acertain definite point of the compass. The cause of this was sought for in vain for some time, but at last it wasnoticed that the critical direction coincided with the bearing of the last house on the parade on the Southport front,which house was at least a mile distant. This shows the long distance to which the effect of an obstacle mayextend. It is difficult to define the terms " gustiness," because it isonly a question of how long a gust lasts Whether it should be called a gust or a squall. The numerical measure of thegustiness of the Wind is obtained from a self-recording anemo- meter by taking the difference between the maximum andminimum velocity for a given time and dividing the difference by the mean velocity for the same time. In the absence ofmarked squalls and for a wind neither increasing nor falling this is a perfectly good measure, but obviously should asquall occur in the chosen interval the ratio may reach quite a high value. With one hour as the interval andusing the pressure tube anemometer as the standard for measuring the velocity of the gusts and lulls, a ratio of £denotes a very steady and a ratio of 3/2 a very gusty wind. The Drift caused by the Wind.—Much misconception appearsto exist as to the effect of a steady wind upon the steering and course of an aeroplane, and since the matter is quitesimple it may be well to set it out, starting from first prin- ciples. In what follows by a steady wind is meant a wind in Whicheach particle of air is travelling with the same uniform velocity and in the same direction as every other particle,and in so far as the drift and course to be set are concerned, a gusty wind has exactly the same effect as a steady windof the same mean velocity. Now as soon as an aeroplane has left the ground, save in the important matter of drift, it isdjiite immaterial to it in what direction and with what velocity the wind is blowing. The aeroplane shares thevelocity of the wind, but in all other respects it is quite un- affected by it. The case is exactly the same as that of ,4man walking about on the deck of a steamer on a calm sea ; he walks about in exactly the same way, whether the steameris anchored or is moving, and if he be in the saloon, out of sight of fixed objects of reference, he cannot tell in theslightest degree in which direction he is moving, neither, save by inference from the noise of the engines, can he tell thespeed. The steamer itself is possibly being carried by a tidal current as well as by its own motive power, and thenavigating officer, so far as his own observation goes, is in the same position with regard to the effect of the tide. Butthere is this very important distinction between the navigating officer of a vessel and the pilot of an aeroplane when bothare out of sight of objects of reference. The officer knows beforehand very closely what current he may expect to findat the precise time and place in which he is, but this know- ledge is very imperfect for the pilot; moreover a tidal currentor a drift of the vessel, though wind is in general quite small in comparison with the motion of the vessel through thewater, whereas the wind which is carrying an aeroplane may have the same or even a greater speed than that of the aero-plane itself. It is this latter fact which raakes the wind, even when steady, such an important element in practical aviation.The rule by which the position is found of an aeroplane having a known velocity through the air—the velocity indi-cated after correction for density by the speedometer—and subject to a wind of known velocity and direction, is quitesimple, and is best shown by a diagram. Suppose the aeroplane to start from A and to be kept bythe rudder and compass apparently heading in the direction AB. Let AB be the distance travelled through the airin a given time, say 1 hour. Then if it were a dead calm, after 1 hour the aeroplane would be at B. Suppose a pilotballoon sent up from A with its lift adjusted to float at the same height as the aeroplane, and suppose that 1 hour laterit has moved to C. Complete the parallelogram ABDC so that D is the corner opposite A, and then D will be the positionof the aeroplane after 1 hour. The practical problem is, starting from a point A with theintention of flying to a point F to know what course to follow. It is necessary for the purpose to know the speed of themachine and the speed and direction of the wind. The geo- metrical solution is as follows :— From a map set out the direction AF and draw AC from Ain the direction of the Wind and of such a length that AC represents the velocity of the Wind in any convenient scale.With centre C and a radius representing in the same scale the speed of the machine, describe a circle cutting AF in D.Then CD or a parallel line AB gives the course to be followed, and AF —• AD will give the number of time units that theflight will take. In general, the circle will cut AF in two points, the onenearest F should be taken, also it may not cut it at all, in which case the wind is too strong to render the proposedflight possible. Since AC may make any angle with AF, in the majority of cases AD will be less than CD, and, therefore, onthe whole, the wind increases the time of flight, and if the return journey is to be made under the same conditions ofwind the whole time is inevitably increased. Of course, in most instances the course from one place toanother is followed by a series of landmarks, but cases arise when it is not possible to see the landmarks, and, therefore,it is of importance to know the strength and direction of the wind. This is a purely meteorological problem, and if it canbe solved, and if also the difficulty about steering by compass can be overcome, two ifs in both of Which there is much virtue,then it will be possible to take an aeroplane from one place to another with the same certainty as there is in taking avessel from one port to another by dead reckoning alone. Estimate of the Wind.—There is no difficulty in determiningthe velocity and direction of the wind at the surface, but it is a knowledge of the wind at greater heights that the pilotof an aeroplane requires. The greater the height the more uncertain must any estimate become, but even at a fewthousand feet the wind in most cases will differ considerably from the surface wind. The question, therefore, is to whatextent can we infer the conditions that prevail at, say, 3,000, 6,000 and 9,000 ft. from surface conditions that we canascertain. The means of observing the upper winds are observationsof clouds, kites, and pilot balloons, but in all these cases the observation is confined to the space below the lowest sheetof clouds and above that sheet we have no power of observa- tion. It is, however, just when the ground is hidden by a lowcloud sheet that the knowledge is most required. Fortunately, there is no reason to suppose that the connection between thesurface and the upper wind is different when it is cloudy and when it is clear ; in my own experience of kite flying I donot remember any break in continuity at the surface of a cloud layer rather than at any other level. If a person stands facing the Wind and there are low cloudsin sight, he will generally find that the clouds are coming somewhat from his right, and he will also infer, and rightly so,that the wind is stronger at the cloud level than at the surface. How much stronger will depend upon the exposure of thestation, but as a rough rule it may be said that at an inland station at 2,000 ft. the wind will have doubled in velocity, 1369
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