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Aviation History
1910
1910 - 0063.PDF
JANUARY a2, 191a experiment that a well-designed aeroeurve will lift more weight per square foot at a given velocity and angle of inclination than can be lifted according to theory. In Fig. 5, taking the under side of the cutting edge as being parallel to the direction of flight, and the trailing edge as making an angle oc with the horizontal, the air is deflected downwards with a velocity of V sine «, where V = the forward velocity in feet per second. The weight of air dealt with = V x breadth of plane x / x •weight of air per cubic foot, and the momentum imparted downwards to the air breadth of plane x L x weight of air per cubic foot x V2 sine <x ~ . g this being the vertical force exerted by the wings on the air, that is, the force tending to lift the planes. Taking a speed of about 40 miles per hour, and the ratio — = 10, it will be found that the theoretical lift is only about '87 lb. per square foot, whereas it has been proved from experiments carried •our by the authors that a well-designed aerocurve under these condi tions will lit nearly 2^ lbs. per square foot. The question of drift to lift, in the design of a plane, should be carefully considered, as the drift of the plane determines the horse power required. In the case of a flat surface, or plane, the ratio drift to lift varies as the sine to the cosine of the angle of incidence, if the pressure is normal to the chord of the plane. In the case of an aerocurve, or arched plane, it has been found that the actual drift is slightly less than that obtained by the above method, and it can only be assumed that with a curved surface the pressure must act in a direction slightly forward of the line normal to the chord. This theory has been upheld by many experimenters, and the authors themselves have found, by their experiments, that with Plane A (Fig. 6) the drift obtained was only slightly in excess of the theoretical drift, and, making allowance for skin friction and head resistance, the actual drift was slightly less than that calculated ; and with Plane C the actual drift was somewhat larger than the theoretical drift; but in practice it is advisable to take the calculated drift, and allow for head resistance and skin frictional losses, when deciding upon the horse power required. The design of such an aerocurve may be obtained in the following manner : The underside of the front edge is given a negative angle of about io° for about an eighth of the length of the section, after which the underside is given a positive angle, gradually increasing to the trailing edge to about 6°, this angle varying slightly with the length of section. The greatest depth of camber should be at about one-third of the length of section from the front edge, and the total depth measured from the top surface to the chord at this point should not be more than one twelfth of the length of section, as planes with a greater arch are liable to be very unstable at small angles of incidence owing to the fact that a large portion of the upper surface is exposed to wind pressure, thus causing a reversal of the centre of pressure. Having arrived at the correct shape of the planes, the amount or lifting surface required to lift a machine of given weight will now fee considered. In finding the amount of area required, it is necessary also to consider the length of cutting edge. It has been found by experi ment that the aspect ratio should not be less than 5:1. By " aspect ratio" is meant the ratio of length to breadth of plane. For an IjliGHT) aspect ratio of from 5 : I to 8: 1, and with reasonable angles of incidence, say, from 2J0 to 15*, the following empirical formula, p _ Vs x tan oc , L • ~6T~~ where P = pressure in lbs. per square foot, V = velocity in miles per hour, has been found by the authors to approximate very closely the results obtained from their own and others' experiments, and may be safely used with well-designed aerocurves under normal con ditions. In Fig. 6 the results obtained from various forms of planes are plotted, and it will be noticed that the points obtained from plane A vary very slightly from the formula given. The planes used in the tests are shown at A, B, C. The aspect ratio in all cases was 5:1. 7 li ** $4 S ^ 1 *^ % a . D — 1 r 1 ,. r •— ,;/ '<' / 7 / 4\ * /' ,'' I — ;< #"<,U The experiments were conducted as follows:—The plane was placed on a carefully balanced arm, which was pivoted on a swinging carriage, so arranged that it> was possible to take the necessary readings with great accuracy. The apparatus was mounted on the front of a motor car, the wind velocity being recorded by means of a flat disc, the constant '0036 being used ; careful deduc tions were made for all frictional resistances. Curve D has been plotted from figures obtained from published data. (To be concluded.) BOOH REVIEWS. Comment l'Oiseau Vole* Comment l'Homme Volera. THIS work by Wilhelm Kress has been translated into French by R. Chevreau, and deals in a theoretical manner with various aspects of the problem oi flight, starting at the fundamental point of considering the action of the wind on bodies exposed to its influence. A certain amount of space is devoted to the subject of soaring flight in birds, and there is a chapter on automatic stability. Some chapters are given over to early experi mental work, but the book does not deal with modern machines. (Paris: L. Vivien. 3 fr. 50 c.) Etudes Experimentales sur les Zoopteres. IN the introduction Dr. Paul Amans explains that the zooptere is a blade having a form based on the geometri cal design of natural wings, and the book is given up to a discussion and investigation of the salient points which ought to be taken into consideration by anyone anxious to produce artificial wings or surfaces having their essential characteristics. The author's study of the subject was evidently of a most painstaking character, and should be of considerable interest to those who wish to start their designs from the standpoint of nature. (Paris : L. Vivien, 2 fr.) 50
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