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Aviation History
1911
1911 - 0079.PDF
JANUARY 28, 1911. edge be bent down as in Fig. 2, the tangent to the leading edge remaining horizontal. The arrangement now constitutes a cambered lifting surface. Yet there can be no cyclic up-currents, for the surface behaves to the air just as the flat plane would, until the air has passed the leading edge. The presence of a dipping front edge in the wings of soaring birds is no proof that it is advisable for aeroplanes, for a bird is primarily an " ornithopter," not an aeroplane, and again, for a bird to " glide" horizontally a natural upward air current is pro bably necessary. Douglas. A. A. GRIFFITH. [This question of the dipping front edge is one that causes a great deal of confusion of thought, mainly arising out of initial difficulties in the assumed premises. The fundamental principle from which the advisability of the dipping front edge is deduced is one that is primarily illustrated by Fig. 3 herewith (this and subsequent diagrams being supplementary to those submitted by our corre spondent). Fig. 3 shows a flat plane normal to an air stream. It will be observed that the stream lines in the vicinity of the edges move to one side as they approach the plane and leak over the edge in a curved path. This represents the state of affairs when the flat plane in question is falling towards the earth face on, or is otherwise experiencing a relative normal wind. Now a flat plane in flight being inclined to the direction 01 flight can either be regarded as virtually falling or in the presence of a virtual normal wind, for it is a fact that the inclined plane causes a downward discharge of air, which is equivalent in effect to an actual motion through the air. It is here that most confusion of thought arises, for in the mind's eye the velocity of flight, being so much greater than the velocity of downward discharge, stands out so pre dominantly as to obscure a proper appreciation of any phenomena that may be associated with the latter factor. And yet it is a very common explanatory phrase to say that an aeroplane is " virtually falling" all the time that it is flying. If, therefore, this statement is true the cylic disturbance in the vicinity of the edges due to the virtual fall cannot be neglected, for they will compound with the horizontal velocity, and produce a relative upward trend in the air, as is represented, for instance, in Fig. 4. The object of a plane, used as an aeroplane in flight, is to maintain a uniform downward acceleration of the air, in order to do which it must essentially begin its operation by receiving the air tangentially at its leading edge without shock, whence we have the condition illustrated in Fig. 5, which shows the dipping front edge. In order to take the matter a little more in detail let us revert once more to Fig. 3, which diagramatically illustrates the cyclic disturbance at the edges, due to a vertical fall. It is more or less immaterial as to what actual cause we attribute this particular dis turbance and we may, for instance, regard the leakage in question as being due to the expansion of air from a region of comparative positive static pressure below the plane into a region of comparative negative static pressure above the plane. We suggest this point of view because we know that many of our readers are much concerned with these differences in the static condition of the air surrounding the plane. In any case, however, the state represented in Fig. 3 is not difficult to believe as true and it can be otherwise deduced as probable from the mere fact that the recognised equation for normal pressure and velocity is quite inadequate to account for the complete interruption of the air stream over the entire area of the plane ; it is therefore necessary to suppose that part of the stream leaks over the edges without contributing its proper value on impact. If the condition represented by Fig. I is true at all, it is true for any velocity of fall however small and it must also obviously be true if the plane in question does not happen to be quite normal to the line of fall, for the fundamental conditions can obviously only be affected in degree, and not in principle, by the angle. Thus, for example, suppose the plane represented in Fig. 4 to be falling with the same speed as that represented in Fig. 3, and to have no other velocity, then the conditions represented in Fig. 3 would obtain in Fig. 4 also. If, again, the plane whilst falling has impressed upon it a velocity in a horizontal direction the fundamental conditions are still unchanged, for there is, at any rate, no prima* facie evidence to show cause for an alteration. Thus we should assume that in the immediate vicinity of the leading edge the compound velocity would give a relative upward trend to the wind, as shown in Fig. 4, itself. As a matter of fact, experiment also serves to show that the stream line does divide below the level of the leading edge, and that the cyclic disturbances are maintained approximately as shown. If the horizontal motion of the plane is accompanied by an actual fall through space the result in practice is known as a glide, but if the angle is sufficiently inclined so that the horizontal velocity infinitely prolongs the actual fall, in other words, the fall is only virtual, then the state in practice represents horizontal flight; but the relative upward trend is there all the same, and the need for a dipping front edge is as great.—ED.] " Atherium." [1037] I am sending you herewith particulars of a new white metal alloy, Atherium, which has recently been brought out, and which I think may interest some of your readers. The properties are rather remarkable. In the first place, it is lighter than aluminium. The specific gravity is 2"4 to 2*57, according to the mixture. Combined with this remarkable lightness the alloy has a tensile strength of i8'66 tons to the square inch. A test made by Mr. R. H. Harry Stanger, of Westminster, on a test piece C628 in. in diameter, showed an elastic limit of 33,712 lbs. per square inch, and an ultimate strength of 41,798 lbs. per square inch. The extension on a 2 inch measured length was 17*5 per cent., and the reduction of area was 39-l per cent. The alloy has the valuable properties of making good sound castings, and works well in rolling and turning. Clean screw threads can be cut, and it can also be soldered, forged, and welded. It does not tarnish or corrode, and withstands the action of sea water. It is also elec trically positive. The conductivity is about 55M. The sale of this alloy is in the hands of Messrs. Pritt, Bowley and Co., 46, Fenchurch Street, London, E.C. Deptford. S. P. HUTTON. Early Aeronautics. [1038] I beg respectfully to take the liberty of submitting for your consideration an extract from a work which is in my possession, and which may prove of interest to your readers. Extract: "Professor Robertson proposes to construct an aero static machine, 150 ft. in diameter, to be capable of raising 72,954 kilogs., equivalent to 149,637 lbs. weight (French). To be capable of conveying all necessaries for the support of sixty individuals, scientific characters, to be selected by the academicians, and the aerial navigations to last for some months, exploring different heights and climates, &c., in all seasons. If from accident, or wear, the machine elevated above the ocean, should fail in its functions, to be furnished with a ship that will ensure the return of the aeronauts." Extract (2). " In the year 1840 Mr. Green, the most celebrated aeronaut of modern times, who has performed several hundreds of aerial voyages, proposed making a voyage in a balloon from the American to the European Continent across the Atlantic. In order to convince the scientific public of the practicability of his propelling or directing a balloon, causing it to ascend or descend without dis charging either gas or ballast, and in a tranquil atmosphere, to move horizontally and in any direction, he commenced a series of im portant experiments at the Polytechnic Institution, London, which excited considerable attention and created a great sensation among the curious in scientific matters. " The machinery made use of by Mr. Green consisted of two propellers attached to a spindle, a ruidder, a guide line, and several appendages. "The propellers appear to have been somewhat like two sails of a windmill, which were whirled round with a rotary motion, and which were intended to produce an effect both on the horizontal progress of the balloon amd likewise in elevating and depressing it. The practicability of Mr. Green's plans appears to have been admitted by many scientific gentlemen, and although he has never yet attempted his daring excursion across the Atlantic, yet it is well known that he performed, along with Mr. Mason, in the great Nassau balloon, an aerial voyage from England across the German Sea to Welbury, in Germany, one of the most daring and extensive voyages hitherto attempted, and which was accomplished without the least danger." The reader will find an account of Mr. Green's experiments in the Polytechnic Journal for January and February, 1840, and likewise in the number of the Mirror for April, &c, 1840, Vol. 35, with an engraving of the proposed balloon. Bun bury, W. Australia. JNO. H. MURRAY. The Season for Prizes. [1030] I am writing to you about a point which I consider is of vital importance. Up till now the closing date ot all the big prizes has been on December 31st. This means that all the aviators are attempting difficult flights towards December, in treacherous winds and foggy weather. There is no reason why the closing date of these prizes should be on December 31st. The football season begins in October and ends in spring ; the school year begins in October and ends in summer. Why, then, should not the aviation year begin in October and end in September, or any other suitable months; this would allow aviators, competing for the prizes, to fly in fairly good weather and prevent such accidents as those to Cecil Grace, and to Laffont and Pola and Lieutenant de Caumont in France. r
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