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Aviation History
1910
1910 - 0051.PDF
JANUARY 15, 1910. suddenly flashes into his brain, reminding him of how he made economy the first consideration in the construction of the machine to which he has entrusted his life. Pilcher, the first among British pioneers of gliding, was killed through the fracture of a main spar. We do not wish to imply by the above that our correspondent is likely to build an unsafe machine ; on the contrary, he distinctly states that it must be " entirely practical." On the other hand, we have sometimes felt that those who write to us about the construction of full-size gliders and flying machines are just a little apt to overlook the above-mentioned result of success, and we are taking this opportunity to remind all whom it may concern. We do not know anyone who is at present making a speciality of supplying detailed drawings of full-sized machines, but possibly some of our readers might be willing to dispose of their own.—F.D.] PROPELLERS. [294] In answer to Mr. Challenger, December 25th, page 838, drawing attention to the error in Maxim's " Artificial and Natural blight," the following calculation with respect to the stationary thrust is more scientific and may prove instructive :— If A is the disc area, V the velocity in feet per sec, -08 the •08 AV* weight of a cubic foot of air, the thrust as , the h.p. •08 A Vs 2 x 32 x 550 In the example the h. p. is given , , -08 x 500 x Vs • '• 363-63 =s 2 . J J J 2x32x550 363-63- V = 68 ft. J/363'63x2x32x550 V -08x550 This means, in a theoretically perfect screw throwing air away rom its full disc area, the maximum possible velocity is 68 ft. per sec. The greatest thrust possible is , •08 x 500 x 68 x 68 "32 5,780 lbs. The actual thrust obtained was = 2,160 lbs. The thrust efficiency : is, therefore, ^'i6.? s 0-38. If the velocity of air ejected by the : ' 5,780 * screw had been measured from the centre to the extremity, it would have been found to vary considerably, being very much less than the pitch multiplied by the number of turns per second, towards the centre, and approaching this figure at about two-thirds of the radius. From this it would always seem best to always consider the full disc area of the screw, but when the pitch multiplied by number of revolutions per second is very much in excess of the theoretical velocity, V, the stationary thrust efficiency is likely to be low. The maximum stationary thrust efficiency obtainable is about 65 per cent. When the screw under consideration is moving forward at 40 miles an hour it becomes more efficient, as it is oapable of acceleration from 60 ft. per second (40 miles per hour) to 100 ft. per second (pitch multiplied by number of turns), which is a very different thing to accelerating the air from o to 100, which the screw tries to do when stationary. ^London, W.C. J. R. PORTER, A.M.I.C.E. 1/liOHT) WRIGHT GLIDER LAUNCHING APPARATUS. [295] REFERRING to the short article in to-day's FLIGHT about the launching apparatus for our Wright glider, I beg to draw your notice to several modifications of interest to other experimenters. In the first place, our second and most successful rail was only 40 ft. long, giving 33 ft. run. The rail was in two sections of 20 ft. each, consisting of 6 in. by ij in. timber, supported every 10 ft. on feet which were 6 ins. high. The feet were set across the rail and f-in. angle iron stakes, of |_-section, held the whole together and pro jected into the ground some 6 ins. These were necessary to take the pull of the weight. The glider ran on 1 in. angle iron of X"sect'on- The rail was usually on a gradient of. 1 in 7, and about 10 ft. away from the derrick, which appears to be about correct in the article. The wire rope was f in. in circumference, and ran over 2 in. pulleys (steel) and was attached to the glider by a ring on to a hook on the front wheel bearer. The glider was held back by a spring catch on the front boom. The weights were 4 discs, 100 lbs. apiece, of cast iron, with a hole in the centre. The wire rope was reeved through a three-fold purchase in the derrick and led out under the rail, round a pulley at the end of the rail, and back to the machine. The weights were hoisted by a four-fold manila fall attached by another short rope to the wire behind the ring by means of a slip-knot. The other end of the fall was fixed to the foot 0/ the derrick. So two men walking down the hill could easily raise the weights, and the ring on the wire rope could be hooked on to the glider without difficulty. If the wind was blowing fairly strongly up the hill, only two of the weights were necessary. It was just possible to get the machine into the air in a calm, using the full run of 33 ft. and four weights. This gliding in a calm, which as a matter of fact we only got on one or two days owing to the bad weather, we found very pleasant and instructive. The arrangements were, no doubt, rather elaborate, but were compensated for by getting many glides of over 400 yards. Camber. ALEC OGILVIE. HOLLOW SPARS. [296] I have read with interest the letter of Mr. Linton Hope in the issue of FLIGHT of the 1st inst. regarding hollow silver-spruce spars. Will Mr. Linton Hope or any of your readers oblige me by giving the address where such hollow spars are made. London, E.C. B. II. BALASSANIAN. "FAKED" FLIGHT PHOTOGRAPHS. [297] I noticed in your issue of Jan. 1st a " faked " photograph ("flying in the clouds") as the frontispiece. I beg to enclose two genuine photographs, taken by myself at Brooklands, of Paulhan flying. I thought, possibly, these might be of some use to you. Maida Vale. F. A. HALFORD. [These interesting photographs sent by our correspondent are a striking object-lesson of the size of a flyer as really seen by the camera at fair heights.—ED.] Two Genuine, not " faked • photos of Paulhan flying high at Brooklands (see Mrs. Halford's letter). 47
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