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Aviation History
1912
1912 - 0902.PDF
Having verified the above three laws (see formulae 2, 3 and 4) we *ee that by carrying out one experiment on any cable of convenient section and length, the constants c and i may be found ; and sub stituting these values in the above formula; the maximum and safe number of revolutions, the maximum and safe torque obtainable, and also the approximate energy over the whole range or up to the safe working limit can be determined for any size of cable of the same rubber, under similar conditions of lubrication, stretch, &c. Series C.—To ascertain the effect of lubrication on a certain skein of rubber. In the general table arc given the results obtained in tests of six different unUibricated skeins of rubber, and also those obtained for similar skeins when lubricated. These tests showing the importance of lubrication, a further investigation was carried out to ascertain more definitely its effect. Although the section chosen was small (four strands of i',;-in. sq.) the results for this small section may, in view of the similarity of the graphs of the full tests in Series A, be taken as quite reliable. The first skein was tested dry, the second was lubricated with water, and all the others with solutions of white powdered Castile soap in water, excepting No. 9, in which the lubricant used was ordinary green soft soap, obtainable at any chemist at about l^d. per oz. The ratio of the energy stored in any lubricated skein to that stored in the dry skein was found in each case—-see Table C. Reference to this table will show that when the Castile soap lubricant was of 10 per cent, strength, the energy stored was double, and when 66 per cent, the energy was treble that stored in the dry skein. A further result noticed on examination of the fractured cable was, that whereas the strands of the unlubricated cable were badly stuck together, and scarred and cut in several places, those of the lubricated cable showed not the slightest tendency to stick together, and there were no subsidiary cuts whatever. Series D.—An investigation as to the effect of initial stretch on a given skein of rubber. It has been frequently stated in the correspondence columns of FLIGHT that it is desirable to give the rubber cable considerable initial stretch and so, in order to ascertain what is the effect of this stretch, seven partial and two full tests were carried out, the initial stretch varying from o to too per cent. The maximum values for the revolutions, torque and end pull were observed, and, together with the energy stored, entered in Table D. These experiments revealed the surprising fact that the only result of an initial stretch is to increase the average end pull—the values at fracture for the revolutions per inch of original length, torque and energy stored not changing to any material extent. Therefore any initial stretch beyond the 5 per cent, or 10 per cent, allowed for the permanent strain of the rubber is inadvisable, since such stretch involves the use of a longer motor and consequently increases the weight of the model. Series E.—To ascertain the deterioration due to continued use of a twisted rubber motor. "-. . First a full test on a skein consisting of twelve strands of A-in. square cord was conducted in order to ascertain the constants, and then the following two experiments were made. i In this, a cable similar to the above was wound up almost to the breaking-point; and then unwound and rewound up to fracture, a full test being conducted on each wind. This experiment showed that winding the rubber almost to fracture permanently injures it, the energy stored in the range R = o to R = 240 (safe working limit) being reduced to 90 per cent, on the second wind. ii. In this test a similar cable was wound up to 240 revolutions (about three-fourths of the maximum), and then unwound, and this repeated twenty times, the twentieth rewind being carried to fracture, and full readings being taken on the first, tenth, and last winding. The results of this" experiment showed that the energy stored over the above range is reduced on the tenth rewind to 76 per cent., and on the twentieth rewind to 71 per cent, of its original amount. From this it may be assumed that after a fair amount of use the energy would drop to about two-thirds of that stored in the first wind, and this result is confirmed by our own experience with models. We have further noticed that if the rubber has, say, a week's rest, the energy stored rises to about four-fifths of its original value, but such recovery is only temporary and disappears after the first wind. The following notes, although outside the original scope of the experiments, are yet of considerable practical utility to model aero experimenters. Note on End Pull.—The question of end pull is one that has considerable effect on the design of model aeroplanes, owing to the fact that the main strut or struts must be made strong enough to withstand it. With regard to this question the results obtained were rather disappointing, but as a rough guide it may be said that other things being equal the maximum end pull varies as the area of cross section (for actual valves, see tables). The average end pull seems to vary with the amount of stretch, being doubled by stretching to twice the original length. Where the amount of initial stretch is small, the end pull at the safe working limit is very nearly two-thirds of the value at fracture. Note on the Various Sections of Rubber.—The whole series of tests showed that there is not much to choose between the various sections of rubber provided those sections are rectangular. Rubber of circular section is, owing to its process of manufacture, unsuitable as a motive power for aero models. Cables composed of a single strand are also undesirable as, although when dry they fracture in the regular manner, when lubricated they generally cut at the knot tying the loop at either end before final values can be reached. This remark applies also to cables containing an odd number of strands. ® ® ® ® THE KITE AND MODEL AEROPLANE ASSOCIATION. OFFICIAL NOTICES. Hand-launched Off ground ... Hydro, off water . Single-tractor screw, hand-launched 477 yards. 89 sees. 84 yards. 30 sees. 25 sees. S4 yards. II SCC: British Mod«l Records, r Distance ... A. E. Woollard ' \ Duration ... A. F. Houlberg (Distance „, F. W. Jannaway . ' I Duration ... G. Rowlands- Duration ... G. P. Bragg-Smith . ,f Distance ... H.R.Weston . ( Duration ... F. W. Jannaway Competition, held on September 28th, on the 100-Acre Field, Greenford, for single-tractor screw models, duration and stability, rising off ground. Prizes : 1st, George 111 drinking-cup (presented by Col. Fullerton); 2nd, silver medal of the Association (also a Mann monoplane, presented by Messrs. Mann and Grimmer) ; 3rd, bronze medal of the Association. The results were as follows:— t. 1. Dollittlc, 8S marks; a. H. R. Weston, 64; 3. F. W. Jannaway, j9. Judges: Messrs. V. E. Johnson, M.A., G. P. Bragg-Smith and E. W. Twining. Considering the gale blowing, the flying was good, and was a severe test for stability. Official Flights.—The first official trials for tractor screw models took place after the above competition, and, owing to the gale increasing, the models had to be hand-launched. Results: H. R. Weston, 84 yards; J. Dollittle, 52 yards ; F. W. Jannaway, n sees. The above are after allowance has been made for wind velocity. Lecture.—The first lecture of the winter session will be given by Col. S. F. Cody, on November 7th, on behalf of the first war kite squadron's equipment. The lecture will be on "What the nation should do to stand first in aviation," and the place will be announced as soon as possible. All members are requested to apply for tickets for disposal among their friends. Members will have tickets for themselves forwarded free on application. Kite Display.—The kite display fixed for to-day has been postponed, but any member who would like to practise with their kites will be welcomed at The Plumes Hotel, Park Royal, at 3 p.m., or to assist with the Baden-Powell team. The reason of the postponement is that the letter from the County of London Territorial Association was not received in time to make suitable arrangements. 27, Victory Road, Wimbledon. W. H. AKEHURST, Hon. Sec. ® ® ® ® MODEL CLUB DIARY AND REPORTS. CLUB reports of chief work done will be published monthly for the future. Secretaries' reports, to be included, must reach the Editor on the last Monday in each month. Aero-Models Assoc. (N. Branch) (15, HIGHGATE AVENUE, N.). OCTOBER 5TH. Flying at Finchlcy. Aldershot Aero Club (37, ALEXANDRA ROAD). OCTOBER 5 TH. Flying, 2.30. October 6th, 11 a.m. BUckheath Aero Club (48, HAFTON ROAD, CATFORD, S.E.). OCTOBKK 5TH. Flying, Grove Park and Blackheath. Croydon and District Aero Club (Sec., I36A, HIGH STREET). OCTOBER JSTH. Open duration contest, Wimbledon Common, 4 p.m. (prizes). Hcndon Model Aero Club (8, MONTAGU ROAD, W. HENDON). OCTOBER 5TH. All-round contest (trophy). Leytonstone and Districts Aero Club (64, LEYSPRING ROAD). OCTOBER 5TH. Meeting, Wanstead Flats, as usual. Paddingrton and Districts (77, SWINDERLY ROAD, WEMBLEY). OCTOBER STH. Sudbury Hill. Tractor competition (Johnson cup and 2nd and 3rd prizes). October 12th. Duration handicap (prize). ScottishAe.S.(ModeI AeroSection) (3, STANMORERD., GLASGOW) OCTOBER STH. Hydro-aeroplane meet, the pond, Maxwell Park, Pollok- shields. October 12th. R.o.g. contest. For venue see notice board. Stony Stratford and District Aero Club (OLD STRATFORD). OCTOBER STH. Impromptu contests, Blackhorse ground, Old Stratford. October oth. General meeting, 8 p.m. Yorkshire Ae.C. (Model Sec.) (53, WEST STREET, LEEDS). OCTOBER 5TH. Hydro-aeroplane meeting, 3.30, small lake, Roundhay Park. 902
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