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Aviation History
1918
1918 - 1380.PDF
DECEMBER 5, 1918 Fig. 12 shows the whole apparatus with the test piece intension crossing the model spar and placed as close as possible to it, the beam of the polarised light beingfocussed on the median plane of the model spar. It is there- fore not focussed on the median plane of the test piece, anda certain lack of clearness in the definition of the dark band neutralised by the test piece is accordingly produced. Thiseffect is kept as small as possible by placing the test piece close up to the model spar. But a better arrangement sug-gested by Major Filon, and actually tried on another model, is obtained by interposing another lens between model sparand test piece at such a distance that the beam of light is again focussed on the median plane of the test piece as-wellas on the median plane of the model spar. In the present experiments the method of placing the test piece close 1othe model spar was used throughout, and it is considered that the blurring of the image inevitable in this method didnot introduce any serious discrepancy. initial stress, even when no load is applied to the bar, and onthis account the colour bands are seriously upset by this initial stress in the neighbourhood of the edge, which is there-fore best avoided when accuracy is required. (9) Method of Observation Errors of Parallax.The test piece was then placed with its length vertical and as close to the specimen as possible. The effect was to shiftthe neutral line from the centre line of the bar up or down through a varying amount, corresponding to the stress -inthe test piece. By adjusting the total pull or thrust in the test piece and also moving it slightly up and down, it wasalways possible to bring the black band with its centre on one of the points I, J. The stress in the test piece was thenread. Now let P be the stress in the specimen due to the loading,P o the permanent stress difference in the specimen at the pointobserved, and let T, T o be corresponding quantities in thetest piece. In practice it was found that the permanent stress (which was small) had in general its axes roughly parallel andperpendicular to the length of the bar—and the same held good for the test piece. We can therefore simply add P to Po and T to Tg to get thetotal effect and we get P + Po = T + To. Another set of readings taken with the framework unloaded,so that P = 0. If T is then found to be T1 Po = T- - T,. /Subtracting P = T — 71 gives a result for the stress due to Practical Help for the R.A.E. Benevolent Fund SOME very useful work has been done by the BenevolentFund in connection with the Royal Aircraft Establishment. One of the most valuable supporters is Mr. H. Proctor, theartist, who gave permission for the fund to use a fine water- colour of an S.E.5A. The reproduction of the picture addedabout fjio to the funds. He has now given another picture, this time an F.E.2B, and it is hoped to raise a similar amount.This material help has considerably assisted the Committee, who are desirous of expressing their appreciation to Mr.Proctor accordingly. ^ Aircraft After the War. AT a concert held at the Queen's Hall on November 26thn aid of the Lord Roberts Memorial Workshops, Mr. J. A. Whitehead gave a short address in which he emphasised thegreat progress which had been made in flying during the War, and pointed out how necessary it was that Great Britainshould turn this supremacy in the air to account in connection with commercial aviation. He stated that Whitehead Parkat Feltham was being organised as a great aerodrome from the loading, which should be "fairly independent of the per-manent stresses in beam and test piece. To get the best possible results the test piece should have been changed fromtension to compression when passing from I to J (or con- versely), as it is found that a number of causes of error) andin particular the possibility that the stress optical coefficient C may be slightly different for large pressures and largetensions) are eliminated if we take care always to have the bar and specimen at right angles, so that at the pointsexamined the two are always under stresses' of the same name. It was not, however, found convenient to do this. Withthe high loads employed a considerable amount of " creep " showed itself in the material. This " creep " itself modifiedthe applied stress system, by increasing the part of the bend- ing moment due to thrust and in this way the stress in thematerial showed progressive increase with time. It was therefore important to make all measurements at I and J asnearly simultaneous as possible, and the time taken in re- adjusting the test piece from tension to compression wasprohibitive. The method employed then was to use the test piece in tension, only placing it at right angles to the specimenwhere tensions were to be measured, but parallel to it to measure pressures. A little" difficulty was experienced owing to the fact thatthe test piece and bar could not be in focus together, so that the graduations of the test pieTe were always somewhatblurred. This difficulty, however, was not such as to seriously affect the observations. A certain amount of overlapping of colours must alsonecessarily occur owing to rays from the same point of the median plane of the specimen passing through differentparts of the test piece. This " parallax " error undoubtedly must have slightly impaired the definition, and it may inaddition have introduced a small systematic error. In later observations all these troubles due to parallaxwere removed by separating the specimen and test piece, and introducing between them a lens, so that the medianplanes of the specimen and test piece were focussed upon each other and the two were finally focussed together uponthe screen. (10) General Accuracy of Optical Observations of Stress. Some idea of the accuracy with which such observationsof stress could be taken with xylonite was obtained by using a specimen strained under flexure without strain, the bendingmoment on which was exactly known. In this case the stress in the specimen can be calculated exactly. By measuringit independently with the test piece, we can form an estimate of the errors made by this process. The measurements were taken with test piece and specimencrossed and parallel, and also with the specimen in tension and with the specimen in compression. By taking means of the four measurements the permanentstresses were eliminated, the same point of the specimen and of the test piece having been used in all four. Altogether the results showed a mean probable erroraccording to the following table :— Range of No. of observa- Mean probable pro-stress, tions. portional error. 100-500 lbs. 95 'O57 (2) 500-1000 108 -032 (3) 1000-1500 1500-2200 60 24 100-2200 287(To be continued.) •031 (4) •045 (1) 28-5 lbs. which aerial services might start, not only for the Continent,but also for various parts of England. In this connectio n he outlined some of the advantages which would accrue tobusiness people from the speed with which journeys from point to point could be accomplished. Lord Cheylesmore, who occupied the chair, made a stirringappeal on behalf of the Lord Roberts Memorial Workshops, and invited anyone interested in the work to visit the work-shops and see the men at their various tasks. • . . Useful Work by the I.A.F. , IN a letter describing the work of looking after theprisoners who are coming back from Germany on foot. Sir Arthur Lawley, British Red Cross Commissioner in Franceand Belgium, mentions that at Nancy where the men came in in hundreds, he got into telephonic communication with theIndependent Air Force, and before he left they had the situa- tion well in hand in caring for the men and seeing that theywere supplied with clothing, food, etc., the American Red Cross, who have a stores depot in Nancy, lending their aid. 1381
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