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Aviation History
1926
1926 - 0422.PDF
SUPPLEMENT TJFLIGHT JUNE 24, 1926 THE AIRCRAFT ENGINEER Determination of Rubber Area and Length of Rubber Column. geometrically similar. If the proportions of the inside and Where :— outside diameters to the thickness remain the same, then as w = static load on rubbers in lbs. far as present tests indicate, the stress-deflection graph W =-- maximum load on rubbers in lbs. • remains unaltered with increase of cross-section area. T = required deflection of column in ins. under W. In order that the outside diameter of the stabilising and A = cross-section area rubbers in sq. ins. separator plates may be determined, it is necessary to know I = length of column in ins. the increase of the diameter of the ring when subject to its The first three values, to, W and T. are always known before maximum compression. This information is contained in the design of the shock absorbing member is commenced. Fig. 7. By way of an example let:— Where w = 2,000 lbs. v — increase of diameter as per cent, of original diameter. W = 10,000 lbs. i _ deflection as per cent, of original length.T =7 ins. The maximum permissible stress on the rubber under r — u /»-(•— ). static load as stated previously is 250 lbs. per sq. in. The most advantageous form of compression rubber depends 2 QQQ entirely on the type of machine for which it is required. For Therefore A = t)r(. = 8 sq. ins. high-speed aircraft, where a reduction of frontal area is of "° primary importance, the " Gloster" type of rubber is and the stress on the rubber under maximum loading ia undoubtedly the best form and lends itself to good stream. 10,000 _ lining. In the case of slower speed aircraft and especially of g = !>2oO lbs. per sq. in. UTe commercial type, where accessibility, ease of replace-. .,.,,,, -i-i T • • -, . , , ment and initial cost are governing factors, then the ring tvpe Assuming that the tvpe of rubber used is similar to that shown , ,, 'J , , c . x• -in- £»"xi- £ ~Ji i a x- -, • ,i i °f compression rubber shows advantages, in bis. 2, then from the stress deflection graph it is seen that T , . ,, ,, ,. ". t ,, , ,,,• . i , i a A- J- -Z. - , In designing rubbers, the proportioning of the rubber isthis stress corresponds to a deflection of o3-o per cent. ,, „ • ," , • A f •<. • *• i+1. +-u u \A A lL J. A i in • - • i the all-important point, for it is essential that the mass should As the total deflection is / ins. then u J- j • •/ j. t n ... r ,- .„ _ ', „ . be disposed in its most favourable position for spreading. 53-o per cent. 1 = 1 ins. T ,, , .,, , ,. £ ir ±- • , ,„ , . In the several illustrations of rubbers, proportions are givenrT>u' , i, " ', i i • i x ,, , j a j.- which are known to be good, but it is not suggested that I he following formulae, derived from the stress-deflection ,, ,. . ? . ', n ,, ,. ,., -ii u * J x i u . ., , - these prorortions are ideal because further investigationgraphs, will be found useful as an alternative method for . r l , .,. , , ,, , , • ,, r , ,% ,r . . ,,, . , • • i ,i is necessarv. and this work should be done bv the rubber determining any one of the four unknown quantities when the - , * r^, -± t J t- •' • i, u*, , fc J . i * manufacturer. The necessity of good proportioning is brought otner tnree^ are given:— out by the foUowillg illustration. If two rubbers of the 8 = deflectwn of column m inches. game • ]itv ^ cross.section are but of m&Tent I = length of rubber column in mches. ,*. . * . i i j i i i- 4i, . ° .... proportions are tested and under equal loading the one gives A = cross-section area rubber m sq. ins. £0 v cent and the ^^ 4Q 4cent defl^tionf then bv L •= load on column in lbs. . ,. c i.i j.u 11 i • * on usiii" the iormer rubber there would be a saving of 20 per For plain type ring, as shown in Fig. 2. cent; in weight over the latter plus a reduction of 20 per / 104 A \ cent- in the length of the rubber column. This represents a 8 = I (0-615 — —j— | (I) considerable saving, both in weight and frontal area. \ / typical compression rubber leg showing the arrangement For Parnall type, Fig. 3. of the various components is illustrated in Fig. 6. Curve 1. In this article no attempt has been made to discuss the z x shock-absorbing qualities of compression rubber. Generally, ^\ z/o>r44 \ /TT\ some form of oleo gear is incorporated in the leg, which \ L / absorbs the major part of the energy due to the vertical „ velocity of the aircraft. With this arrangement the compres- sion rubbers are used as a means of spring suspension only. ^ (Ill)_ ' DURALUMIN Curve 3. By LESLIE AITCHISON, D.Met., B.Sc, F.I.C. M.I.A.E. o" = nO-59 — —=— ) (IV) (Continued from page 56.) * I The corrosion of Duralumin may profitably be considered For " Gloster " type, Fig. 4. under three headings : Firstly, the evidences of corrosion : / QK.C »\ secondly, the ways in which the corrosion may be accelerated ; § = I (0-56 — -—K ) (V) an<^' thirdly, the methods which may suitably be employed \ ^ / for the prevention of its occurrence. The corrosion which These formulae cease to hold good below deflections of does occur on Duralumin shows its presence by the formation 35 per cent. °f a white, somewhat powdery, deposit on the surface of the To illustrate the use of the above formulae : Take the case metal- The distribution of the corrosion over the surface of of the example previously considered where Duralumin is approximately similar to that of the rust that j> J_ „ . forms on steel in the early stages of an attack. It is unusual o — ' ins. ^or ^e surfaCe of attacked Duralumin to become covered with -A- = 8 ins. sq. a uniform film of corrosion products such as occurs on steel. L = 10,000 lbs. The products of corrosion gather in spots, and unless the Rubbers of the same type (i.e., as Fig. 2). From formula I. corrosion has gone on for a very considerable time the surface / i of the metal never becomes even approximately wholly 7 ins. = Z ( 0-615 — x J covered. As a result of this method of attack, such corrosion \^ 10,000 J as occurs tends to spread into the metal by the formation of = 0-5318 I P'^s anc* no^ ^v an attack on the whole of the surface of the . / _ no.o • metal. " ' ' ~ mS- The rate at which Duralumin suffers corrosion is not high, This agrees to within 1 per cent, of the length previously and a comparatively small amount of corrosive reaction results calculated. . in the formation of a very noticeable quantity of corrosion It must be understood that the stress-deflection graphs product. The presence of this corrosion product can be given for the various types of rubbers only apply to rubbers detected very readily, and if the metal is cleaned as soon as 362/
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