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Aviation History
1913
1913 - 0684.PDF
(2) The intensity of stress due to the lateral force. This is denoted in the diagram by the letters B M. The sum of (i) and (2) gives the total intensity of stress in the spur. In Fig. 14, the depths of the respective spars in the various machines calculated have been drawn to scale. Fig. 13 shows the cioss section of the spar. Objection might be raised to the dimen sions of this cross section, but for the purpose of comparison an alteration in this respect will not materially affect the result. Finally, there is the stress diagram for the wing cross section. The distribution of air pressure across the plane section is that obtained on the plane No. 13 it's by Eiffel.* From this distribution ffi w-aaih pf *»ei run I'm 7 Some Aerodynamical and General Comparisons.—So lon°- as structural considerations do not limit the size of spars or in other ways affect the design, the monoplane will outclass the biplane by reason of the superiority of its lift for a given wing area, plane area section and angle of inclination. The lift drift ratio will, there fore, also be better, and generally the monoplane will be the more efficient type In the large monoplane size to obtain sufficient strength it is not only necessary to increase the spar area consider ably, but also the span of the machine has to be curtailed so as to bring its size within reasonable limits. This entails a larger chord, and, therefore, a smaller aspect ratio. For planes of equal aspect ratio span, cross section, and angle of inclination, the monoplane has about 15 per cent, more ,,*„ lift than the biplane for equal speeds through the air, Afjf 5ut this advantage may be lost if the aspect ratio had to be reduced from 7 to 4. Area of Machines 1104 150XI.fr 510b 275 9 to 12 40P' neti »• 3 Sllbs per tn&i run Jection ofJparj fiwAram jhou'tno relative' Jtzes of spars J Flit* Total E>.tl. 1 r! > ' ^^^ t 3703) 2$^ htm "Hi _^^^ <•"" T 90' Diagram showing b.H.on part of spar between supports nt) 11 6 FDtafcram for rib Fl6.11 E>HDiagram far rib Comparison of monoplanes and biplanes, Figs. 11 to 16. curve the resultant centre of prcsure is obtained at a point '31 01 the chord away from the frcnt edge. The total load per unit length of span on this plane section is taken by the two spars in the inverse ratio of their distances from the centre of pressure. 6. An Analysis of the Stress Diagrams.—In looking through the figures for the intensity of stress on the various wing spars it is at once seen that the determining factor in their design is the lateral load causing bending of the spar, not that due to bending owing to the compression. This is especially the case with the large size monoplane, the spar depth necessary to give the requisite strength increasing at a very rapid rate, as will be seen from the lines for 8 and 12 in Fig. 14. This is not quite so pronounced with the monoplanes with ordinary bracing, and could probably be decreased in the king-post type by the use of a slightly increased length of the king-post. From a structural point of view the aerodynamical advantages of a monoplane of very large size would thus be outweighed by the increased weight necessary to give the required spar strength. The question of providing adequately for the drift force acting on the wing is carefully allowed for in all modern aeroplanes. The ribs give an effective increase of width to the spar (lange. Other wise, the moment of ineitia of the section would be small when taken alwut an axis perpendicular to the one previously considered, and were the drift per unit length of span of large dimensions, there would be here a serious source of plane structural weakness. * Kiffcl, La Rtsittmnctdt tAhrgiPAinatiem, 1911, Plate 16/'/jr. Referring again to the diagram of relative depths of wing spars (See Fig. 14), there is little to choose between the spars for either monoplane or biplane of the smallest size, and the decided aero dynamical advantages of the monoplane type more than point to it as the superior type for this size. The second size would appear to be equally suited, whether as a monoplane or biplane. This is for a machine carrying about 1,400 to 1,500 lbs. load. For the largest size of machine the balance would seem to lie in favour of the biplane. When the results are generally reviewed from this diagram it would appear that a limit will be first reached for monoplanes. As more load is carried a limit would be reached with biplanes, and it would be more economical to employ the multiplane machine. This point has not yet been reached in practice. In many other ways this distribution of the types would appear to fit in well with the requirements. A small machine is essentially one that can easily be carried about, and very quickly erected. There is not the same necessity for the larger size of machines to be so easily packed, and consequently there is not much advantage in a monoplane from this point of view in the larger sizes. A biplane has a greiter resistance than the corresponding mono plane, but the difference is not very great. What in the monoplane one gains by having no plane struts is lost by the increased size of the chassis members for a given diameter and arrangement of propeller. For machines of area up to 250 or 275 sq. ft. a monoplane is the most economical type. From this point onward the biplane holds the field. 7IO
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