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Aviation History
1919
1919 - 0278.PDF
FEBRUARY 27, 1919 might be of advantage aerodyuamically and that the flexibility itself might improve the strength, or more accurately perhaps decrease the danger of failure in the air though not under sand loading. Plato IV. We now come to the question of the economic position for points of support along the main wing spars. Firstly, we must decide on how the load may be considered to be distributed along the spar. In the top figure is drawn in full lines what is a fair average to take for the grading off of the load at a wing tip. AC represents a length equal to the chord of the aerofoil. In dotted lines is drawn what I call an " equivalent rectangle." The area of this rectangle and the moment of its area about BC are practically the same as are area and moment of area about BC of area below grading curve. The end of the rectangle is . 2 of the aerofoil chord short of the spar end. It appears to be sufficiently accurate therefore to consider a spar as uniformly loaded to a distance of .2 of the aerofoil chord length from its end. 1 use this assumption in the few spar strength calculations which follow in this paper. The question of economic positions for points of support is a very highly complicated one ; it should mean properly that disposition which will give as a whole the lightest com bination of spars, 'tween-wing struts and wire bracing, for some required strength. I have considered only what dis position of supports will give approximately the same maxi mum stresses in a spar of uniform cross section throughout. There is ho reason to claim that this will give the lightest structure, it might prove more economical to use other positions for points of support and stiffen up the stress locally ; but it was impossible in the scope of this paper to undertake a very lengthy investigation on this point, and I believe that the uniform section spar disposition is probably about as light as any other. The five figures below give respectively the positions of points of support for the top and bottom wings of a single bay biplane, for the top wing of a two-bay biplane, for the bottom wing of a two bay biplane, for the top wing of a three- bay biplane and for the bottom wing of a three-bay biplane. At each point of support are given the values of the bending moment and of the vertical re-action, the approximate values of maximum bending moment between points of support are given also. In the single bay type the disposition is the same for top SPAR WEIGHTS FDR 'STANDARD AEROFOIL SECTtOM CP TAlCEhl AT '2^ POW rftONT SPAR LO*Pr fc AT '&Q FXX? WEAR SPAR LOAD. ! L/Nl t. 111- WmgflfStftrf' fp* REAR »TAR TWCKI AS -7g THAT FOR neONTSRM?. MCANpgTTH OTSPAR TWEH AS, frHICKNE3S OF SECTION ON SPAKCKNTR^ IfoAWNHJivtTHlCKNeSScg^SCCTK)^^. SPINf S TAKE^I AS OF 'gTAfJOARC? roRM of CROSS SgCTtOfs). -4.fi IQTAL SPARWEISHTS, (Wg+-Wg\ REAR SPAR- vVElSHTS^Wg^ FRQN r SPAR- WE>GHT 5j(W) CURVED PRAWN t*J PirPERENTT TYPES OF UNiE THUS;- FOR rUDKT SPAR CCH~ *E AT '\<q CHORD F*otA L'E . • m » n *» M *» n « v> • n n m '\A- « r> *YQ. n ,> -IO » n -Oft « M '0& « »» >» • M •» n » *» • » •*"* -SO -54- -38 SCALE OTQlSTANCe OF CENTRE Or REAR SPAR FROM L.E. •6fc •6G» TO •74CHom
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