10 LANDER, Stresses in the Main Spars of Monoplanes. 



The process of finding the bending moments on an 

 aeroplane spar with non-axial attachments may therefore 

 be summarised as follows : — 



i°. Divide the length into spans equal to the distance 

 between the intersections of the wires with the axis of 

 the spar. 



2°. Draw Claxton Fidler's bending moment diagram 

 for this arrangement. 



3°. Taking the moments thus found for these imagi- 

 nary points of support, calculate the pull on the wires by 

 equating the moments of external loads and forces on the 

 wires to these values. 



4°. Returning now to the actual spar, draw the dia- 

 gram of bending moments induced by the components of 

 the pulls in the wires in a direction parallel to the lines 

 joining the actual points of attachment to the fuselage 

 end of the spar. 



5°. Set up parabolas representing the moments upon 

 supported beams of spans equal to the distance between 

 the attachments, these parabolas being drawn from the 

 upper lines of the diagram in section 4 as base. 



6°. Find the characteristic points of this diagram and 

 draw the true base line. 



In order to render the method clear the example 

 previously taken for a spar loaded with 40 lbs. per foot 

 run and supported at three equidistant points 4' 6" apart 

 has been worked out, on the assumption that the points 

 of attachment of the wires lie two inches below the 

 neutral axis of the beam, the other end of the wires being 

 attached to a point 4 8" below the end of the spar where 

 attached to the fuselage. 



If P x P n P^ be produced as described, we obtain three 

 spans of 4' 8" each, together with a cantilever portion of 



