200 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



Although this wing was a great improvement in every way over any of the 

 previous constructions, it was felt that it was too weak for the large aerodrome. 

 Further experiments were therefore made in order to secure a form of cross- 

 rib which would meet the rigorous requirements imposed. An inordinate amount 

 of time was spent in the construction and tests of various forms of rib, but as 

 a result a satisfactory cross-rib was at last constructed of the form shown in 

 Plate 66, Figs. 4-8, the dimensions at the three principal points, viz., first, where 

 the cross-ribs join the front rib; second, where they cross the mid-rib; and third, 

 at the rear tip, being given both for the intermediate cross-ribs and the end 

 cross-ribs. 



Following the plan employed by Nature in the construction of the bamboo 

 pole, small partitions, approximately one millimetre thick, were placed every 

 three inches in the thin, hollow rib to keep it from being crushed. The parti- 

 tions were glued in place when the hollow rib was glued together on the form 

 around which it was bent and clamped until the glue dried. Longer blocks were 

 also inserted in each of the intermediate ribs at the point where it crossed the 

 main rib and also at the front end where it was attached to the front rib. In 

 the end ribs blocks were also inserted at the points where the cross-braces were 

 fastened to them for resisting the end stress due to the cloth covering. 



Upon making up one of these ribs and testing it, it was found to possess 

 remarkable stiffness, so much so that it was thought probable that it was as 

 stiff in proportion to its size as the best thing that Nature had produced in the 

 bird's wing. A large quill from the wing of a harpy eagle was therefore 

 si lipped and the large end clamped in a special holder, and measurements were 

 made of the deflection produced by weights at various distances from the clamp. 

 As the main mid-rib of the wing of the aerodrome is placed approximately at 

 the point -of the center of pressure, the bending action on the cross-ribs may be 

 assumed to act on a lever arm from the mid-rib towards the front, and from 

 the mid-rib towards the rear in the cases of the pressure on the front and rear 

 portions of the wing, respectively. In testing these cross-ribs, therefore, against 

 the quill, the rib was (damped at the point where it crosses the mid-rib of the 

 wing, and measurements were made of the deflection produced by weights placed 

 at various distances from the point of clamping both front and rear. 



The quill on which the measurements were made was ID.") inches long and 

 had a gradual curve, the highesi point of the curve being about the center of 

 the length of the quill, and the depth of curvature being about 2 inches. When 

 the butt of this quill was placed in the clamp the tip stood 17 cm. above the 

 horizontal. The hollow spruce rib, when clamped at a point 5 feet from the 

 tip (the pninl from which it tapers in both directions) had its tip 2.2cm. above 

 the horizontal, there being very little curve in that portion of the rib. The 

 quill weighed 4 grammes when stripped and IS inches of it projected from the 



