NO. 3 LANGLEy MEMOIR OH MECHANICAL FLIGHT 197 



Before proceeding with I lie construction of any more complete wings, an 

 extended series of experiments was made in order to secure ribs of proper light- 

 ness and strength. Various forms of metal tubes were tested; but, although 

 aluminum seemed at one time to promise good results, it was found that hol- 

 low ribs could be constructed of spruce which were much stronger thaii aluminum 

 tubes of the same weight. In order to determine more accurately whal mode 

 of construction would give the greatest stiffness and strength for a minimum 

 weight, it was decided to make up some test pieces of different forms before 

 making up complete ribs. For convenience in construction, these test pieces 

 were made straight and shorter than the large cross-ribs. Each piece was 

 tested by fastening it in the testing clamp with 1 metre of its length projecting 

 horizontally, and attaching at its end a weight of 1 kilogramme. The deflec- 

 tion from the horizontal gave an index of the stiffness of the piece under 

 examination. 



The first test piece was a hollow square, 17 mm. length of side on the ex- 

 terior, and 11 mm. length of side on the interior, the walls thus being 3 mm. 

 thick. This weighed 73 grammes per metre and had sinall internal stiffening 

 pieces, like the partitions in bamboo, glued into it 4 inches apart. A weight of 

 1 kilogramme at the distance of 1 metre gave a deflection of 56mm. The sec- 

 ond test piece was a duplicate of the first one, except that it had no internal 

 stiffening pieces, and the weight per metre was made the same, 73 grammes, as 

 formerly, by leaving the walls a fraction thicker. The deflection in this case 

 was, as would be expected, exactly the same as in the first one. The first test 

 piece, however, was superior to the second one in that it was stiffer against be- 

 ing crushed in by accident. The third test piece was a hollow cylinder, 22 nun. 

 outside diameter and 17 mm. inside diameter, the walls thus being 2.5 mm. thick. 

 The weight per metre was 91 grammes, and the deflection was 46mm. The 

 fourth test was made by taking two of the original solid cross-ribs, 12 mm. in 

 diameter, and fastening them in the clamp side by side, with a length of 1 metre 

 projecting. The weight per metre for the two ribs was 10.") grammes, and the 

 deflection produced on the two by 1 kilogramme at 1 metre distance was 11.". mm. 

 The fifth test piece was an I-beam of spruce, having a depth of 25mm., with 

 the flanges 12.5 mm. wide and the web 3 mm. thick. The weight per metre was 

 65 grammes, and the deflection was 26 mm. All of these test pieces were made 

 of carefully selected straight-grained spruce. 



It is readily seen that the test piece having the I-beam section weighed less 

 than the hollow square in the first and second tests, and had a deflection of less 

 than half. This I-beam section, however, did not show up so well when a Longer 

 piece was tested, for as soon as the length was made appreciably greater than 

 a metre it began to twist, the twisting becoming more and more serious the 



