NO. 3 LANGLEY MEMOIB ON MECHANICAL FLIGHT L95 



more than it would be subjected to in flight. This grade of percaline weighed 

 approximately one and a half times as much as a grade of silk, which on tesl 

 was found to have a slightly greater tensile strength than I lie percaline, though 

 the latter did not " flute " or " pocket " nearly as much as the silk. More 

 over, the cost of the percaline was only about one-third thai of the silk, ami 

 it was chiefly for this reason that percaline was adopted in place of silk. Al- 

 lowing for necessary seams and extra material to he turned over al the front 

 and rear edges of the wings, the percaline covering, which under the original 

 plans comprised approximately 1000 square feet, was therefore calculated to 

 weigh approximately 7000 grammes, exclusive of the necessary cords for lacing 

 the coverings to the wooden frames of the wings. 



As the one hundred and twenty pounds allowed for the four wings per- 

 mitted only thirty pounds per wing, and as the cloth covering, lacing cords, 

 etc., were found to weigh something over four pounds, there remained only 

 about 25 pounds as the permissible weight of the wooden framing, including 

 the necessary metal clips, secondary guy-wires, etc., for each wing. With the 

 relative proportions of the various parts of the wooden framing of the wings 

 of the models as a basis, it was decided to make the main ribs of the large 

 wings 1.5 inches in diameter for one-half their length, and have them taper from 

 this size to one inch in diameter at the extreme point. After making allow- 

 ance for the weight of these ribs, it was found that, if the cross-ribs were to 

 be spaced no farther than ten inches apart, and the two end ones were to be 

 made at least as wide as 1.5 inches in order to resist the end strain due to the 

 stress of the cloth, the twenty-six intermediate cross-ribs could be only seven- 

 sixteenths of an inch in diameter at the point where they crossed the main rib, 

 and that they must be tapered to three-eighths of an inch in diameter at the 

 front end and to one-fourth of an inch in diameter at the rear tip. 



A trial wing, whose total weight was 30 pounds '2 ounces, was made up with 

 the various parts of its frame of the above dimensions. Even upon inspection 

 it appeared to be too flimsy to withstand the sudden gusts of wind which were 

 certain to be met in actual practice. In order, however, to gel some definite 

 data as a guide, the wing was inverted and guyed in the same way thai it was 

 proposed to guy it on the aerodrome, and a uniform thickness of sand was then 

 sprinkled over it to such a depth as to give it a load of 0.7 pounds per square 

 foot. Even before one-quarter of the sand was sprinkled over it, it was seen 

 that the wing was rapidly going out of shape, and it was feared that the full 

 amount of sand would not only seriously distort it, but would even break it. 

 The full quantity of sand, however, did not break it, but distorted it to such 

 an extent that, had the pressure been due to its being propelled through the 

 air its serious change in form would have rendered it worse than useless. 



