Before proceeding with the construction of any more complete wings, an extended series of experiments was made in order to secure ribs of proper lightness and strength. Various forms of metal tubes were tested; but, although aluminum seemed at one time to promise good results, it was found that hollow ribs could be constructed of spruce which were much stronger than aluminum tubes of the same weight. In order to determine more accurately what 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 deflection 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 exterior, and 11 mm. length of side on the interior, the walls thus being 3 mm. thick. This weighed 73 grammes per metre and had small 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 56 mm. The second 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 being crushed in by accident. The third test piece was a hollow cylinder, 22 mm. 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 46 mm. 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 105 grammes, and the deflection produced on the two by 1 kilogramme at 1 metre distance was 115 mm. The fifth test piece was an I-beam of spruce, having a depth of 25 mm., 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 [p198] greater the length, until with a piece 11 feet long, the full length of a cross-rib, the twisting was so serious as to make the rib practically useless. It was at first thought that this twisting might be overcome by making the webs slightly wider, and it would to a certain extent, but in looking ahead and planning how the cross-ribs were to be fastened to the main ribs, the I-beam section was seen to present so many difficulties that it was thought hardly worth while to spend time on further experiments with it. This decision was made all the more imperative by foreseeing the difficulty of bending the I-beam section to the curve which the cross-ribs were to have. In fact it had been found by experience that while many different forms of ribs could be bent to the proper curve by steaming and clamping them over a form and then drying them out while still clamped to the form, yet the grain of the wood varied so in different ribs, that of a dozen steamed and bent over the same form it was seldom that as many as three would have approximately the same amount of curvature when removed from the form after drying. If, however, the curve was formed in the ribs by making them in two parts, which were glued together and clamped up on the form while the glue dried, practically any number could be made which would have the same curvature when thinned down to the proper thickness of wall.
It was recognized at all times that the gluing together of the ribs not only entailed extra work, but introduced an element of uncertainty unless some kind of a varnish for the ribs could be found which would prevent any possibility of the glue becoming soft from moisture in the atmosphere or from the wings actually coming down into the water when the aerodrome was tried in flight. A search was therefore made for a varnish that was water-proof. A large number of different varnishes were tried, and one was finally found which, after repeated tests, seemed to be thoroughly good. Several test ribs were given three coats of this varnish, and were then kept immersed in water for 24 hours without the glue showing any signs of softening. It was therefore decided to follow the plan of gluing the ribs together and protecting them with three coats of this varnish, which seemed to possess the remarkable properties of being not only impervious to water, but also unaffected by the application of concentrated ammonia or of gasoline, either of which produces immediate softening when applied to ordinary varnishes.
Following the indication of these tests that the hollow, round section, 22 mm. outside diameter by 17 mm. inside diameter, would probably give the best cross-rib for the weight that it seemed possible to allow, a set of cross-ribs of this form was constructed and put in place in the large experimental wing, in which the former solid ribs had been tested. The wing was inverted and fastened into two posts at the angle it would have in flight, the guy-wires from the [p199] lower guy-posts of the aerodrome being represented by wires stretched from the posts. In actual use on the aerodrome it was proposed to have three main guy-wires running from each of the main cross-ribs to the lower guy-post, but in the test, which is now to be described, the wires which would have come nearest the body of the machine were left off to see what effect their removal would have on the wing.
The weight and dimensions of the wing, as set up, were as follows: Length of the main ribs, 24 feet; length of the cloth covering, 22 feet; width of the cloth covering, 11 feet; total weight, 29 pounds. The two main ribs (front rib and mid-rib) were solid, 3.5 cm. in diameter at butt, 2.5 cm. in diameter at tip, and tapering from the middle to the tip. There were twelve regular cross-ribs set 50.8 cm. (20 inches) apart, each rib being as above described, 11 feet long, 22 mm. outside diameter by 17 mm. inside diameter at the butt, and tapering from where they were attached to the mid-rib to the tip, and each weighing 300 grammes. There were two extra cross-ribs, one at the inner end next to the body of the machine and the other at the outer end. These were solid strips of wood 3.8 cm. wide by 1.2 cm. thick, made extra wide and stiff in order to withstand the strain of stretching the cloth covering. There was also a thin, flat strip at the rear edge, which connected together all the tips of the cross-ribs, holding them a uniform distance apart, and also serving to fasten the cloth. The main mid-rib was stiffened in a vertical direction by a system of small guy-wires drawn over short guy-posts about 6 inches high. With the wing inverted and fastened in the way above described, a weight of 2 kilogrammes placed at the inner rear corner produced a deflection of 26.7 cm. When the inner rear corner was pulled up by a spring balance until the balance registered 2 kilogrammes, there was an upward deflection of 41.3 cm. When the main mid-rib was held at the inner end, the pull of 2 kilogrammes, applied to the inner corner as before, caused an upward deflection of 25.4 cm. instead of 41.3 cm. This wing was afterwards given a sanding test under a weight of 0.7 pound per square foot. With fine guy-wires fastened from the front of the cross-ribs to the tip and drawn just taut, the ribs showed an average deflection of 9 inches at the tip under the above weight. When a small wooden guy-post was added under each of these small guy-wires, the same weight produced an average deflection of 5 inches at the tip of each rib under the same load. In a previous test of the wing, using hollow cross-ribs 16 mm. outside diameter by 10 mm. inside diameter at the butt, and only half as far apart as the later ones, a load of 1 pound per square foot on the wing produced an average deflection of 9 inches at the tip of each rib when the cross guy-wires on each rib were held up by short guy-posts, but when these short guy-posts were removed, the same load produced a deflection of nearly 25 inches at the tip of each rib. [p200]
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 partitions 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 stripped 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 clamped 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.
