PL. 66. DETAILS OF RIBS AND FITTINGS FOR WINGS◊lgr
PL. 66. DETAILS OF RIBS AND FITTINGS FOR WINGS◊lgr
PL. 67. CROSS SECTION OF RIBS◊lgr
PL. 67. CROSS SECTION OF RIBS◊lgr
The quill on which the measurements were made was 19.5 inches long and had a gradual curve, the highest 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 point from which it tapers in both directions) had its tip 2.2 cm. above the horizontal, there being very little curve in that portion of the rib. The quill weighed 4 grammes when stripped and 18 inches of it projected from the[p201]clamp which held it during the tests. The rear portion of the spruce rib projected 5 feet from the clamp, being thus 3.3 times as long as the quill, and it weighed 120 grammes, the weight for the larger size having, therefore, increased slightly less than the cube of the length.
The results of the tests of both the quill and the rib are given in the following table. The approximate cross-section of the quill at the point of clamping, the middle and the tip are shown in diagramsA,BandC, respectively, of Plate67. The cross-sections of the rib at the corresponding points are shown in diagramsD,EandF. The cross-sections of the quill, enlarged five times, are shown in diagramsA′,B′, andC′.
QUILLFROMTHEREMIGESOFHARPYEAGLE.Weight, 4 grammes; length, 45 cm.; tip, 17 cm. above butt when the latter is horizontal.Point of application of weight in terms of length.Absolute weight in grammes.Weight in terms of greatest weight.Deflection in terms of length.0.3910501.0.380.4456050.58.380.564050.39.380.752100.20.380.95770.075.38
HOLLOWSPRUCERIB.Weight, 120 grammes; length 153 cm.; section, rectangular; tip, 17 cm. above butt.Point of application of weight in terms of length.Absolute weight in grammes.Weight in terms of greatest weight.Deflection in terms of length.0.3915,0001.0.110.44511,4000.76.110.567,9000.53.110.754,0000.27.110.952,0000.135.11
In each case the unit of length was the portion extending beyond the clamp; the unit of weight, the greatest weight employed to produce the deflection. It should be noted, however, that the relative deflection was quite different in the two comparisons. In the case of the quill the deflection was 17 cm. in 45 cm., or 38 per cent; in the case of the rib it was 17 cm. in 153 cm., or 11 per cent. In the case of the rib at the point 0.39 the absolute weight was 15,000 grammes, the relative weight unity and the deflection in terms of length 0.11. While no rigorous comparison can be instituted, since the rib was not deflected nearly as much proportionately as the quill, yet the general inference is that while the rib was not intended to be, and was not as elastic proportionately as the quill, it was probably at least as strong in proportion to its weight. Briefly summarizing these results it will be noted that the spruce rib was about 3.3 times the length and 30 times as heavy, while it was 15 times as stiff near the butt and 26 times as stiff at the tip, as the quill.
As this test on the rib for the large wings had apparently shown that the plan of constructing the ribs in the form of a hollow square secured maximum strength for minimum weight, it was decided to construct a few sample ribs after the same plan for the wings of the new quarter-size model of the large aerodrome, and to test, these ribs in a similar manner. The following table shows the results of the test on one of these ribs:
Total length of rib = 80 cm. Curve = 1 in 18. Highest point of curvature = 0.25 from front. Section of rib = 10 mm × 14 mm. at the point of attachment to mid-rib, tapering to 8 mm. × 12 mm. at the front point and to 7 mm. × 2 mm. at the tip. The rib was clamped with the tip projecting[p202]46 cm. and was weighted at different percentages of its length to such an extent that it was deflected 11 per cent of its length, or 5 cm. The weight of the 46 cm. length of rib which projected from the clamp was 11 grammes, the whole rib weighing 22 grammes and balancing on a knife edge placed at the point where it was clamped.
Total length of rib = 80 cm. Curve = 1 in 18. Highest point of curvature = 0.25 from front. Section of rib = 10 mm × 14 mm. at the point of attachment to mid-rib, tapering to 8 mm. × 12 mm. at the front point and to 7 mm. × 2 mm. at the tip. The rib was clamped with the tip projecting[p202]46 cm. and was weighted at different percentages of its length to such an extent that it was deflected 11 per cent of its length, or 5 cm. The weight of the 46 cm. length of rib which projected from the clamp was 11 grammes, the whole rib weighing 22 grammes and balancing on a knife edge placed at the point where it was clamped.
Point of application of weight in terms of length.Absolute weight in grammes.Weight in terms of greatest weight.Deflection in terms of length.0.3976801.00.110.44559800.780.110.5636800.480.110.7523000.300.110.9511000.1430.111.009300.1210.11
A lighter rib than the above, which was constructed at the same time, was also tested with the results shown in the following table. This rib was also 80 cm. long, but was only one-half the linear dimensions in section of the rib previously tested. The rear portion of it projected 46 cm. from the clamp. The total weight of the rib was 11 grammes, or 5.5 grammes for the 46 cm. on which the measurements were made.
Point of application of weight in terms of length.Absolute weight in grammes.Weight in terms of greatest weight.Deflection in terms of length.0.3914001.00.110.44511000.7850.110.567000.500.110.754000.2750.110.952500.1780.111.002200.1570.11
A still lighter rib of the same length, weighing 9 grammes, suitable for use in the wings of the quarter-size model, was constructed and a set of tests was made on it with the following results. As in the above test, 46 cm. of the rear portion of it projected from the clamp which held it.
Point of application of weight in terms of length.Absolute weight in grammes.Weight in terms of greatest weight.Deflection in terms of length.0.3914501.00.110.44511500.7950.110.567400.510.110.753800.2620.110.952100.1450.111.001800.1240.11
Among quite a number of different forms of cross-ribs which were constructed of a size suitable for use in the model aerodrome, but made primarily for use in tests to determine the best form to employ, may be mentioned the following, in which both ribs were seven-sixteenths of an inch outside diameter and five-sixteenths of an inch inside diameter. One was filled with elder pith, formed up into a round rod that just fit the interior of the hollow rib, and was glued into it when the rib was glued up. The other rib was left hollow. Upon testing these by suspending weights at different points, the rib without[p203]the pith showed a slightly less deflection than the one with it, it happening probably that the wood in one case was a little stiffer than in the other, although they were carefully selected to be as nearly alike as possible. The rib with the pith in it weighed 34 grammes and the one without it weighed 30 grammes. It was inferred from this test that the placing of a light pithy material in the interior of the ribs would have no good effect, and would not only add weight, but also complicate the construction. The reason for making this test with pith in one of the ribs was that it was thought probable that the rib flattened out somewhat when it was deflected under a load, and that the pith stiffened with the glue with which it was fastened in, might lessen this.
As the cross-rib described above, which was tested on October 23, 1899, seemed in every way suited for use in the wings of the large aerodrome, a complete wing equipped with similar ribs but of slightly changed dimensions, as shown in Plate66, Fig. 5, was immediately constructed. As previous tests had shown that the wing covering did not “flute” or “pocket” to any considerable extent even when the ribs were as much as thirty inches apart, only ten cross-ribs were used in this wing. The eight intermediate cross-ribs were of the form described above, but the ribs at either end of the wing were made of a larger cross-section and otherwise stiffened in order to resist the strain of the tightly stretched cloth covering.
On April 13, 1900, a final sanding test was made on this wing, guyed in a manner similar to that used in the aerodrome, in which the following results were obtained:
SANDINGTESTOFLARGEWING.Area, 260 sq. ft.; weight of wing, 29 pounds; weight of sand on wing, 231 pounds; total weight supported by wing, 260 pounds, or one pound per square foot.Deflection of cross-rib, numbering from inner edge to extreme outer edge of wing—Number of rib.Deflection. Inches.1 (Heavy end rib)5.529.5311.75412.25512.5612.75712.9813.0912.010 (Heavy end rib)9.75
Area, 260 sq. ft.; weight of wing, 29 pounds; weight of sand on wing, 231 pounds; total weight supported by wing, 260 pounds, or one pound per square foot.
Deflection of cross-rib, numbering from inner edge to extreme outer edge of wing—
The weight of sand put on the wing in this test was 1.5 times as great as the pressure which at this time it was expected would be imposed upon it in flight, and was in fact 1.2 times as great as the normal pressure when supporting the aerodrome as finally constructed. Even under this weight the greatest deflection noted in terms of the total length of the rib was less than 0.10, showing that the elastic limit of the rib was far from being reached.[p204]
As this test seemed to indicate that the wings constructed in this manner were certainly strong and rigid enough for use on the aerodrome, and that immediate further improvement could hardly be made, three similar wings were at once constructed to complete the set. Somewhat later two additional wings were provided, so that when the large aerodrome was taken to Widewater on the Potomac in 1903 one and a half complete sets of wings were on hand, which seemed to be ample to provide for any emergencies that might arise.
Each of these wings had, as is clearly shown in the drawings, Plates53and 54, two main ribs, which formed the main strength of the framework and gave the wing longitudinal rigidity. To the main front rib were attached the cross-ribs and the pieces for the curved extension later described. The mid-rib extended across the cross-ribs, parallel to and about 5 feet behind the front rib, this being approximately the line in which lay the center of pressure of the wing. It was upon this rib, therefore, that the greatest strain would fall.
The mid-rib, Plate66, Fig. 2, was 731.5 cm. (24 ft.) long, having at the butt an outer diameter of 38 mm. (1.5 in.) and an inner diameter of 25 mm. (1 in.), the walls being, therefore, approximately 6.5 mm. (0.25 in.) thick. From the butt to the middle point the section was uniform, but from this point it had a taper of one-twenty-fourth of an inch to the foot, so that at the tip it had an outer diameter of 25 mm. (1 in.), the thickness of the wall being unchanged. At the butt end a wooden block 8 inches long was glued inside the rib, and at uniform distances of 75 mm. (30 in.) 10 smaller blocks were glued in where the cross-ribs were attached. The main front rib was of the same form and size, except that it was some 2 inches shorter and had no blocks, except the long one at the butt, glued in it.
To these main ribs were attached, in the manner later described, the 10 cross-ribs, to which the cloth cover was attached. The 8 intermediate cross-ribs have already been described in connection with the tests. The cross-ribs at the end of the wings, upon which greater lateral strains would come from the stretching of the cloth, were made of the larger cross-section shown in Fig. 8 of Plate66. Additional longitudinal stiffness was provided by gluing a strip 2 mm. thick between the upper and lower halves, as shown in the section. These end ribs, as well as those next to the ends, had small blocks glued into them where they were crossed by the diagonal braces, in addition to the small partitions 1 mm. thick, which were glued into the ribs every 3 inches to prevent crushing, and the blocks 2.5 and 3 inches long respectively, where they were attached to the front rib and to the mid-rib. At the extreme rear edge of the wing the cross-ribs were attached to the small “D”-rib, which served to hold the ribs at equal distances and to keep the cloth cover stretched tight. This “D”-rib, as shown in Plate66, Fig. 3, had semi-circular walls 4 mm. thick, 21 mm. in diameter, to the edge of which was glued a flat strip 3 mm. thick.[p205]
As originally designed the wings had a curve of only 1 in 18, the main front rib forming the leading edge of the wing. Later, however, it seemed desirable to “quicken” the curve and at the same time give the wing a sharper leading edge. This was accomplished by attaching to the front rib, at the points where the cross-ribs joined it, properly curved wooden pieces of the form shown in Plate66, Fig. 10, over which the cloth cover of the wing was stretched. The curve of the wing after the addition of this extension is shown in Plate66, Fig. 4, and was a curve having a rise of approximately 1 in 12, with the highest point .25 from the front end.
On account of the large size of these wings and the consequent difficulty in handling them it was necessary to construct them in such a manner that they could be easily taken apart, rolled up, transported to the house-boat or any other point where they might need to be used, and then quickly reassembled. After much experiment as to the best means of constructing them, the following plan was devised. The cloth covering was permanently fastened to the front rib, to which were attached the front extension pieces by means of small metal clips secured by small wood screws. On the rear edge of the front main rib, at a uniform distance of 30 inches apart, 10 small metal horns of 1-mm. tubing, 5 cm. long, each brazed to an independent clamping thimble, as shown in Fig. 9 of Plate66, were fastened. The front end of each of the cross-ribs was slightly rounded out to fit the front main rib, and in the wooden block which was glued in this end of the cross-rib a hole was bored to fit these horns. Each of the cross-ribs was then pushed over its proper horn and against the front main rib, and the cloth covering then drawn back toward the rear tips of the cross-ribs. In the extreme rear edge of the cloth covering a seam was made, and in this was inserted the “D”-rib already described. The cloth was then tightly stretched and a wood screw forced through the “D”-rib and into and through the metal ferrule at the tip of the cross-rib. Near the inner and outer edges of the cloth covering eyelets were placed about 6 inches apart, through which small cords were then inserted and tied to the end cross-ribs. The main or mid-rib was then placed on top of the cross-ribs and fastened to them with wood screws, and the cross-braces were then fastened on the top of the wing, as shown in Plate54. The frame of the wing was stiffened horizontally by cross guy-wires which passed from each cross-rib, at the point where the mid-rib crossed it, to the adjoining cross-rib, at the point where it was connected to the front rib. Each of the main ribs was individually guyed, in the manner clearly shown in Plate52, in order to stiffen it in the vertical direction, the fittings for these guy-wires being shown in detail in Figs. 11–15 of Plate66. Finally, small guy-wires were run from the front end of the cross-ribs over a guy-post 12 inches high at the point where the cross-rib crossed the mid-rib to the rear tip of the cross-rib. These cross guy-wires were regulated in[p206]tightness by raising and lowering a screw in the slot of the head of which they rested, and which was threaded in the end of the small guy-post. Upper and lower guy-wires, running from the main ribs to the guy-posts on the aerodrome, as already described, and as is clearly shown in the drawings, Plates52and54, completed the guy-wire system for the wings, except for the “drift wires,” which for the front wings were run from the lower side of the mid-rib to the bowsprit at the front of the machine, and for the rear wings to the main frame.
Each wing when completely assembled weighed approximately 29 pounds, and had a rectangular surface 22.5 by 11.5 feet (measured on the chord of the curve), or 260 square feet, making the weight per square foot equal about 50 grammes, rather less than 1.5 times as much per square foot as the wings for the steam-driven models. The total supporting surface of the aerodrome was 1040 square feet, and as the aerodrome when equipped for flight weighed, including the aviator, 850 pounds this gave 1.22 square feet to the pound, or 0.82 pound to the square foot. Although this was a somewhat larger proportion of weight to supporting surface than it had originally been expected to have, there is every reason to believe that it was sufficient, for the quarter-size model, when weighted so that it had 1.22 square feet to the pound, flew well, as will later appear.