The general plan for the large aerodrome was never a matter of uncertainty. At the time when the first general designs were made there had been in the history of mankind only one type of machine, that of the steam-driven Langley models, which had proved capable of flight for any considerable distance. Furthermore, the selection of this type had been the result not of sudden fancy or of purely theoretical consideration, but of years of the most careful experimentation, in the course of which nearly every conceivable style of machine had been tested with some form of power. It would have been worse than folly, therefore, if the one clear path had been left to seek some unknown way.
It was fully realized from the first, however, that the increase in size alone would make necessary in the design for the large aerodrome a great many modifications from the designs of the steam-driven models. It was not possible here, as in nearly every other kind of structure, simply to magnify uniformly the parts and proportions of the small machine in order to obtain a successful large one. This is particularly true in the case of the aerodrome, because the rapid increase of weight in the larger structure is out of all proportion to the increase in strength, while it is very desirable that the more expensive machine which is designed to carry a human being shall be relatively even stronger than the easily replaced model. This problem of increasing size without sacrificing strength and stability, it was known from the beginning, would be encountered in a particularly difficult form in designing the frame of the large machine, and was to be solved not by the discovery of some new and wonderfully strong material, but by improvements both in the general plan and the details of the machine. Here, as is often the case, it was not the large changes in the design but the improvements in small and sometimes seemingly unimportant details which demanded the most careful consideration and, as a whole, contributed most to the final result. For this reason, as well as because the large changes, when pointed out, are usually easily understood, the present chapter is for the most part a description of the improvement of details.
From the experience gained in the construction of the frames of the several steam-driven models, it was decided that the frame for the large aerodrome must consist essentially of two principal parts. First, a rigid backbone was required, extending from the point of attachment of the front wings to the point of attachment of the rear wings; and this backbone, for convenience designated [p165] the “main frame,” must support the second principal part, the “transverse frame,” which formed a cross with the main frame, and at the ends of which the propellers were mounted. While it was necessary that this transverse frame should have considerable rigidity and strength in a vertical direction, yet its main strength and stiffness was required in the horizontal plane for withstanding the thrust of the propellers. It had been possible to construct the frames of the later steam-driven models stiff enough, and at the same time light enough, by the use of properly proportioned steel tubing, but calculation very soon showed that in order to secure sufficient rigidity for the frame of the large aerodrome and at the same time keep the weight within the permissible limit, it would be necessary to depend very largely on guy-wires and to use tubing only for forming the struts against which the guy-wires should act. But this obviously introduced a new series of problems. The extensive system of guy-wires necessary would add materially to the head resistance of the aerodrome, and this might conceivably be so great as to require more propulsive power than would be required for a frame heavier but unincumbered by the head resistance of the wires. It became necessary to consider these problems, but no data were accessible from which the head resistance could be computed with any confidence. The coefficient of resistance for a cylindrical body moving through the air in a direction perpendicular to its length may in general be taken as one-half that of a flat body of the same cross-section; but it was thought very certain that, owing to the fact that tightly stretched wires are in constant vibration when the aerodrome is in the air, the resistance of the wires must be considerably greater than would be calculated from treating them as cylinders having a coefficient of 0.5. Unfortunately, no data on the resistance of vibrating wires were at hand. Before proceeding with the designs for the guying of the frame, therefore, the following brief series of tests was made in November, 1898, on the whirling table, in order to learn approximately the resistance that the proposed system of guy-wires for the large aerodrome would offer:
MEASUREMENTS OF THE RESISTANCE OF GUY-WIRES, USING FRAME ATTACHED TO “BALANCE.”
| RESISTANCEOFFRAMEWITHOUTWIRES. | |||
| Frame consists of: 4 tubes, 1 cm. diameter, 14.5 cm. long; 2 tubes, 1 cm. diameter, 41 cm. long; 2 tubes, 1 cm. diameter, 101 cm. long. | |||
| Revolutions of turn-table per minute. | Velocity of frame. Feet per minute. | Resistance. Grammes. r. | Calculated resistance of frame. Grammes. |
| 6.75 | 608 | 11.5 | 14.2 |
| 9.75 | 877 | 34.0 | 29.6 |
| 12.0 | 1080 | 51.8 | 44.8 |
| 16.35 | 1475 | 97.0 | 83.8 |
| 19.75 | 1775 | 134.0 | 121.3 |
| 22.7 | 2045 | 168.0 | 161.2 |
| 25.5 | 2290 | 205.0 | 202.0 |
[p166]
| RESISTANCEOFFRAMEWITH 1STSETOFWIRES. | ||||
| First set of wires: 16 wires, 0.6 mm. diameter, 102 cm. long; 6 wires, 0.6 mm. diameter, 42 cm. long. | ||||
| Revolutions of turn-table per minute. | Velocity of wires. Feet per minute. | Resistance of frame and wires. Grammes. R1. | Resistance of wires. R1−r=r1. | Calculated resistance of wires. Grammes. |
| 9.75 | 877 | 47.5 | 13.5 | 8.88 |
| 12.0 | 1080 | 73.5 | 21.7 | 13.47 |
| 13.75 | 1237 | 93.5 | 25.5 | 17.65 |
| 17.25 | 1550 | 144.0 | 37.0 | 27.7 |
| 20.25 | 1822 | 187.0 | 45.5 | 38.4 |
| 22.50 | 2025 | 216.0 | 47.5 | 47.4 |
| 22.875 | 2060 | 225.0 | 52.0 | 49.0 |
| 24.56 | 2215 | 250.0 | 56.5 | 56.7 |
| RESISTANCEOFFRAMEWITH 2DSETOFWIRES. | ||||
| Second set of wires: 15 wires, 1.2 mm. diameter, 102 cm. long; 2 wires, 1.2 mm. diameter, 42 cm. long. | ||||
| Revolutions of turn-table per minute. | Velocity of wires. Feet per minute. | Resistance of frame and wires. Grammes. R2. | Resistance of wires. R2−r=r2. | Calculated resistance of wires. Grammes. |
| 9.25 | 833 | 54.0 | 26.5 | 15.35 |
| 9.35 | 841 | 55.0 | 27.0 | 15.4 |
| 11.3 | 1018 | 82.0 | 36.75 | 22.65 |
| 11.5 | 1035 | 82.0 | 35.25 | 23.4 |
| 13.0 | 1170 | 104.5 | 43.0 | 29.9 |
| 13.15 | 1185 | 105.0 | 42.5 | 30.60 |
| 16.7 | 1505 | 160.0 | 59.0 | 49.5 |
| 16.75 | 1510 | 160.0 | 58.0 | 49.9 |
| 19.5 | 1755 | 196.0 | 64.0 | 67.4 |
| 19.7 | 1770 | 203.0 | 69.5 | 68.5 |
| 21.60 | 1945 | 236.0 | 77.0 | 82.6 |
| 21.65 | 1950 | 237.0 | 77.75 | 83.2 |
| 21.75 | 1957 | 235.0 | 75.5 | 83.7 |
| RESISTANCEOFFRAMEWITH 3DSETOFWIRES. | ||||
| Third set of wires: 15 wires, 2 mm. diameter, 102 cm. long; 2 wires, 2 mm. diameter, 42 cm. long. | ||||
| Revolutions of turn-table per minute. | Velocity of wires. Feet per minute. | Resistance of frame and wires. Grammes. R3. | Resistance of wires. R3−r=r3. | Calculated resistance of wires. Grammes. |
| 9.25 | 833 | 65 | 37.5 | 25.2 |
| 11.55 | 1040 | 91 | 43.5 | 39.35 |
| 11.55 | 1040 | 101 | 53.5 | 39.35 |
| 15.25 | 1375 | 160 | 75.0 | 68.7 |
| 18.1 | 1630 | 203 | 86.5 | 96.6 |
| 19.25 | 1735 | 221 | 92.0 | 109.5 |
| 19.25 | 1735 | 216 | 87.0 | 109.5 |
| 20.63 | 1860 | 237 | 90.5 | 125.8 |
The last column of these tables is calculated for a coefficient of form equal to 0.5, which has been found to be approximately correct for a rigid cylindrical body.