NO. 3 LANGLEY MEMOIR ON MECHANICAL FLIGHT 77 



edly light frame and the elevation of the transverse frame 12 centimeters above 

 the midrod, whereby the position of the line of thrust was raised so that it was 

 20 centimetres from the center of pressure, wbicli from theory seemed to be 

 very nearly its correct position. The total flying weight was but 6400 grammes 

 (14 pounds), with a total supporting surface of fifty-four square feet, equivalent 

 to very nearly four square feet per pound. It was hoped that with this extremely 

 light construction the " dead lift " would amount to a large percentage of the 

 flying weight, and as much as sixty per cent was actually lifted on the pendulum. 

 As, however, the aerodrome approached completion it became more and more 

 evident that the construction was hopelessly fragile, the frame being scarcely 

 able to support itself in the shop. By November this conclusion became certain, 

 and this aerodrome (New No. 4) was never put to an actual test in the field. The 

 very expensive set of wings covered with gold beater's skin, which were also 

 constructed at this time for this model, proved so weak under test that they were 

 entirely abandoned. 



When Mr. Langley returned to Washington in the fall, many important 

 points, which had been under special consideration during the past year, partic- 

 ularly those relating to the disposition of sustaining surfaces, and the pro- 

 vision of automatic equilibrium, were still not definitely determined. It was 

 not yet decided whether two sets of wings of equal area should be used for the 

 aerodrome, or what the efficiency per unit of area of the following surfaces was 

 in comparison with the leading surfaces. To aid in determining these and other 

 important points concerning the relative position of the center of gravity and 

 the center of pressure in the horizontal planes, he had several small gliding mod- 

 els made, which could be used with either one or two pairs of wings, and afforded 

 an opportunity for testing and comparing several types of curved surfaces. 



These models were built so that the center of gravity could be adjusted to 

 any desired point, and had in addition, as a means of assisting in preserving 

 equilibrium, a small tail-rudder, shaped somewhat like a child's dart, which was 

 intended to support no part of the weight. 



The tests with these models were very satisfactory and aided greatly in the 

 final development of what is known as the " Langley type." Indeed, in the 

 single month of November all the points, which had hitherto been more or less 

 indefinite, were finally decided upon, and the tests of the following spring proved 

 these decisions correct. 



Two sets of wings of equal area were hereafter provided for every aero- 

 drome, which not only greatly increased the stability, but also overcame the 

 difficulty hitherto experienced in bringing the CP over the CG. The tail-rudder, 

 formed of planes intersecting at right angles, was adopted as the means of con- 

 trol. In use on the aerodromes it was set at a negative angle, and given a certain 



