NO. 3 LANGLEY MEMOIB ON MECHANICAL FLIGHT 61 



ciently when it was inside tlie hull. Finally such a degree of success was attained 

 that the burners could be kept lighted even when the aerodrome was placed in a 

 considerable artificial breeze, created by a blower in the shop. 



In connection with these tests of the engines and boilers, some method was 

 desired, in addition to the Prony brake tests, by which the thrust of the propel- 

 lers when driven by the engines at various speeds could be measured accurately 

 and in terms which would be readily available in judging whether the aerodromes 

 were ready to be given an actual trial in free flight. Such a method was found 

 in the use of an apparatus known as the " pendulum," which was introduced 

 near the end of 1892, but was not generally used until the end of 1893. After 

 this time, however, this test was made a condition prerequisite to taking any of 

 the aerodromes into the field, and proved of the greatest assistance in estimat- 

 ing the probable outcome of the trials. 



The apparatus used, which is diagrammatically shown in Fig. 10, was ex- 

 tremely simple both in theory and operation. It consisted primarily of a hori- 

 zontal arm AC carrying the knife-edge B by which it is pivoted on each side on 

 supporting beams not shown. Depending from AC is the light vertical arm DE, 

 rigidly joined to it and carrying the lower horizontal arm FG, all of which are 

 braced together so as to maintain the arm DE constantly perpendicular to AC. 

 To this arm FG the model was rigidly attached with its center of gravity in line 

 with the vertical arm DE and its weight increased by the addition of properly 

 disposed flat weights, in order to make the angle of lift for a given thrust of the 

 propellers smaller and less likely to interfere with the working of the boiler and 

 separator. 



Before the actual test of the " lift " could be made, it was necessary to know 

 the exact distance of the vertical center of gravity of the model and the extra 

 weights from the knife-edge B. This was determined by the following method: 

 A known weight was suspended from the arm AB at some arbitrarily selected 

 distance from the point B. This weight caused the perpendicular arms AB and 

 DE to rotate through an angle, 0, which was measured on the scale KL. Know- 

 ing, then, the weight on the arm AB, its point of application, the weight of the 

 aerodrome suspended on the arm DE, and the angle of rotation, it is easy, by a 

 simple application of trigonometric functions, to determine the distance of the 

 center of gravity of the model from the point B. 



In a test of Aerodrome No. 6 made on September 23, 1898, the weight sus- 

 pended from AB was 10,000 grammes, its point of application 50 cm., the model 

 was weighted to 20,450 grammes, and the angle of rotation, 6, was 7° 2'. Letting 

 y equal the distance of the CG from B, we may equate the balanced forces thus : 



10,000x50 cos 7° 2' = 20,450 XT/ sin 7° 2' 

 10,000 x 50 cot 7° 2' = 20,450 y 

 y = 198.2 cm. 



