NO. 



LANCF.KV MF.MOTH (IX MECHANICAL FLIGHT 161 



the aerodrome at the extreme rear cud just preparatory to launching, and the 

 latter showing- the front end of the track folded down with the hinged prop 

 standing outward in its downward path and the aerodrome jus! launched. These 

 photographs will be more particularly referred to later, but attention is here 

 called to them so that the description immediately following may be more 

 easily understood. 



Although this method of launching the aerodrome seemed to Mr. Langley, 

 both theoretically and from the experience with the models, to be a satisfactory 

 aud feasible plan, there were two very important respects in which it seemed 

 from the very first open to objection. In the first place, it was necessary that 

 the aerodrome should be launched as nearly at its soaring speed as possible, 

 because either an excess or deficiency of speed interfered to some extent with 

 the equilibrium of the machine. So many factors were involved in the deter- 

 mination of what this final velocity should be that it seemed almost impossible 

 to he sure of the results until at least one test of the aerodrome had been made. 

 In the second place it was not known whether the rapid acceleration of the car 

 would seriously interfere with the equilibrium of the aviator. 



In reference to the first question it was, of course, known that a freely 

 falling body acquires a speed of 32 feet per second at the end of the first sec- 

 ond after having fallen a distance of 16 feet. It was proposed to launch the 

 aerodrome at approximately 35 feet per second; and, since the distance over 

 which the car would pass in acquiring this speed was approximately 60 feet, the 

 rate of acceleration would, of course, be less than that for a freely falling body. 

 The conditions in the two cases, however, are quite different. In the case of 

 the freely falling body there is the constant force of gravity which causes the 

 acceleration. In the case of the aerodrome the car is initially standing still but 

 ready to be acted upon by the combined force of the thrust of the propellers 

 . and the tension of the springs. The propeller thrust is approximately 450 pounds 

 at the moment of releasing the car, while the spring tension adds approximately 

 400 pounds more pull, giving a total pull of 850 pounds acting on the car at the 

 start. The weight of the aerodrome including the aeronaut being approximately 

 850 pounds, and the weight of the car being approximately 450 pounds, the total 

 weight to be accelerated is 1300 pounds. The resistance of the car and the aero- 

 drome is zero at the moment the car is released, and increases approximately as 

 the square of the velocity until it reaches approximately 300 pounds at the soar- 

 ing speed of the aerodrome; while on the other hand the spring tension de- 

 creases uniformly from 400 pounds at the start to approximately 76 pounds at 

 the end of the track, and the thrust due to the propellers decreases from 450 

 pounds at the start to approximately 250 pounds at the moment of launching. 

 Consequently, it is in a general way clear that the rate of acceleration of the 

 aerodrome and car decrease, probably in a geometric ratio, the rate of accelera- 



16 



