WEBSTER— DYNAMICAL ASPECTS. 



165 



Suppose we now know the law of the force exerted by the air 

 current on the plane, and the position of the center of pressure. 

 We have now to apply an elementary principle of equilibrium of 

 rigid bodies. If a body is submitted to the action of three forces 

 the lines of action of these forces must pass through a common 

 point. Thus if we consider a single plane supporting a machine, 

 with the resultant pressure R, Fig. 4, with weight W concentrated 

 at the center of gravity of the whole machine G, the thrust of the 

 propeller D, which is nearly horizontal, must pass through the in- 

 tersection of R and W. The second principle is that if we draw 

 lines representing by their length and direction the three forces in 



Fig. 4. 



question, these lines must form a closed triangle, Fig. 5. Thus 

 knowing the weight IV, we may find D, the thrust required from 

 the motor, as well as R, the force required, and a the angle of 

 attack. 



I shall illustrate the preceding principles by a very simple ex- 

 periment, which I think well shows all the leading ideas involved 

 in the dynamics of the aeroplane. I have here a heavy card fastened 

 by a hook at the middle of one side to this rubber cord. I now need 

 a very brave assistant, whom I request to hold the end of the rub- 

 ber cord at the height of his shoulder. I strongly stretch the 

 cord, holding the card in my hand, both card and string being hori- 

 zontal. We are thus in a position to perform the Wilhelm Tell 

 experiment, with the apparent probability that, since there are ab- 

 solutely no upward forces present, the card will cut Walther's head 

 ofif. On releasing the card you see that no such thing happens, 

 but the card soars several feet above my assistant's head, although 

 the cord is actually pulling down all the time. The reason is that 



