398 Introduction to the Study of Science 



How? Can a kite be made to fly from a moving steamer's deck 

 during a calm? If a steamer were moving at the rate of ten miles 

 an hour, and the wind were blowing at the same velocity in the same 

 direction, would a kite fly? 



Skin friction. Besides the drift due to the fact that some of 

 the kite surface is presented horizontally to the air, there is 

 also a drift due to the friction of the air moving over its surfaces. 

 This is called " skin friction." Whether this be little or much 

 depends upon the form and roughness of the surfaces. This 

 factor must be taken into account in the selection of material 

 for the planes of an airplane. 



Forms of vanes of airplanes. In the building of airplanes 

 much careful experimental work has been done to determine 



what form of vanes will give 



the greatest lift with the 



least drift. The drift in the 



"FIG. uo. -cross section of plane and airplane represents the pres- 

 horizontai air current. Compare with sure to be maintained in the 



section of a bird ' s f rward direction b y the 



thrust of the propellers. The 

 lift is the power of the machine to sustain itself and its load. 

 Curiously enough the supporting vanes are now shaped some- 

 what like the curve shown in Fig. 140. The resemblance to the 

 form of the bird 's wings is apparent . Experiment has shown that 

 this form is best adapted to flying, a fact already established 

 by natural selection in the evolution of flying creatures. 



Stability. Besides the form of the planes there are other 

 problems of even greater importance in the airplane. The 

 machine must have stability, equilibrium, or balance in all 

 directions. Otherwise it might tip to its destruction in any 

 one of four ways. The tipping forward and backward can be 

 controlled by horizontal rudders, fastened some distance astern. 

 If the plane starts to dive forward, it is automatically righted 

 by the increased air pressure of the upper surface of the horizontal 

 tail piece. 



