852 The Outline of Science 



on a windy day are often tilted off their balance and have either 

 to try again or "land" badly. In the air they may be noticed 

 adjusting themselves tc the "bumps" caused by sudden altera- 

 tions in the centre of air pressure. 



So far as flying is concerned, the wings are the most im- 

 portant part of an aeroplane. There may be one, two, or more 

 sets of planes according to the type of machine monoplane, 

 biplane, triplane, etc. These planes are slightly curved, the apex 

 of the curve being nearer the front edge than the rear edge of 

 the wing. The thickness of the plane also varies, for it gains 

 in breadth somewhat abruptly from the front to the apex of the 

 curve, and then narrows down gradually to the rear of the plane. 

 When in flight this wing is not absolutely parallel to the path 

 of flight, but is slightly tilted so that the wind blows against its 

 under-surface. The rush of air round the wing sets up pressure 

 on the under-side and suction on the top surface, the so-called 

 "lift" of a wing being about two-thirds suction and one-third 

 pressure. In order to keep up the flow of air round the wings 

 an aeroplane has to be fitted with a motor, an internal com- 

 bustion engine built on similar principles to the engine of a 

 motor-car. This engine revolves the air-screw which either pulls 

 or pushes the wings through the -air and so causes the necessary 

 lift to be set up. 



What the Pilot Does 



The control of an aeroplane is simple in the extreme. There 

 are two levers for the use of the pilot, one an upright control 

 lever known as the "joy-stick," which works the elevator and 

 ailerons or wing-flaps, the other a rudder bar, set near the floor 

 of the machine and operated by the pilot's feet. In addition 

 there are the ordinary switch, and ignition, and throttle controls 

 for the engine, which at present is always of the internal com- 

 bustion type. 



The principle of the internal combustion aero-engine 



