35*^ THE CONQUEST OF THE AIR. 



design the necessary speed, the solution of flying- is solved. This 

 speed is obtained by means of so-called propellers, also designed 

 on the principle of the inclined plane. In this way we obtain the 

 aeroplane. 



The action of the aeroplane is as follows : — By setting the 

 motor going the propellors throw air backwards, which causes a 

 reactive force that pushes the aeroplane forward. The latter com- 

 mences to run if allowed to do so. The arched sustaining sur- 

 faces thereby throw air downwards, which produces a reactive 

 upward force ; and when the lateral speed is high enough, so that 

 the reactive upward force is equal to the weight of the whole 

 machine, including pilot, the machine rises. We see that the 

 aeroplane cannot rise on the spot, but is obliged to make a start 

 by first having a run. If the velocity increases still further the 

 machine keeps on rising. Each machine is, however, limited to 

 a certain height independently of the steering mechanism. The 

 horizontal travelling of an aeroplane may be compared with the 

 hill climbing of a motor car. If the motor is stopped the aero- 

 plane would glide down the natural angle of descent. As T = | G, 

 the angle of descent will be about f . 



To keep horizontal the motor has to overcome gravity. To 

 make it rise, additional work must be done, i.e. the speed of the 

 motor must be increased. This speed is limited. Moreover the 

 higher the regions, the thinner is the air ; hence, in order to move 

 the same mass of air, the speed must be still further increased if 

 the machine is to keep on rising. Consequently, no further rising 

 will take place when the increase in speed, which the motor can 

 give, does not more than counterbalance the reduction in the 

 weight of the air moved. If the same motor is to lift the aero- 

 plane still higher, the only remedy lies in the cutting of the wings, 

 i.e. the sustaining surfaces. 



Consider now the stability and steering. We distinguish 

 between longitudinal and lateral stability. A machine is stable 

 longitudinally when the centre of wind pressure coincides with 

 the centre of gravity. The latter is largely fixed, except for the 

 pilot. The former varies with the speed. If the latter is increased, 

 the centre of wind pressure moves forward. This the pilot has 

 to watch. He can alter the non-stability somewhat by moving 

 his body. To make the stability more secure, aeronauts use 

 elevators and fixed tails, and in most cases warp the wings. The 

 action of the elevators is as follows : — 



When the speed is sufficiently high the elevators are so placed 

 that the air strikes underneath. This manoeuvre takes place to 

 make the aeroplane rise. To descend, the reverse takes place. 

 With a tail the same result is obtained or improved. The warping 

 of the wings is done for similar reasons. If the wings are flat- 

 tened, the angle of incidence reduced, the wing drops there. If 

 it is warped so that the air strikes underneath, the wing rises 

 here. It will be obvious that such warping may also be employed 

 for lateral stability. This warping is also resorted to for turning. 

 By Increasing the angle of incidence the resistance is increased 



