Global Transportation 83 



High-altitude planes are greatly aided by hydromatic or 

 electrically controlled propellers, which automatically control 

 the pitch of the blades to permit the engine to operate at its 

 greatest efficiency. For take-off the blade pitch is low to let 

 the engine attain full power. In high pitch the blades bite into 

 the air at a greater angle, and thus maintain thrust in thinner 

 air. The high-pitch blades also prevent overspeeding of the 

 engine when the plane is in level flight. 



The speed at which the tip of the propeller blade turns must 

 be kept below the speed of sound about seven hundred and 

 fifty miles per hour or there will be an enormous increase 

 in drag. The air fails to behave in its usual manner. The pres- 

 sure generated at the forward point cannot get out of the 

 way and so must be carried along by the moving object. To 

 make it possible for both the propeller and the engine to turn 

 at their most suitable speeds, a set of reduction gears is built 

 into the nose section of the engine. The gears slow down the 

 propeller to fifty to seventy-five revolutions for each one 

 hundred of the engine. 



How can the pilot of a high-altitude plane be protected? 

 We have noted that a person's lungs are flexible enough for 

 him to compensate for the thinness of air at high altitudes by 

 breathing in more air. This "supercharging" operation works 

 satisfactorily within certain limits. A healthy pilot has no 

 trouble getting enough air in his lungs at ten thousand feet. 

 But at twelve thousand feet a mask is needed to supply oxy- 

 gen or the flyer loses efficiency. Because of the reduced air 

 pressure, the higher the pilot goes the more need there is for 

 oxygen to keep him normal. Above twenty-five thousand feet 

 the air is so thin that it is impossible to whistle. At thirty-three 

 thousand feet the pilot must be breathing 100 per cent pure 

 oxygen or he will die. 



If the pilot continues to go higher, he does not get enough 



