GLIDING 15 



and 5° respectively would mean 70 and 90 miles 

 per hour. Even 1 horse-power, with an angle of 

 5° gives a velocity of 43 ; in other words, 1 horse- 

 power, if the plane is set at an angle of 5°, is more 

 effective than 3 horse-power when the angle is 30°. 

 In fact, if you have a good aeroplane and the skill 

 to use it well, you require less power to fly fast than 

 to fly slow. But if you reduce the angle of inclina- 

 tion below 5° you find that the tables are turned 

 upon you. So far from economising power, you 

 would have to use it in lavish style, to overcome 

 friction. 



Curve of Wings. 



We must now turn to the question of curves. 

 The curved surface is undoubtedly superior to the 

 flat. A toy paper glider, being a mere feather- 

 weight, can dispense with curves, but without 

 curved surfaces — cleverly designed ones too — 

 aviation would be out of the question. An aeroplane 

 presents concave surfaces to the air ; its " planes " 

 curve from front to back, and a bird's wings have 

 concavities that are probably better adapted for 

 flight than anything that human ingenuity has 

 designed. On this subject even an umbrella can 

 tell us something. When its ample concavity is 

 turned towards the wind its efficiency in catching 

 and holding the air is so great that it speedily 

 becomes a wreck of wires. Lilienthal, a distin- 

 guished flight pioneer, whose experiments in gliding 

 did much to make aviation practicable, fully ap- 

 preciated the value of the curved surface. Some- 

 times when he was carrying his glider to a little 



