Fig. 50. A model aeroplane
"Please, father," said Fred, "do keep on." George, also, wanted to hear more, so Mr. Gregg decided to continue.
Fig. 51. Section model aeroplane
"I have given you an outline of the reason why an aeroplane can be made to rise from the ground and navigate the air; but I have not told you of all the kinds of machines that can be made to fly, for there are many others than those I have spoken of. One is the glider, which does not carry an engine, but, as its name indicates, glides along in the air at a distance not far from the earth. These are not capable of travelling very far and, therefore, are not likely to come into general use. They have to be started either by gliding off a high tower, by sliding down a hill or by being propelled by hand or towed by some rapidly moving machine. Some day, perhaps, a machine will be evolved on the same or similar lines as the glider, that can be propelled by natural forces, but the time is not yet. Beside the monoplane and the biplane, there is the triplane, constructed on the same lines as the other flying planes, that is to say, the three planes used on the machine are made the same as the planes on the others, each having a convex and concave side of different curvatures.
Fig. 52. Blade of propeller