"The second kind of flier is the aeroplane, which, as its name indicates, is supplied with 'air planes,' that give it the power of rising and falling at the will of the operator when the machine is in motion. These planes play a very important part in the successful operation of the machine, as I will explain later. The first type of machine is classed as a 'lighter-than-air' machine or a balloon, while the planes of all kinds are classed as 'heavier-than-air' machines. Among other types of 'fliers,' there is the helicoptere, which is raised by screws or propellers on vertical shafts. These revolve rapidly, and drive the machine upward, just as the propeller on the Caroline drives her forward when in rapid motion. Another type, nearly abandoned, is called the ornithoptere, or 'wing flyer.' These machines are built to operate like the wings of a bird, and are provided with the necessary contrivances to work the wings, both vertically and horizontally. This type, like the helicoptere, is not considered practicable, and is virtually abandoned, so that the field is now left altogether to the 'lighter-than-air,' and the aeroplane machines. I do not intend giving you any instruction regarding balloons, or dirigibles, as I think such is unnecessary, but will confine myself altogether to the discussion of aeroplanes.
Fig. 44. Aero-curves
"It must not be supposed because of the name aeroplane, that the so-called plane is a real plane; it is not. The front edge of an air-ship plane must always be curved, as shown in [Fig. 44], so that the air strikes the under surface and is forced under the plane, to buoy up the machine as it moves forward; or, to put it another way, there must be a current of air either natural or artificial on which the machine must float, or it will be drawn by gravitation to the earth. While we cannot see air or wind, we know from experience that it has great power, and for thousands of years ships have been propelled across the seas by this force, acting on sails of some kind. We know how difficult it is to travel against a high wind, and it is this quality in the air that makes it possible to travel through it. The resistance of the atmosphere makes it possible for the aviator to hold his machine suspended in opposition to the laws of gravity, and to drive it forward and upward by means of the revolving propeller acting against this resistance, the motor acting on the same principle and manner as the wheel or propeller of a boat when it is urged forward. If, as I have seen George do, we take a flat stone, a piece of slate, or flat metal, and throw it along the face of the river, in such a manner that its flat surface strikes the surface of the water, it will skim along, striking the water at intervals in its course, until the force given by the hand that threw it is exhausted, when it will drop and sink. The water, though lighter in equal bulk than the stone, is aided by the force given by the hand to buoy up the stone until the force is expended. The curve on the front edge of the planes, when the machine is in motion, really takes in more air than the space allowed for it under normal conditions, and it may be said to be compressed to some extent. If the wind be blowing in the 'teeth' of the machine, the resistance of the air will be greater, and the buoyancy of the machine increased. So, also, if the machine is travelling rapidly, the motion will increase the resistance and the buoyancy at the same time. The moment the propellers stop, gravitation grasps the machine, and if the planes are kept evenly balanced it will quietly and gently descend to the earth. You must particularly bear in mind that wind blowing in the face of a machine tends to hold it up, and that a machine flying rapidly makes its own wind, so that the results are the same.
"The curve on the front of the planes may be called an 'aero-curve,' and much of the success of the machine depends on this curvature of the planes, which gives to the inside of the plane a concave shape of a peculiar character, and to the outside a convex form.
Fig. 44a. Maxim's aero-curve
"If you examine the rough drawing I made for you on the blackboard ([Fig. 44]) you will notice that the upper or convex curve is different from the under or concave one, and it is upon this difference in curvatures that many of the flying qualities of the machine depend. This little section showing the different curves is the one used by many of the successful aviators, though some prefer the form invented by Sir Hiram Maxim, shown in [Fig. 44a], which does not differ very materially from the previous section shown. In all cases, however, the accepted plane is one of a curved vertical section in which the convex side is uppermost and the upper surface more curved than the lower. Although different authorities disagree as to why this shape of plane is best, all agree that it is so. Sir Hiram Maxim's theory is that the air follows both the upper and lower surfaces of the plane, as shown in [Fig. 44a], while Phillips holds that the air follows the lower surface of the plane, and, striking the hump, shown at A, [Fig. 44], is reflected off the upper surface of the plane, thus forming a partial vacuum on the upper surface, which gives an additional upward pull to the plane. There is, however, little doubt that most of the work is done by the force exerted on the lower surface of the plane.