Fig. 62.—This shows an aeroplane of great thickness, placed at the highest angle that will ever be used—1 in 4—and even with this the air follows the upper and lower surfaces. No eddies are formed, and the direction that the air takes after leaving the aeroplane is the resultant of the top and bottom angles.
Fig. 63.—Section of a screw blade having a rib on the back. The resistance caused by this rib is erroneously supposed to be skin friction.
Fig. 64.—Shows a flat aeroplane placed at an angle of 45°, an angle which will never be used in practical flight, but at this angle the momentum of the approaching air and the energy necessary to give it an acceleration sufficiently great to make it follow the back of the aeroplane are equal, and at this point, the wind may either follow the surface or not. Sometimes it does and sometimes it does not. See [experiments with screws].
Fig. 65.—The aeroplane here shown is a mathematical paradox. This aeroplane lifts, no matter in which direction it is driven. It encounters air which is stationary and leaves it with a downward trend; therefore it must lift. However, if we remove the section b, and only subject a to the blast, as shown at [Fig. 66], no lifting effect is produced. On the contrary, the air has a tendency to press a, downwards. The path which the air takes is clearly shown; this is most important, as it shows that the shape of the top side is a factor which has to be considered. All the lifting effect in this case is produced by the top side.