Engineering Wonders of the World
Volume III

6 ENGINEERING WONDERS OF THE WORLD. and more air until a state of equilibrium is again attained. We must observe, however, that this increase of angle means also a great increase in drift proportionately to lift. If the descent of the kite had been caused, not by decrease in wind velocity, but by the addi- tion of weight to the kite, the increase in the pull on the string would have been very noticeable. Fig. 1.—DIAGRAM TO SHOW THE FORCES ACTING ON A KITE. It is the aim of the kite-maker as well as of the aeroplane builder to design surfaces which shall use the wind pressure most effi- ciently—that is, extract a maximum of lifting force, and be subject to a minimum of drift. If the string of a kite breaks, the equilibrium of forces is destroyed ; drift and gravity take command, and the kite either tumbles or glides to earth backwards. If it were possible to attach to the kite at the moment of rupture a weightless engine and propeller, exerting a horizontal windward push equal to the drift, the kite would remain stationary. Again, were the wind to drop suddenly, and the engine to give the kite a forward velocity equal to that of the wind, the kite would move forward—assuming that it were able to main- tain its stability—and be a true aeroplane or self - supporting heavier - than - air apparatus. Under usual conditions a kite is not strictly self-supporting, in that it depends on the resist- ance of a string anchored to a fixed point. Lilienthal, the great German experimenter, Octave Chanute, the brothers Wright, and other seekers after aerostatical knowledge, made use of man - bearing « T-I „ ... r “ Gliders. gliders, either free or an- chored, of large area, as well as of laboratory tests on surfaces of various forms, from which was derived the preliminary knowledge neces- sary to the construction of mechanical self- propelled and self-sustaining machines. With- out going into wearisome details, it may be stated that the shape and the arrangement of surfaces to give the greatest lifting power and stability were the chief objects of their search. It. was proved conclusively that (a) a true plane had not, area for area, so great a sustain- ing power as a slightly curved surface, convex on the upper side. Horatio Phillips, and subsequently Shape of Maxim, demonstrated by elabo- s“PPortin£ rate tests that (6) an aeroplane (we here apply the term to a sustaining sur- face, not to a machine) with the upper surface more curved than the lower, and inclining downwards in front so as to give a “ negative entering angle ” (see Fig. 2), was most efficient. Fig. 2.—SECTION OF A DECK WHICH GIVES GOOD LIFTING POWER. Tho arrows indicate the direction of the wind. Tests showed that (c) depth fore and aft was not so important as length of transverse enter- ing edge ; that, in fact, a number of narrow aeroplanes, arranged one over the other, Vene- tian blind fashion, were much more effective than a single aeroplane of equal length and of a breadth totalling that of the narrow aero- planes. It has been established that (d) in the case of well-made aeroplanes the lift in- creases, within certain limits, in direct propor- tion to the angle of inclination or incidence : thus, a plane making an angle of 10° with the horizontal has twice the lift of one inclined at 5° to the horizontal. Also that (e) the drift