Engineering Wonders of the World
Volume III

THEORY AND PRINCIPLES OF THE AEROPLANE. 9 The fact that some flying machines give a much better lift per horse-power is due to a naturally better (more acute) gliding angle, to good design as regards minimizing fric- tional resistances, to high engine and propeller efficiency, or to a combination of all three. A Wright machine, weighing 950 lbs., is pro- pelled at 40 miles per hour by a 24 horse- power engine, which works out at over 40 lbs. carried per horse-power. THE MAINTENANCE OF STABILITY. The flying machine, as at present consti- tuted, is able and liable to topple in any direc- tion. As flight necessitates high speed and considerable elevation above the earth’s sur- face, the maintenance of stability is literally of vital importance. Even under favourable conditions early experimenters found it ex- tremely difficult to counteract the tendency of a glider or power-driven machine to execute unpremeditated dives and somersaults. The history of flight is punctuated by records of more or less disastrous spills resulting directly from the failure of the aviator to keep the machine in such a position that the centre of air-pressure should lie over or coincide with the centre of gravity of the mass in motion. The problem of balancing an aeroplane is a peculiar one. Hold a sheet of paper hori- zontally and let it fall. It darts first one way and then another. You can only guess at the direction which it will take finally before alighting. If launched horizontally, it be- haves in a most erratic manner. Even a more scientifically designed paper “ glider,” instead of following a steady downward course, dips up and down, as if influenced by a horizontal rudder. This phenomenon is due to the fact that the pressure on a surface of Pressure. movin§ obliquely through the air varies in strength at differ- ent points on that surface, being greater at the front than at the back edge. The centre of pressure—that is, the point at which the total pressure may be considered to act___is normally situated, in the case of a curved “ deck,” about one-third of the width, of the deck from its front edge; or the pressure may be regarded as affecting the deck on a line drawn transversely through this point. An increase of speed moves the centre of pressure nearer to the front edge of the oblique surface ; a decrease causes it to recede to- wards the rear edge. A paper glider, as it swoops downwards, is tilted up in front be- cause, though the centre of gravity remains unchanged, the centre of pressure has worked forwards, and the air gets an upward leverage at the front. The tilt gives the glider extra lift, but also slows it; the speed decreases, the centre of pressure recedes, and the original angle of descent is resumed. This cycle of variations may recur many times in the course of a glide. To keep an aeroplane from pitching longi- tudinally, provision must be made whereby the centre of pressure may be kept close to the centre of gravity at varying speeds. All biplanes are fitted Front with an auxiliary movable hori- zontal surface or surfaces in front of the main decks, and under control of the pilot. Move- ments of the elevator vary the average angle of incidence of all the sustaining surfaces. Thus, if the aeroplane is gliding downwards, and the pilot wishes to take a horizontal course, he raises the front edge of the elevator. This gives the elevator a greater upward leverage, and increases the angle of incidence of the main decks. To cause a descent, the elevator is tilted downwards, and the general angle of incidence decreased. Gusts of winds coming headways on are counteracted by a proper manipulation of the elevator. It should be understood, however, that the elevator has but little effect in making the machine take a steady upward course. For this an increase in the motive force is re- quired.