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tional to the weight. If aeroplanes of all sizes were constructed of the same materials and geometrically similar in all parts, the weight of the structure would increase with increasing size as the cube of the linear dimensions, that is, as the 3/2 power of the total weight. This does not in fact obtain, because geo- metric similarity would give greater strength to the larger aeroplane; also the design may be elaborated and materials worked to relatively finer dimensions; and moreover, large aeroplanes are not designed to have the same strength as smaller craft, as they are less sharply manoeuvred. Nevertheless, the weight of the structure is to be expected and is in fact found to become a larger proportion of the total weight as the size increases. It is therefore disadvantageous to increase size in- definitely and there is in fact a best size depending upon the duty to be done.

To carry an indivisible unit of cargo, such as a large bomb, an aeroplane of at least a certain size is required; hence we find size increasing. Sometimes it is preferable to carry a total load in a smaller number of larger aeroplanes, because the weight of the crew becomes less in proportion to the cargo carried, so that every square foot of wing and every unit of engine power of a fleet carries more useful load. Initial outlay and fuel consump- tion are reduced and there is further an economy of pilots. At some point the larger aeroplane requires a larger crew, and for war the larger " bomber " must carry a number of gunners and offensive armament for defence against more mobile attackers. The optimum size for a commercial service with a sufficient volume of traffic is what would be termed to-day a large aero- plane (say 7,000 Ib. at least). The actual size depends to some extent upon the speed of the service, which governs the relative costs of fuel and personnel, and also upon the distances.

The first large aeroplane flown was the Russian Sykorsky in 1913. Large aeroplanes were demanded in 1915 for bombing and were increasingly used during the war. The Handley Page (13,000 Ib. gross) was extensively used by the British. The " Gotha " and others were used for raids on London. The same Handley Page aeroplanes and a subsequent design were em- ployed on a passenger service between London and Paris through- out 1919 and 1920. The " Vimy " (12,500 Ib. gross) crossed the Atlantic, flew from Cairo to the Cape, and from Europe to Australia, and has been used on a London-Paris commercial service.

Controlling Surfaces. Stability in aviation is discussed in Section III. Complete inherent stability is obtainable by a proper dis- tribution of weight and subsidiary surfaces and suitable arrangement of the main planes. The planes are commonly inclined upwards from root to tip to secure a righting couple if one wing tip falls and the aeroplane begins to sideslip. A vertical surface at the rear, known as a fin, is general although the rudder may entirely replace this surface. The travel of the " centre of lift " of the wings is such as to produce instability, and a subsidiary horizontal surface is required either in front or in the rear. To secure " longitudinal " stability, the centre of gravity must be sufficiently, forward in rela- tion to the main planes, and the load on the subsidiary surface main- tains equilibrium. The aeroplane has three degrees of angular freedom and has almost invariably employed three means of con- trol: elevators, to produce a " pitching ' motion, and so govern the angle of attack of the wings and the speed of flight ; rudders to pro- duce motion about the vertical axis ; and warp or ailerons, to secure lateral balance and adjust the angle of " bank." The early Voisin aeroplanes had no control for lateral balance. The aeroplane when turning has a natural tendency to bank, which is accentuated or reduced by sideslip outwards and inwards respectively if the wings are inclined upwards from root to tip or fitted with a vertical surface above the centre of gravity. The Voisin aeroplane carried curtains between the planes to provide this righting couple and was sufficiently controllable for the requirements of the pioneer content to achieve flight. " Lateral " control is desirable and is clearly necessary for rapid manoeuvring. The Wrights obtained this by twisting or " warping " the wings, and this method was extensively used up to the end of 1914. Control has been more generally obtained by means of hinged portions of the wings at the rear near the wing tips.

Elevators have been placed both in front and in the rear: rudders always in the rear. They have constituted the whole, or only a part of, the necessary stabilizing surfaces. Control with a single rudder requires an effective " keel " surface, which is adequately provided by the body of the aeroplane and the exposed struts of the structure. The tendency of design towards the " tractor " type places elevators and rudders most conveniently at the rear end, and this gives a

clear field of view forwards. The early biplanes with an elevator in front and rudder at the rear disappeared about 1914 ; the monoplanes conformed to the modern usage. Both elevators and rudders are usually hinged portions of fixed surfaces, but in some cases the entire surface has been movable and constituted the elevator or rudder. The latter arrangement has not provided stability if the controls were abandoned. Later the fixed horizontal surface was made adjustable by the pilot during flight and known as a " trimming tail plane," a device much used by the British from 1916 onwards. It enabled the flier to vary the speed of flight at which no pressure upon the con- trolling lever was required, and effectively increased the range of control resulting from the application of a definite force.

The arrangement of control levers or wheels, at first very diverse, became standardized in 19156, and consists of a " rudder bar " operated by the feet and a hand lever whose fore-and-aft movement operates the elevators and whose lateral movement provides latera, control. The rudder bar and the lever are moved in the direction in which it is desired to move the aeroplane. In larger aeroplanes rotation of a wheel mounted on the fore-and-aft lever actuates the ailerons, the fore-and-aft control remaining as before. The lever or wheel is generally connected to the control surfaces by steel cables, although shafts in torsion and tension or compression mem- bers have also been used.

Balanced control surfaces, although in use from an early date, only became necessary as the size of aeroplanes increased. A part of the surface to be balanced is carried in front of the hinge and this surface is most frequently the rear portion of a fixed element, the part brought forward of the hinge being extended beyond the end of the fixed element. This so-called " horn " balance proved un- satisfactory. If a large " horn " were used (adequate to give ease in normal flight), there was overbalance at low speeds, or when the aeroplane sideslipped, and the controls would then tend to " take charge." A more uniform effort results if the balancing projection is run the full span of the hinge, which must then be set back behind the fixed element. The front edge of the balanced surface is sharp and its movement takes place behind the bluff end of the fixed element. Alternatively separate balancing surfaces in advance of the hinge have been rigidly attached to the moving element and placed above the fixed element.

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EXAMPLE OF HORN BALANCE

FIG. 14.

EXAMPLE OF SET-BACK HINGE BALANCE FIG. 143.

Two Methods of Balancing Ailerons.

The imperfection of balancing obtained has led to the develop- ment of relay motors to reduce the effort. In these, power derived from the air by a small windmill is brought into play whenever the flier attempts to move the controls. Relay motors had been but little used up to 1921.

Chassis or Undercarriage. The Wright aeroplane alighted upon skids. It was launched by a catapult. The French pioneers took the air under their own power, and the Farman and Bleriot used wheels. From 1909-14 combined wheels and skids were used. The wheels were commonly sprung by means of 'rubber cord. The skids might be brought into action if the alight- ing were imperfectly executed, and were carried well forward to prevent the aeroplane from turning over forwards when land- ing. Sometimes additional wheels were fitted in a forward position in place of the skids for this purpose. Under the tail a wheel was often fitted, but a small skid was used alternatively. Wing-tip wheels or more commonly light skids were used to protect the wing tips from contact with the ground. In Bleriot's