undercarriage the wheels were mounted as castors to facilitate landing across the wind. This was subsequently abandoned.
The common arrangement of undercarriage comprises a pair of wheels a little forward of the centre of gravity of the aeroplane and a small tail skid. The wheels, of wire-spoke construction with pneumatic tires, are carried on an axle of steel tube at- tached to two V-struts from the aeroplane by rubber cord. The tail skid is also sprung by means of rubber and is mounted on a swivel. Steering on the ground was improved in 1912 by arranging the tail skid to be moved by the rudder -bar. The use of skids and wheels ahead of the main wheels was generally abandoned early in the war, except in the case of large aero- planes.
Steel springs have been used, but rubber is superior to steel because it stores more energy for a given weight. Hysteresis in rubber is also much greater than in steel. To avoid bouncing after the first shock the energy received on impact should be restored as little as possible. This requirement led to the design of undercarriages containing a combination of steel spring and oil dashpot, such as the " Oleo " design fitted to the Breguet and to the Royal Aircraft Factory's " BE-2 " in 1914. This form of " shock absorber " was chiefly useful for night flying.
Methods of Construction. The first experimenters built their aeroplanes of wood and fabric with metal at joints and in the form of piano-wire bracing. The aeroplane of to-day uses spruce for beams and struts and steel for joints and tension members, the latter in the form of stranded cable, or " rafwires," i.e. rods rolled to a " streamline " section. Wings and body are covered with linen, pulled taut by " dope," and varnished or painted for protection from sunlight and moisture. Frames composed entirely of metal were used as early as 1911, but wood remains in general use, except for the tropics. Steel tubes have been extensively used in parts, notably for the part of the body to which the engine is attached, for struts between the planes, and in the undercarriage. The use of steel tubes for the engine- bearers gave place to wood owing to the greater absorption of vibration obtained.
The wings in the common type of biplane contain two wood spars of I or box section forming the flanges of a truss braced by wood or steel struts and cables or solid wires. To these spars are attached transverse ribs which give the shape of the wing and a light wood edge completes the frame. The linen covering is sewn on to this with a seam along the rear edge; stitched to every rib since 1914. The body is most often a frame of wood compression members and wire bracing. Bodies built of three-ply wood, with or without reinforcing members, have also been used. These retain their shape better and, being infinitely redundant structures, have perhaps some advantage against rifle fire; but the former have been preferred apparently as being more easily repaired and in- spected and allowing of a more certain calculation of stresses.
Metal construction advanced further in Germany than in other countries. Junker produced aeroplanes without external bracing, strength being obtained by the use of thick wings. These contained in place of the usual two spars a number of steel tubes interconnected by tubes forming triangles. The wings were covered with aluminium sheet corrugated so that the air flowed along the corrugations. The interconnecting tubes and the corrugations replaced the usual ribs. Great Britain has experimented with spars and ribs of steel and duralumin, and secured the necessary strength without increase of weight; but metal construction is still in the experimental stage. The principal difficulty in the use of steel lies in the prevention of local buckling due to the thin gauge of metal required to secure a light structure. Welding is unreliable owing to the impossibility of detecting weakness in the finished part, and joints are made by rivets or bolts. Bodies have been made of duralumin on the same lines as those built of three-ply wood.
The Strength Required in the Structure. The aeroplane structure is subjected to a very variable load. In straight flight the wings support the weight of the craft. A sudden gust, or change in the direction, or speed of the relative wind, momentarily increases or decreases the load. To estimate the extent of this, the proportion which any possible gust bears to the speed of flight must be known. On a banked turn or when returning to level flight after diving, the wings must provide an accelerating force, depending upon the rate of turn and the speed of flight. The pioneers were content to fly warily, and the accelerations necessary when they turned were small. The larger variations in loads were due to gusts. They flew only in the calmest weather, but their speed was slow. As soon as the aeroplane was used for trick flying, the effect of gusts became relatively insignificant, and the accelerations due to manoeuvres
became the necessary basis of design. In an aerial combat the wings may have to sustain over three times the normal load, and it is not practicable to design a fighting aeroplane for the accelerations which could be produced by flattening out too rapidly from a steep dive, in which a speed of over 200 m. an hour may be reached.
The determination of the load variation possible is one part of the problem of specifying the strength required of the wing structure. We must also know how this load is distributed over the surface, along and across the wing, and how it is shared by the different members of the structure. The important factor is the variation of the " centre of pressure " on the wing. As the angle between the wing and the direction of motion decreases the centre of pressure moves backward with increasing rapidity. It may be noted here that in a nearly vertical dive at high speed, although the lift of the wings is small, there is a large couple acting upon them tending to twist them and to turn the aeroplane over on its back; this is resisted by the action of the tail.- A number of the early accidents occurred in the course of a " vol pique," or steep dive.
Rough calculations were probably made of the strength of the early aeroplanes, and in 19112 those supplied to the Government were tested by inverting them and loading the wings with sand. Spars of wings were also tested separately, but as a rule both the strength required and the strength realized were uncertain quanti- ties. A number of accidents to monoplanes led to this type becoming suspect. Early in 1912 B16riot forwarded a suggested explanation to the French War Office, which resulted in the suspension for a few months of the use of monoplanes by the French army. Later in the year accidents to monoplanes in England led to a suspension of their use by the War Office, although the navy continued to use them. A committee was appointed and reported early in 1913. It decided that the accidents were due to the construction of these monoplanes, but not to anything inherent in the monoplane system. They recommended that the wings should be braced internally against drag (a remarkable omission previously), the main bracing wires duplicated and made independent of the undercarriage, and the fabric well fastened to the ribs, especially on the upper surface. Makers were to supply evidence of strength; official inspection and investigation of accidents were instituted; and the question of sta- bility and the danger of the " vol pique " and recovery were to be investigated.
Prior to this, efforts had been made in England to impose a factor of strength based on the load in straight level flight through steady air. The same factor has since been termed the " load factor." In 1914 the British Advisory Committee for Aeronautics issued a report on " factors of safety," regarding the load factor as the product of two factors, one representing the number of times maximum load might exceed the normal load, and the other a factor to cover possible faults of material and workmanship. The first factor is based on the acceleration due to a banked turn combined with a gust, and to recovery from a dive. Forty-five degrees was the steepest angle of bank considered advisable and it is recommended that to secure safety aeroplanes should not be dived to a speed exceeding the nor- mal by more than 20 per cent. The committee advised that the structure should have a factor of safety of at least 2 under the acceleration so obtained. A factor of from 6 to 8 (which had been worked to by the Royal Aircraft Factory since 1912) was recom- mended, to be increased to 12 if this should become possible. There is no record of the obligatory use of such factors in France or Germany at this date.
During the war the problems involved were investigated both mathematically and by experiment. Loops and mock fights were carried out at the Royal Aircraft Factory by aeroplanes fitted with an accelerometcr and with tension meters on the wires. The distribu- tion of pressure^ over wings has been measured in wind tunnels (first by Eiffel in Paris) and on aeroplanes in flight at Farnborough. It is now possible to specify the strength of the various members of an aeroplane with sufficient accuracy for any manoeuvres required. The " load factor " demanded has never risen to 12, but now ranges from 4 to 8, the lower factor for the large aeroplane which is not so violently manoeuvred. The adequacy of these factors has been confirmed by experience.
The need for extreme lightness precludes the use of the factors of safety currently used in other branches of engineering, and instead accuracy of stress calculation and careful inspection and testing of materials are imposed. It became the practice of the British Govern- ment to check by its own officials the strength of each design by detail calculations of stresses and by a proof load on one aeroplane of a type. Other governments followed. Since 1918-9 Great Britain requires that an " air-worthiness certificate " be obtained before a type may be used for commercial purposes. Drawings are sub- mitted by the applicant from which calculations of stresses are made by the Air Ministry.
The calculation of stresses proceeds upon the usual lines, com- mon to other branches of engineering, but with rather greater ac- curacy of detail. The theorem of Three Moments is applied to the spars, which require treatment as beams continuous through a number of supports and subjected to end load. Aeronautical practice has somewhat extended this theorem. A theory of the strength of struts of tapering section has been evolved. Knowledge of the mechanical properties of timber has been much extended.