In certain isolated instances, the fuselage is built up of steel tubing, and on one machine of recent design the joints throughout are effected by welding: a detail of the attachment of the vertical and cross struts to the longerons is shown by [Fig. 84]. It will be noticed that a small quadrant-shaped piece of tube or rod is welded to the struts, and from this are taken the bracing wires. As the welded joints impart a certain rigidity to the structure, the fact that the wires are exerting a side pull on the struts may be of little consequence, although this method could hardly be used in conjunction with the fuselage construction of average English machines. A rather unusual feature may be noticed in the attachment of the bracing wires, which are not finished off with the orthodox wire ferrule, but are arranged as a loop, the turnbuckle forming the anchorage for the two ends. The trend of design in this country seems to incline towards the clip stamped out from sheet steel and bent up. This class of fitting can be produced accurately and quickly, and, in the writer’s opinion, is by far the best manufacturing proposition. Aluminium castings are quite obsolete, and the built-up fitting, involving welding or brazing, does not seem greatly in vogue.
CHAPTER X.
UNDERCARRIAGE TYPES.
The present chapter deals with the general arrangement of the different types of undercarriages, as distinct from the details of construction. The principles of design embodied in the undercarriage are necessarily a compromise, this position being due to the fact that its construction has to be considered from two distinctly opposed view-points, and undue attention to the requirements of either does not produce the best results. Thus, on the one hand, we have the desirability of great strength to withstand landings on very rough ground, ploughed fields, and the like; and on the other hand, we have the considerations of aerodynamical efficiency in flight, which, taken to one extreme, would be best satisfied if the undercarriage did not exist, and at most calls for a system in which the head resistance is brought to an irreducible minimum. By the ordinary process of evolution the agglomeration of ideas existing in the early days of flying with regard to the most suitable form of landing gear, have given place to something which, for machines of modern attainments, approaches finality. This has resulted from improvements along the line of (1) simplification of general design, (2) the reduction of head resistance and weight without a consequent diminution in its powers as an alighting gear. A better impression of the distinguishing points of the various types will be gathered if we consider the desiderata of an ideal undercarriage.
Principles of Design.
One of the most important points is that rolling shocks should be completely absorbed, and the least possible strain transmitted to the fuselage or main structure, this calling for a good system of wheel suspension. It must be capable of standing the considerable strains sustained in alighting, not the least of which are those attendant upon landing in a side wind; should offer the least possible head resistance, while the weight must be reduced to a minimum. Cross-country flying, which more often than not means “getting off” in a restricted space, requires that the machine shall attain flying speed in the shortest time, and conversely in alighting the machine should come to rest in the quickest time. Innumerable smashes have been caused after a perfectly good landing by failure to pull up before a hedge, fence, or ditch. These are the main principles involved, and at least they indicate how and why the undercarriage is necessarily a compromise.
Fig. 85.
It is clear that in landing the speed of the machine relative to the ground should be as low as possible, without developing into the operation generally known as “pancaking,” or stalling, and the usual method of accomplishing this is to bring the machine into the wind, which, if of a moderate velocity, materially reduces the speed relative to the earth. In ordinary circumstances, landing would be accomplished by gradually increasing the angle of incidence until the maximum, or angle of no lift, is reached, which is practically stalling point. To satisfy this consideration, the heights of the main rolling wheels and tail skid should be arranged to allow the wings to lie at an angle a little in excess of this. With modern wing sections the angle of maximum lift is between 14° and 16°, so that the angle of 18°, as shown in [Fig. 85], is usually sufficient. This has additional value in restricting the length of run after contact with the ground, the wings acting as air-brakes. It will be realized that reduction in height of the undercarriage, desirable as it is from the aspect of head resistance, cannot be carried beyond a certain point without the sacrifice to some extent of the foregoing qualities. So far we have taken the principles of design as affecting the disposition of the undercarriage members in a longitudinal direction, but, of course, there are several details to be considered in its arrangement laterally. A fundamental point is that the track of the wheels, i.e. the distance, centre to centre, should be of ample width, but several constructional difficulties tend to restrict this to certain limits. Where the undercarriage is of the type in which the main rolling wheels are mounted on a single axle, it is clear that the wheel base is limited to the greatest length the steel or duralumin tube can be used without buckling under landing shocks. If this is to be exceeded a bigger diameter tube of thicker gauge will be necessary, and this means additional weight. Again, the fuselage width for the tractor machines now in vogue does not greatly exceed 3 ft., being more usually under that figure, so that a very wide base would mean raking the struts at a flat angle, which would therefore require to be made of larger section than would be the case if the wheel base was narrower; or, if the same section strut is used, the strength is reduced. A wide wheel base therefore means an undesirable increase in weight and resistance. To make up for the deficiencies of the almost unavoidable narrow wheel base, it is usual to make use of the wing tips by fitting skids of malacca cane or laminated ash, which are brought into action when the machine is excessively canted over sideways. At one time the wing tips were almost invariably used to assist the undercarriage, the wing tips of the Nieuport monoplane being specially constructed for the purpose, and no skids were fitted. Earlier still the R.E.P. monoplane had only one central rolling wheel, a smaller wheel being attached to each wing tip. The wing tip wheels of the Cody biplane performed similar functions, although these were used in conjunction with two main wheels.
Undercarriage Types.
The type of landing gear in use to-day does not vary in principle to any great extent, the differences usually occurring in the choice of material, the system being that usually known as the Vee type, from the fact that viewed in side elevation, the struts form a V. While this type has much to commend it from the points of low head resistance and great strength for weight, there are other systems, some of which have been tried-out, while others still exist, incorporating features designed for some specific purpose. Of these the Farman type is an example of a landing gear designed for the requirements of school work, consisting of two long ash skids, which, extended from the rear end of the nacelle, being gradually bent upwards to carry the front elevator. This was the arrangement on the “Longhorn” machine, but on the “Shorthorn,” produced at