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.
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
a later date, the skids, as shown by Fig. 86, terminated in short bends. Each skid carried a pair of rolling wheels, attached to a short axle, this being bound to the skids by rubber bands. The wheel base being almost 9 ft., this type gave excellent results. In the case of big machines, where it is desired to keep the load on the tail skid as light as possible, three wheels are sometimes used, two main rolling wheels and a light pilot wheel in the front. This enables the main rolling wheels to be placed under the centre of gravity, the pilot wheel preventing the consequent tendency to pitch forward when rolling. A further development of this system dispenses with the tail skid, two main wheels being placed under the centre of gravity, and two smaller wheels a little forward of
the propeller, as in Fig. 87. The skids were sometimes continued back behind the rear struts, and saw-kerfed to increase the resiliency. The base of support was formed by the rear wheels and the ends of the skids, the machine being pulled on to the front wheels by the thrust of the propeller. The short wheel base is bad for rolling on bumpy ground, and frequent skid replacements are necessary with this system. A similar type with no tail skid has the wheels disposed forward of the C.G., while a single central skid, connected to the fuselage by
a series of V struts, replaced the double skids, as in Fig. 88. This type was used on the original Nieuport monoplane, and with minor modifications on the Avro 80 h.p. Gnome tractor biplane. Its chief advantage is low head resistance, but unfortunately with this system a narrow wheel base, with the attendant defects, is inevitable. A very distinctive system was that favoured by Bleriot, and used with minor alterations on all the Bleriot monoplanes. This is shown, diagrammatically, in side elevation, by Fig. 89, and was unusual in that the wheels were arranged to swivel, this being an attempt to counteract the side strains set up when landing in a side wind. Although in the hands of some of our most famous exhibition pilots this has functioned excellently, it is complicated and somewhat heavy.
Recent Developments.
During the last three years the vital necessity of speed and climb, and more speed and climb, has resulted in the gradual elimination of skids, struts, and wires, until to-day the chassis for machines of average dimensions is almost invariably the V type (Fig. 90). The wheels are placed about a foot in front of the C.G., as, owing to the absence of any forward skid, no other provision exists to counteract the tendency to pitch over. In the actual construction of the Vee undercarriage, some diversity of practice exists with regard to the material chosen. In some cases the struts forming the Vees are constructed of a streamline section steel tubing, in others round tubing, the streamline section being obtained by a wooden fairing bound on, while a number of constructors use wood for the struts.