CHAPTER XI.
UNDERCARRIAGE DETAILS.
The details of construction associated with the undercarriage are those concerned with the forming of the struts and main members, and the suspension of the axle. As noted in the previous chapter the Vee undercarriage is greatly in favour at present, but the fact that with this type no forward support exists to prevent pitching over when obstructions are met in rolling, will almost certainly result in some arrangement of wheels and skids for the touring machines of the post-war period. Machines are now designed for air performance pure and simple, so that an undercarriage of the simple Vee type is all that is permissible; but in the post-war machine general utility will be the desideratum sought for by designers. At one time the majority of the undercarriage arrangements incorporated one or more skids. The material most suited for this purpose is hickory, although some designers prefer ash, steamed to the desired curve, and generally channelled out between the points of intersection of the struts, fittings, etc., in a similar manner to longerons and wing spars.
Where the bend is sharp, and therefore difficult to obtain by steaming, it is usual to form the skid from a number of strips, or laminations, glued together. Quite a good method of stream-lining the curved toe of the skid is shown by Fig. 91, consisting of a spruce block attached to the skid by screws, and it has additional value in ensuring permanency of curve. Where the design is such that the rear end of the skid performs the functions of a tail skid it is saw-kerfed, as in Fig. 92, the laminations so formed being stepped back, and the bottom layer shod with a plate, or claw fitting, acting as a brake, and also preventing wear produced by contact with the ground. At one time this constituted popular practice, but it is a matter of some difficulty to prevent the saw-cuts from developing into fractures. As a matter of fact, on one type of machine replacements were so frequent that eventually the skid end was left solid.
Methods of Suspension.
In the preliminaries of design referred to in the last chapter, it was observed that the action of rolling and alighting called for a good system of suspension and shock absorption, and this is accomplished on modern machines by binding the axle to the main members of the structure with either rubber cord (this being a number of strands of rubber about 1/16 in. square, compressed and bound together with a woven twine casing) or plain rubber rings. The latter are more or less obsolescent, at least in this country, the reason being found in the better lasting qualities of the cord, which will also withstand a much higher ultimate stress, the fabric covering contributing largely to this. In a number of cases, and generally for heavy machines, steel helical springs are fitted. Various attempts right from the beginning of successful flight have been made to utilize steel springs for suspension, but hitherto very few machines have successfully incorporated them, and but a brief examination will show that their use on machines of the average modern type is attended with some unsatisfactory features. Firstly, they are much heavier than rubber, but this in itself is no great disadvantage, as ease of attachment probably compensates for this; but what is of moment is the fact that steel springs are not nearly so efficient shock-absorbers as the rubber variety, while even the efficiency of the latter is capable of considerable improvement. If we take the case of a machine rolling over bumpy ground, all that is required of the suspension is that the wheel movement over the inequalities shall not be transmitted to the whole machine. So far both steel springs and rubber cord satisfy these conditions, but in the operation of alighting the machine not infrequently strikes the ground with some force, sometimes the result of gusts or pancaking. With steel springs, and to a lesser degree those of rubber, the energy of landing is not absorbed, but is stored up, being given out again in the form of a rebound. With rubber, elongation and its consequent depreciation of ultimate tensile strength prevents any energy of moment being returned to the aeroplane, which is why, for light machines of modern design, say, up to 2500 lbs. total weight, rubber is the better material. Steel springs being deficient in the power to damp out shocks, it becomes necessary to use these in conjunction with some other medium possessing this quality, and one of the most suitable arrangements extant is that known as the oleo-pneumatic gear, consisting of a combination of helical coil spring and oil plunger. It is usual to arrange the main compression members in two halves, the upper half forming a piston, and the lower, attached to the wheels, constituting the cylinder, is filled with oil. The weight of the machine is taken normally during rolling by the helical spring, wound round the upper half of the telescopic tube. Excessive shocks cause the oil to be forced through a spring valve, adjusted to open at a certain pressure, into the upper half, a back-pressure valve enabling the oil to gradually return to the cylinder. The Breguet biplane, a pre-war machine of original design, embodied in the undercarriage arrangement a system analogous to the foregoing.
Shock Absorbing Effect of Tyres.
The assistance rendered by tyres of large diameter must not be overlooked. The merits of the large tread are quite well known in the sphere of the motor-car, and they are no less beneficial to the aeroplane. It is of interest to record that a pre-war racing machine had no other suspension and shock-absorbing medium than that provided by the very large tyres fitted to the wheels, the axle being fixed rigidly to the undercarriage struts. A similar arrangement existed on a machine of much more recent date. One does not advocate this system, as it can be of very little use for rough ground, the instance being cited to emphasize the assistance so rendered to the ordinary type of suspension.
Connections.
Various methods exist for connecting the rubber to the main members, a typical arrangement with the Vee undercarriage
of steel being shown by Fig. 93, and a variation of this, when wood is the material, is indicated by Fig. 94. The web plate in Fig. 93 forms a means for guiding the axle in its upward travel, and is another version of the one-time popular
radius rod. It is not considered necessary, in many instances, to fit either web plate or radius rod, the movement of the axle
being of no great extent. Another system is shown by Fig. 95, this being the method of suspension adopted for the Farman
machines. In this case rubber bands are attached to the main skids, the short axle passing between the two. A similar arrangement in general outline is shown by Fig. 96, although in this case the rubber takes the form of cord.
A method greatly in vogue in America is that indicated by Fig. 97, known as the bridge type, and a characteristic Wright detail, the rings being approximately two inches wide by two inches long. The fact that very few examples of this system exist in this country may be ascribed to the inferiority of rubber bands compared with the rubber cable.
Axle Fairings.
It is now the practice to streamline the compression tubes between the vees of the undercarriage with a fairing of aluminium or three-ply. This is so arranged that in flight the axle lies in a slot formed in the fairing, which appreciably reduces head resistance. A typical arrangement is indicated by Fig. 98. The axle is usually formed of steel or duralumin tube, and in the majority of undercarriage arrangements is divided and hinged in the centre, a wire or wires from this point to the fuselage accounting for any strain. Duralumin tube is especially suited for this item, as a much stiffer axle is possible for a given weight, although, unfortunately, this is slightly discounted by the fact that duralumin does not form a good bearing surface for the wheel hubs, and it therefore becomes necessary to fit either sleeves or stub-axles of steel.
Undercarriage Brakes.
Additional means for restricting the length of travel after contact with the ground is sometimes found in the employment of brakes of various types. A very simple and widely used arrangement is to terminate the tail skid in a claw fitting, as Fig. 99, so that in alighting the tail is shoved hard down, bringing the skid into contact with the ground. The disadvantage is that undesirable strains may be carried to the fuselage members.
Another version recently patented is to construct small planes to conform to the wing curve, and hinged so that by a system of wires and pulleys, actuated from the pilot’s seat, they could be adjusted to offer a normal surface to the direction of flight. The efficiency of this arrangement at low speeds is not very great, moreover a landing with the wind renders them quite useless. The best form of brake is undoubtedly one acting direct on the main wheels, either of the rim or band type, a good example of the latter being the system used on the 70 h.p. Bristol biplane. Closely allied to the question of brakes is that of steering, and the requirements of this latter item are fairly well satisfied by pivoting the tail skid and working it in conjunction with the rudder from the foot-bar or wheel.
Housing of Undercarriage during Flight.
Numerous suggestions, ideas, and patents exist, having as their object the housing of the undercarriage in the fuselage during flight, with a resultant reduction in resistance; and excellent as the principle is, its practical application is difficult of achievement—at least, for machines of the present. In flight the undercarriage is a useless encumbrance, adding weight and head resistance, so that an arrangement whereby this component could be folded into the main structure would apparently effect a saving in resistance. This would mean that the fuselage would be of larger cross-sectional area, the natural sequence being extra weight and resistance. It does not appear that the saving effected in resistance, when the undercarriage is folded during flight, would account for the additional weight of the operating mechanism and the increased head resistance of the fuselage, so that altogether the advantages of any so-called disappearing landing gear are very much more apparent than real. There is also the very great possibility of the undercarriage folding up or disappearing when it would be least required to do so. In the construction of the problematic air-liners of the future it may be possible to economically effect the housing of the undercarriage.