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.