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.
[Illustration: FIG. 91.--Streamlining curved toe of skid.]
[Illustration: FIG. 92.--Laminated skid end.]
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
[Illustration: FIG. 93.--Arrangement of suspension on steel tube
undercarriage.]
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
[Illustration: FIG. 94.--Arrangement of suspension on wood
undercarriage.]
radius rod. It is not considered necessary, in many instances, to
fit either web plate or radius rod, the movement of the axle
[Illustration: FIG. 95.--Farman type axle suspension.]
being of no great extent. Another system is shown by Fig. 95,
this being the method of suspension adopted for the Farman
[Illustration: FIG. 96.--Rubber cord suspension.]
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.
[Illustration: FIG. 97.--Bridge type suspension.]
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.
[Illustration: FIG. 98.--Axle fairing.]
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.
[Illustration: FIG. 99.--Tail skid with claw fitting.]
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.