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.
[Illustration: 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
[Illustration: FIG. 86.--Side view of Farman landing gear.]
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
[Illustration: FIG. 87.--Side view of four-wheeled landing gear.]
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
[Illustration: FIG. 88.--Side view of Nieuport undercarriage.]
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.
[Illustration: FIG. 89.--Bleriot undercarriage.]
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.
[Illustration: FIG. 90.--Vee type undercarriage.]