48
FLIGHT
JANUARY 13TH,
1944
Tricycle L a n d i n g Gear is
A Review of Design and Performance Criteria
• G R O U P CLASSIFICATION:
T
B.3
HE susceptibility of the conventional tail wheel type
tion at the nose-wheel may be found, in turn, from the
of landing gear to nosing over under a strong braking
shock absorber travel and the aircraft sinking speed; unlike
effort, its inherent tendency to ground-loop, particuthe three-wheel landing case, the total wheel base has no
larly in a side-wind, and the necessity of making a fullcritical effect on the nose-wheel reaction.
stall landing to prevent bouncing have made it less and less
Load Distribution
satisfactory as landing speeds have increased. If properly
designed, the tricycle landing gear will eliminate these
The general load distribution of the aircraft naturally
undesirable characteristics, but it may introduce other
plays a decisive rdle in the determination of landing gear
objectionable features peculiar to its type. Among these
loads. In order to illustrate these conditions, it is possible
are porpoising, longitudinal instability with the nose-wheel
to represent the aircraft by an arrangement of two concenoff the ground, shimmy of the nose-wheel and failure of
trated masses, one* of which is located directly over the
the nose-wheel strut from execessive dynamic loads. In
nose-wheel and corresponds to the load produced by the
addition, it should be remembered that the elimination
landing impact in a nose-wheel first landing, whereas the
of the faults in the convensecond is located at a distional gear will result in
tance aft of the e.g. which
increased loads on the trimay be determined by the
*THIS is the fourth in the series of articles based on
cycle gear and airframe
distance of the nose-wheel
abstracts from the world's scientific and technical Press as
structure. The removal of
from the e.g., and the air--. _
compiled by R.T.P.3 Section of the M.A.P.
the possibility of nosing
craft's radius of gyration. A .
The subjects dealt with in the series are divided into the
over will lead to faster
calculation carried out with
following groups :—
taxying and manoeuvring
a single-engined, s i n g l e A. Aerodynamics and Hydronamics.
on the ground ; the eliminaseater tricycle machine,
b. Aircraft and Airscrews.
tion of the tendency to
driven by an engine and
C. Engines and Accessories.
ground-loop will result in
tractor airscrew mounted in
D. Materials and Methods.
more cross-wind landings,
E. Instruments and Devices.
the nose has revealed a
F. Production.
and the lack of any tenradius of gyration of nearly
dency to bounce will result
G. Physiology and War Medicine.
18.4 per cent, of the aircraft
in more landings at high
Each article appearing will carry its classification group
length, whereas the distance
sinking speeds.
letter followed by a number indication that it is the 2nd, 4th,
of the nose-wheel from the
7th, etc., in that group to be published.
e.g. is between 20 and 35
A paper read before the
The articles will not necessarily appear in the alphabetical
per cent, of the aircraft .
U.S. Institute of the Aero
order of their groups.
length.
nautical Sciences by Jenkins
and Donovan, dealt with
This means that .between
the design and perform46 and 22 per cent, of the
ance aspects of tricycle undercarriages in a most
aircraft's weight acts on the nose-wheel; since the static
comprehensive manner. In the authors' opinion many of
percentage is usually only about 15 per cent., the error in
the investigations so fai carried out, both theoretic and
designing from the load in the three-wheel attitude
ma^,
experimental, are erroneous, since they are only an adaptaexceed 200 per cent., assuming the same sinking speed in
tion of conventional landing gear criteria to the geometry
both conditions. These figures apply only to the type of
of tricycle gears, or have not been reviewed in the light of
aircraft outlined above, and they vary-widely in other
practical experience. For example, the effect of the wheeltypes, depending upon loading conditions, size, etc.; the
base, i.e., t i e distance between the nose-wheel and main
ratios between nose-wheel first, and three-wheel percentwheels' axles, on the characteristics of the landing gear
ages of weight acting on the nose-wheel have been found to
cannot be determined without considerations of its location
vary from 4 : 1 to I | : I .
relative to the centre of gravity and the distance of the
In order to reduce the weight actually on the nose-wheel,
main wheels aft of the aerodynamic centre.
it is useful to select the largest possible distance between
the nose-wheel and the e.g.; it is wise, furthermore, to
M o d e - o f - L a n d i n g Effect
make the travel of the nose-wheel considerably greater than
The distance of the nosewheel from the aircraft's e.g.
that of the main wheels, by which means the acceleration
principally affects the nosewheel load and also has an influin a nose-wheel first landing and the loads on the noseence on the resistance to porpoising and the stability against
wheel and structure will be reduced.
overturning. The size of the loads placed on the nosewheel
Of much greater importance than the location of the
by the landing impact, in turn, depend on the type of
nose-wheel is the distance of the main wheels aft of the
landing made, e.g., (i) three-wheel landing, (ii) main wheels
e.g., because this affects more of the properties than any
first, or (iii) nose-wheel first. In the three-wheel landing,
other single characteristic. It influences not only the
the load distribution between the front and rear wheels
directional and longitudinal stability, the length of takeis usually assumed to be the same as the static distribuoff run and. the static reaction on the nose-wheel, but also
tion ; an assumption which can be considerably modified by
the ability to land with excess air speed and high sinking
the effect of the shock absorber on the nose-wheel assembly.
speed without bouncing.
This distance is determined
partly by the requirement that the aircraft's e.g. must be
In a main-wheels-first landing, the proximity of the rear
ahead of the main wheels even when the tail buffer is
wheels to the e.g. will result in most of the energy being
against the ground. Thus the vertical forces acting at the
absorbed there. The loads on the nose-wheel produced by
main wheels will tend to decrease the angle of attack of
the subsequent dropping of the aircraft's nose may therethe aircraft, reducing the lift and preventing it from rising
tore be expected to be less than in a three-wheel landing.
into the air again.
In a nose-wheel first landing, the nose wheel striking the
ground tends to rotate the aircraft about its lateral axis,
As it is normally desirable to make the take-off with the
so that the load on the nose-wheel depends on the pitching
nose-wheel off the ground, the aircraft must be stable
moment of inertia of the particular aircraft. The acceleralongitudinally when running only on the rear wheels, other-