LANDING GEAR - PURPOSE AND TYPES

peletonwhoopUrban and Civil

Nov 26, 2013 (3 years and 8 months ago)

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LANDING GEAR
-

PURPOSE AND TYPES

The
landing gear

in
aviation

is the structure that supports an
aircraft

on the ground and
allows it to
taxi
,

takeoff and to land. It is a complex structure capable of

reacting the largest
local loads on the aircraft. In one brief moment, the landing gear must
make the best of returning
the aircraft from its natural environment to hostile environment
-
the earth.



PURPOSE
:


The purpose of landing gear is



To provide smooth ride

and steering the aircraft
on taxiing



To absorb
kinetic
shock
s

during landing touchdown



To stop the aircraft
with wheel brakes
during runway operation



Provide adequate tail down angle for takeoff rotation &



To provide the stable support while ground parking

TYPES OF LANDING GEARS:



The

basic types of landing g
ear

are



TRAINGULATED



TRAILING LINK OR LEVERED



CANTILEVER



ARTICULATED



OLEO
-
PNEUMATIC


TRIANGULATED:



The triangulated gear is similar to the levered bungee gear . When the triangulated gear is
deflected, an oleo

pneumatic shock absorber is compressed.
This provides a leveraged effect in
which the oleo can be shorter than the required wheel travel. This is especially useful for carrier
-
based aircraft that require large amounts of wheel travel to absorb the carrier
-
landing impact
loads. On a triangulated
gear, the oleo can be replaced without removing the wheel assembly.
The wheel lateral and braking loads are carried by the solid gear legs, which reduces the oleo
weight. However, the complete triangulated gear is usually a little heavier than the oleo sho
ck
-
strut gear. Also, there is a tire
-
scrubbing effect that shortens tire life.

The triangulated gear is
sometimes seen on smaller aircraft using rubber blocks or springs in compression instead of an
oleo pneumatic shock absorber. The rubber blocks or sprin
gs can be inside the fuselage which
streamlines the exposed part of the gear but requires the gear leg to support the aircraft’s weight
in a cantilevered fashion. This increases the gear weight.

TRAILING LINK OR LEVERED:

The trailing
-
link, or levered, gear

resembles the triangulated gear, but with the solid gear
leg running aft rather than laterally. This gear is common for carrier
-
based aircraft where it
provides the large amounts of gear travel required for carrier landings. Typically the pivot point
of t
he lower gear leg is slightly in front of the tire, less than one tire radius in front of the tire.
The levered gear allows the wheel to travel aft as it deflects. This is very desirable for operations
on rough fields. If a large rock or other obstacle is
encountered, the aft motion of the wheel gives
it more times to ride over the obstacle. The triangulated gear and levered gear provide a
mechanical lever effect that reduces the deflection of the shock
-
absorber oleo. However, this also
increases the forces

on the shock
-
absorber oleo, which increases its required diameter. The
mechanical advantage of the triangulated or levered gear is determined from the actual
dimensions of the gear layout, and is used to size the shock absorber.

CANTILEVER
:

A cantilever c
onfiguration is most widely used, and it is without question the most cost
and weight efficient. The name comes from the fixit of shock strut cylinder to aircraft. It supports
drag and side loads.

ARTICULATED
:

An articulated gear finds application where th
e ground

clearance is low, or where
s
towage is limited. It offers maintenance advantage, since the shock strut can be removed in the
field without major effort. It is pin ended and does not support drag and side load semi
-
articulated gear is similar to ful
ly articulated except the cylinder also acts as a structural member.

Apart from this, there are main land
ing gear and nose landing gear

OLEOPNEUMATIC:

The oleo
-
pneumatic shock strut
or “oleo,” is the most common type of shock
-
absorbing
gear in use today.

The oleo combines a spring effect using compressed air with a damping effect
using a piston which forces oil through a small hole (orifice). For maximum efficiency, many
oleos have a mechanism for varying the size of the orifice as the oleo compresses (me
tered
orifice).

When used as a
shock
-
strut, the oleo itself must provide the full required amount of
wheel deflection, w
hich can lengthen the total landing gear height. Also, the oleo strut must be
strong enough to handle the lateral and braking loads of t
he wheels. To repair or replace the oleo
strut, the entire assembly must be removed because it is attached to the bottom of the strut.

Based on the arrangements
, Landing gear are mainly classified as



CONVENTIONAL



TRI
-
CYCLE GEAR



TANDEM WHEEL



SINGLE WHEEL
GEAR


CONVENTIONAL LANDING GEAR
:

Conventional landing gear also known as “Tail wheel” or “Tail dragger” type.

The main
landing gear wheels are located at each side of the center line, ahead of center of gravity, with
the steerable tail wheel located aft near the rudder. This type of landing gear was frequently
noted for its ability to land ‘tail first’ i.e., Grou
nd loop. This design places the main wheels on a
line running about 16.5
o

ahead of center of gravity with the airplane in a level altitude. For
aircraft having a high thrust line, it will be necessary to increase the angle by 4
o
or 5
o

to prevent
nose
-
over

during engine run up and rough field operation.

TRICYCLE LANDING GEAR
:

Tricycle landing gear also known as the “nose wheel” type. The main wheels are located
at each side of centerline behind the center of gravity, with a free swivel or steerable nose whe
el
mounted on centerline forward. This configuration is noted for ease of ground handling and will
not ground loop unless steered into a skid. Approach stability, longitudinal trim and tricycle gear
combine to make landing almost automatic with ground hand
ling no longer a major problem for
the occasional pilot.


The layout of tricycle landing gear is even more complex. The length of the landing gear
must be set so that the tail does not hit the ground on landing. This is measured from the wheel in
the sta
tic position assuming an aircraft angle of attack for landing that gives 90% of the
maximum lift. This ranges from about 10
-
15 deg for most type of aircraft. The “tipback angle”
is the maximum aircraft nose
-
up attitude with the tail touching the ground an
d the strut fully
extended. To prevent the aircraft from tipping back on its tail, the angle off the vertical from the
main wheel position to the c.g. should be greater than the tipback angle or 15 deg, whichever is
larger.


TANDEM
-
WHEEL LANDING GEAR:

Two

wheels mounted one behind the ot
her on the airplane centerline. T
he larger wheel is
usually located just behind the center of gravity,

with a smaller wheel forward in a nosewheel
position.

T
his arrangement has been carried to some sort of ultimate in bomb
er design which has
duel sets of tandem wheels retracting into the fuselage ahead and behind the bomb bay,plus
outrigger wheels retracting into each outboard wing panel.

T
andem
gear is difficult to taxi in a
crosswind,

e
ven with a steerable front wheel.


SINGLE
-
WHEEL GEAR
:

Single wheel gear u
sually mounted just forward or aft of the center of gravity with a
landing skid to protect the fuselage.

T
his type of gear is found on most sailplanes,

with the wheel
being manually retractable on higher performance
designs.








LOCATION OF LANDING GEARS ON THE AIRCRAFT

Main gear is located at the best position for structural pick up. It is located depending on
the stability criteria i.e. if the wing span is long then main gear is located on the wings, if the
wing
span is less it is located on the fuselage. the main gear tires cannot be selected until the
static load has been determined, and to do this nose landing gear must be selected. It should be
place as far as possible to minimize its load, minimize elevator p
ower required for takeoff
minimize main gear load for maximum retardation during braking, maximize flotation, and
maximize stability. Conversely, the load should not be too light either, in that event steering
would be difficult. The static and dynamic loa
ds acting are calculated using the following
formulae.

Let W= max
imum gross weight


Maximum static main gear load (per strut)


=

W (F
-
M)

/

2F


Maximum static nose gear load

=

W (F
-
L)

/

F


Minimum static nose gear
load

=

W (F
-
N)

/

F


Maximum braking nose gear load

=

Max.
Static load +

10H .W
/

32.2

F


(With 10 ft/sec
2

deceleration)

If the minimum static nose is too gear load small, the nose gear must be moved

aft or the main
gear must be moved aft. If the maximum static nose gear load is too high, the rev
erse procedure
must be used i.e.,

move the nose gear forward or move the main gear forward. Hence, it is
necessary to move both nose and main gear to obtain a

satisfactory overall compromise in the
overall loading.




PARTS OF

LAN
DING GEAR



Landing gear generally consists of
main cylinder, castor arms, shock absorbers, wheel
fork,

drag strut,
retraction actuators, up
-
lock, down lock

on nose landing gear and main cylinder,
shock absorber, wheel arm on main landing gear
. The description of the abov
e
-
mentioned parts
is as follows:



MAIN CYLINDER:

The main cylinder is
a blind bored cylinder which includes the shock absorber element.
The top of the main cylinder has a transverse boss for the pintle pin, which attaches the main
under carriage unit to the main plane structure. At a position below the transverse boss are
re
plenishment and bleed screws for shock absorbers. The bottom of the main cylinder is made
into an offset forked arm in which the wheel arm pivots. It also includes a mechanical lock and
shock absorbers. The mechanical lock includes a mechanical lock and sh
ock absorber. The
mechanical lock includes a spring loaded plunger. The plunger extends through the top of the
main cylinder and engages with the up lock and down lock brackets.

CASTOR ARM :


The castor arm is cylindrical at the top and forked at the
bottom. A gland nut which
engages with thread on the inside of the main cylinder attaches the castor arm to the bottom of
the main cylinder. The castor arm is attached on a bearing and linear, which lets the castor arm
castors. The wheel fork is attached t
o the forked end of the castor arm. A warning label is bonded
to the front face of the castor arm and red warning lines are painted on the forked end of the arm.

WHEEL ARM :


The wheel arm is a hollow type and it is attached to the main cylinder and it inc
ludes the
wheel axle, attachment points for the wheel brake units and a bracket to protect the brake piping.
A pivot pin attaches to the wheel arm to offset forked arm on main cylinder. Transverse bolts
hold the pivot pins in position. An Oleo link, which
includes spherical bearings is connected
between the wheel arm and the closed end of the plunger tube. The wheel speed transducers for
the anti skid system is fitted inside the wheel axle. Bolts attach the wheel axle to the wheel arm.


SHOCK ABSORBERS
:


A

requirement of all aircraft shock absorbers is that they absorb or dissipate the energy of
descent or transient or vertical shocks without transferring them to the vehicle or aircraft
structure

and also

ease and stability for ground maneuvering
.

The vario
us shock absorbing system are as follows :

a.

SHOCK CHORD

-

It is a simple and cheap way of controlling wheel deflection to absorb
vertical velocity. But except for friction developed between elastic components, no
impact energy is absorbed by shock chord. Of

course, axle displacement permitted by the
shock chord plus tire deflection reduces vertical impact velocity. This in turn reduces the
landing gear load factor and airframe loading, as shown in the spring strut design.





Figure: Oleo
-
pneumatic shock absorber


b.

RUBBER DISC

-

Rubber disc quite act like a shock cord, dissipating some energy by
generating internal friction and possibly surface friction as well through contact with a
cylinder wall. Again the resulting

wheel and tire deflections combine to reduce gear
impact loads imposed upon the airplane structure.


c.

FLUID

(alone)
-

Although oil is considered to be virtually incompressible fluid, Dowty
developed a liquid spring strut many years ago based on trailing be
am design. As fluid
pressure increases, the containing cylinder deflects, permitting the liquid strut to
compress and the wheel to move up. However the high pressure involve preclude use of
this type of absorber on small aircraft because of high cost and w
eight


d.

LEAF SPRING STRUTS
-

The combination of oil, a compression spring instead of air,
and an orifice used to be quite popular. This approach eliminated the need to monitor air
pressure for correct operation,
but the cylinder had to be fluid tight nonetheless.
Unfortunately, coil compression springs tend to set with time and would frequently
bottom with a bang if the oil level become too low. Spring struts are used very much
today. While leaf spring do not prov
ide the energy control of an air/oil orifice system,
while they have the distinct advantage of extreme simplicity and virtually maintenance
free operation. Steel is the most effective material for leaf spring landing gear because of
its strength characteri
stics.


e.

OLEO PNEUMATIC
-

Air and oil can be used to provide a desired rate of strut
deflection by controlling flow through an orifice. The air chamber above the oil is
pumped up to a pressure sufficient to support the static load on the strut when the ai
rplane
at rest. Impact loads experienced during taxiing over rough ground or when landing will
compress the air so increase strut deflection, forcing oil up through the orifice. The action
dissipates some energy while controlling the rate of strut deflecti
on, and so reduces
landing loads by eliminating shock peaks. A low sink rate at wheel contact will require
only a small deflection of the oleo strut to accept the landing loads which would be
similar to the static loading; while a high sink rate at ground
contact will cause large strut
deflections at a rate controlled by orifice design. To provide the low impact load factor
desired, orifice characteristics must be developed and proven by drop test. The metering
pin may not be necessary to regulate orifice a
rea vs. displacement for very light airplanes
since a simple drilled hole may be adequate, but it is mandatory for heavier models due to
the great variations in landing loads possible with larger aircrafts. The oleo strut is an
ideal method of controlling
impact loads, as evidenced by its use on carrier based aircraft.
Since an oleo requires more maintenance and more expensive than simple spring leg
strut.



Figure: Various types of shock absorbers


Most aircraft use oleo
-
pneumatic shock absorbers with e
fficiencies as high as 90
percent, it is almost a perfect device for absorbing the kinetic energy due to sink speed and also
best in energy dissipation.


Shock absorber
operation
: shock absorber undergoes both compression and expansion.

Shock absorber
Compression:

It is a two

tube arrangement having a cylinder and piston. The nitrogen chamber is
inside the piston and is charged at a particular pressure. The oil chamber is
between cylinder and
piston. A
separator separates the nitrogen and oil. When the

piston is compressed, the oil from
chamber is forced through a constant orifice by compressing the nitrogen during closure.
Compression stops when nitrogen force balances the load on the wheel.

Shock absorber Extension:

During
recoil, the nitrogen energy
stored during compression pushes the oil back into the
chamber through a constant area recoil orifice. The recoil orifice slows down extension by
restricting oil flow. The extension stops when the load on the wheel balances the decreasing
nitrogen force.

S
TROKE DETERMINATION:


The required deflection of the shock absorbing system (“the stroke”) depends upon the
vertical velocity at touch down, the shock absorbing material, and the amount of wing lift still
available after touch down. As a rule
-
of
-
thumb, the

stroke in inches approximately equals
vertical velocity at touch down in (ft/s). The vertical velocity (“sink speed”) at touch down is
established in various specifications for different types of aircraft. Most aircraft requires 10 ft/s
vertical velocity
capability. This is substantially above the 4
-
5 ft/s, that most pilot consider a
“bad” landing. While most Air force aircraft require only 10ft/s. Air force trainer aircraft requires
13 ft/s. Carrier based naval aircraft requires 20 ft/s or more which is m
uch like controlled crash.


The vertical energy of the aircraft which must be absorbed by the aircraft during the
landing is defined by the equation below,


KE
vertical

= ( 1 / 2 ) ( W
landing

/ g ) V
2
vertical


Where,


W = total aircraft weight


This kinetic energy is absorbed by the work of deflecting the shock absorbed and tire.


The actual energy absorbed by deflection is


KE
absorbed

=
ŋ
LS


Where,


Ŋ
= shock
-

absorbing efficiency


L = average total load during deflection



S = stroke


The stroke can be calculated by,


S = [( V
2
vertical

) / ( 2 x g x
ŋ
x N
gear

)]


[(
ŋ
T

/
ŋ
) S
T

]


The stroke calculated by the above equation should be increased by about 1 inch[3cm] as
a sa
fety margin. Also, a stroke of 8 inch [20cm] is usually considered a minimum, and at least
10
-
12inch[25
-
30inch] is desirable for most aircraft.

OLEO SIZING:


The actual dimension of oleo shock absorber or shock strut can now be estimated. The
total oleo stroke is known. For most types of aircraft the static position is approximately 66% of
the distance from the fully extended to the fully compressed position. F
or a general aviation
aircraft, the static position is typically about 60% of stroke above the extended position. The total
length of the oleo including the stroke distance and the fixed portion of the oleo will be
approximately 2.5 times the stroke.

For
an aircraft with the desired gear attachment point close to the ground, this minimum
oleo length may require going to a levered gear. The oleo diameter is determined by the load
carried by the oleo. The oleo carries its load by the internal pressure of com
pressed air, applied
across a piston. Typical an oleo has an internal pressure of 1800 psi.

The external diameter is typically 30% greater than the piston diameter, so the external
oleo diameter can be approximated by





D
oleo =
1.3

4L
oleo
/ Pπ where, L
oleo
= Load on the oleo.

LOCKS:


After retraction of the landing gear, doors close the bay. Depending on the location of the
retraction bay, the retraction can be forward retrac
tion.

The landing gears are to be kept in the
retracted position by means of locks.

There are two types of landing gear locks. Type
s

are




Uplock



Downlock



Ground safety lock


These two locks
, Uplock and Downlock

may be external or internal. If th
e locks are
provided external to the jack, such locks are called external
locks. I
f locks are provided internal
to the jack, such locks are called internal locks. Hence, retraction landing gears are held with the
help of up locks. Bef
ore the landing of the

aircraft, t
he bay doors have to be up locked and doors
open, and subsequently the landing gear up locks are released and the actuator extend
s the
landing gear.

After extension it has to be kept locked in the extended position, with the help of
down lock.


Down
locking of the landing gear shall be done by an internal lock in retraction actuator.
Up
-

locking shall be done through a hydro mechanical lock in retraction actuator. up

locking
shall be done through a hydro mechanical external up
-
lock unit mounted on the aircraft structure.
An up
-
lock pin on the landing gear strut shall engage with the hook on the up
-
lock limit. An
emergency release system must be provided to unlock the
up locks if the normal release system
fails.

GROUND SAFETY LOCKS
:


They must be provided to ensure that the gear cannot be retracted while the aircraft is on
the ground. These are usually pins, which are placed in a knuckle joint such as that prevailing in
the elbow of a folding side brace. The only requirement that migh
t be applied to the ground
locks is that they must be located in an easily accessible position and they must be caple of
withstanding the full retraction loads
.

BRAKES :


Modern aircraft are fitted with hydraulic disc brakes because this type has fewer
ten
dencies to fade than drum brakes and will immediately wipe water, slush and mud from
braking surface. A steel brake disc is fitted to the inside face of the cast magnesium or aluminum
wheel, while the friction

producing brake ling material is fitted onto
the facing of small pistons.
Hydraulic pressure moves these pistons to clamp the brake disc in a caliper like action whenever
the master cylinder is operated in the cabin.



Brakes are

mainly

used



To stop

an aircraft

on groun
d



To steer the aircraft by differential action



To hold the aircraft stationary when parked



To hold the aircraft while running up engines prior to take off and



To control speed properly while taxiing

The design of the brakes and its components are explained

in detail.

a.

HEAT SINK
:
In stopping an aircraft, kinetic energy is transformed to heat energy by the
heat sink. It comprises rotors, stators, and wear pods. The rotors are keyed to the wheel
and are therefore rotating. The stators are keyed to the torque tu
be, which is attached to
the axle, and the friction pads are attached to both sides of the rotors and stators. The
pads have high thermal conductivity to ensure that the entire heat
-
sink mass functions as
one unit.


b.

TORQUE PLATE OR HOUSING: It functions as

a pressure vessel to transmit fluid
pressure to actuate the brake, it transmits brake torque forces to the landing gear, it
houses and positions the brake pistons.

c.

PISTON AND CYLINDER LINERS:
The brake piston assemblies are housed in
aluminum alloy cylind
er that are screwed into the torque plate. The cylinder bore and
mating piston surfaces should be hard
-
anodized to provide long wear and the cylinder
should be honed to a superfine finish to minimize bore wear and increase seal life.

T
he
cylinder are seale
d with the torque plate by O
-
rings.


d.

BRAKE WEAR INDICATOR: This item is installed to provide visual indication of the
degree of brake wear.


e.

TORQUE TUBE:

Torque loads are transferred from the stators to the torque plate(and
hence the axle) by means of a to
rque tube.


f.

PRESSURE PLATE:

This part is used to transmit the thrust force from the pistons to heat
sink. A friction pad is attached to one side of the pressure plate so that it is also part of
the total heat sink.

BRAKE OPERATION:

When hydraulic fluid in
the cylinder is pressurized, the pistons move and apply a direct
thrust to the pressure plate. This thrust is transmitted to the stators and rotors, which are clamped
together

to set up frictional resistance to the rotation of rotors, which are driven by t
he wheel. As
friction pad wear takes place during service, the pistons move progressively out of the cylinder
liners to apply thrust. Whenever the pressure is released, the pressure plate is returned to the off
position through the medium of the retracting

pins and adjuster springs. In this way, adequate
working clearance is maintained throughout the life of the brake unit.

WHEEL DESIGN:

Wheel are usually formed from forged aluminum alloy, such as 2014
-
T6. The rim
contour is in accordance with international

standards. Static and fatigue loads design the flanges
bead ledge and wheel

well area, with the flange acting as a torsion ring to hold the tire bead in
position. The flange must also distribute the shear loads, from ground reaction into the rest of the
wheel.

The two wheel halves are joined together by a number of tie bolts. This area of the wheel is
designed for high stiffness. At the center of the wheel, the hub is designed to house the wheel

bearings. The bearings are of the taper
-
roller type and are
sealed to ensure that their grease is not
ejected at high speed, as well as the bearing from contamination.





A standard tire inflation valve is installed in the outboard wheel, usually near the tie bolt
flange. Fusible thermo sensitive pressure
release plugs are also installed in the wheel in this area.
These plugs release the tire pressure if local temperature reaches a predetermined level and also
prevent blowouts due to excessive pressure. Each plug is sealed by an O
-
ring and consists of a
hol
low casing housing a eutectic insert, a solid piston ,and rubber seal. Other items that have to
be considered include the rotor drive keys or blocks, a heat shield if required and a possibly a tire
change counter. Heat shields are sometimes provided to min
imize heat transfer from the brake,
and the tire change counter is sometimes specified to tire changes.

Non
-
frangible wheels must be
used to prevent aircraft damage due to disintegration of wheel after the failure.

STEERING:

Aircraft is steered by either d
ifferential braking or by turning the nose gear. the former is
satisfactory for tail
-
wheel aircraft and for some light planes, although it is now common practice
to equip even the light planes with a form of nose gear steering.

Nose wheels may be turned by

the rudder pedal or by a wheel or bar in the cockpit, or by
a combination of both.

Shimmy damping must be provided, and the steering system is often used
as a part of this damping system. The steering can contribute to shimmy damping by restricting
motion

in the steering actuator or motor. A wheel is said to shimmy when it oscillates about is
caster axis. It can be caused by lack of torsional stiffness in the gear, inadequate trail, and
improper wheel balancing. Steerable nose wheels are particularly susc
eptible to shimmy. It is
prevent in all the aircrafts. Hence to reduce shimmy it is first desirable to provide high torsional
stiffness. This is possible when the trail is greater than the radius of the wheel. The is not
possible to design. Hence, only an
efficiency of 85%

is maintained.


TIRES:


The tires are sized to carry the weight of an aircraft. Typically the main tires carry about
90% of the total weight of an aircraft. Nose tires carry only about 10% of the static load but
experiences higher dynamic

loads during landing. Nose tires can be assumed to be about 60
-
100% the size of main tires. The front tires of the bicycle or quadricycle gear aircraft are usually
the same size as the main tires. As a tire ages, it loses ability to withstand its own int
ernal
pressure. This causes it to swell in size about 2 or 3% in diameter and 4 % in width. This
swelling should be allowed for in designing the wheels and retraction geometry. A tire supports s
load almost entirely by its internal pressure. The load carry
ing ability of the side walls and tread
can be ignored. Operating a tire at a low internal pressure will greatly improve tire life. Hence
operating at half of its maximum rated load will increase the number of landings obtained from
the tire by a factor of

six. Sometimes the diameter of tires is set by the braking requirements.
Aircraft brakes are similar to automobile disk brakes and are usually placed inside the wheels in
all aircraft. The wheel typically has a rim diameter of about half the total diamete
r of the tire
mounted on it.


The landing weight is not as same as the weight at the end of the design mission. To
allow an emergency landing shortly after take off, the landing weight should be approximated as
80
-
100% of the take off weight.

The main fu
nctions of tires are as follows



For towing without any power source from aircraft on ground.



Facilitate directional control and allowing ground maneuver.



To stop the aircraft by applying brakes when aircraft on ground



To absorb the energy during landing to
uchdown on rough surfaces



TYPES OF TIRES :

a)

TYPE ΙΙΙ(LOW PRESSURE):

It is used on

piston
-
driven aircraft today

and it has wide tread and low internal pressure
.

A type III tire is permitted a dynamic load of 1.4 times the static value.

The section width is
relatively wider in relation to the bead diameter. This provides lower pressure for improved
cushioning and flotation.

Typical size:

9.50
-
16

==
(section width)

(rim diameter)

b)

TYPE VII ( HIGH PRESSURE):

These types are generally

used

on
today’s jet aircraft. Their conventional shape and very
high load capacities characterize them.

They are designed for such higher landing speeds and to
operate under higher internal pressure which reduces their size.

Typical size: 39 x
13

== (outside
diameter) x
(section width)

c)

TYPE VIII (EXTRA HIGH PRESSURE):

The newest and highest pressure tires were called
Type

VIII, and they

are designed for higher
take off speeds. All are tubeless for nose and main wheels.

Typical size: 26 x
8

-
14= (outside diamet
er) x (section width)
-

(rim diameter)

TIRE SELECTION:

As the load are known a preliminary tire selection can be made, and it is first necessary to
determine how many tires will be used on each strut. All aircrafts between 60,000 and 175,000
pounds weight
seem to have two main gear struts

and two main tires per strut. A
ircraft between
235,000 and 400,000 pounds, a decision has to be made whether two or four tires will be used on
each main gear. As the aircraft approaches 500,000 pounds, it becomes increasi
ngly necessary to
consider runway loading, even on high strength concrete surfaces. The loading cannot be
alleviated by merely increasing the number or tires per strut or by increasing their spacing. The
only way is to increase the number of strut.

TYPES O
F LANDING

LIMIT LANDING:


The landing, which occurs according to the designed sink rate values, is limit landing.
During this landing there is no damage to the aircraft structure.

RESERVE LANDING:


The landing, which occurs at sink rates about 20% greater than limit landing is reserve
landing. This is the accepted limit upto which landing is satisfactory.

HARD LANDING:


All landings with sink rates greater than that which occurs during reserve landin
g are
hard landings. Damage to aircraft structure is more during this landing.

ADVANTAGES OF THE HEAVY LANDING INDICATOR:


Due to hard landing there is a possibility of failure to the components of aircraft.
Therefore maintenance after each hard landing is

required. Thus to indicate to the ground crew
on the occurrence of a hard landing, an indicator is required.


The main function of the heavy landing indicator is to give an indication to the ground
crew whenever the aircraft experiences a heavy landing. It senses the vertical deceleration during
touch down and indicates by a protruding pin whenever the deceleratio
n exceeds a preset value.


Heavy landings may result in structural distortion, fractures, skin wrinkling and failure of
bolted and riveted joints. The extent of damage if any will depend upon the nature of the load
imposed.


The effects are more localized
and are associated with heavy impact. Forces transmitted
through the landing gear structure may result in shearing and bending forces being applied at the
landing gear attachments points and transmitted to the fuselage and wing.


Heavy landing checks are c
arried out to ascertain the extent of damage, if any, and
restore the aircraft to flight worthy condition.

To minimize these checks and to carry them out
only when a heavy landing occurs, an indicator is essential.