Landing gear

Landing Gear Components & Equipment:
1.Struts
2.Steering Linkages
3.Tow Bars
Main Components:
1.Torque Links/Torque Arms/Torsion Links
(Scissors Assembly)
2.Drag Link/Drag Struts
3.Side Brace Link/Side Struts
4.Overcenter Link/Downlock Struts/Jury Struts
5.Trunnion
6.Shock Struts
7.Shimmy Damper
8.Actuator/Jack/Actuating Cylinder
9.UpLock Cylinder/UpLock Actuator
10.DownLock Cylinder/DownLock Actuator
11.Indicator(microswitches/ground safety switch/squat switch)
12.Centering Cam
13.Steering Actuator
14.Landing Gear Door Actuator
15.Emergency Lowering Mechanism

1.Torque Links/Torque Arms/Torsion Links
(Scissors Assembly)
• It Maintain wheel and axle in a correct alinged position in relation to the strut
(misalignment corrected by adding/installing a spacer or shim of different thickness).
• Prevent ram of a piston turning in the struts.
• Restrict the extension of the piston during the extension of the struts.
FUNCTION:
• The hinged link between the piston and cylinder of an oleo-type landing gear shock
absorber.
• The torque links allow the piston to move freely in and out of the landing gear
cylinder, but prevent it rotating.
• The torque links can be adjusted to achieve and maintain the correct wheel
alignment.
(Fig.1)
2.Drag Link/Drag Struts:
• Stabilizing the Landing gear and support the aircraft structure longitudinally.
FUNCTION:
• A drag strut is used to restrain against the pivot action built into the trunnion
attachment

(Fig.2)
3.Side Brace Link/Side Struts:
• Stabilizing the Landing Gear and support the aircraft laterally
(Fig.3)
4.Overcenter Link/Downlock Struts/Jury Struts:
• Holds the drag link and the side brace in the ‘DOWN’ and ‘LOCKED’ position by
applying pressure to the center pivot joint in a drag or side brace link.
• Operates hydraulically by bungee cylinder or mechanically by bungee springs.
FUNCTION:
• The side brace assembly includes a jury strutvwhich functions to lock the side brace
assembly with the landing gear in the extended position.

(Fig.4)
5.Trunnion:
• The trunnion is a fixed structural extension of the upper strut cylinder with bearing
surfaces that allow the entire gear assembly to move.
• It is attached to aircraft structure in such a way that the gear can pivot from the
vertical position required for landing and taxi to the stowed position used during
flight.
• Supported at its end by bearing/bushing assemblies,which allow the gear to pivot
during retraction and extension (the struts form the cylinder for the Oleo pneumatic
shock absorber).
FUNCTION:
• The purpose of a trunnion is to structurally support the landing gear in bearing the
weight of the aircraft when it is not flying.
(Fig.5)

6.Shock Struts
• A typical pneumatic/hydraulic shock strut uses compressed air or nitrogen combined
with hydraulic fluid to absorb and dissipate shock loads.
• It is sometimes referred to as an air/oil or oleo strut. A shock strut is constructed of
two telescoping cylinders or tubes that are closed on the external ends.
• The upper cylinder is fixed to the aircraft and does not move.
• The lower cylinder is called the piston and is free to slide in and out of the upper
cylinder.Two chambers are formed.
• The lower chamber is always filled with hydraulic fluid and the upper chamber is
filled with compressed air or nitrogen. An orifice located between the two cylinders
provides a passage for the fluid from the bottom chamber to enter the top cylinder
chamber when the strut is compressed.
• Vertical member of the landing gear assembly that contains the shock absorbing
mechanism.
FUNCTION:
• Absorbs and dissipating landing shocks incorporates of a telescopic tube mechanism
which absorbs landing shocks but resist re-bound and at the same time protects the
aircraft structure.
• Controlling the re-bound(spring back or recoil)movement.
• Providing cushioning effects during taxiing.
(Fig.6)

7.Shimmy Damper
• A shimmy damper uses a cylinder filled with hydraulic fluid or a rubber/lubricant
combination to prevent rapid movement of the nosewheel, while not interfering with
slower operations.
• Hydraulic snubbing unit that reduces the tendency of the nose or tail wheel oscillate
from side to side (rapid oscillation).
FUNCTION:
• A shimmy damper uses a cylinder filled with hydraulic fluid or a rubber/lubricant
combination to prevent rapid movement of the nosewheel, while not interfering with
slower operations.
• It turns the airplane, and the fluid squirts from one end of the cylinder to the other. At
higher speeds, the fluid can’t get through as fast as the nosewheel wants to shimmy,
and the nosewheel is stabilized, preventing shaking.
(Fig.7)
8.Actuator/Jack/Actuating Cylinder
• Landing Gear Retraction Actuators are designed to provide retraction and extension
forces while maintaining light weight and reliability.
• Raise and lowering the landing gear.May also used as a Downlock
mechanism(continous application of pressure).
• Retraction actuators are manufactured from complete hog outs or finish machined
from forgings to a one piece body. Housings are commonly made out of stainless
steel for strength and anti-corrosive properties.

• High strength and wear resistant materials are utilized for the piston, end gland, and
bearings to ensure longevity and reliable service without the need for overhauls or
scheduled maintenance.
FUNCTION:
• The landing gear is rotated about a pivot by an extend/retract actuator.
• To retract the landing gear, an unlock actuator pulls the lock-stay from over-center,
which enables the extend/retract actuator to retract the landing gear to the stowed
position. Both the extend/retract and unlock actuators are typically hydraulically
powered.
(Fig.8)
9.UpLock Cylinder/UpLock Actuator
• Locking the landing gear in ‘UP’ and ‘LOCK’ position
10.DownLock Cylinder/DownLock Actuator
• Locking the landing gear in ‘DOWN’ and ‘LOCK’ position
(Fig.9)

11.Indicator(microswitches/ground safety switch/squat switch)
• Means of providing cockpit indication with regards to the landing gear position,either
‘UP’ and ‘LOCKED’,’DOWN’ and ‘Locked’ or in transition and ‘NOT LOCKED’.
• A landing gear squat switch, or safety switch, is found on most aircraft.
• This is a switch positioned to open and close depending on the extension or
compression of the main landing gear strut.
• The squat switch is wired into any number of system operating circuits. One circuit
prevents the gear from being retracted while the aircraft is on the ground.
• There are different ways to achieve this lock-out. A solenoid that extends a shaft to
physically disable the gear position selector is one such method found on many
aircraft.
FUNCTION:
(Squat Switch)
• The squat switch is essentially a sensor that senses if the weight of the aircraft is
resting on the gear.The squat switch is wired into any number of system operating
circuits. One circuit prevents the gear from being retracted while the aircraft is on the
ground.
(Ground Safety Switch)
• A switch in the landing gear that prevents the inadvertent retraction of landing gear
on the ground. This switch usually is located in the port landing gear.

(Fig.10)
12.Centering Cam
• Aligning the nose wheel before retracted to ‘UP’ and back to its wheel well
compartment
• A cam in the nose-gear shock strut that causes the piston to center when the strut
fully extends. When the aircraft takes off and the strut extends, the wheel is
straightened in its fore-and-aft position so it can be retracted into the wheel well.
FUNCTION:
• The centering cam forces the nosewheel straight back with the strut before it is
retracted into the nose wheel well.
(Fig.11)

13.Steering Actuator
• Allowing the pilot to control or steer the aircraft by means of NLG wheel assembly
for taxiing.
• Nose Wheel Steering Actuators work in tandem to provide linear push/pull forces for
aircraft steering. Actuators are sized for regional jets and large business jets.
(Fig.12)
14.Landing Gear Door Actuator
• Allowing the opening or closing of landing gear door (main and nose landing gear).
FUNCTION:
• When the doors are fully open, the door actuator engages the plungers of both
sequence valves to open the valves.

(Fig.13)
15.Emergency Lowering Mechanism
• Providing the means of lowering down the landing gear in the event of normal
extension system fails (under FAR 23.729).
FUNCTION:
• A device in an aircraft landing gear system that absorbs the landing shock, which
occurs when an aircraft touches down when landing or during taxiing.
(Fig.14)

Materials Of Landing Gear
• The materials used for these components of landing gear are composed of mainly
High Strength Steels,Titanium,Aluminium And Magnesium.
High Strength Steel
• High Strength Steel have strength and stiffness.
• High Strength to volume ratio is the main factor which makes it important for the
landing gear.
Titanium
• The titanium has high strength and low density used as for weight saving.
• The main titanium alloy is Ti-5Al-5V-5Mo-3Cr,which is used in the main
components of landing gear.
Aluminium
• Modern aircraft consists of approximately 70% of aluminium alloys.It has relatively
low cost,light in weight and can be heat treated to Stength level.
• Aluminium 7075 is used for the Landing Gear in helicopters because it is strong,with
a strength comparable to many steels, and has good fatigue strength.

Possible Failures Of Landing Gear
1.Fatigue Cracking Failure
2.Stress Corrosion Cracking
3.Dynamic Failure
4.Landing Gear Spring Failure
5.Wheel Failure
1.Fatigue Cracking Failure:
• Usually aircrafts and military experience serious damages and the fatigues
.
(Fig.15)
Equations used for analyzing:
1.Paris Equation:
2.Forman Equation:
3.Walker Equation:
Effective Stress=(1-R)m
X Maximum Stress

2.Stress Corrosion Cracking:
• SCC is defined as the crack propogation caused by a synergy between a corrosive
environment and a mechanical tensile stress.
(Fig.16) Stress Corrosion Crack Growth
3.Dynamic Failure
• When an aircraft lands on the tricycle landing gear(the nose wheel and both landing
gear) and the load affected by the ground/pavement response are distributed on them
• Numerical test showing stress analysis to define the dynamic characteristics of the
landing gear,with the vertical-drop test
• Problem of contact between mating components and surfaces of fractures
• Investigation into kinematics of the landing gear.
• Investigation into the problem of dissipation of energy in the whole system.
• Checking of possible failure influence on the structure behaviour.
(Fig.17)

4.Landing Gear Spring Failure
• The Micro cracks acted as stress concentration as well initiation crack site leading the
springs to fracture due to fatigue.
(Fig.18)
5.Wheel Failure
• During landing,wheels are exposed to a lot of pressure.
• Through this,overpressure may occur.This could have the effect that the thread is
thrown off.
(Fig.19) Investigation photos show the damage was limited to tries and rims only

Landing gear

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