Aircraft Landing Gear

• Landing gear systems involve both hydraulic and pneumatic power systems.

Landing Gear

• Landing gear systems involve different configurations of the aircraft landing gear.
• Both the main and nose landing gear have specific operating principles.
• Landing gear relies on components like struts, torque links, drag links, side struts, and shimmy dampers.
• Axles, wheels, and tires are also vital components of the landing gear.
• Shock absorption is part of the landing gear design.
• The landing gear involves aircraft steering, along with normal and emergency extension/retraction systems.
• Safety devices, indication, and warning systems are crucial for landing gear operation.

Hydraulic Power and Landing Gear System

• The main landing gear is the primary support during landing and taxiing.
• Brakes are installed on the main wheel for slowing or stopping the aircraft.
• Number of landing gears, wheels, and brakes is based on the aircraft design, weight and load.
• The nose landing gear supports the aircraft's weight and load.
• Nose landing gear is equipped with a steering mechanism for ground maneuvering.

Types of Landing Gear Arrangement

• Tail or conventional configuration
• Tandem configuration
• Tricycle configuration

Conventional vs Tricycle Configuration

• The tail wheel configuration is for older aircraft and operations on rough surfaces.
• The tricycle configuration has benefits over conventional types.
• Brakes can be applied more forcefully without nosing. This enables higher landing speeds.
• Tricycle gear provides better visibility during landing and ground maneuvering.
• Ground-looping is prevented due to the center of gravity being forward of the main gear.

Main and Nose Landing Gear Sub-components

• Aircraft wheels are a vital part of the landing gear system for supporting the aircraft.
• Aircraft wheels are usually made of aluminum alloy.
• Aircraft tires support the weight, absorb shock, provide runway grip, and discharge static electricity.
• A trunnion attaches the landing gear to the airframe.
• The trunnion is supported at its ends by bearing assemblies and allows the gear to pivot during retraction and extension
• The strut is a vertical member of the landing gear assembly.
• Drag links support shock struts and stabilize them longitudinally.
• Side struts stabilize the landing gear laterally.
• Overcenter links prevent gear collapse during ground operation by preventing pivoting.
• Overcenter, or "Downlocks" can be hydraulically retracted to allow retraction.
• A "downlock" locks the landing gear in the down position.
• The uplock mechanism holds the main landing gear in the UP position.
• Axles support and house the main wheels.

Shock Absorption

• Landing gear must support the forces of impact during landing and taxi.
• This can be achieved by altering energy transfer to the airframe.
• Conversion of energy into heat energy is another way of shock absorption
• Flexible spring steel, aluminum, or composite struts are used on many aircraft to receive the impact of landing.
• These struts return energy to the airframe to dissipate it in a controlled manner.

Shock Absorbing Struts

• Pneumatic/hydraulic shock struts combine nitrogen gas with hydraulic fluid.
• Shock struts consist of two telescoping cylinders or tubes.
• The upper cylinder is fixed, and the lower cylinder (piston) slides inside it.
• The lower chamber always contains hydraulic fluid and the upper with nitrogen.
• An orifice between cylinders allows fluid to flow from the bottom chamber to the top when strut is compressed.

Shock Strut Operation

• The compression stroke starts when the aircraft wheels touch the ground.
• The center of mass moves downwards, compressing the strut and forcing the piston upwards.
• The metering pin is moved up through the orifice.
• As the volume of gas decreases, the pressure increases, while the hydraulic fluid volume remains constant.
• The initial landing shock is cushioned by hydraulic fluid that is forced through the metered opening.
• As pressure and temperature increase in the cylinder, vertical aircraft speed decreases.
• Eventually, the pressure will halt vertical aircraft motion.
• Gas pressure then becomes sufficient to recoil the aircraft upwards.
• During recoil, the shock strut extends until the gas pressure supports the aircraft's weight.
• The compressed air then acts a shock absorber during taxiing.

Gear Type

• Fixed landing gear exposes the gears to airflow, thus increasing drag.
• Retractable mechanisms reduce drag but add weight.
• Retractable gears are fitted as the small weight is considered small compared to drag as aircraft fly faster.

Landing Gear Retraction and Extension

• Main gear extends/retracts via a handle in the flight deck connected to the selector valve.
• The flight crew can set the handle to "UP", "OFF" (neutral), or "DOWN".
• Setting gear to “UP” supplies hydraulic pressure to unlocking the unlatch and door actuator, unlocking the downlock actuator, retracting the retractactuator, and closing the wheel well doors.
• The landing gears are kept in the “UP” position by a up-lock mechanism
• Setting gear to “OFF” returns all “UP” and “Down” systems to the return line of the hydraulic system.
• Landing gear is kept in the “UP” position by a up-lock mechanism.
• Selecting “DOWN" releases pressure from hydraulics via an internal circuit in the selector valve and is used for unlocking the wheel well doors, unlocking the up-lock, extending the landing gear, and closing the wheel well doors.

Retraction and Extension Force Analysis

• Assume a maximum force is 53,000N, an actuator stroke of 700mm, the gear must be fully retracted in 10 seconds, two main and one nose landing gear identical to main landing gear, maximum pressure is 207 bar, and cap end diameter is 500mm with a piston rod diameter of 300mm to calculate the pump power.
• Volume to retract 3 actuators = [3 x (π/4 x (0.5)² - π/4 x (0.3)²) x 0.7] = 0.263 m³ = 263 litres
• Flow rate to retract 3 actuators = (0.263/10) = 0.0263m³/s
• Power Required = (0.0263 x 207 x 10^5) / 0.85 = 640KW

Downlock Mechanism

• The mechanism prevents undesired retraction in the “DOWN” position.
• “Overcenter links" are between the strut and side brace.
• Overcenter links ensure the side brace cannot pivot when in the “overcenter” position.
• The Overcenter link remains “overcenter" due to the spring force of "bungee springs".
• During ground maintenance, the overcenter mechanism is locked by landing gear lock pins.
• When the landing gear retracts, the downlock actuator pulls the overcenter links from the “overcenter" position.
• The side brace can then pivot when the landing gear is pulled up by the retract cylinder.

Uplock Mechanism

• The uplock mechanism includes a hook which secures the landing gear in a retracted position.
• When unlocked, the landing gear extends due to its mass, aided by the bungee springs.
• Hydraulic fluid which flows from the retraction actuator slows the process to reduce the "down" shock.

Ground Steering

• Aircraft steering during taxiing happens via the nose wheel steering system.
• Steering is controlled from the flight deck by using a small wheel, tiller, or joystick mounted on the left side wall.

Nose Landing Gear Alignment

• Most struts utilize torque links or torque arms.
• One end attaches to the upper cylinder fixed in place, with the other to the piston.
• Nose gear struts have locating cam assemblies to keep the gear aligned.
• A cam protrusion attaches to the lower cylinder, and a cam recess attaches to the upper cylinder.
• These cams align the wheel and axle assembly in a straight-ahead position when the shock strut is fully extended.
• Alignment helps the nose wheel enter the wheel well, which prevents damage.
• Alignment lines up the wheels with the aircraft's longitudinal axis before landing.
• Nose gear struts use attachments for external shimmy dampers.

Nose Landing Gear Steering System

• Pressure from the hydraulic system passes through the safety shutoff valve.
• The pressurized fluid is then routed through port A and into the steering cylinder via metering valve.
• Pressure in the one-port steering cylinder causes the piston to extend.
• Because the piston is connected to the shock strut, it pivots at point X.
• The piston extends to turn it gradually towards the right.
• As the nose wheel turns, fluid is forced out of, into port B of the metering valve, through a compensator and into the aircraft.
• The compensator routes this return fluid into the aircraft's hydraulic system return manifold.

Nose Landing Gear Shimmy Dampers

• Torque links from the strut's upper cylinder to the lower moveable cylinder cannot prevent the tendency to rapidly oscillate at certain speeds.
• Shimmy dampers control nose wheel shimmy through hydraulic damping.
• A piston shimmy damper's case attaches firmly to the upper shock strut cylinder.
• Strut cylinder connects to to to a piston inside the shimmy damper.
• The lower strut cylinder tries to shimmy, hydraulic fluid goes through a bleed hole inside the piston.
• The fluid dampens oscillation through restricted flow through the bleed hole.

Emergency-Extension System

• An emergency extension system lowers landing gear when the main power system is down.
• Some aircraft have an emergency release handle in the flight deck, connected through a mechanical linkage to the gear uplocks.
• When operated, the uplocks release and the gear free-falls to the extended position due to gravity.
• Other aircraft use non-mechanical backups, such as pneumatic power, to unlatch the gear.

Safety Switch

• Most aircraft utilizes a landing gear squat or safety switch.
• This switch opens and closes based on its reaction to the main landing gear strut.
• The strut can be wired to any amount of operating circuits to prevent the gear from being retracted while the aircraft is on the ground.
• Switch closes and allows current to flow, energizing the solenoid, when the gear extends to retract the lock-pin from the selector while taking off.

Ground Locks

• Ground locks are used to prevent the gear collapsing when the aircraft is on the ground.
• These locks can be as simple as a pin inserted pre-drilled holes.
• All ground locks should be be made visible by red streamers.
• Typically ground crew place them after landing and before a walk-around to be removed.

Gear Indicator

• Gear indicators consist of micro or proximity switches on the up-lock and down-locks connected to a landing gear position indicator on the instrumental panel.
• Green lights indicate that each gear is down and locked.
• Red lights show that the gear is in transit.
• No light indicates that all gears are up and locked.

Warning Horns

• When landing gear is unlocked because the aircraft is on, pilots will hear a warning horn.
• During aircraft landing approach, the horn sounds and the the gear is not down & locked, a red light will illuminate.

Explore the advantages of tricycle landing gear over tailwheel configurations. Learn about mitigating ground-looping, the function of brakes, and the purpose of the nose landing gear. Discover how the center of gravity placement enhances braking efficiency.