Aircraft Landing Gear Systems

• Concerns hydraulic and pneumatic power systems in aircraft, and landing gear systems

Landing Gear Systems

• Main task involves understanding various configurations, operating principles, components, shock absorption, steering, extension/retraction, and safety devices in landing gear.
• Main landing gear supports aircraft during ground operations via absorbing download forces during landing and taxiing.
• Brakes are usually installed on the main wheel for deceleration or stopping.
• Number of landing gears, wheels, and brakes is determined by design considerations related to aircraft weight and load.
• Nose landing gear also supports aircraft weight, is equipped usually with steering mechanisms for ground maneuvering.

Landing Gear Arrangement

• Tail wheel or conventional configuration used in older aircraft meant for rough field operations.
• Tricycle configuration allows more forceful braking without nosing over, supports higher landing speeds.
• Tricycle provides enhanced visibility from the flight deck (especially during landing and ground maneuvering) and prevents ground-looping as the aircraft's center is forward of the main gear.

Main and Nose Landing Gear: Sub-components

• Aircraft wheels support the entire aircraft weight when tires are mounted, and are made from lightweight, strong aluminum alloy.
• Aircraft tires support weight, absorb shock from landing and taxiing, provide runway grip, and discharge static electricity.
• Trunnion attaches the landing gear assembly to the airframe, is supported by bearing assemblies for gear pivoting during retraction and extension.
• The strut is the vertical member of the landing gear assembly
• Drag links or struts support shock struts, which stabilizes them longitudinally
• Side struts or side brace links stabilize the landing gear laterally
• Overcenter links prevent joint pivoting (except when retracted), which prevents gear collapse during ground operation.
• Downlocks lock the main gear, are sometimes called "Downlock" and is hydraulically retracted to allow gear retraction.
• Downlocks lock landing gear in the down position; uplock mechanisms hold the main landing gear in the UP position.
• Axles support and install the main wheels.

Shock Absorption

• The forces of impact on an aircraft during taxi and landing are absorbed by the gears.
• In shock absorption, shock energy is either transferred through the airframe at a different rate/time or converted into heat energy.
• Non-shock absorbing landing gear uses flexible spring steel, aluminum, or composite struts to receive landing impact and return it to the airframe at a non harmfull rate.
• Pneumatic/hydraulic shock struts absorb/dissipate shock loads with nitrogen gas combined with hydraulic fluid.
• Composed of two telescoping cylinders/tubes closed on the external ends.
• The upper cylinder is fixed to the aircraft, and the lower cylinder (piston) slides in and out of the upper cylinder, forming two chambers each filled with hydraulic fluid and nitrogen.
• An orifice between cylinders allows fluid passage from the bottom to top chamber when the strut is compressed.

Shock Strut Operation

• Compression stroke begins as the aircraft wheels touch the ground.
• Center of mass moves downward during compression, and lower cylinder/piston gets forced upward into the upper cylinder.
• The metering pin moves through the orifice.
• Gas volume decreases, increasing the pressure while hydraulic fluid volume remains constant.
• Initial landing shock gets cushioned when the hydraulic fluid is forced through the metering opening.
• As pressure and temperature in the cylinder increase, the aircraft's vertical speed decreases
• Cylinder pressure increases until it stops vertical motion, and gas pressure energy recoils the aircraft upwards.
• During recoil, the shock strut extends fully and the compressed air acts as a shock absorber until the aircraft is taxiing.

Landing Gear Types

• Fixed landing gear exposes the gears to the airflow during flight, increasing drag.
• Retractable landing gear reduce drag, which adds weight.
• Aircraft are fitted with retractable gears, the added weight is a small sacrifice as compared to the drag as aircraft are flying faster.

Retraction and Extension

• Main landing gear can be extended/retracted via the landing gear handle in the flight deck.
• The handle is mechanically connected to the selector valve: the air/ground crew can set it to “UP”, “OFF” (neutral), or "DOWN".
• When set in “UP”, an internal selector valve circuit supplies hydraulic pressure to opening the wheel well, unlock landing gears, and lock the landing gears in the “UP” position.
• When set to “OFF”, all landing gear operation system components are connected to the return line of the hydraulic system
• Landing gears are kept in the “UP” position by an up-lock mechanism.
• When put to “DOWN”, pressure is released via the internal circuit in the selector valve.
• This is used for unlocking the wheel well, unlocking the up-lock, extension of the landing gear, and closing of the wheel well doors

Equations

• Volume to retract 3 actuators = [3 x (π/4 x (0.5)^2 - π/4 x (0.3)^2) x 0.7] = 0.263 m^3 = 263 litres
• Flow rate to the retract 3 actuators = 0.263/10 = 0.0263 m^3/s
• Power Required, P = (0.0263 x 207 x 10^5)/0.85 = 640 KW

Downlock Mechanism

• Prevents undesired landing gear retraction when in the DOWN position.
• "Overcenter links" between the strut and side brace ensure side brace cannot pivot when in "overcenter" position
• Overcenter link will remain "overcenter" in position by the spring force of bungee springs"
• In ground maintenance, the overcenter mechanism is locked via landing gear lock pins (for safety).
• When landing gear is retracted, the downlock actuator pulls the overcenter links from overcenter position so the side brace can pivot as gear is pulled up by the retract cylinder.

Uplock Mechanism

• The uplock mechanism consists of a hook securing the landing gear in the retracted position.
• When the landing gear is unlocked it extends (due to mass) and reaches the "down and locked" position with the help of the bungee springs.
• Hydraulic fluid flows away from the retraction actuator and slows the process to reduce the "down" shock

Steering System

• Aircraft can be steered or turned during taxying via nose wheel steering system and/or differential braking
• Steering is controlled from the flight deck using a small wheel, tiller, or joystick (typically on the left sidewall)
• Aircraft hydraulic system pressure is directed through open safety shutoff valve into a line leading to the metering valve.
• Metering routes pressurized fluid out of port A and is directed through the right turn alternating line, and into steering cylinder A
• Cylinder pressure extends the piston and connecting rod to the nose steering spindle.

Nose Landing Gear Alignment

• Most shock struts are equipped with torque links or torque arms that attach to the upper fixed cylinder and other ends attached to the lower cylinder (piston) to prevent rotation and keeps wheels aligned.
• Nose gear shock struts are provided with a locating cam assembly in order to keep gear aligned
• Allows the nose wheel to enter the wheel well (preventing structural damage) when the nose gear is retracted
• Aligns wheels with the aircraft's longitudinal axis before the strut is fully extended, and during landing.
• Most nose gear shock struts have attachments to install an external shimmy damper.

Nose Landing Gear Shimmy Dampers

• Torque links attached from the stationary upper cylinder of a nose wheel strut to bottom moveable cylinder/piston are not sufficient to prevent most nose gear from shimmying at certain speeds.
• Vibration controlled through a shimmy damper; it controls nose wheel shimmy via hydraulic damping.
• Piston-type shimmy damper is attached firmly to the upper shock strut cylinder with its shaft attaching to the lower shock strut cylinder to a piston inside the shimmy damper.
• The lower strut cylinder attempts to shimmy and hydraulic fluid is forced through a bleed hole in the piston which restricts flow to dampen oscillation.

Emergency Extension Systems

• Emergency extension systems lower the landing gear when the main power system fails.
• Some aircraft have an emergency release handle in the flight deck that connects through a mechanical linkage to the gear uplocks.
• Operating handle releases uplocks so gear free-falls to the extended position because of gravity.
• Other (non-mechanical) aircraft use a back-up using pneumatic power to unlatch the gear.

Safety Switch

• Safety switches are present on most aircraft, opens/closes depending on the main landing gear strut compression/extension
• Wired in system operating circuits, and prevents gear retraction when aircraft is on the ground.
• The landing gear strut extends during takeoff and the safety switch closes, which allows current to flow the safety circuit.
• The solenoid energizes the landing gear and retracts the lock-pin from the selector handle which then permits the gear to be raised

Ground Locks

• Ground locks serve as additional safety devices and prevent gear collapse when aircraft is on the ground.
• Includes pins which are placed into holes on the gear components to keep the gear from collapsing.
• These locks feature red streamers so they are visible and can be easily removed before the aircraft is sent to flight.

Gear Indicator (Landing Gear Safety Device)

• The gear indicator consists of micro switches or proximity switches on the up-lock and down-locks connected to landing gear position indicator on the instrumental panel.
• Green light usually represent where a gear is down and locked.
• Green indicates the landing gear is down and locked: the red lights indicate gears are in transit: no light indicates all gears are up.

Warning Horn

• Informs the pilot if the gear isn’t locked/down, particularly during a possible retracted-gear landing.
• During a landing approach the horn goes off if the landing gear is in any position other than down and locked.

Explore aircraft landing gear systems, focusing on design considerations, tricycle gear advantages, and braking systems. Understand the roles of nose gear, tail wheel configurations, and center of gravity effects. Improve your knowledge about landing gear and its types.