Aircraft Landing Gear Design

• Aircraft systems covered deal with hydraulic and pneumatic power systems.
• Landing gear systems will be detailed.

Landing Gear System Scope

• Describe the different configurations of aircraft landing gear.
• Explain the operating principles of the main and nose landing gear.
• Landing gear components covered include: struts, torque links, drag links, side struts, shimmy dampers, axles, wheels, and tires.
• Construction and operation of the shock absorbing element in the landing gear will be described.
• Also covered are: aircraft steering, normal and emergency extension/retraction systems, safety devices, and indication/warning systems.

Main Landing Gear

• Provides main support by absorbing large forces of the aircraft during ground operations.
• Brakes are installed on the main wheel to enable slowing or stopping the aircraft.
• The number of landing gears, wheels, and brakes depends on the aircraft's weight and load.

Nose Landing Gear

• Provides support for the aircraft's weight, load, and is equipped with a steering mechanism.
• Enables the aircraft to maneuver on the ground.

Landing Gear Types

• Tail wheel or conventional configuration
• Tandem configuration
• Tricycle configuration

Tail Wheel Configuration

• Used on older aircraft for landing on rough fields.

Tricycle Configuration

• Provides more forceful application of brakes without nosing over, enabling higher landing speeds.
• Provides better visibility from the flight deck.
• Prevents ground-looping of the aircraft due to the center of gravity being forward of the main gear.

Sub-Components of Landing Gear

• Aircraft Wheels:
  • Important part of the landing gear system.
  • Support the entire weight of the aircraft during taxi, takeoff, and landing when tires are mounted upon them.
  • Made from aluminum alloy; lightweight and strong
• Aircraft Tires:
  • Support the aircraft's weight.
  • Absorb shock from landing and taxiing.
  • Provide gripping contact with the runway surface.
  • Discharge static electricity.
• Trunnion:
  • Part of the assembly that is attached to the airframe.
  • Supported at its ends by bearing assemblies.
  • Allows the gear to pivot during retraction and extension.
• Strut:
  • The vertical member of the landing gear assembly.
• Drag Link/Strut:
  • Provides support to the shock strut.
  • Stabilizes the shock strut longitudinally.
• Side Strut:
  • Stabilizes the landing gear laterally.
• Overcenter Link:
  • Prevents the link from pivoting at the joint except when retracted.
  • Prevents gear collapse during ground operation.
  • Locks the main gear in the down position and called "Downlock".
  • It is hydraulically retracted to allow gear retraction.
• Lock Mechanism:
  • A "downlock" locks the landing gear in the down position.
  • Mechanism holds the main landing gear in the UP position.
• Axles:
  • Where the main wheels are supported and installed.

Shock Absorbing Landing Gear

• Supports the aircraft, and absorbs the impact of landing.
• Forces of impact are absorbed by altering and transferring shock energy and converting the energy into heat.
• Non-Shock Absorbing Landing Gear:
  • Flexible spring steel, aluminum, or composite struts are used to receive the landing impact.
  • Dissipates energy without causing harm.

Key Features: Shock Absorbing Struts

• Pneumatic/Hydraulic Shock Strut:
  • Uses nitrogen gas combined with hydraulic fluid to absorb and dissipate shock loads.
• Construction:
  • Uses two telescoping cylinders or tubes closed on the external ends.
• Operation
  • The upper cylinder is fixed, and the lower cylinder (piston) slides in and out.
  • Two chambers are formed; the lower is filled with hydraulic fluid, and the upper with nitrogen.
• Orifice:
  • Passage located between the two cylinders.
  • Allows fluid from the bottom chamber to enter the top chamber when the strut is compressed.
• Compression Stroke:
  • Begins when the aircraft wheels touch the ground.
  • Center of mass moves downward, compressing the strut and forcing the lower cylinder upward.
    • Metering pin moves through the orifice.
  • Gas volume decreases, increasing pressure while hydraulic fluid volume remains constant.
  • Initial shock cushioned by the hydraulic fluid being forced through the metered opening.
  • As pressure and temperature increase, vertical speed decreases.
  • Pressure increases until it stops the aircraft's vertical motion until energy in the gas pressure recoils the aircraft upwards.
• Recoil:
  • Extends the strut until gas pressure supports the aircraft's weight.
  • Compressed air acts as a shock absorber during taxiing.

Fixed and Retractable Landing Gear

• Aircraft with fixed landing gear expose the gears to airflow, increasing drag as speed increases.
• Retractable gear reduces drag but adds weight.
• Retractable gears mitigate drag over sacrificing the added weight as aircraft are flying faster.

Main Landing Gear Retraction and Extension

• Landing gear handle in flight deck controls this.
• Handle is mechanically connected to the selector valve.
• Aircrew and ground crew can set the handle to "UP", "OFF", or "DOWN".
• In the "UP" position:
  • Internal circuit in the selector valve supplies hydraulic pressure to unlock/open wheel well doors, unlock landing gears, retract gears, and close wheel well doors.
  • Gears kept in "UP" position by an up-lock mechanism.
• In the "OFF" position:
  • All components on the "UP" and "DOWN" sides connected to the hydraulic system's return line, maintaiqning the uplocked position.
• In the "DOWN" position:
  • Pressure from the hydraulic system used to unlock wheel well doors, unlock the uplock, extend the gear, and close the doors.

Example Calculations to Retract Landing Gear

• Formula to Calculating volume required to retract 3 actuators: [3x(π/4 x 〖(0.5)〗^2-π/4 x 〖(0.3)〗^2) x 0.7] =0.263 m^3=263 litres
• Formula to calculate flow rate to retract 3 actuators: (0.263)/10 = 0.0263m^3/s
• Formula to Calculate power Required: (PQ)/η((0.0263X207X〖10〗^5))/0.85 =640KW

Downlock Mechanism

• Prevents undesired retraction of landing gear.
• Overcenter links between the strut and side brace ensure that the side brace cannot pivot.
• Overcenter link remains "overcenter" due to bungee springs.
• Locked by landing gear lock pins during maintenance as a safety measure.
• In retraction, the downlock actuator pulls the overcenter links from the "overcenter" position allowing the side brace to pivot.

Uplock Mechanism

• Hook mechanism secures the landing gear in the retracted position.
• When unlocked, the landing gear extends due to its mass plus the bungee springs.
• Hydraulic fluid flow from the retraction actuator slows the process to reduce "down" shock.

Nose Landing Gear Steering

• During taxiing, the aircraft can be steered using the nose wheel steering system or differential braking.
• Steering control is from the flight deck via a small wheel, tiller, or joystick typically mounted on the left sidewall.

Nose Landing Gear Alignment

• Most shock struts have torque links or arms.
  • End of the links attached to upper cylinder, while other end attached to lower cylinder to keep wheels aligned.
• Nose gear shock struts have a locating cam assembly.
  • Cam protrusion attached to the lower cylinder.
  • Mating lower cam recess attached to the upper cylinder.
  • When shock strut fully extended the cams line up the wheel and axle assembly.
  • Nose wheel enters the wheel well on retraction without structural damage, and wheels align when the strut is fully extended.
• Also attachments for an external shimmy damper.

Nose Landing Gear Steering Operation

• Pressure from the aircraft hydraulic system travels through the open safety shutoff valve to a line leading to the metering valve.
• Metering valve routes pressurized fluid out of port A and into steering cylinder A initiating piston extension.
• The extension turns the nose steering spindle toward the right at point X because steering spindle is connected on the nose gear shock strut .
• Fluid forces through steering cylinder B into port B of the Metering Valve as the turn is initiated.
• Metering Valve directs the return flow into the Compensator that routes this fluid to the hydraulic system's return manifold.

Nose Landing Gear Shimmy Dampers

• Torque links often insufficient to prevent nose gear oscillation or shimmy at certain speeds.
• Shimmy damper controls nose wheel shimmy through hydraulic damping.
• Piston-type shimmy damper's case attached firmly to the upper shock strut cylinder, the shaft is attached to the lower shock strut cylinder and to a piston inside the shimmy damper.
• As the strut cylinder shimmies, hydraulic fluid is forced through a bleed hole in the piston to dampen oscillation.

Emergency Extension Systems

• Lowers the landing gear if the main power system fails.
• Some aircraft have an emergency release handle that is connected through a mechanical linkage to the gear uplocks.
• When the handle operates it. releases the uplocks.
• Aircraft uses a non-mechanical back-up, like pneumatic power, unlocks the gear, and allows the gear to free-fall to extended position.

Safety Switch (Landing Gear Safety Device)

• The landing gear squat or safety switch is positioned to open and close based on main landing gearstrut.
• Switch wired into system operating circuits.
• Prevents the gear from being retracted when the aircraft is on the ground.
• Strut extends on takeoff closing allowing current to energize the solenoid.
• The solenoid energizes, retracts the lock-pin, and permits the gear to be raised.

Ground Locks (Landing Gear Safety Device)

• Most aircraft prevents gear collapse when aircraft is on the ground.
• A pin placed into pre-drilled holes keep gear components from collapsing.
• Red streamers show that the locks are installed.

Gear Indicator (Landing Gear Safety Device)

• Consist of micro switches or proximity switches on the up-lock and down-locks connected to a landing gear position indicator panel.
• Indicator consists of green light indicating locked position or Red light in transit.
  • No light indicates locked/all gears up

Warning Horn (Landing Gear Safety Device)

• Sounds when the landing gear is not down and locked to prevent landing with retracted gear for safety precaution.

Aircraft landing gear design depends on landing conditions. Tricycle gear offers advantages over tailwheel, including reduced risk of ground loops and forceful braking, nose wheel steering improves maneuverability. Main landing gear must support weight.