AS Chpt 7

• The landing gear system is made of 2 main parts:

  • Main leading gear
  • Nose landing gear
• Main landing gear:

  • Provides the main support of the aircraft
  • Absorbs large downward forces acting on the aircraft during ground operation (landing & taxiing)
• Main landing gear:

  • Brakes are installed on the main wheel - To allow the aircraft to slow down or stop, as required
  • No. of landing gear, wheels & brakes depends on the initial design of the aircraft's weight & load it carries
• Nose landing gear:

  • Provides support for the aircraft's weight & the load it carries
  • Generally equipped with a steering mechanism, so that the aircraft can be maneuvered on the ground
• Types of landing gear arrangement:

  • Tailwheel/Conventional configuration
  • Tandem configuration
  • Tricycle configuration
• Tailwheel/Conventional configuration:

  • Used on older aircraft for landing operations on rough fields
• Tricycle configuration - used over conventional type landing gear:

  1. Allows more forceful application of the brakes without nosing over when braking, which enables higher landing speed
  2. Provides better visibility from the flight deck, especially during landing and ground manoeuvring
  3. Prevents ground-looping when landing (i.e rapid yawing of the aircraft while on the ground)
• Tricycle configuration:

  • In this configuration, the aircraft's centre of gravity is forward of the Main Gear
  • When the aircraft lands, the forces acting on the centre of gravity tend to keep the aircraft moving forward, rather than looping (such as with a tailwheel configuration)
• Aircraft wheels:

  • Typically made from aluminium alloy
  • Lightweight & strong
  • Important component of the landing gear system:
  • Tires are mounted onto the wheels
  • Together, they support the entire weight of the aircraft during taxi, takeoff & landing
• Aircraft tyres:

  • Support the aircraft's weight
  • Absorb shock from landing & taxiing
  • Provides gripping contact with the runway surface
  • Discharge static electricity
• Trunnion:

  • Part of landing gear assembly attached to the airframe - Is supported at its ends by bearing assemblies
  • Allows the landing gear to pivot during retraction & extension
• Strut:

  • Is the vertical member of the landing gear assembly
• Drag link / Drag strut:

  • Provides support to shock strut
  • Stabilise the shock strut longitudinally
• Side strut / Side brace link

  • Stabilise the landing gear laterally
• Overcenter link / Downlock:

  • Sometimes called a "Downlock"
  • Locks the main gear down position
  • Prevents the landing gear from collapsing during ground operation
  • In order to retract the landing gear, the link must be hydraulically retracted first
• Uplock mechanism:

  • Holds the main gear in the UP position when it is fully retracted
• Axles:

  • Where main wheels are installed on
  • Supports the main wheel
• Shock absorption in landing gear: To ensure that the aircraft is able to safely land & taxi, the landing gear must be able to absorb the forces of impact experience when the plane is landing
• 2 ways the landing gear can be absorbed:

  • Force from landing impact is transferred throughout the airframe & gradually dissipated -> Use non-shock absorbing landing gear
  • Force from landing impact is converted into heat energy -> Use shock absorbing landing gear
• Non-shock absorbing landing gear: The strong, single pulse of the landing gear is transferred throughout the airframe, & gradually dissipated at a different rate & time
• Non-shock absorbing landing gear consists of struts made of:

  • Flexible spring steel
  • Aluminium
  • Composite material
• Non-shock absorbing landing gear:

  • Upon receiving the landing impact, the struts of the landing gear flex
• Non-shock absorbing landing gear:

  • The landing gear then transfers the forces (from landing) to the airframe at a different rate (that will not harm the aircraft structure)
• Non-shock absorbing landing gear:

  • The force of the landing impact will gradually be dissipated, and the landing gear will eventually return to its original (unflexed) position
• Shock absorbing landing gear: The landing gear contains shock struts that help to absorb the shock loads from landing, & dissipate it as heat energy
• Shock strut: A typical pneumatic/hydraulic shock strut uses nitrogen gas combined with hydraulic fluid to absorb & dissipate shock loads
• A shock strut is constructed out of 2 telescoping cylinders/tubes that are closed on the external ends:

  • Upper cylinder is fixed to the aircraft, & does not move
  • Lower cylinder is called piston, & is free to slide in & out of the upper cylinder
• Due to this configuration, two chambers are formed.

  • Lower chamber is always filled with hydraulic fluid
  • Upper chamber is filled with Nitrogen gas
• An orifice is located between the two cylinders

  • It provides a passage for the fluid from the bottom chamber to enter the upper chamber, when the strut is compressed
• When the aircraft wheels touch the ground (during landing), the compression stroke of the shock strut begins.

  As the aircraft's center of mass moves downward,

  • The shock strut compresses
  • The piston is forced upward into the upper cylinder
  • This causes the metering pin to move up through the orifice
• • The compression of the Shock Strut will cause the volume of gas (in the upper cylinder) to decrease
• • This will increase the pressure in the shock strut (while the volume of hydraulic fluid remains the same)
• • The initial shock of landing is cushioned by the hydraulic fluid being forced through the metered opening
• • As the pressure and temperature in the cylinder increases, vertical speed of the aircraft decreases
• • The pressure in the cylinder will increase until it is sufficient to stop vertical motion of aircraft
• • At this point, the energy in the gas pressure is sufficient to recoil the aircraft upwards
• • During recoil (when the strut begins to extend), the shock strut extends until the gas pressure is just enough to support the weight of aircraft
  • The compressed air then acts as a shock absorber while the aircraft is taxiing
• Fixed landing gear: Aircraft fitted with fixed landing gear have their gear exposed to the airflow when flying

  • When the aircraft flies faster, the drag also incraeses
• Having a mechanism to retract and stow the landing gear helps to reduce drag on the aircraft

  • However, this mechanism adds weight
• Generally, aircraft fitted with retractable gears consider the mechanism's added weight a small sacrifice, especially when the aircraft is flying at faster speeds (where there is a larger increase in drag)