LANDING GEAR SYSTEMS

Landing Gear Configurations

Two main types: Conventional, and Tricycle

Tricycle

Has nose wheel, which may be steerable

Main gear, on either side

Example: Cessna

Keeps aircraft level during take-off and landing

The most important advantage is its ease of ground handling.

Conventional

Two main wheels

One tail dragger wheel

Reduced drag in the air

Reduced landing gear weight

Requires more skill in ground taxiing

The most important advantage is the ability to operate the aircraft over rough terrain.

Classification of Landing Gear

Main landing gear

Cushions landing impact

Heavily stressed area

Main Landing Gear consists of the main weight-bearing structure

Auxiliary landing gear includes tail wheels, skids, nose wheels, etc.

Nonabsorbing Landing Gear

Includes Rigid landing gear, Shock-cord landing gear, Spring landing gear

Rigid: helicopters, sailplanes. No flexing other than the structure.

Shock cord system: uses “Bungee” cords

Spring type uses spring steel (some Cessna’s)

Shock-Absorbing Landing Gear

Dissipates landing energies by forcing fluid through a restriction

This fluid generates heat, dissipated into the atmosphere

Two types: Spring Oleo, and Air-Oil Oleo

Spring Oleo is history by now

Air Oleos are all very similar: a needle valve restricts fluid flow

Air in the oleo holds the weight of the a/c on the ground

Air Oleos present in both retractable and fixed gears

Fixed Gear

Non retractable, usually bolted on to the structure

Often uses fairings or wheel pants

Cessna 152

Advantages:

Lighter weight

Less compex

Least costly

Retractable Gear

Designed to eliminate drag (the greatest advantage)

Can be either fully or partially retractable

Direction of retraction depends on airframe model

Methods of retraction: hydraulic, electric, mechanical, pneumatic

Critical area of aircraft maintenance for safety reasons

Hulls and Floats

Can be single float, or multiple

Definition may include floating hulls (ex. “Lake” aircraft)

Floating hulls may only require wing tip floats

Skis used for snow and ice (wood, metal, composites)

Skis may use shock cord to assist angle of ski attack

Skis are mounted on the same strut as tires

Landing Gear Components

Exact definitions of some components will vary

The Oleo strut is the widely used form of shock absorption on aircraft landing ge

ar.

Trunnions

Portion of the landing which attaches to the airframe

Supported at the ends by bearings

Landing gear traditionally extends from the center

Struts

Vertical member, contains the shock absorbing mechanism

Top of the strut mounts onto the trunnion

Strut forms the cylinder for the oleo (“outer” cylinder)

Piston is the moving portion (aka piston rod, tube or inner cylinder)

Oil is forced from the lower portion of the strut to the upper

Oil flow is restricted or varied according to a metering pin

Final weight of a/c rests on air in the top of the strut

Snubbers are used to prevent a sudden dropping of gear on takeoff

Metering pin controls the flow of fluid between the chambers.

The shock of landing is absorbed by the fluid being forced through a metered orifice. The metering pin gradually reduces the size of the orifice as the shock strut extends, which avoids a rapid extension after the initial shock of landing and related bounce.

Chevron seals are used in shock struts to prevent the oil from escaping

On nose wheel struts, a cam is built into the strut for the purpose of straightening

the nose wheel before retraction.

Filling a shock strut: “exercise” the strut in order to seat the seals, and remove air bubbles from the fluid.

Most shock strut oil levels are checked by releasing the air, bottoming the strut, and checking to see if the oil is at the level of the filler plug.

Information about shock struts: see:

Manufacturer’s maintenance manual

Information decal located on the strut

Mfr’s overhaul manual

Torque Links

Also called scissors assembly

Two A-frame members

Connects and aligns upper and lower cylinders

Connects the strut cylinder to the piston

Restricts extension of piston during retraction

Correctly aligns axle to the strut

Trucks

Located at the bottom of the strut piston

Axles are mounted on the truck

Trucks can tilt fore or aft to allow for a/c attitude changes

Drag Links

Stabilizes landing gear longitudiannly

May be hinged to allow retraction

Also called a drag strut

Side Brace Links

Stabilize gear laterally

May be hinged to allow retraction

Can be called a side strut

Overcenter Links (aka downlock mechanism)

Use to apply pressure to the center pivot joint in a drag or side brace link

Overcenter link is hydraulically retracted to allow gear retraction

Also called a downlock, and/or a jury strut

Swivel Glands

Flexible joint with internal passages

Route hydraulic fluid to the wheel brakes

Used where space limitation eliminate flex hoses

Shimmy Dampers

Hydraulic snubbing unit

Reduces tendency of nose wheels to oscillate

Piston type dampers

Piston and rod filled with hydraulic fluid

Piston has an orifice restricting speed of travel

Slow movement has no restriction

Large shimmy dampers incorporate temperature compensation

Vane type dampers

Employ stationary vanes and rotating vanes

Small passages restrict fluid movement

Central shaft rotation is restricted from moving quickly

Damper Inspections

Check for leakage & effectiveness of operation

Check mounting bolts and hardware

Most dampers are fairly reliable