Landing Gear Type and Configurations
Landing
gear provides support and directional control of the aircraft while on the
ground, and is a means for the aircraft to transition from the ground to the
air. During landing and taxiing, the gear will provide a cushion effect that
absorbs shock. A landing gear assembly consists of a shock strut, actuating
cylinders, side and drag brace, torque links, and a wheel and brake assembly.
When the landing gear is retracted during flight, drag is reduced. On most aircraft,
the landing gear will be enclosed in an opening either in the nacelle,
fuselage, or wings, and streamlined with doors.
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Points discussed:
Landing gear type and configurations, bicycle landing gear, modified tricycle landing gear, quadra-cycle landing gear, shock strut of landing gear, shock strut shock strut, Trunnion of landing gear, nose landing gear, torque links of landing gear, steering system of landing gear, nose wheel steering switch, rudder pedals/steering wheel, centering cams/motors, type of wheels, wheel markings, torquing instructions, arresting systems, drag chute,
Go to Lesson 2 : PRINCIPLES OF AERODYNAMICS
Go to Lesson 3 : AIRFOIL CHARACTERISTICS
BICYCLE Landing Gear,
Figure
1-37, Bicycle Type Landing Gear
TRICYCLE.
The most common type of landing gear pattern is the tricycle type. Figure 1-38.
This arrangement consists of a nose landing gear and two main landing gears.
The main landing gears are located slightly aft of the center of gravity (the
forward and aft balance point of the aircraft). The distance between the main
landing gears will vary with the size of the fuselage and wings.
Figure
1-38, Tricycle Type Landing Gear
MODIFIED TRICYCLE Landing Gear,
The
C–5 landing gear is a fully retractable modified tricycle type with four main
landing gear shock struts with six wheels mounted on bogie type units that
retract into pods on each side of the aircraft. Figure 1-39. The nose gear is a
four-wheel steerable unit, which retracts aft into the fuselage nose wheel
well.
Figure 1-39, Modified Tricycle Type Landing
Gear
QUADRA-CYCLE LANDING GEAR,
The
quadra-cycle landing gear, Figure 1- 40, is used on the B–52 aircraft. It
consists of four main gears mounted in the fuselage in the form of a rectangle
and two outriggers or wing tip protection gears, as shown. The wing tip
protection gear is located near the outboard end of each wing to provide
lateral stability.
The quadra-cycle landing gear retracts into
the fuselage, which allows the use of a thinner wing design and results in
greater speeds.
Figure
1-40, Quadra-cycle Landing Gear
Aircraft
Shock struts:
SHOCK STRUT of Landing Gear,
The purpose of the shock strut is to absorb
shock during take off, landing, and ground operation. The shock strut is a
pneudraulic unit that consists of several components. Each component serves a
specific purpose. Refer to Figure 1-41 as we discuss each of these components.
OUTER
AND INNER CYLINDER of Landing Gear,
The
inner cylinder or piston is on the lower part of the strut and the axle is at
the lowest point. The inner cylinder slides inside the outer cylinder to give
the desired shock absorbing qualities. This shock strut is serviced with
hydraulic fluid and nitrogen or dry air. A set of seals at the lower part of
the outer cylinder keeps the unit from leaking oil or the air charge.
Figure 1-41, Shock Strut
TRUNNION of Landing Gear,
At the top of the outer cylinder is the
trunnion. Figure 1-42. This is the point at which the landing gear is attached
to the aircraft structure. It is also the pivot point for extending and
retracting the landing gear.
Figure
1-42, Landing Gear Trunnion
Nose landing gear
TORQUE LINKS of Landing Gear,
The torque links are attached at the base of
the outer cylinder and just above the axle on the inner cylinder. The torque
links are hinged in the middle and at their attaching points. This allows the
inner and outer cylinder to telescope. Torque links also keep the inner
cylinder from rotating or spinning.
STEERING SYSTEM of Landing Gear,
Most aircraft have steerable nose wheels. Nose
wheel steering provides a means of directional control when the aircraft is
taxiing, during takeoff and on landing roll. Certain conditions must be present
for nose wheel steering to operate. There must be DC electrical power,
hydraulic pressure, and the ground safety switch must be closed.
NOSE WHEEL STEERING SWITCH,
To energize the nose wheel steering system on
some fighter aircraft, the nose wheel steering switch must be depressed,
opening a solenoid controlled shutoff valve. This switch is located on the
control stick grip. On most heavy type aircraft, the nose wheel steering is
automatically engaged when weight is on the landing gear or when the landing
gear is down and locked.
RUDDER PEDALS/STEERING WHEEL,
On fighter type aircraft the rudder pedals
allow the pilot to select the direction and degree of travel. The rudder pedals
are connected to the nose gear steering system by mechanical linkage and
electrical circuits. These allow the steering power unit/servo valve to control
nose wheel steering. On many large
aircraft, the nose wheel steering system is controlled by a steering wheel
located to the left and forward of the pilot. The steering wheel controls the
direction and degree of travel of the nose wheel. When small amounts of
movement are required for corrections on takeoff and landing, the rudder pedals
will give the pilot up to 8° of steering. This minor amount of steering aids in
high speed ground movement because the normal steering wheel control could
easily over control the aircraft.
CENTERING CAMS/MOTORS,
One
notable difference between main and nose gear shock struts is the centering
device found in nose struts. Two types of centering devices are shown in
Figures 1-43 and 1-44. The upper cam is attached to the top of the piston
(inner cylinder), and the lower cam is connected to the inside bottom of the outer
cylinder. When the weight of the aircraft is removed from the gear, the shock
strut piston extends and forces the upper cam into the lower cam. The seating
of the two cams aligns the wheel for proper retraction into the wheel well.
When the weight of the aircraft is on the gear, the upper cam is unseated from
the lower cam and the wheel is free to swivel.
Figure 1-42, Figure 1-43,
V Type Centering Device Nose Strut,
Centering (Cam and Lobe Type)
Wheels/Tires,
TYPE OF WHEELS,
One
type of aircraft wheel is the split type wheel mostly used on large aircraft.
This type of wheel is in two halves and it is bolted together. When bolted
together both halves must have identical part numbers and manufacturer. The
second type of aircraft wheel used is the split rim (removable flange) wheel;
this is primarily used on fighters. The flange portion of the wheel is held on
with a locking ring, this type of wheel is faster in changing out the rubber
(tire), making it go back into service faster. Aircraft wheels are usually made
of aluminum, magnesium, or steel alloy. These materials are susceptible to
corrosion. To help reduce corrosion and failure, all aircraft wheels will be
cleaned, inspected and repaired. This maintenance will be accomplished at the
time of tire change.
WHEEL MARKINGS,
On
each wheel there are markings very much like those on a tire. Each wheel is
made for a specific type of tire. Each half of a split wheel has a part number
and manufacturer stamped on it. Wheel halves must be matched with respect to
wheel part number and manufacturer when assembled.
•
Size. The wheel size is stamped on each half of the split wheel.
•
Serial Number. Each wheel half has a serial number stamped on it. This number
will be used when turning in the wheel after a wheel and tire change
•
Disassembly Warning. A warning is stamped on the wheel which reads: Deflate
tire before loosening tie bolts. The tie bolts hold the wheel halves together
on a split wheel. If tie bolts are loosened before the tire is deflated, the
wheel and tire may explode.
• Torquing Instructions,
The
tire shop puts the wheel halves together. The bolts must be torqued in a
specific way and to a specific torque. These instructions are stamped on the
wheel.
TIRE
SHOP,
The
tire shop is where the tires and wheels are put together and inflated. A modern
tire shop uses pneumatic tools for disassembly an reassembly of the wheels. The
tire shop will also have vats of solvent for cleaning the used parts. This is
so a through inspection can be made to determine if the part can be reused.
Arresting Systems,
Arresting
gear is usually installed on fighter type aircraft. The arresting gear is made
to stop the aircraft on the runway in case of an emergency. The arresting gear
is located underneath and at the rear of the aircraft. DO NOT walk or crawl
under the arresting gear. This is a very dangerous area and can cause serious
injury or death. Always make sure the arresting gear is safely pinned when the
aircraft is stationary.
DRAG CHUTE,
With
high landing speeds, drag chutes are often installed to assist the aircraft
braking system. These drag chutes allow aircraft to land on shorter runways at
higher speeds and weights.