Landing Gear

Study of Evolution and Details of Landing Gear

NIMRA INSTITUTE OF SCIENCE & TECHNOLOGY, A.E - 1 -

INDEX CHAPTER 1

1.1.1 INTRODUCTION 3

1.1.2 EVOLUTION 3

CHAPTER 2

2.1 DIFFERENT TYPES OF LANDING GEARS 6

2.1.1 TRICYCLE LANDING GEAR 7

2.1.2 CONVENTIONAL LANDING GEAR 7

2.1.3 UNCONVENTIONAL LANDING GEAR 8

2.2 DIFFERENCE BETWEEN MAIN AND NOSE LANDING GEAR 9

2.3 SHOCK STRUTS 10

2.3.1 TYPES OF SHOCK STRUTS

1. METERING PIN TYPE 10

2. METERING TUBE TYPE 11

3. NOSE GEAR STRUTS 12

4. DOUBLE-ACTING SHOCK ABSORBER 12

2.4 OPERATION OF SHOCK STRUTS 13

CHAPTER 3

3.1 HYDRAULIC SYSTEM FOR AIRCRAFT LANDING GEAR 15

3.2 LANDING GEAR EXTENSION AND RETRACTION 15

3.2.1 LANDING GEAR EXTENSION AND

RETRACTING MECHANISMS 15

3.3 EMERGENCY SYSTEMS 16



CHAPTER 4

4.1 BRAKING SYSTEM IN LANDING GEAR 18

4.2 DIFFERENT TYPES OF BRAKES AND THEIR EVOLUTION

4.2.1 CARBON AND BERYLLIUM BRAKES 18

4.2.2 AUTO-BRAKE AND BRAKE-BY-WIRE SYSTEM 19

4.3 DESCRIPTION OF A HYDRAULIC BRAKING SYSTEM 20

4.4 ADVANCED BRAKE CONTROL SYSTEM (ABCS) 21

4.5 PNEUMATIC BRAKING 21

4.6 DIFFERENTIAL BRAKING 22

CHAPTER 5

LUBRICANTS USED IN LANDING GEAR 23

CONCLUSION 24

REFERENCES 25

FIGURES

FIG. 1 LANDING GEARS IN THE INITIAL STAGES 26

FIG. 2 BASIC TYPES OF LANDING GEARS 26

FIG. 3 TU-144 MAIN LANDING GEAR 27

FIG. 4 TRACK-TYPE GEAR 27

FIG. 5 THE ITALIAN BONMARTINI TRACK GEAR 28

FIG. 6 THREE COMMON TYPES OF LANDING GEARS 28

FIG. 7 TRICYCLE LANDING GEAR 29

FIG. 8 LA-4 AIR CUSHION GEAR 29

FIG. 9 MAIN LANDING GEAR 29

FIG. 10 NOSE GEAR ASSEMBLY 30

FIG. 11 SHOCK STRUT WITH A METERING PIN 31



FIG.12 SHOCK STRUT WITH A METERING TUBE 32

FIG. 13 SIMPLE NOSE GEAR STRUTS 33

FIG. 14 DOUBLE-ACTING SHOCK ABSORBER 33

FIG. 15 OPERATION OF SHOCK STRUT 34

FIG. 16 OLEO-PNEUMATIC SHOCK STRUT TYPES 35

FIG. 17 BASIC HYDRAULIC SYSTEM 35

FIG. 18 THE HYDRAULIC LANDING GEAR SYSTEM INSIDE

THE WHOLE AIRPLANE 36

FIG.19. TYPICAL BRAKE AND ITS RELATIONSHIP TO

THE LANDING GEAR 37

FIG. 20 BERYLLIUM BRAKE 38

FIG. 21 CARBON BRAKE 39

FIG. 22 ESTIMATED NUMBER OF STOPS VS.

KINETIC ENERGY PER POUND. 39

FIG. 23 BRAKE MATERIALS: SPECIFIC STRENGTH VS.

TEMPERATURE 40

FIG. 24AUTO-BRAKE AND BRAKE-BY-WIRE SYSTEM 40

FIG. 25 EMERGENCY HYDRAULIC BRAKING SYSTEM 41

FIG. 26 PAIRED WHEEL HYDRAULIC BRAKING SYSTEM 41

FIG. 27 INDIVIDUAL WHEEL HYDRAULIC BRAKE CONTROL SYSTEM 42

TABULAR FORM

TABLE 1 PROPERTIES OF DIFFERENT HEAT SINK MATERIALS 42



CHAPTER 1

1.1 INTRODUCTION

The landing gear is that portion of the aircraft that supports the weight of the aircraft

while it is on the ground. The landing gear contains components that are necessary for taking

off and landing the aircraft safely. Some of these components are landing gear struts that

absorb landing and taxiing shocks; brakes that are used to stop and, in some cases, steer the

aircraft; nosewheel steering for steering the aircraft; and in some cases, nose catapult

components that provide the aircraft with carrier deck takeoff capabilities.

The landing gear is the principle support of the airplane when parked, taxiing, taking

off, or when landing. The most common type of landing gear consists of wheels, but

airplane0s can also be equipped with floats for water operations, or skis for landing on snow.

[Figure 1-9] The landing gear consists of three wheels—two main wheels and a third wheel

positioned either at the front or rear of the airplane. Landing gear employing a rear mounted

wheel is called conventional landing gear. Airplanes with conventional landing gear are

sometimes referred to as tailwheel airplanes. When the third wheel is located on the nose, it is

called a nosewheel, and the design is referred to as a tricycle gear. A steerable nosewheel or

tailwheel permits the airplane to be controlled throughout all operations while on the ground.

1.2 EVOLUTION

The first wheeled landing gears appeared shortly after the Wright Brothers' maiden

flight in December 1903. Santos-Dumont's "No. 14 bis" had a wheeled landing gear; this

airplane made the first flight in Europe in October 1906. This was followed quickly by

wheeled aircraft designed. Then came World War I, by which time the configurations had

more or less settled down to tail wheel types, employing fairly rugged struts attached to the

fuselage and landing gears that had some degree of shock absorption through the use of

bungee cords wrapped around the axles, as illustrated in figure 1.

The Sopwith Camel was shown in fig. 1(a), SE5 shown in fig. 1(b) and SPAD VIL

shown in fig1. (c) Were typical World War I fighter/scout aircraft. Both the Camel and

SPAD had axles that pivoted from the spreader bars, the main difference being in the location

of the bungee that restrained the axle from moving. The Camel's bungees were at the extreme

ends of the spreaders and permitted 4 in. of wheel travel. The SPAD's shock cords permitted



3–4 in. of travel (depending on the model), but were located inboard of the gear support

struts.

In the 21 years between World Wars I and II, landing gear design developed as fast as

airframe design. The latter changed from braced wood and fabric biplanes to aluminum alloy

monoplanes and the landing gears became retractable, employing a variety of shock-

absorbing systems. Increased shock absorption became necessary in order to accommodate

the constantly increasing aircraft weights and sink speeds. Although the shock absorber

stroke is not a function of aircraft weight, it was important to increase that stroke in order to

lower the landing load factors and thereby minimize the structure weight influenced by the

landing loads.

Larger-section tires provided some of the desired shock absorption, but size

limitations and relatively low (47%) efficiency prevented a major contribution from this

source. Therefore, shock-absorbing support struts were devised. These ranged from rubber

blocks and compression springs to leaf springs, oleo-pneumatic struts, and liquid springs.

To decrease drag in flight some undercarriages retract into the wings and/or fuselage

with wheels flush against the surface or concealed behind doors; this is called retractable

gear.

The earliest retractable landing gear is that used on the Bristol (England) Jupiter

racing aircraft of the late 1920's. In the United States, Lockheed's Model 8D Altair, which

first flew in 1930, had a fully retractable landing gear.

The landing gear consists of two dual wheel main gears and one dual nose gear, each

main gear is equipped with Disk brakes, anti skid protection and thermal tire deflators

(fusible plugs).

The landing gear is positioned hydraulically as selected by the landing gear lever in

the cockpit on the center instrument panel. Door and gear sequencing is automatic. Except for

the nose gear, which is mechanically opened and closed by the movement of the gear, there is

a door release handle in each main gear well for ground access.



CHAPTER 2

2.1 DIFFERENT TYPES OF LANDING GEARS

Airplanes require landing gear for taxiing, takeoff, and landing. The earliest airplane

Wright Flyers used skids as their landing gears. Soon, wheels were attached to the skids.

Since that time, various arrangements have been used for wheels and structures to connect

them to the airplane. Landing gears are generally categorized by the number of wheels and

their pattern. Figure 2 illustrates the basic types.

This terminology is rapidly gaining worldwide acceptance. For instance, the

USAF/USN Enroute Supplements define the strength of 11 a given field as T-50/TT-100,

indicating that the airfield is cleared to accept aircraft weighing 50,000 lb with a twin-wheel

gear or 100,000 lb with a twin-tandem gear.

There are also hybrid arrangements such as the 12-wheel arrangement used on the

Soviet TU-144 supersonic transport depicted in the figure 3 and the track gears that were

tested on the Fairchild Packet, Boeing B-50, and Convair B-36 the latter is illustrated in the

figure 4. The objectives of the track gear were to reduce the weight and size attributable to

the tires and to improve flotation by having a larger contact area.

Track gears did have higher flotation by keeping the contact pressures as low as 30

psi, but there was no weight reduction. In fact, aircraft weight was increased by about 1.8%

(1.78% on the Packet and 1.87% on the B-36). Maintainability and reliability were also

degraded substantially because of the complicated mechanism (multiple shock absorbers in

the track bogie), low bearing life, low belt life, and high spin-up loads.

The Italian Bonmartini track gear was also tested successfully, but it too was heavier

than a conventional gear. It used a pneumatic belt to encompass the two wheels, as shown in

the figure 5.

Today, there are three common types of landing gears namely conventional landing

gear shown in figure 6(a), Tricycle landing gear shown in figure 6(b), and Unconventional

Landing Gears as shown in figure 6(c).



2.1.1 TRICYCLE LANDING GEAR

Tricycle gear describes an aircraft undercarriage, or landing gear, arranged in a

tricycle fashion. The tricycle arrangement has one gear strut in front, called the nose wheel,

and two or more main gear struts slightly aft of the center of gravity.

Several early aircraft had primitive tricycle gear, notably the Curtiss Pushers of the

early 1910s. Tricycle gear is essentially the reverse of conventional landing gear or

taildragger. Tricycle gear aircraft have the advantage that it is nearly impossible to make

them 'nose over' as can happen if a taildragger hits a bump or has the brakes heavily applied.

In a nose over, the airplanes tail tips up, burying the propeller in the ground and causing

damage. Tricycle gear planes are also easier to handle on the ground and reduce the

possibility of a ground loop. This is due to the main gear being behind the center of mass.

Tricycle gear aircraft are easier to land because the attitude required to land on the

main gear is the same as that required in the flare, and they are less vulnerable to crosswinds.

As a result, the majority of modern aircraft are fitted with tricycle gear. Almost all jet-

powered aircraft have been fitted with tricycle landing gear, to avoid the blast of hot, high-

speed gases causing damage to the ground surface, in particular runways and taxiways. The

few exceptions have included the Yakovlev Yak-15, the Supermarine Attacker, and

prototypes such as the Heinkel He 178, the Messerschmitt Me 262 V3, and the Nene powered

version of the Vickers VC.1 Viking.

The taildragger configuration does have advantages. The rear wheel means the plane

naturally sits in a nose-up attitude when on the ground; this is useful for operations on

unpaved surfaces like gravel where debris could damage the propeller. Additionally, on the

ground the wing naturally sits at a higher angle of attack, permitting a shorter takeoff roll

than an equivalent tricycle design. The simpler main gear and small tailwheel result in both a

lighter weight and less complexity if retractable. Likewise, a fixed-gear taildragger exhibits

less interference drag and form drag in flight than a fixed-gear aircraft with tricycle gear. A

typical tricycle landing gear is shown in figure 7

2.1.2 CONVENTIONAL LANDING GEAR (OR) THE TAILDRAGGERS

It consists of two wheels forward of the aircraft's center of gravity and a third small

wheel at the tail as shown in figure 6(b). This type of landing gear is most often seen in older



general aviation airplanes. The two main wheels are fastened to the fuselage by struts.

Without a wheel at the nose of the plane, it easily pitches over if brakes are applied too soon.

Because the tailwheel is castered free to move in any direction. The plane is very difficult to

control when landing or taking off. The tailwheel configuration offers several advantages

over the tricycle landing gear arrangement.

Due its smaller size the tailwheel has less parasite drag than a nosewheel, allowing

the conventional geared aircraft to cruise at a higher speed on the same power. Tail wheels

are less expensive to buy and maintain than a nosewheel.

Taildraggers are considered harder to land and take off (because the arrangement is

unstable, that is, a small deviation from straight-line travel is naturally amplified by the

greater drag of the mainwheel which has moved farther away from the plane's center of

gravity due to the deviation), and usually require special pilot training. . The taildragger

arrangement was common during the early propeller era, as it allows more room for propeller

clearance. Landing a conventional geared aircraft can be accomplished in two ways.

Normal landings are done by touching all three wheels down at the same time in a

three-point landing. This method does allow the shortest landing distance but can be difficult

to carry out in crosswinds.

The alternative is the wheel landing. This requires the pilot to land the aircraft on the

main wheels while maintaining the tail wheel in the air with elevator to keep the angle of

attack low. Once the aircraft has slowed to a speed that can ensure control will not be lost,

but above the speed at which rudder effectiveness is lost, then the tail wheel is lowered to the

ground.

2.1.3 UNCONVENTIONAL LANDING GEAR

Usage of skids during and after World War II has been an endeavor to reduce the

landing gear weight below the normal 3–6% of gross weight and, to a great extent, this has

been accomplished. However, in most cases, the aircraft must use a trolley beneath the skids

for takeoff, with the trolley being retrieved after the aircraft has left it. A typical

unconventional landing gear with skids is shown in figure 6(c).



Air cushion systems are another type of unconventional gear, which have been

pioneered by Bell-Textron in the United States. The LA-4 was their first venture; it was a

small aircraft shown in the below figure, that operated successfully on plowed ground, over

tree stumps up to 6 in. high, over 3 ft wide ditches, on soft muddy ground, and over both sand

and water. Further details of this and other systems, including the ACLS Buffalo, are also

provided in later chapters.

Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder

to land and take off (because the arrangement is unstable, that is, a small deviation from

straight-line travel is naturally amplified by the greater drag of the mainwheel which has

moved farther away from the plane's center of gravity due to the deviation), and usually

require special pilot training. Sometimes a small tail wheel or skid is added to aircraft with

tricycle undercarriage, in case of tail strikes during take-off.

The Boeing727also had a retractable tail bumper. Some aircraft with retractable

conventional landing gear have a fixed tailwheel, which generate minimal drag (since most of

the airflow past the tailwheel has been blanketed by the fuselage) and even improve yaw

stability in some cases.

2.2 DIFFERENCE BETWEEN MAIN AND NOSE LANDING GEAR

A main landing gear assembly is shown in figure 9. The major components of the

assembly are the shock strut, tire, tube, wheel, brake assembly, retracting and extending

mechanism, and side struts and supports.

A typical nose gear assembly is shown figure 10. Major components of the assembly

include a shock strut, drag struts, a retracting mechanism, wheels, and a shimmy damper.

The nose gear shock strut, drag struts, and retracting mechanism are similar to those

described for the main landing gear. The shimmy damper is a self-contained hydraulic unit

that resists sudden twisting loads applied to the nosewheel during ground operation, but

permits slow turning of the wheel. The primary purpose of the shimmy damper is to prevent

the nosewheel from shimmying (extremely fast left-right oscillations) during takeoff and

landing. This is accomplished by the metering of hydraulic fluid through a small orifice

between two cylinders or chambers.



Most aircraft are equipped with steerable nose-wheels and do not require a separate

self-contained shimmy damper. In such cases, the steering mechanism is hydraulically

controlled and incorporates two spring-loaded hydraulic steering cylinders that, in addition to

serving as a steering mechanism, automatically subdue shimmy and center the nosewheel.

For more information concerning landing gear components (shock struts, shimmy dampers,

power steering units, and brakes), you should refer to chapter 12 of this TRAMAN.

2.3 SHOCK STRUTS

Shock struts are self-contained hydraulic units. They carry the burden of supporting

the aircraft on the ground and protecting the aircraft structure by absorbing and dissipating

the tremendous shock of landing. Shock struts must be inspected and serviced regularly for

them to function efficiently. This is one of your important responsibilities.

Each landing gear is equipped with a shock strut. In addition to the landing gear shock

struts, carrier aircraft are equipped with a shock strut on the arresting gear. The shock strut’s

primary purpose is to reduce arresting hook bounce during carrier landings.

Because of the many different designs of shock struts, only information of a general

nature will be included in this chapter. For specific information on a particular installation,

you should refer to the applicable aircraft MIM or accessories manual.

2.3.1 TYPES OF SHOCK STRUTS

1. METERING PIN TYPE

A typical pneumatic/hydraulic shock strut (metering pin type) is shown in figure 11. It

uses compressed air or nitrogen combined with hydraulic fluid to absorb and dissipate shock,

and it is often referred to as the "air-oil" type strut. This particular strut is designed for use on

the main landing gear. As shown in the illustration, the shock strut is essentially two

telescoping cylinders or tubes, with externally closed ends. When assembled, the two

cylinders, known as cylinder and piston, form an upper and lower chamber for movement of

the fluid. The lower chamber is always filled with fluid, while the upper chamber contains

compressed air or nitrogen. An orifice (small opening) is placed between the two chambers.



The fluid passes through this orifice into the upper chamber during compression, and returns

during extension of the strut.

Most shock struts employ a metering pin similar to that shown in figure 11 to control

the rate of fluid flow from the lower chamber into the upper chamber. During the

compression stroke, the rate of fluid flow is not constant, but is controlled automatically by

the variable shape of the metering pin as it passes through the orifice.

2. METERING TUBE TYPE

On some types of shock struts now in service, a metering tube replaces the metering

pin, but shock strut operation is the same. An example of this type of shock strut is shown in

figure 12.

Some shock struts are equipped with a dampening or snubbing device, which consists

of a recoil valve on the piston or recoil tube. The purpose of the snubbing device is to reduce

the rebound during the extension stroke and to prevent a too rapid extension of the shock

strut, which would result in a sharp impact at the end of the stroke.

The majority of shock struts are equipped with an axle that is attached to the lower

cylinder to provide for tire and wheel installation. Shock struts not equipped with axles have

provisions on the end of the lower cylinder for ready installation of the axle assembly.

Suitable connections are also provided on all shock struts to permit attachment to the aircraft.

A fitting, which consists of a fluid filler inlet and a high-pressure air valve, is located

near the upper end of each shock strut to provide a means of filling the strut with hydraulic

fluid and inflating it with air or nitrogen.

A packing gland designed to seal the sliding joint between the upper and lower

telescoping cylinders is installed in the open end of the outer cylinder. A packing gland wiper

ring is also installed in a groove in the lower bearing or gland nut on most shock struts to

keep the sliding surface of the piston or inner cylinder free from dirt, mud, ice, and snow.

Entry of foreign matter into the packing gland will result in leaks. The majority of shock

struts are equipped with torque arms attached to the upper and lower cylinders to maintain

correct alignment of the wheel.



3. NOSE GEAR STRUTS

Nose gear shock struts are provided with an upper centering cam that is attached to

the upper cylinder and a mating lower centering cam that is attached to the lower cylinder as

shown in figure 13. These cams serve to line up the wheel and axle assembly in the straight-

ahead position when the shock strut is fully extended. This prevents the nosewheel from

being cocked to one side when the nose gear is retracted, preventing possible structural

damage to the aircraft. These mating cams also keep the nosewheel in a straight-ahead

position prior to landing when the strut is fully extended. Some nose gear shock struts have

the attachments for installation of an external shimmy damper.

Nose and main gear shock struts are usually provided with jacking points and towing

lugs. Jacks should always be placed under the prescribed points. When towing lugs are

provided, the towing bar should be attached only to these lugs.

All shock struts are provided with an instruction plate that gives, in a condensed form,

instructions relative to the filling of the strut with fluid and inflation of the strut. The

instruction plate also specifies the correct type of hydraulic fluid to use in the strut. The plate

is attached near the high-pressure air valve. It is of the utmost importance that you always

consult the applicable aircraft MIMs and familiarize yourself with the instructions on the

plate prior to servicing a shock strut with hydraulic fluid and nitrogen or air.

4. DOUBLE-ACTING SHOCK ABSORBER

Double-acting shock struts improve shock absorption characteristics during taxi

conditions over rough or unpaved fields. If such conditions are an important aspect of the

aircraft's requirements, then this type of strut should be considered since its secondary

chamber (shown in Fig. 14) substantially reduces loads beyond the static position; they

generally have lower overall efficiencies than single-acting struts; they are also more

expensive and somewhat heavier.

The following are the specifications of the double-acting shock absorber shown in fig. 14

1. Landing gear attachment to airframe for cantilever strut arrangement centerline

2. Drag strut retraction actuator centerline

3. Axle centerline for twin-wheel tires and brakes



4. Oil charge and bleed plug

5. Oil charging valve

6. Oil drain plug

7. Oil/air separator pistons

8. First-stage damping

9. Second-stage damping

10. Charge valve, first-stage nitrogen

11. Charge valve, second-stage nitrogen

12. Pressure gage

13. Brake hydraulic manifold

14. Weight-on wheel switch subassembly

2.4 OPERATION OF SHOCK STRUTS

Figure 15 shows the inner construction of a shock strut and the movement of the fluid

during compression and extension of the strut. The compression stroke of the shock strut

begins as the aircraft hits the ground. The center of mass of the aircraft continues to move

downward, compressing the strut and sliding the inner cylinder into the outer cylinder. The

metering pin is forced through the orifice, and by its variable shape, controls the rate of fluid

flow at all points of the compression stoke. In this manner, the greatest possible amount of

heat is dissipated through the walls of the shock strut. At the end of the downward stroke, the

compressed air or nitrogen is further compressed, limiting the compression stroke of the strut.

If there is an insufficient amount of fluid and/or air or nitrogen in the strut, the compression

stroke will not be limited, and the strut will "bottom" out, resulting in severe shock and

possible damage to the aircraft.

The extension stroke occurs at the end of the compression stroke, as the energy stored

in the compressed air or nitrogen causes the aircraft to start moving upward in relation to the

ground and wheels. At this instant, the compressed air or nitrogen acts as a spring to return

the strut to normal. At this point, a snubbing or dampening effect is produced by forcing the

fluid to return through the restrictions of the snubbing device (recoil valve). If this extension

were not snubbed, the aircraft would rebound rapidly and tend to oscillate up and down

because of the action of the compressed air. A sleeve, spacer, or bumper ring incorporated in

the strut limits the extension stroke.



Oleo-pneumatic shock struts shown in the below figure, absorb energy by "pushing" a

chamber of oil against a chamber of dry air or nitrogen and then compressing the gas and oil.

Energy is dissipated by the oil being forced through one or more orifices and, after the initial

impact; the rebound is controlled by the air pressure forcing the oil to flow back into its

chamber through one or more recoil orifices. If oil flows back too quickly, the aircraft will

bounce upward; if it flows back too slowly, the short wavelength bumps (found during

taxiing) will not be adequately damped because the strut has not restored itself quickly

enough to the static position. Different types of oleo-pneumatic shock struts are shown in the

figure 16.



CHAPTER 3

3.1 HYDRAULIC SYSTEM FOR AIRCRAFT LANDING GEAR

A hydraulic system for raising and lowering aircraft landing gear includes an actuator

which is extendible and retractable to operate the landing gear, the actuator including a

movable member in a casing, the movable member being moved relative to the casing in a

first direction to extend the actuator when fluid under pressure is supplied to a first side of the

movable member while fluid is exhausted from a second side of the movable member, and

the movable member being moved in a second direction to retract the actuator when fluid

under pressure is supplied to the second side of the movable member while fluid is exhausted

from the first side of the movable member, and there being selector valve selectively to

supply pressurized fluid to the first or second side of the movable member, and a check valve

to permit exhausted fluid from at least one of the first and second sides of the movable

member to augment the supplied fluid from the selector valve and thus be directed with the

supplied fluid, to the second or first side respectively of the movable member as shown in

figure 17.

There are multiple applications for hydraulic use in airplanes, depending on the

complexity of the airplane. For example, hydraulics is often used on small airplanes to

operate wheel brakes, retractable landing gear, and some constant-speed propellers. On large

airplanes, hydraulics is used for flight control surfaces, wing flaps, spoilers, and other

systems. A basic hydraulic system consists of a reservoir, pump (either hand, electric, or

engine driven), a filter to keep the fluid clean, selector valve to control the direction of flow,

relief valve to relieve excess pressure, and an actuator. The hydraulic fluid is pumped through

the system to an actuator or servo. Servos can be either single-acting or double-acting servos

based on the needs of the system. This means that the fluid can be applied to one or both

sides of the servo, depending on the servo type, and therefore provides power in one direction

with a single-acting servo. A servo is a cylinder with a piston inside that turns fluid power

into work and creates the power needed to move an aircraft system or flight control. The

selector valve allows the fluid direction to be controlled. This is necessary for operations like

the extension and retraction of landing gear where the fluid must work in two different

directions. The relief valve provides an outlet for the system in the event of excessive fluid

pressure in the system. Each system incorporates different components to meet the individual

needs of different aircraft. A mineral-based fluid is the most widely used type for small



airplanes. This type of hydraulic fluid, which is a kerosene-like petroleum product, has good

lubricating properties, as well as additives to inhibit foaming and prevent the formation of

corrosion. It is quite stable chemically, has very little viscosity change with temperature, and

is dyed for identification. Since several types of hydraulic fluids are commonly used

3.2 LANDING GEAR EXTENSION AND RETRACTION

Gear Doors. Each gear is sequenced automatically with its gear door; opening of the

door is controlled by the gear lever. The main gear cannot extend or retract unless the gear

door is open and cannot close unless the gear is locked in the up or down position all due to

sequence valves being installed. The nose gear is controlled mechanically by linkages to the

gear. The forward doors are closed in both the gear up and down positions but the aft doors

remain open when the gear is down.

Gear Air-Ground Logic. Air ground sensing for various systems is provided by safety

switches on the left main gear and nose gear. These are actuated by the extension (air logic)

or compression (ground logic) of the left main gear and nose gear.

3.2.1 LANDING GEAR EXTENSION AND RETRACTING

MECHANISMS

Some aircraft have electrically actuated landing gear, but most are hydraulically

actuated. Figure 18 shows a retracting mechanism that is hydraulically actuated. The landing

gear control handle in the cockpit allows the landing gear to be retracted or extended by

directing hydraulic fluid under pressure to the actuating cylinder. The locks hold the gear in

the desired position, and the safety switch prevents accidental retracting of the gear when the

aircraft is resting on its wheels

3.3 EMERGENCY SYSTEMS

If the landing gear fails to extend to the down and locked position, each naval aircraft

has an emergency method to extend the landing gear. Emergency extension systems may

vary from one aircraft to another. The methods used may be the auxiliary/ emergency

hydraulic system, the air or nitrogen system, or the mechanical free-fall system. An aircraft

may contain a combination of these systems. For example, the main landing gear emergency



extension may be operated by the free-fall method and the nose gear by the

auxiliary/hydraulic system method.

The nitrogen storage bottle system is a one-shot system powered by nitrogen pressure

stored in four compressed nitrogen bottles. Pushing in, rotating clockwise and pulling out the

landing gear control handle actuates the emergency gear linkage connected to the manually

operated release valve on the nitrogen bottle. The release valve connects pressure from the

bottle to each release valve of the remaining three bottles. The compressed nitrogen from the

manually operated bottle repositions the shuttle valve in each of the other three nitrogen

bottles and permits nitrogen pressure to flow to the extend side of the cylinders. When the up

lock hooks are released, the main gear drops by gravity, and the nose gear extends by a

combination of gravity and nitrogen pressure. Each gear extends until the down lock secures

it in the down position. At this time, the cockpit position indicator shows the down wheel,

and the transition light on the control panel goes out. When the landing gear control handle is

actuated in the emergency landing gear position, a cable between the control and the

manually operated nitrogen bottle opens the emergency gear down release valve on the

bottle. Nitrogen from this bottle actuates the release valves on the other three bottle so that

they will discharge.

This action causes the shuttles within the shuttle valve on the aft door cylinders, and

on the nose gear cylinder, to close off the normal port and operate tie cylinders. The nose

gear cylinder extends and unlocks the up lock and extends the nose gear. The nitrogen

flowing into the aft door cylinders opens the aft doors. Fluid on the closed side of the door

cylinders and the up side of the nose gear cylinder is vented to return through the actuated

dump valves. Nitrogen from another bottle actuates the shuttle valves on the up lock

cylinders. Nitrogen flows into the up lock cylinders and causes them to disengage the up

locks. As soon as the up locks are disengaged, the main gear extends by the force of gravity.

Fluid on the up side of the main gear cylinders is vented to return through the actuated dump

valves, preventing a fluid lock. When the gear fully extends, the down lock cylinder’s spring

extends its piston and engages the down lock.



CHAPTER 4

4.1 BRAKING SYSTEM IN LANDING GEAR

Brakes, in conjunction with a skid control system (if provided), are used to stop, or

help stop, an aircraft. They are also used to steer the aircraft by differential action, to hold the

aircraft stationary when parked and while it is running up its engines, and to control speed

while taxiing. Most aircraft use disk brakes. The primary variables to consider are disk

material and diameter and the number of disks.

Skid control systems are used to minimize stopping distance and to reduce the

possibility of excessive tire wear and blowout caused by excessive skidding. The systems do

this by constantly sensing the available degree of friction coefficient and by monitoring brake

pressure to provide a fairly constant brake force almost up to the skidding point. Figure 19 is

provided to show more details of a typical brake and its relationship to the landing gear

4.2 DIFFERENT TYPES OF BRAKES AND THEIR EVOLUTION

4.2.1 CARBON AND BERYLLIUM BRAKES

Until about 1963, most brake heat sinks were made from steel. Beryllium was

selected for the Lockheed C-5A to save about 1600 lb on the aircraft's 24 brakes. It is also

used on other aircraft such as the Lockheed S-3A and the Grumman F-14. More recently,

carbon has been introduced (e.g., C-5B, Boeing 757, and Concorde). The below graph

compares the weight and volume of different heat sink materials. It was reported in 1986 that

the substitution of carbon for beryllium brakes on the C- 5B saved 400 lb per aircraft and that

they gave equal or better performance. In addition, overhaul time for the carbon brakes was

37% less than the beryllium brakes. Figure 20 shows the beryllium brake. The carbon brake

is shown in figure 21.

Characteristics of current heat sink materials are provided in table 1. As shown,

carbon has properties that make it highly desirable as a heat absorber. Its high specific heat

reduces brake weight. High thermal conductivity ensures that heat transfer, throughout the

disk stack, is more uniform and occurs at a faster rate. It is obvious, therefore, that there are

several factors other than weight to consider; in the case of beryllium, one of its problems is



the toxicity of beryllium oxide. This requires special precautions when handling the material.

In particular, the rubbing of beryllium against any other material must be avoided to prevent

formation of a toxic dust.

Another aspect in the carbon vs. beryllium comparison is their relative strengths at

high temperatures. Figure 21 compares the specific strengths of candidate brake materials as

a function of temperature, where specific strength equals ultimate tensile strength (psi)

divided by density (lb/in.3). It shows how carbon retains its strength at high temperature.

Relative to a steel heat sink, the beryllium and carbon heat sinks require a larger volume of

brake, which sometimes causes design problems.

To illustrate some of the economics, it was estimated in 1971 that on the Concorde

carbon would probably allow 3000 landings vs. 500–600 landings for steel before brake

refurbishment and would save 1200 lb weight, equivalent to 5% of the estimated transatlantic

payload.

4.2.2 AUTO-BRAKE AND BRAKE-BY-WIRE SYSTEM

Some details of a typical auto brake system were provided in this section. Automatic

brakes are applied typically by the wheel spin-up signal and the subsequent deceleration is

controlled by a pilot operated switch such as that described above. The primary objective,

when used in the landing mode, is to reduce ground run. In some cases, this reduction

amounted to 200 ft. Side benefits is increased passenger comfort due to controlled

deceleration and smooth braking, as well as reduced pilot workload. Figure 24 illustrates a

system that incorporates an auto brake.

This provides a comprehensive review of hydraulic brake systems applicable to

modern commercial and military aircraft. In addition to describing the overall systems, it

describes and diagrams the various components such as antiskid valves and auto brake

valves. are taken from that report to show systems of gradually increasing complexity.

The system described above is that used on the Boeing 757 and 767 aircraft. In

addition to providing skid control, it also includes an auto brake



4.3 DESCRIPTION OF A HYDRAULIC BRAKING SYSTEM

The system comprises a control unit, a wheel speed transducer on each of the eight

main gear wheels, two valve modules for the normal braking system, and two for the

alternate system. Each normal system valve module contains four antiskid control valves,

while each alternate system module contains two. In addition to these components provided

by Hydro-Aires, Boeing provides the auto brake control panel, auto-brake hydraulic module,

annunciates, status displays, and associated hardware.

The control unit contains four identical and interchangeable main wheel cards, in

addition to an auto brake card, BITE (built-in test equipment) card, BITE interface card;

interconnect harness, front panel display, and various switches. Braking of each wheel is

controlled by an independent skid control channel. Each card controls two channels, i.e.,

wheels 1 and 5 are controlled by a single card, wheels 2 and 6 by another, and so on. Each

card channel accepts a wheel velocity input from its associated wheel transducer. After

calculating wheel slip, the channel supplies brake pressure correction signals to its respective

skid control servo valve.

Transducers are mounted in each of the eight main wheel axles and are driven by

wheel hubcap rotation. Transducer output signals are routed through shielded wiring to the

control unit, where the wheel speed data are converted from analog to digital form. The

information is processed and analyzed so that correction signals can be produced.

These brake pressure corrections are converted back to analog form and smoothly

varying correction currents are sent from the control unit to each control valve, where brake

pressure is varied to maintain optimum braking efficiency. Figure 25 shows the emergency

hydraulic braking system. Figure 26 shows the schematic diagram of the paired wheel

hydraulic braking system. Figure 27 shows the schematic figure of individual wheel control

system.



4.4 ADVANCED BRAKE CONTROL SYSTEM (ABCS)

The Advanced Brake Control System is currently under development. It integrates the

nose gear steering, rudder, and braking controls to provide improved automatic ground

handling, particularly during high crosswinds and slippery runway operation. Configurations

have already been developed for the F-4, F-16, and F-111 aircraft.

When landing on a slippery surface under crosswind conditions, the pilot must apply

sufficient control to prevent the aircraft from sliding off the runway. The ABCS helps the

pilot by coordinating all of the systems related to directional control and by applying

corrective action far more quickly than it could have been applied manually. Tendencies to

overcorrect are also avoided.

Problems may occur at any time during the landing ground roll. For instance,

immediately after touchdown, the aircraft is at high speed and fast action is required to

correct any deviations from the desired heading. In this case, the rudder is the most effective

control. At low speed, rudder control is poor, so steering control becomes the predominant

control.

The control panel in the flight station comprises the following items: a switch to

select fully automatic (hands-off), semiautomatic, or manual control, a runway heading

indicator, and a runway friction indicator. After selecting, say, automatic control, the pilot

inputs the runway heading and the expected runway friction coefficient. A heading trim

control is also provided to make minor corrections.

4.5 PNEUMATIC BRAKING

The pneumatic braking system is an alternate system and is a way of providing

pressure to main brakes in the event of hydraulic system failure. There is no anti skid or

differential braking available from the pneumatic source. A pneumatic brake control valve

operated by a handle on the captain's instrument panel opens and modulates air bottle

pressure to a transfer tube. Pressurized hydraulic fluid from this tube is routed to a shuttle

valve on each main wheel brake. The shuttle valve moves to block the hydraulic pressure port

of the main brake line and permits fluid from this tube to apply the brakes. Pneumatic

braking is only used when hydraulic pressure is lost.



4.6 DIFFERENTIAL BRAKING

Differential braking depends on asymmetric application of the brakes on the main

gear wheels to turn the aircraft. For this, the aircraft must be equipped with separate controls

for the right and left brakes (usually on the rudder pedals). The nose or tail wheel usually is

not equipped with brakes. Differential braking requires considerable skill. In aircraft with

several methods of steering that include differential braking, differential braking may be

avoided because of the wear it puts on the braking mechanisms. Differential braking has the

advantage of being largely independent of any movement or skidding of the nose or tail

wheel.



CHAPTER 5

LUBRICANTS USED IN LANDING GEAR

The ordinary greases that have been used for years are petroleum-based. Like oil, they

thicken at cold temperatures until they freeze solid at about 25°F. In the last two decades, Shell and

Mobil have developed synthetic greases which are relatively unaffected by temperature. Today,

synthetic greases are offered by Mobil, Shell and others for use on aircraft, in two types: diester-

based and synthetic hydrocarbons.

These greases have a wide temperature range; Aero shell 7 is a diester-based grease that is

good for 85°F to 300°F. Unlike petroleum-based greases, these greases do not thicken with

temperature changes: Aero shell 7 has the same consistency at 60°F that it does at 250°F. Mobil 28,

a synthetic hydrocarbon, goes even higher. Aero shell 7 is tan in color and is to be used in the

landing gear retraction gearbox.

Aero shell 7 is used in the landing gear motor gearbox, changed at 500 hour intervals.

Aeroshell 17 on the exposed gears and screw jacks of the retraction system at 100 hour intervals, and

Mobil 28 on the landing gear grease fittings, wheel bearings, torque links, side load struts, etc. at 100

hour intervals.

The synthetic hydrocarbon Mobil 28 is used on the landing gear since it is more dirt-resistant,

and it stands solvents and detergents well-although you should relube the landing gear with grease

after washing with high-pressure spray and strong detergents or solvents. Mobil 28 is red in color.

Aeroshell 17 is Aeroshell 7 with 5% molybdenum disulfide-"moly"-for extreme pressure.

Moly is a crystalline lubricant like graphite, but moly can also be an abrasive in concentrations

above 5%. Moly works best with steel and bronze and is not normally recommended for aluminum.

It is used on extreme pressure situations because when a bearing surface is under high stationary

pressure, the grease can be squeezed out. The moly provides the first lubrication until the grease film

is restored by rotation. Moly does not make the grease any more slippery. Because of the moly,

Aeroshell 17 is black. It is very important that the synthetic hydrocarbon Mobil 28 not be mixed with

the diester-based Aeroshell 7 or 17, since the combination forms an acid.

Aviation Consumables is a specialist in aviation lubricants and is a major supplier of grease and

other lubricants to the aviation maintenance industry.



CONCLUSION

1. The study of evolution of landing gear.

2. Classification details of landing gear.

3. Internal construction of the landing gear is done.



REFERENCES

1. FAA Pilot's Handbook of Aeronautical Knowledge

2. Aircraft design: A conceptual approach – Daniel P. Raymer

3. Aviation Maintenance Administration

4. Aviation Structural Mechanics

5. Dictionary of Aeronautical Terms

6. Landing gear – Hilmerby

7. Landing gear design - Norman S. Currey



FIGURES

Fig. 1 Landing gears in the initial stages

Fig. 2 Basic types of landing gears



Fig. 3 TU-144 Main Landing Gear

Fig. 4 Track-Type gear



Fig. 5 The Italian Bonmartini track gear

Fig. 6 Three common types of landing gears



Fig. 7 Tricycle landing gear

Fig. 8 LA-4 air cushion gear

Fig. 9 Main landing gear



Fig. 10 Nose Gear Assembly



Fig. 11 Shock strut with a metering pin



Fig.12 shock strut with a metering tube



Fig. 13 Simple nose gear struts

Fig. 14 Double-acting shock absorber



Fig. 15 Operation of shock strut



Fig. 16 Oleo-pneumatic shock strut types

Fig. 17 Basic hydraulic system



Fig. 18 The hydraulic landing gear system inside the whole airplane



Fig.19. typical brake and its relationship to the landing gear

Fig. 20 Beryllium brake



Fig. 21 carbon brake

Fig. 22 Estimated number of stops vs. kinetic energy per pound.



Fig. 23 Brake materials: specific strength vs. temperature

Fig. 24Auto-brake and brake-by-wire system



Fig. 25 emergency hydraulic braking system

Fig. 26 paired wheel hydraulic braking system



Fig. 27 individual wheel hydraulic brake control system

TABULAR FORM

Property

Carbon

Beryllium

Steel

Desired

Density 0.061 0.066 0.283 High

Specific heat at 5000 F 0.310 0.560 0.130 High

Thermal conductivity

at 5000 F

100 75 24 High

Thermal expansion at

5000 F

1.500 6.400 8.400 low

Thermal shock

resistance index

141 2.700 5.500 low

Temperature limit, °F 4000 1700 2100 low

Landing Gear

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