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Landing Gear

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Study of Evolution and Details of Landing Gear

INDEX
CHAPTER 1
1.1.1 1.1.2 INTRODUCTION EVOLUTION 3 3

CHAPTER 2
2.1 DIFFERENT TYPES OF LANDING GEARS 2.1.1 TRICYCLE LANDING GEAR 2.1.2 CONVENTIONAL LANDING GEAR 2.1.3 UNCONVENTIONAL LANDING GEAR 2.2 DIFFERENCE BETWEEN MAIN AND NOSE LANDING GEAR 2.3 SHOCK STRUTS 2.3.1 TYPES OF SHOCK STRUTS 1. METERING PIN TYPE 2. METERING TUBE TYPE 3. NOSE GEAR STRUTS 4. DOUBLE-ACTING SHOCK ABSORBER 2.4 OPERATION OF SHOCK STRUTS 10 11 12 12 13 6 7 7 8 9 10

CHAPTER 3
3.1 HYDRAULIC SYSTEM FOR AIRCRAFT LANDING GEAR 3.2 LANDING GEAR EXTENSION AND RETRACTION 3.2.1 LANDING GEAR EXTENSION AND RETRACTING MECHANISMS 3.3 EMERGENCY SYSTEMS 15 16 15 15

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CHAPTER 4
4.1 BRAKING SYSTEM IN LANDING GEAR 4.2 DIFFERENT TYPES OF BRAKES AND THEIR EVOLUTION 4.2.1 CARBON AND BERYLLIUM BRAKES 4.2.2 AUTO-BRAKE AND BRAKE-BY-WIRE SYSTEM 4.3 DESCRIPTION OF A HYDRAULIC BRAKING SYSTEM 4.4 ADVANCED BRAKE CONTROL SYSTEM (ABCS) 4.5 PNEUMATIC BRAKING 4.6 DIFFERENTIAL BRAKING 18 19 20 21 21 22 18

CHAPTER 5
LUBRICANTS USED IN LANDING GEAR 23

CONCLUSION REFERENCES 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

24 25

26 26 27 27 28 28 29 29 29 30 31

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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 TABLE 1 PROPERTIES OF DIFFERENT HEAT SINK MATERIALS

32 33 33 34 35 35

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37 38 39

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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

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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 shockabsorbing 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.

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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).

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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 jetpowered aircraft have been fitted with tricycle landing gear, to avoid the blast of hot, highspeed 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

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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).

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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.

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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.

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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.

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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 straightahead 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

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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.

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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.

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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
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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

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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.

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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

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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

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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.

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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.

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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.

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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: diesterbased 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.

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CONCLUSION
1. The study of evolution of landing gear. 2. Classification details of landing gear. 3. Internal construction of the landing gear is done.

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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

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FIGURES

Fig. 1 Landing gears in the initial stages

Fig. 2 Basic types of landing gears

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Fig. 3 TU-144 Main Landing Gear

Fig. 4 Track-Type gear

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Fig. 5 The Italian Bonmartini track gear

Fig. 6 Three common types of landing gears

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Fig. 7 Tricycle landing gear

Fig. 8 LA-4 air cushion gear

Fig. 9 Main landing gear

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Fig. 10 Nose Gear Assembly

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Fig. 11 Shock strut with a metering pin

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Fig.12 shock strut with a metering tube

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Fig. 13 Simple nose gear struts

Fig. 14 Double-acting shock absorber

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Fig. 15 Operation of shock strut

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Fig. 16 Oleo-pneumatic shock strut types

Fig. 17 Basic hydraulic system

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

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

Fig. 20 Beryllium brake

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Fig. 21 carbon brake

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

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Fig. 23 Brake materials: specific strength vs. temperature

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

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Study of Evolution and Details of Landing Gear

Fig. 25 emergency hydraulic braking system

Fig. 26 paired wheel hydraulic braking system

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Study of Evolution and Details of Landing Gear

Fig. 27 individual wheel hydraulic brake control system

TABULAR FORM

Property
Density Specific heat at 5000 F Thermal conductivity at 5000 F
Thermal expansion

Carbon
0.061 0.310 100

Beryllium
0.066 0.560 75

Steel
0.283 0.130 24

Desired
High High

High

at

1.500

6.400

8.400

low

5000 F
Thermal shock resistance index Temperature limit, °F

141 4000

2.700 1700

5.500 2100

low

low

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