Aircraft Landing Gear Systems PDF

ME3531 Aircraft Systems Hydraulic and Pneumatic Power Systems Scope Landing Gear System State and describe different configuration of the aircraft landing gear. Describe the operating principle of the main and nose landing gear. Describe the functions of variou...

ME3531 Aircraft Systems Hydraulic and Pneumatic Power Systems Scope Landing Gear System State and describe different configuration of the aircraft landing gear. Describe the operating principle of the main and nose landing gear. Describe the functions of various components of the landing gear such as struts, torque links, drag links, side struts, shimmy dampers, axles, wheels and tyres. Describe the construction and operation of the shock absorbing element in the landing gear. Describe the aircraft steering. Describe the normal and emergency extension/retraction systems of the landing gear. Describe the landing gear safety devices, indication and warning systems. Page  2 Hydraulic Power Landing Gear System Main Landing Gears The main landing gear provides the main support of the aircraft by absorbing large download forces of the aircraft during ground operations such as landing and taxying. The brakes are also installed on the main wheel to allow the aircraft to slow down or stop as required. The number of landing gears, wheels and brakes depends on the initial design of the aircraft’s weight and load it carries. Nose Landing Gears The nose landing gear also provides support of the aircraft’s weight and the load it carries. It is generally being equipped with a steering mechanism so that the aircraft can be manoeuvre on the ground. Page  3 Hydraulic Power Landing Gear System Types of Landing Gear Arrangement Tail or conventional Tandem configuration Tricycle configuration configuration Page  4 Hydraulic Power Landing Gear System Tail wheel or conventional configuration This configuration is used on older aircraft for landing on rough field operations. Tricycle configuration Tricycle-type is used over conventional type landing gear aircraft with the following benefits: 1. Allows more forceful application of the brakes without nosing over when braking, which enables higher landing speeds. 2. Provides better visibility from the flight deck, especially during landing and ground manoeuvring. 3. Prevents ground-looping of the aircraft. Since the aircraft center of gravity is forward of the main gear, forces acting on the center of gravity tend to keep the aircraft moving forward rather than looping, such as with a tail wheel- type landing gear. Page  5 Hydraulic Power Landing Gear System Sub-components of the Main and Nose Landing Gear Aircraft Wheels Aircraft wheels are an important component of a landing gear system. With tires mounted upon them, they support the entire weight of the aircraft during taxi, takeoff, and landing. The typical aircraft wheel is lightweight, strong, and made from aluminum alloy. Aircraft Tyres Its function includes supporting the aircraft’s weight, absorbing shock from landing and taxying, providing gripping contact with the runway surface and discharging static electricity. Page  6 Hydraulic Power Landing Gear System Sub-components of the Main and Nose Landing Gear Trunnion It is a part of the landing gear assembly attached to the airframe which is supported at its ends by bearing assemblies and allow the gear to pivot during retraction and extension. Sturt The strut is the vertical member of the landing gear assembly. Page  7 Hydraulic Power Landing Gear System Sub-components of the Main and Nose Landing Gear Drag Link or Drag Strut To provide support to shock strut and stabilises the shock strut longitudinally. Side Strut or side brace link To stabilise the landing gear laterally. Overcenter link To prevent the link from pivoting at the joint except when the gear is retracted, thus preventing collapse of the gearduring ground operation. It locks the main gear only in the down position. It is sometimes called a “Downlock”. The link is hydraulically retracted to allow gear retraction. Page  8 Hydraulic Power Landing Gear System Sub-components of the Main and Nose Landing Gear Lock Mechanism A “downlock” locks the landing gear in the down position and the main landing gear is held in the UP position by the uplock mechanism. Axles The axles is where the main wheels are supported and installed on it. Page  9 Hydraulic Power Landing Gear System Shock Absorbing and Non-Shock Absorbing Landing Gear To support the aircraft for taxi, the forces of impact on an aircraft during landing must absorbed by the gears. This impact is absorbed in two ways: 1) The shock energy is altered and transferred throughout the airframe at a different rate and time than the single strong pulse ofimpact. 2) The shock is absorbed by converting the energy into heat energy. Non-Shock Absorbing Landing Gear Many aircraft use flexible spring steel, aluminium, or composite struts that receive the impact of landing and return it to the airframe to dissipate at a rate that is not harmful. The gear flexes initially and forces are transferred as it returns to its originalposition. Page  10 Hydraulic Power Landing Gear System Shock Absorbing A typical pneumatic/hydraulic shock strut uses nitrogen gas combined with hydraulic fluid to absorb and dissipate shock loads. A shock strut is constructed of two telescoping cylinders or tubes that are closed on the external ends. The upper cylinder is fixed to the aircraft and does not move. The lower cylinder is called the piston and is free to slide in and out of the upper cylinder. Two chambers are formed. The lower chamber is always filled with hydraulic fluid and the upper chamber is filled with nitrogen. An orifice located between the two cylinders provides a passage for the fluid from the bottom chamber to Main Landing Gear Nose Landing Gear enter the top cylinder chamber when the strut is Shock Strut Shock Strut compressed. Page  11 Hydraulic Power Landing Gear System Shock Strut Operation The compression stroke of the shock strut begins as the aircraft wheels touch the ground. As the center of mass of the aircraft moves downward, the strut compresses, and the lower cylinder or piston is forced upward into the upper cylinder. The metering pin is therefore moved up through the orifice. This cause the volume of the gas to decrease hence increasing the pressure with the volume of the hydraulic fluid remains the same. The initial shock of landing is cushioned by the hydraulic fluid being forced through the metered opening. As the pressure and temperature in the cylinder increases, vertical speed of the aircraft decreases. The pressure in the cylinder will increase until it is sufficient to stop vertical motion of aircraft. At this point, the energy in the gas pressure is sufficient to recoil the aircraft upwards. During recoil (when the strut begins to extend), Shock strut extends until gas pressure is just enough to support the weight of aircraft. The compressed air then acts a shock absorber during the time that the aircraft is taxying. Page  12 Hydraulic Power Landing Gear System Fixed and Retractable landing gear Aircraft fitted with fixed landing gear will have the gears expose to the airflow when flying. As the aircraft fly faster, the drag also increases. The introduction of mechanisms to retract and stow the landing gear reduce drag however this mechanism adds Fixed Landing Gears weight. The aircraft are generally fitted with retractable gears with the added weight is considered a small sacrifice as compared to the drag as aircraft are flying faster. Retractable Landing Gears Page  13 Hydraulic Power Landing Gear System Retraction and Extension of Main Landing Gear The main landing gear can be extended or retracted via the landing gear handle located in the flight deck. This handle is connected mechanically connected to the selector valve and the aircrew and ground crew can set the handle to “UP”, “OFF” (neutral) or “DOWN” position. If the landing gear is set in the “UP”, an internal circuit in the selector valve supplies hydraulic pressure from the hydraulic system to the following: Unlocking and opening the wheel well doors via the unlatch and door actuator. Unlocking the landing gears via downlock actuator Retracting the landing gears via retract actuator Closing the wheel well doors via the door and unlatch actuators The landing gears is kept in the “UP” position by Page  15 a up-lock mechanism. Hydraulic Power Landing Gear System Retraction and Extension of Main Landing Gear If the landing gear is set in the “OFF”, all components of the landing gear operation system on the “UP” as well as “DOWN” side are connected to the return line of hydraulic system hence the landing gears is kept in the “UP” position by a up-lock mechanism. If the landing gear handle is put in the “DOWN” position, pressure from the hydraulic system is released via the internal circuit in the selector valve. This pressure is used for: Unlocking and opening the wheel well doors Unlocking the up-lock Extension the landing gear Closing of the wheel well doors Page  15 Hydraulic Power Landing Gear System Retraction and Extension of Main Landing Gear The main landing gear of an aircraft with a force analysis that shows the maximum force exerted by the retract actuator is 53,000N and the actuator stroke is 700mm. Assume the following: The gear must be fully retracted 10 seconds There are two main landing gear and a nose landing gear and the nose landing gear is identical to the main landing gear. Maximum system pressure is 207 bar The cap end diameter is 500mm and piston rod diameter is 300mm Calculate the power in KW to drive the pump, assuming the overall efficiency of the pump is 85%. Page  16 Hydraulic Power Landing Gear System Retraction and Extension of Main Landing Gear Volume require to retract 3 actuators 𝜋 𝜋 = [3 x( 4 x ( 0.5)2 - 4 x ( 0.3)2 ) x 0.7] = 0.263 𝑚3 = 263 litres Flow rate to retract the 3 actuators 0.263 =( ) = 0.0263𝑚3 /s 10 Power required 𝑃𝑄 5 = = 0.0263X0.85 207 X 10 𝜂 = 640KW Page  17 Hydraulic Power Landing Gear System Downlock Mechanism It prevents undesired retraction of landing gear, when it is in the “DOWN” position. “overcenter links”, which are between the strut and the side brace, ensure that the side brace cannot pivot when it is in the “overcenter” position. The overcenter link will remain “overcenter” position by the spring force of “bungee springs” During ground maintenance, the overcenter mechanism is locked by landing gear lock pins for safety reasons. If the landing gear is retracted, the downlock actuator pulls the overcenter links from the “overcenter” position and side brace can pivot when the landing gear is pulled up by the retract cylinder. Page  18 Hydraulic Power Landing Gear System Uplock Mechanism The uplock mechanism consists of a hook in which the landing gear is secured in the retracted position. When the landing gear is unlocked, it extends due toits mass and reaches the “down and locked” position with the help of the bungee springs. The hydraulic fluid which flows away from the retraction actuator slows this process down slightly to reduce the “down” shock Page  19  Hydraulic Power Landing Gear System Nose Landing Gear Steering System During aircraft taxying, the aircraft can be steered or turn using nose wheel steering system and/or differential braking. The control of the steering is from the flight deck through the use of a small wheel, tiller, or joystick typically mounted on the left side wall. Page  20 Hydraulic Power Landing Gear System Nose Landing Gear Alignment Most shock struts are equipped with torque links or torque arms. One end of the links is attached to the fixed upper cylinder. The other end is attached to the lower cylinder (piston), so it cannot rotate. This keeps the wheels aligned. Nose gear shock struts are provided with a locating cam assembly to keep the gear aligned. A cam protrusion is attached to the lower cylinder, and a mating lower cam recess is attached to the upper cylinder. These cams line up the wheel and axle assembly in the straight-ahead position when the shock strut is fully extended. This allows the nose wheel to enter the wheel well when the nose gear is retracted and prevents structural damage to the aircraft. It also aligns the wheels with the longitudinal axis of the aircraft prior to landing when the strut is fully extended. Many nose gear shock struts also have attachments for the installation of an external shimmy damper. Page  21  Hydraulic Power Landing Gear System Nose Landing Gear Steering System The pressure from the aircraft hydraulic system is directed through the open safety shutoff valve into a line leading to the metering valve. The metering valve then routes the pressurized fluid out of port A, through the right turn alternating line, and into steering cylinder A. This is a one-port cylinder and pressure forces the piston to begin extension. Since the rod of this piston connects to the nose steering spindle on the nose gear shock strut which pivots at point X, the extension of the piston turns the steering spindle gradually toward the right. As the nose wheel turns, fluid is forced out of steering cylinder B through the left turn alternating line and into port B of the metering valve. The metering valve directs this return fluid into a compensator that routes the fluid into the aircraft hydraulic system return manifold. Page  22 Hydraulic Power Landing Gear System Nose Landing Gear Shimmy Dampers The torque links attached from the stationary upper cylinder of a nose wheel strut to the bottom moveable cylinder or piston of the strut are not sufficient to prevent most nose gear from the tendency to oscillate rapidly, or shimmy, at certain speeds. This vibration must be controlled through the use of a shimmy damper. A shimmy damper controls nose wheel shimmy through hydraulic damping. A piston-type shimmy damper’s case is attached firmly to the upper shock strut cylinder. The shaft is attached to the lower shock strut cylinder and to a piston inside the shimmy damper. As the lower strut cylinder tries to shimmy, hydraulic fluid is forced through a bleed hole in the piston. The restricted flow through the bleed hole dampens the oscillation. Page  23 Hydraulic Power Landing Gear System Emergency Extension Systems The emergency extension system lowers the landing gear if the main power system fails. Some aircraft have an emergency release handle in the flight deck that is connected through a mechanical linkage to the gear uplocks. When the handle is operated, it releases the uplocks and allows the gear to free-fall to the extended position under the force created by gravity acting upon the gear. Other aircraft use a non-mechanical back-up, such as pneumatic power, to unlatch the gear. Page  24 Hydraulic Power Landing Gear System Safety Switch (Landing Gear Safety Device) A landing gear squat switch, or safety switch, is found on most aircraft. This is a switch positioned to open and close depending on the extension or compression of the main landing gearstrut. The squat switch is wired into any number of system operating circuits. One circuit is to prevents the gear from being retracted while the aircraft is on the ground. At takeoff, the landing gear strut extends. The safety switch closes and allows current to flow in the safety circuit. The solenoid energizes and retracts the lock-pin from the selector handle. This permits the gear to be raised. Page  25 Hydraulic Power Landing Gear System Ground Locks (Landing Gear Safety Device) Most aircraft is equipped with additional safety devices to prevent collapse of the gear when aircraft is on the ground. A ground lock can be as simple as a pin placed into the pre-drilled holes of gear components that keep the gear from collapsing. All ground locks should have red streamers attached to them, so they are visible and removed before flight. Ground locks are typically carried in the aircraft and put into place by the flight crew during the post landing walk-around. Page  26 Hydraulic Power Landing Gear System Gear Indicator (Landing Gear Safety Device) The gear indicator usually consists of micro switches or proximity switches on the up-lock and down-locks connected to landing gear position indicator on the instrumental panel. The indicators generally consists of green light for each gear to indicate that they are down and locked position. The red light for each gear indicates that they are in transit and no light indicates when all gears are up and locked. Page  27 Hydraulic Power Landing Gear System Warning Horn (Landing Gear Safety Device) The pilot will hear a warning horn when the landing gear is not down and locked and this is to ensure that the aircraft does not land with a retractedgear. Therefore when aircraft is performing a landing approach, the horn sounds and red light will illuminates if the landing gear is on any position other than down andlocked. Page  28