This invention relates to a hydraulic system for raising and lowering an aircraft landing gear, and more particularly but not exclusively useful for a kind of aircraft landing gear known as a side brace, where the landing gear is movable between a generally vertical condition for landing, and a generally horizontal condition for stowage e.
A particular feature of such side brace landing gears is that landing loads act through an actuator which is provided to raise and lower the landing gear. Accordingly such actuators have to be more substantial than would be required simply for raising and lowering the landing gear.
Typically such actuators are piston and cylinder arrangements, and the piston diameter is made larger than is necessary just for raising and lowering the landing gear. As a result, th actuator demands a large volume of hydraulic fluid for operation. Particularly during landing, when it is desired to lower the landing gear, other aircraft services will be demanding hydraulic fluid, for example flap lowering actuators may require fluid, which can place high demands on the hydraulic pump s supplying pressurised hydraulic fluid to the various services.
By virtue of the system of the present invention, the demand on the hydraulic pump s is thus reduced during landing gear operation as exhausted fluid is recirculated to augment the fluid supply, thus reducing the volume of fluid required from the hydraulic pump s to operate the landing gear. In one arrangement the means which permit exhausted fluid to augment the supplied fluid includes a check valve which is opened as tile movable member of the actuator moves relatively in the casing in the first direction to extend the actuator and lower the landing gear.
The check valve mast be adapted to open to permit exhausted fluid from the second side of the movable member to augment the supplied fluid in response to the pressure of the fluid supplied to the first side of the member or alternatively in response to a pressure build up in a line carrying exhausted fluid from the second side of the movable member.
In each case preferably closure means are provided positively to close the check valve when pressurised fluid is supplied by the selector salve to the second side of the movable member. Preferably means are provided to relieve exhausted fluid which is not recirculated from the at least one of the first and second sides of the movable member as the movable member reaches tile end of travel in the casing.
Thus there is no risk of trapped fluid interfering Faith the propel operation of the landing gear. In one arrangement the hydraulic system includes a first fluid supply line to the first side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a first position, and a second supply line to the second side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a second position, and the means which 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 means and thus be directed with the supplied fluid, to the second or first side respectively of the movable member, permitting the exhausted fluid to flow from the second supply line to the first supply line.
To ensure that exhausted fluid is available to augment the supplied fluid to extend the actuator, the second supply line may include non-return means at least to restrict the flow of exhausted fluid from the hydraulic system. However desirably a restrictor means is provided to enable a restricted flow of exhausted fluid to by-pass the non-return means so that fluid which is not recirculated, is not trapped in the second supply line which could interfere with the proper operation of the landing gear.
The selector valve means may be movable to a first position to permit the flow of fluid therethrough from a source of pressurized fluid to the first side of the movable member, and to a second position to permit the flow of fluid therethrough from the source to the second side of the movable member, and to a rest position in which the source is isolated and fluid may pass from the system tank. According to a second aspect of the invention we provide an aircraft having landing gear which is raised and lowered by a hydraulic system according to the first aspect of the invention.
According to a third aspect of the invention we provide a valve including, a valve member and a piston each received in a passage in a valve body, the valve member and piston being biased apart by resilient means such that the valve member is urged towards a valve seat towards one end of the passage, and the piston is urged towards a stop towards an opposite end of the passage, a fluid inlet and a fluid outlet, the pressure of fluid at the inlet when sufficient, acting to move tile valve member against the force of the biasing means off the valve seat to permit fluid flow from the inlet, past the valve seat, to the outlet, and the piston being movable in the passage away from the stop in response to a pilot pressure delivered to a pilot pressure port of the body against the force of the biasing means to a position in die passage in which the piston engages the valve member and restrains the valve member against movement off the valve seat in response to the inlet pressure.
Means may be provided to permit fluid pressure at the outlet to be communicated to an intermediate region of the passage between the valve member and the piston at least when the piston is engaged with the stop. Such a valve in accordance with the third aspect of the invention may be a check valve to permit the flow of exhausted fluid from at least one of the first and second sides of the movable member of the actuator of the hydraulic system according to the first aspect of the invention to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable members.
Referring first to FIG.
The landing gears 1112 are operable by means actuators 18 which are extendible and retractable by means of pressurized hydraulic fluid. Referring now to FIG. It can be seen that in this example each actuator 18 has a movable member or piston 19 in this example, which moves inside a casing or cylinder 20as is well known in the art of hydraulic systems.PA44 180HP HYDRAULIC SYSTEM (LANDING GEAR )
When pressurized hydraulic fluid is supplied to a head end 11 of the actuator 18 at a first side of the piston 19the piston 19 moves in the cylinder 20 so as to extend the actuator 18at the same time forcing fluid at a second opposite side of the piston 19to be exhausted from an actuator rod end 23 of the actuator Conversely, when pressurized fluid is supplied to the actuator rod end 23 of the actuator 18the piston 19 moves in the cylinder 20 so as to retract the actuator 18at the same time forcing fluid to be exhausted from the head end Hydraulic fluid is fed to the actuator 18 via a selector valve means 25 to which a source of fluid under pressure i.
The pump 26 draws hydraulic fluid for pumping, from a tank The selector valve means 25 is movable between three positions in this example. When in a first raised position, i. In the first supply line 30 there is a flow regulating means 31 which controls the pressure of fluid which is supplied to the head end 22 of the actuator Also there is a by-pass one way valve 32 which enables fluid from the first supply line 30 to flow freely to tank 27 as hereinafter described.
When the spool 29 of the selector valve means 25 is in an intermediate or rest position as shown and indicated at R, the pressurized fluid source i.
When the spool 29 of the selector valve means 25 is in a second lowered position as indicated at II in FIG. The second supply line 34 includes a one way valve 35 through which pressurized fluid may freely flow to the rod end 23 of the actuator 18and a by-pass restrictor 36 which allows fluid to by-pass the one way vale 35 as hereinafter described. Between the first 30 aid sec id 34 supply lines, there is a check valve In FIG. Starting with the actuator 18 in a retracted position in which the landing gear 11 or 12 is stowed, when it is desired to lower the landing gear for landing, the selector valve means 25 is moved to the first position 1.
Fluid thus flows along the first supply line 30through the flow regulating means 31 to the head end 22 of the actuator 18 and the piston 19 is caused to commence movement to extend the actuator 18 to lower the landing gear 11 or Fluid flow from the rod end 23 of the actuator 18 is however restricted to a small flow through the by-pass restrictor 36 from where the exhausted fluid passes to tang 27 via the selector valve means How are you?
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The doors which are fitted to the landing gear struts are operated mechanically by the gear. The nose gear main doors operate hydraulically. The fixed doors aft doors and leg doors are operated by the gear. Normal braking is powered by the green hydraulic system. Alternate braking, which includes the parking brake, is powered by the yellow hydraulic system. It is hydraulically operated by the green hydraulic system and electrically controlled by the BSCU.
On the A, the nose landing gear is hydraulically operated by the yellow hydraulic system. A downlock safety pin must be installed before maintenance operations. On the A, as the alternate braking becomes electrically controlled and thanks to the new EIS, more information is displayed on the nose wheel steering and braking systems. The anti-skid and nose wheel steering switch controls both functions.Copy embed code:. Automatically changes to Flash or non-Flash embed.
WordPress Embed Customize Embed. URL: Copy. Presentation Description No description available. They are attached to primary structural members of the aircraft. Functions Of Landing Gear: Functions Of Landing Gear Support static load of an aircraft on ground Absorb the landing shocks Dampen the Vibrations Facilitate aircraft for take-off and landing Providing the aircraft the braking and steering function Arrangement Of Landing Gear: Arrangement Of Landing Gear Tail wheel type Conventional Tandem loading gear longitudinally Aligned Tricycle type landing gear Alignment : Alignment This is set by the manufacturer and only requires occasional attention such as after a hard landing.
Tow-in and tow-out refer to the path a main wheel would take in relation to the airframe longitudinal axis or centreline if the wheel was free to roll forward. Three possibilities exist. The landing gear handle actuates a switch that turns on the hydraulic pump motor in the power pack so that it turns in the direction shown by the arrows in figure A.
Fluid flow through the passage and check valve on the right side of the pump and around the outside of the gears. The output from the pump moves the gear-up check valve piston to the right an unseat the gear-up check valve. Fluid flow into the down side of the three actuating cylinder and forces the pistons out. The nose gear is much easier to move than the main gears, so the fluid flows into and out of the nose gear actuating cylinder through restrictor.
Advantages and Disadvantages: Advantages and Disadvantages Advantages It is lighter in weight than alternate existing systems. It is reliable; either it works or doesn't. It can be easily maintained. It is not a shock hazard; it is not much of a fire hazard. It can develop practically unlimited force or torque. Slide Disadvantages The possibility of leakage, both internal and external, may cause the complete system to become inoperative. Contamination by foreign matter in the system can cause malfunction of any unit.
Cleanliness in hydraulics cannot be overemphasized. Html H. Follow us on:.The Mini-IMP features retractable landing gear to improve performance. The nosewheel gear of the Mini-IMP is constructed of cut and drilled aluminum extrusion and tube. Aluminum plate is cut on a metal cutting bandsaw to provide bearing mounts and nylon flanged bushings are used for mount bearings.
The nosewheel is equipped with a shimmy damper, which is easily constructed as part of the nosewheel fork. The nosewheel leg is a heat treated, and bent spring steel rod which bolts in to an aluminum mount block.
An assist spring, not only helps retract the nosewheel, but also provides over center locking of the gear in the DOWN position. A nosewheel UP lock is provided to assure that the gear will not be displaced during aerobatic maneuvering. The nosewheel retracts forward into the fiberglass cone ahead of the nosewheel bulkhead. Retraction and extension on the prototype is mechanical through a single lever on the left side of the cockpit. The main gear is retracted mechanically through cables, which run to suitable chain sprockets, which rotate the main gear outward and up into wheel wells on the bottom of the wing outboard of the fuel tank.
The main gear legs lay in recesses on the side of the fuselage and pass in front of the air ducts to the engine in such a way as to provide straightening vanes for the cooling air. The gear system is also equipped with both mechanical and electrical gear position indicators so that the pilot can be assured that the gear is not only where he wants it, but is also locked there.
The prototype was originally fitted with a fully manual, mechanical retraction system. However, the aircraft can be fitted with a pneumatic or hydraulic retraction system to power the main gear. Either gear retraction system can be fitted and it is entirely a matter of personal preference as to which system is the more desirable.
The nosegear can also be fitted with powered retraction if desired and several builders have done this.
The main wheels are fitted with extremely effective disc brakes, which are individually controlled by toe pedals. Ground steering is easily accomplished through use of full castering nosewheel and differential brake steering.
The full swivel nosewheel permits pushing the aircraft backward with out nosewheel impairment to motion to the rear as is common with most nosewheels. The nose gear is tied-in to these systems so all three wheels are easily actuated.
The nosegear is locked down due to an over-center positioning of the actuator. With the pneumatic or hydraulic systems, the nosegear is operated exactly like to manual system but the main gears are moved with the power cylinder and are BOTH locked in the UP and DOWN positions mechanically with the power system only doing the actual gear movement. The electric pumps only operate while the main gear is in motion. Flight evaluations have shown that nosewheel doors are extremely desirable and the drawings show a suitable nosewheel door installation.
The nosewheel itself has been changed in the prototype from the original 5 inch wheel the same as the main wheels to a 4 inch wheel to further lighten the nosewheel installation and facilitate retraction and nosewheel door operation. The special geometry of the Mini-IMP landing gear is such that, despite the use of rather small wheels and tires, the gear moves slightly AFT under loading.
The extreme smoothness of the prototype landing gear which has been obtained without resort to complicated oleos, dampers, etc. The nose wheel is equipped with an adjustable shimmy damper, which can be built in a few minutes. Extensive testing has shown that the prototype exhibits absolutely no tendency for nosewheel shimmy.There are multiple applications for hydraulic use in aircraft, depending on the complexity of the aircraft.
For example, a hydraulic system is often used on small airplanes to operate wheel brakes, retractable landing gear, and some constant-speed propellers. On large airplanes, a hydraulic system 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-drivena filter to keep the fluid clean, a selector valve to control the direction of flow, a relief valve to relieve excess pressure, and an actuator.
The hydraulic fluid is pumped through the system to an actuator or 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. Servos can be either single-acting or double-acting, 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. A single-acting servo provides power in one direction. The selector valve allows the fluid direction to be controlled. This is necessary for operations such as the extension and retraction of landing gear during which the fluid must work in two different directions. Each system incorporates different components to meet the individual needs of different aircraft.
A mineral-based hydraulic fluid is the most widely used type for small aircraft. This type of hydraulic fluid, a kerosene-like petroleum product, has good lubricating properties, as well as additives to inhibit foaming and prevent the formation of corrosion. It is chemically stable, has very little viscosity change with temperature, and is dyed for identification. Since several types of hydraulic fluids are commonly used, an aircraft must be serviced with the type specified by the manufacturer.
The landing gear forms the principal support of an aircraft on the surface. The most common type of landing gear consists of wheels, but aircraft can also be equipped with floats for water operations or skis for landing on snow.
Landing gear employing a rear-mounted wheel is called conventional landing gear. Airplanes with conventional landing gear are often 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.
Figure The landing gear supports the airplane during the takeoff run, landing, taxiing, and when parked. Nosewheels are either steerable or castering. Steerable nosewheels are linked to the rudders by cables or rods, while castering nosewheels are free to swivel. In both cases, the aircraft is steered using the rudder pedals. Airplanes with a castering nosewheel may require the pilot to combine the use of the rudder pedals with independent use of the brakes.
Tailwheel landing gear airplanes have two main wheels attached to the airframe ahead of its CG that support most of the weight of the structure. A tailwheel at the very back of the fuselage provides a third point of support. With the CG located behind the main landing gear, directional control using this type of landing gear is more difficult while on the ground. This is the main disadvantage of the tailwheel landing gear.
For example, if the pilot allows the aircraft to swerve while rolling on the ground at a low speed, he or she may not have sufficient rudder control and the CG will attempt to get ahead of the main gear, which may cause the airplane to ground loop.
Diminished forward visibility when the tailwheel is on or near the ground is a second disadvantage of tailwheel landing gear airplanes. Because of these disadvantages, specific training is required to operate tailwheel airplanes. Landing gear can also be classified as either fixed or retractable.
Fixed landing gear always remains extended and has the advantage of simplicity combined with low maintenance. Retractable landing gear is designed to streamline the airplane by allowing the landing gear to be stowed inside the structure during cruising flight. Fixed left and retractable right gear airplanes.
Airplane brakes are located on the main wheels and are applied by either a hand control or by foot pedals toe or heel.Aircraft Hydraulic System. Today, the meaning has been expanded to include the physical behavior of all liquids, including hydraulic fluid. Hydraulic systems are not new to aviation. Early aircraft had hydraulic brake systems. As aircraft became more sophisticated, newer systems with hydraulic power were developed.
Hydraulic systems in aircraft provide a means for the operation of aircraft components.
Small and Large Aircraft Landing Gear Retraction
The operation of landing gear, flaps, flight control surfaces, and brakes is largely accomplished with hydraulic power systems.
Hydraulic system complexity varies from small aircraft that require fluid only for manual operation of the wheel brakes to large transport aircraft where the systems are large and complex. To achieve the necessary redundancy and reliability, the system may consist of several subsystems. Each subsystem has a power generating device pump reservoir, accumulator, heat exchanger, filtering system, etc.
System operating pressure may vary from a couple hundred pounds per square inch psi in small aircraft and rotorcraft to 5, psi in large transports. Hydraulic systems have many advantages as power sources for operating various aircraft units; they combine the advantages of light weight, ease of installation, simplification of inspection, and minimum maintenance requirements.
Hydraulic operations are also almost percent efficient, with only negligible loss due to fluid friction. Email This BlogThis!
Newer Post Older Post.An arresting gearor arrestor gearis a mechanical system used to rapidly decelerate an aircraft as it lands. Similar systems are also found at land-based airfields for expeditionary or emergency use. Typical systems consist of several steel wire ropes laid across the aircraft landing area, designed to be caught by an aircraft's tailhook. During a normal arrestment, the tailhook engages the wire and the aircraft's kinetic energy is transferred to hydraulic damping systems attached below the carrier deck.
There are other related systems which use nets to catch aircraft wings or landing gear. These barricade and barrier systems are only used for emergency arrestments for aircraft without operable tailhooks.
Arresting cable systems were invented by Hugh Robinson [ when? These early systems had cables run through pulleys and attached to dead weights, such as sandbags. Modern U. Prior to the introduction of the angled flight decktwo systems were used in addition to deck cables to keep landing aircraft from running into parked aircraft further forward on the flight deck: the barrier and the barricade.
Landing Gear System
If the aircraft tailhook failed to catch a wire, its landing gear would be caught by a 3—4 foot high net known as the barrier. If the aircraft caught a wire upon touchdown, the barrier could be quickly lowered to allow aircraft to taxi over it. The final safety net was the barricadea large, 15 foot 4. Barriers are no longer in use, although ground-based arresting gear are sometimes called "barriers". Barricades are still in use aboard carriers, but they are only rigged and used in emergencies.
A normal arrestment is accomplished when the arresting hook of an incoming aircraft engages one of the deck pendants. As the deck pendant and the purchase cable are pulled out by the aircraft being arrested, the kinetic energy of the aircraft is transferred to mechanical energy of the cables, and the arresting engine transfers the mechanical energy of the cables to hydraulic energy.
Landing Gear Alignment, Support, and Retraction (Part Two)
This classic system of hydraulic arrest is now being supplanted by one using electromagnetics where the energy absorption is controlled by a turbo-electric engine. The arresting engine brings about a smooth, controlled stop of the landing aircraft. At the completion of the arrestment, the aircraft arresting hook is disengaged from the deck pendant, which is then retracted to its normal position.
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