The extension stroke occurs at the end of the compression stroke as the energy stored in the compressed air causes the aircraft to start moving upward in relation to the ground and wheels. At this instant, the compressed air acts as a spring to return the strut to normal. It is at this point that a snubbing or damping effect is produced by forcing the fluid to return through the restrictions of the snubbing device. If this extension were not snubbed, the aircraft would rebound rapidly and tend to oscillate up and down, due to the action of the compressed air. A sleeve, spacer, or bumper ring incorporated in the strut limits the extension stroke.
For efficient operation of shock struts, the proper fuid level and air pressure must be maintained. To check the fluid level, the shock strut must be deflated and in the fully compressed position. Defating a shock strut can be a dangerous operation unless servicing personnel are thoroughly familiar with high-pressure air valves. Observe all the neces.
sary safety precautions. Refer to manufacturer's instructions for proper deflating technique.
Two of the various types of high-pressure air valves currently in use on shock struts are illustrated in figure 9-7. Although the two air valves 5/8-inch hex nut Care 3/4-inch hex body - Valve core housing - - Yellow Compression stroke - Packing -O-ring
• Air orifice Valve assembly (A) Yellow valve cap - 3/4-inch hex nut - Air 3/4 inch hex body - Stem Extension stroke Hydraulic fluid Air orifice FICURE 9-6. Shock strut operation.
• Back-up-ring O-ring If there was an insufficient amount of fluid and/or air in the strut, the compression stroke would not be limited and the strut would "bottom." FIGURE 9-7. High-pressure air valves.
are interchangeable, there are some important dif- jacks to service the shock struts.)
ferences in their construction. One valve (figure 9-7A) contains a valve core and has a 5/8-in. 9-8A).
swivel hex nut. The other air valve (figure 9-7B) has no valve core and has a 3/4-in. swivel hex nut.
Servicing Shock Struts ose hyruts osiorkthe on (F) FICURE 9-8. Steps in servicing shock struts.
(6) Release the air pressure in the strut by slowly turning the swivel nut counterclockwise (figure 9-8E).
(7) Ensure that the shock strut compresses as the air pressure is released. In some cases, it may be necessary to rock the aircraft after deflating to ensure compression of the strut.
(8) When the strut is fully compressed, the air valve assembly may be removed (figure 9-8F).
(9) Fill the strut to the level of the air valve opening with an approved type of hydraulic fluid.
(10) Re-install the air valve assembly, using a new O-ring packing. Torque the air valve assembly to the values recommended in the applicable manufacturer's instructions.
(11) Install the sir valve core.
(12) Inflate the strut, using a high-pressure source of dry air or nitrogen. Bottled gas should not be used to inflate shock struts.
On some shock struts the correct amount of inflation is determined by using a high-pressure air gage. On others it is determined by measuring the amount of extension (in inches) between two given points on the strut. The proper procedure can usually be found on the instruction plate attached to the shock strut, Shock struts should always be inflated slowly to avoid excessive heating and over inflation.
(13) Tighten the swivel hex nut, using the torque values specified in the applicable manufacturer's instructions.
(14) Remove the high-pressure air line chuck and install the valve cap. Tighten the valve cap fingertight.
Bleeding Shock Struts If the fluid level of a shock strut has become extremely low, or if for any other reason air is trapped in the strut cylinder, it may be necessary to bleed the strut during the servicing operation.
Bleeding is usually performed with the aircraft placed on jacks. In this position the shock struts can be extended and compressed during the filling operation, thus expelling all the entrapped air. The following is a typical bleeding procedure:
(1) Construct a bleed hose containing a fitting suitable for making an airtight connection to the shock strut filler opening. The base should be long enough to reach from the shock strut filler opening to the ground when the aircraft is on jacks.
(2) Jack the aircraft until all shock struts are fully extended.
(3) Release the air pressure in the strut to be bled.
(4) Remove the air valve assembly.
(5) Fill the strut to the level of the filler port with an approved type hydraulic fluid.
(6) Attach the bleed hose to the filler port and insert the free end of the hose into a container of clean hydraulic fluid, making sure that this end of the hose is below the surface of the hydraulic fluid.
(7) Place an exerciser jack (figure 9-9) or other suitable single-base jack under the shock strut jacking point. Compress and extend the strut fully by raising and lowering the jack until the flow of air bubbles from the strut has completely stopped.
Compress the strut slowly and allow it to extend by its own weight.
(8) Remove the exerciser jack, and then lower and remove all other jacks.
(9) Remove the bleed hose from the shock strut.
(10) Install the air valve and inflate the strut.
-Hvdraulic fluid bleed hose Exerciser jack FIGURE, 9-9. Bleeding a shock strut using an exerciser jack.
Shock struts should be inspected regularly for Aircraft structural leakage of fluid and for proper extension. Exposed member portions of the strut pistons should be wiped clean daily and inspected closely for scoring or corrosion.
MAIN LANDING GEAR ALIGNMENT, SUPPORT, RETRACTION Trunnion The main landing gear consists of several compo- support nents that enable it to function. Typical of these are the torque links, trunnion and bracket arrange- brackets ments, drag strut linkages, electrical and hydraulic gear-retraction devices, and gear indicators.
Alignment Torque links (figure 9-10) keep the landing gear pointed in a straight-ahead direction; one torque link connects to the shock strut cylinder, while the Shock LTrunnion other connects to the piston. The links are hinged at strut the center so that the piston can move up or down in the strut. cylinder FICURE 9-11. Trunnion and bracket arrangement.
The upper end of the drag strut (figure 9-12) connects to the aircraft structure, while the lower end connects to the shock strut. The drag strut is hinged so that the landing gear can be retracted.
Trunnion bearing surface Drag strut FICURE 9-10. Torque links.
Support To anchor the main gear to the aircraft structure, a trunnion and bracket arrangement (figure 9-11) is usually employed. This arrangement is constructed to enable the strut to pivot or swing forward or backward as necessary when the aircraft is being steered or the gear is being retracted. To restrain this action during ground movement of the Retracting mechanism aircraft, various types of linkages are used, one being the drag strut. FIcURE 9-12. Drag strut linkage.
Manual control torque tube Landing gear motor Manual control gearbox • Drag strut Gearbox Trunnion support Retracting mechanism Shock strut
• Drag strut Universal joints FIGURE 9-13. Electrical retraction system.
Electrical Landing Gear Retraction System An electrical landing gear retraction system, such as that shown in figure 9-13, has the following features:
(1) A motor for converting electrical energy into rotary motion.
(2) A gear reduction system for decreasing the speed and increasing the force of rotation.
(3) Other gears for changing rotary motion (at a reduced speed) into push-pull move-
(4) Linkage for connecting the push-pull movement to the landing gear shock Basically, the system is an electrically driven jack for raising or lowering the gear. When a switch in the cockpit is moved to the "up" position, the electric motor operates. Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages. Thus, the gear retracts and locks. If the switch is moved to the "down" position, the motor reverses and the gear moves down and locks. The sequence of operation of doors and gears is similar to that of a hydraulically operated landing gear system.
Hydraulic Landing Gear Retraction Systems Devices used in a typical hydraulically operated landing gear retraction system include actuating cylinders, selector valves, uplocks, downlocks, sequence valves, tubing, and other conventional hydraulic components. These units are so interconnected that they permit properly sequenced retraction and extension of the landing gear and the landing gear doors.
The operation of a hydraulic landing gear retraction system is of such importance that is must be covered in some detail, First, consider what happens when the landing gear is retracted. As the selector valve (figure 9-14) is moved to the "up" position, pressurized fluid is directed into the gear up line. The fluid flows to each of eight units; to sequence valves C and D, to the three gear downlocks, to the nose gear cylinder, and to the two main actuating cylinders.
Notice what happens to the fluid flowing to sequence valves C and D in figure 9-14. Since the sequence valves are closed, pressurized fluid cannot flow to the door cylinders at this time. Thus, the doors cannot close. But the fluid entering the three downlock cylinders is not delayed; therefore, the gear is unlocked. At the same time, fluid also enters the up side of each gear-actuating cylinder and the gears begin to retract. The nose gear completes 349
Selector valve From main hydraulic Down system pressure manifold
• To return manifold check valve Orifice Sequence valves Orifice check valve Gear cylinder Gear cylinder Up Up Up-lock Up-lock Close Down-lock Down-lock Door cylinde Door cylinder Sequence valves D Up-lock Nose gear Down-lock cylinder Code Pressure manifold Return manifold Up gear up, door close line Up Gear down, door open line FIGURE 9-14. Hydraulic landing gear retraction system schematic.
retraction and engages its uplock first, because of the small size of its actuating cylinder. Also, since the nose gear door is operated solely by linkage from the nose gear, this door closes. Meanwhile, the main landing gear is still retracting, forcing fluid to leave the downside of each main gear cylinder.
This fluid flows unrestricted through an orifice check valve, opens the sequence check valve A or B, and flows through the landing-gear selector valve into the hydraulic system return line. Then, as the main gear reaches the fully retracted position and engages the spring-loaded uplocks, gear linkage strikes the plungers of sequence valves C and D.
This opens the sequence check valves and allows pressurized fuid to flow into the door cylinders, closing the landing gear doors.
accomplished using compressed air which is directed to uplock release cylinders.
In some aircraft, design configurations make emergency extension of the landing gear by gravity and airloads alone impossible or impractical. In such aircraft, provisions are included for forceful gear extension in an emergency. Some installations are designed so that either hydraulic fluid, or compressed air provides the necessary pressure; while others use a manual system for extending the landing gears under emergency conditions.
Hydraulic pressure for emergency operation of the landing gear may be provided by an auxiliary hand pump, an accumulator, or an electrically powered hydraulic pump, depending upon the design of the aircraft.
LANDING GEAR SAFETY DEVICES Wing Landing Gear Operation A typical wing landing gear operating sequence is illustrated in figure 9-15. The wing landing gear retracts or extends when hydraulic pressure is applied to the up or down side of the gear actuator.
The gear actuator applies the force required to raise and lower the gear. The actuator works in conjunction with a walking beam to apply force to the wing gear shock strut, swinging it inboard and forward into the wheel well. Both the actuator and the walking beam are connected to lugs on the landing gear trunnion, The outboard ends of the actuator and walking beam pivot on a beam hanger which is attached to the aircraft structure. A wing landing gear locking mechanism located on the outboard side of the wheel well locks the gear in the "up" position. Locking of the gear in the "down" position is accomplished by a downlock bungee which positions an upper and lower jury strut so that the upper and lower side struts will not fold.
Accidental retraction of a landing gear may be prevented by such safety devices as mechanical downlocks, safety switches, and ground locks. Mechanical downlocks are built-in parts of a gear-retraction system and are operated automatically by the gear-retraction system. To prevent accidental operation of the downlocks, electrically operated safety switches are installed.
Safety Switch A landing gear safety switch (figure 9-16) in the landing gear safety circuit is usually mounted in a bracket on one of the main gear shock struts. This switch is actuated by a linkage through the landing gear torque links. The torque links spread apart or move together as the shock strut piston extends or retracts in its cylinder. When the strut is compressed (aircraft on the ground), the torque links are close together, causing the adjusting links to open the safety switch, During takeoff, as the weight of the aircraft leaves the struts, the struts and torque links extend, causing the adjusting links to close the safety switch. As shown in figure 9-16, a ground
EMERGENCY EXTENSION SYSTEMS is completed when the safety switch closes. The ing gears if the main power system fails. Some aircraft have an emergency release handle in the cockpit, which is connected through a mechanical solenoid then energizes and unlocks the selector valve so that the gear handle can be positioned to raise the gear.
Ground Locks linkage to the gear uplocks. When the handle is In addition to this safety device, most aircraft are operated, it releases the uplocks and allows the equipped with additional safety devices to prevent gears to free-tall, or extend, under their own collapse of the gear when the aircraft is on the weight. On other aircraft, release of the uplock is ground. These devices are called ground locks. One 351
Up and locked Inboard In transit Downlock bungee Gear actuator
- Jury strut (upper and lower) Upper side Downlock actuator strut Drag brace Lower (ommitted side strut for clarity) Down and locked FIcURE 9-15. Wing landing gear operating sequence, 352
Control handle - Warning light (off) - Landing Lock release solenoid selector gear UP UP UP 28V valve de bus Spring - bar Tab type indicators (A) Landing Gear "Up and Locked" Safety switch Lock-pin FICURE 9-16. Typical landing gear safety circuit. Warning light (on) - common type is a pin installed in aligned holes drilled in two or more units of the landing gear support structure. Another type is a spring-loaded clip designed to fit around and hold two or more units of the support structure together. All types of ground locks usually have red streamers permanently attached to them to readily indicate whether or not they are installed.
Goar Indicators To provide a visual indication of landing gear position, indicators are installed in the cockpit or flight compartment.
Gear warning devices are incorporated on all retractable gear aircraft and usually consist of a horn or some other aural device and a red warning light.
The horn blows and the light comes on when one or more throttles are retarded and the landing gear is in any position other than down and locked.
Several designs of gear position indicators are available. One type displays movable miniature landing gears which are electrically positioned by movement of the aircraft gear. Another type consists of two or three green lights which burn when the aircraft gear is down and locked. A third type (figure 9-17) consists of tab-type indicators with markings "up" to indicate that the gear is up and locked, a display of red and white diagonal stripes to show when the gear is unlocked, or a silhouette of each gear to indicate when it locks in the "down" position.
Nosewheel Centering Centering devices include such units as internal Diagonal red and fluorescent stripes (B) Gear unlocked and in an intermediate position Warning light (off) → Silhouette of wheels (C) Landing gear "Down and Locked" FIcURE 9-17. A typical gear position indicator and warning light.
centering cams (figure 9-18) to center the nose wheel as it retracts into the wheel well. If a centering unit were not included in the system, the fuselage wheel well and nearby units could be damaged.
During retraction of the nose gear, the weight of the aircraft is not supported by the strut. The strut is extended by means of gravity and air pressure within the strut. As the strut extends, the raised area of the piston strut contacts the slopping area of the fixed centering cam and slides along it. In so doing, it aligns itself with the centering cam and rotates the nose gear piston into a straight-ahead direction.
The internal centering cam is a feature common to most large aircraft. However, other centering 353
Heavy Aircraft Rod connected Large aircraft, with their larger mass and a need to cylinder strut for positive control, utilize a power source for nose- Cylinder wheel steering. Even though large aircraft nosewheel steering system units differ in their construction features, basically all of these systems work in approximately the same manner and require the same sort of units. For example, each steering sys.
tem (figure 9-19) usually contains:
(1) A cockpit control, such as a wheel, handle, lever, or switch to allow starting, stopping, and to control the action of the system.
(2) Mechanical, electrical, or hydraulic connections for transmitting cockpit control movements to a steering control unit.
Centering cam Lobed area Shock strut piston FIcURE 9-18. Cutaway view of a nose gear internal centering cam.
devices are commonly found on small aircraft.
Small aircraft characteristically incorporate an external roller or guide pin on the strut. As the strut is folded into the wheel well on retraction, the roller or guide pin will engage a ramp or track mounted to the wheel well structure. The ramp/track guides the roller or pin in such a manner that the nose wheel is straightened as it enters its well.
In either the internal cam or external track arrangement, once the gear is extended and the weight of the aircraft is on the strut, the nose wheel may be turned for steering.
NOSEWHEEL STEERING SYSTEM Light Aircraft Light aircraft are commonly provided nosewheel steering capabilities through a simple system of mechanical linkage hooked to the rudder pedals. Most common applications utilize push-pull rods to connect the pedals to horns located on the pivotal portion of the nosewheel strut.
354:
(3) A control unit, which is usually a metering or control valve.
(4) A source of power, which is, in most instances, the aircraft hydraulic system.
(5) Tubing for carrying fluid to and from various parts of the system.
(6) One or more steering cylinders, together with the required linkages, for using pressurized fluid to turn the nose gear.
(7) A pressurizing assembly to keep fluid in each steering cylinder always under pressure, thereby preventing shimmy.
(8) A followup mechanism, consisting of gears, cables, rods, drums, and/or bellcranks, for returning the steering control unit to NEUTRAL, and thus holding the nose gear at the correct angle of turn.
(9) Safety valves to allow the wheels to trail or swivel, in the event of hydraulic failure.
Steering emergency forces the piston to start extending. Since the rod of release switch this piston connects to the nose steering spindle, which pivots at point X, the extension of the piston Steering wheel turns the steering spindle gradually toward the Metering valve Steering right. This action turns the nose gear slowly toward drum Differential link the right, because the spindle is connected to the
• Pedestal nose gear shock strut. As the nose gear turns right, Differential arm fluid is forced out of cylinder B, through the leftsompensator turn alternating line, and into port B of the meter- Differential • Pulleys ing valve. The metering valve sends this return fluid assembly Steering cables into a compensator, which routes the fluid into the aircraft system return manifold.
Followup cables Thus, hydraulic pressure starts the nose gear Followup drum Pulleys turning. However, the gear should not be turned too far. The nose gear steering system contains Safety shutoff valve - Scissor followup links and hold it there. devices to stop the gear at the selected angle of turn Orifice rod Centering cams Steering " cylinder "A" Steering spindle Steering cylinder "B Point X Gear yoke FICURE 9-19. Nosewheel system mechanical and hydraulic units.
Nosewheel Steering Operation The nosewheel steering wheel connects through a shaft to a steering drum located inside the cockpit control pedestal. The rotation of this drum transmits the steering signal by means of cables and pulleys to the control drum of the differential assembly. Movement of the differential assembly is transmitted by the differential link to the metering valve assembly, where it moves the selector valve to the selected position. Then hydraulic pressure provides the power for turning the nose gear.
As shown in figure 9-20, pressure from the aircraft hydraulic system is directed through the opened safety shutoff valve and into a line leading to the metering valve. This metering valve then routes 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 Followup Linkage As already pointed out, the nose gear is turned by the steering spindle as the piston of cylinder A (figure 9-20) extends. But the rear of the spindle has gear teeth which mesh with a gear on the bottom of the orifice rod. Thus, as the nose gear and its spindle turn, the orifice rod also turns, although in the opposite direction. This rotation is transmitted by the two sections of the orifice rod to the scissor followup links (figure 9-19), located at the top of the nose gear strut. As the followup links return, they rotate the connected followup drum, which transmits the movement by cables and pulleys to the differential assembly. Operation of the differential assembly causes the differential arm and links to move the metering valve back toward the neutral position.
The compensator unit (figure 9-21) which is a part of the nosewheel system keeps fluid in the steering cylinders pressurized at all times. This hydraulic unit consists of a three-port housing, which encloses a spring loaded piston and poppet. The left port is an air vent, which prevents trapped air at the rear of the piston from interfering with movements of the piston. The second port, located at the top of the compensator, connects through a line to the metering valve return port. The third port is located at the right side of the compensator. This port, which is connected to the hydraulic return 355
Orifice rod - From hydraulic system Safety shutoff valve pressure manifold Nose steering Steering cylinder "B" Poppet- - Spring spindle Emergency bypass valve Metering Left turn line valve Return port To hydraulic system return Point X manifold Right turn line Compensator Port "A Port 'B" Steering cylinder "A" za Pressurized fluid Return fluid FICURE 9-20. Nosewheel steering hydraulic flow diagram.
manifold, routes the steering system return fluid into the manifold when the poppet valve is open.
The compensator poppet opens when pressure acting on the piston becomes high enough to compress the spring. This requires 100 p.s.i.; therefore, fluid in the metering valve return line contains fluid trapped under that pressure. Now, since pressure in a confined fluid is transmitted equally and undiminished in all directions (Pascal's law), 100 p.s.i. also exists in metering valve passage H and in chambers From pressure E, D, G, and F (figure 9-21). This same pressure is also applied in the right- and left-turn alternating lines as well as in the steering cylinders themselves.
SHIMMY DAMPERS A shimmy damper controls vibration, or shimmy, through hydraulic damping. The damper is either attached to or built integrally with the nose gear and prevents shimmy of the nosewheel during taxiing, landing, or takeoff. There are three types of shimmy dampers commonly used on aircraft: (1) The piston type, (2) vane type, and (3) features incorporated in the nosewheel power steering system of some aircraft.
manifold Piston-Type Shimmy Damper Drilled passage "H" G Return port 9-22 consists of two major components: (1) The The piston-type shimmy damper shown in figure cylinder "A" From = cylinder From Metering valve der. lower end of the nose gear shock strut outer cylin- cam assembly and (2) the damper assembly. The shimmy damper is mounted on a bracket at the "B" The cam assembly is attached to the inner cylin- Spring Popper der of the shock strut and rotates with the nose- wheel. Actually, the cam consists of two cams that Air • are mirror images of each other. Lobes on the cams vent Compensator are so placed that the damping effect offers the Housing retum manifold To hydraulic centered. The cam follower crank is a U-shaped greatest resistance to rotation when the wheel is casting which incorporates the rollers that follow FICURE 9-21. Cutaway view of metering valve and the cam lobes to restrict rotation. The arm of the compensator. crank connects to the operating piston shaft.