TM 1-410. Technical Manual, Airplane Structures 1941: Section 8 - Landing Gear

SECTION VIII: LANDING GEAR

25. General.-Landing gear of the airplane consists of main and auxiliary, both of which may be retractable or nonretractable. The main landing gear consists of that portion of alighting gear which forms the principal support of aircraft when on land or water. It may include any combination of wheels, floats, skids, shock-absorbing mechanisms, brake and steering operating mechanisms, retracting mechanisms with their controls and warning devices, fairings and framing or structural members necessary to secure any of above to the primary structure. The auxiliary landing gear consists of tail or nose landing wheel installations, outboard pontoons, skids, etc., and any necessary cowling or bracing members or structural reinforcement added to or incorporated in the aircraft to facilitate or safeguard landings and ground or water handling.

26. Main.-a. The nonretractable landing gear is rigidly attached to structural members of the airplane and is generally equipped with cowling to reduce air resistance. This fairing, as shown in. figure 28, is assembled in sections and motion occurs between these sections; abrasion shoes are used to prevent fouling or excessive wear. At high speeds exposure of the landing gear creates a considerable loss of power by its resistance or drag. This resistance has been decreased or eliminated by use of retractable landing gear which is drawn up into wells in the wings or recesses in the sides or bottom of the fuselage.

On some airplanes, the lower section of the wheels and sometimes part of the landing gear fairing remain exposed, while on others the gear is completely retracted and covered with fairing. Figure 29 shows such a landing gear in extended and retracted positions. In this case retraction is rearward, however, on some airplanes the gear folds either inward or outward in a lateral direction.

FIGURE 28.-Non retractable landing gear.

b. Retraction of gear is accomplished by one of three means, manual, electric, or hydraulic. Usually manual control is provided along with electric or hydraulic operation as a. safety precaution. An indicator is provided to show position of the gear "up" or "down" and in some cases intermediate positions. Locks which are controllable from the cockpit are provided for locking the gear in its "down" or extended position. In some cases, the gear is also locked by a safety pin or lock managed from the ground which precludes any possibility of the gear being retracted inadvertently from the cockpit while the airplane is at rest on the ground.

(1) Extended.

(2) Retracted.
FIGURE 29. - Retractrable landing gear.

c. Warning signals are generally used in conjunction with retracting mechanism. Signals usually consist of an electric horn, an electric vibrator mounted on the rudder pedal, or a red warning light mounted in a conspicuous position in the cockpit. These warning devices are wired to the engine throttle in such a manner that if the landing gear is not fully extended and positively locked in such position the warning signal will operate when the throttle is pulled back to idling position.

d. Use of landing skis permits the airplane to be operated in deep snow where it would be impossible to operate with wheels only.

FIGURE 30-Landing ski installation.

The type of ski in general use is usually of all-metal construction and so designed that the wheel is not removed from the axle for its installation. It is equipped with an opening through which the wheel protrudes a given distance and a means of attachment to the landing gear on each side of the wheel. The installation is shown in figure 30. This design proves more satisfactory for general operation of airplanes in snow than those which are equipped with skis only.

27. Auxiliary.-a. Auxiliary landing gear may also be retractable or nonretractable, the former being used on most high speed airplanes. It usually retracts with the main landing gear and employs the same controls and similar position indicators. Figure 31 shows a tail wheel in extended and retracted positions.

b. All tail wheels are designed to swivel and in most instances are controllable with the rudder through its range of movement. An automatic disengaging device or release mechanism, shown in figure 32, disengages the control just at or slightly before limit of rudder movement This assures control of tail wheel for taxying, allows full rudder movement even though the tail gear might bind, and beyond

(1) Extended.

(2) Retracted.
FIGURE 31.-Retractable tail gear.

control range permits free swiveling for ground handling of the airplane. This release also serves as a safety device to prevent injury to the pilot's legs and to the fuselage structure resulting from violent twists imposed on the gear by rough ground.

c. Most tail gears are provided with an antishimmy device to minimize tendency of the tail wheel to oscillate violently during landing and taxying. This device usually consists of two friction disks held in contact with each other by a coil spring, also shown in figure 31. One disk is carried by and rotates with the tail gear spindle while the other is fixed to the spindle support, and friction between the two serves to dampen oscillations. Sometimes the tail wheel is restrained toward the trailing position by a centering arm, and in some cases a latch operated from the pilot's compartment is used to lock the tail wheel in the trailing position during take-off and landings. Tail gear control cables incorporate a shock unit (coil spring) connected either in series or parallel with the cable, as shown in figure 23. This unit absorbs shocks caused by oscillation of the tail gear, preventing their transmission to rudder control cables.

28. Shock struts.-a. Shock struts in most general use for main and auxiliary landing gear are of the air-oil type. Although internal construction of the several models is somewhat different, their operation is essentially the same. Figure 33 shows the cross section views of a typical air-oil strut in deflated and extended positions. This unit consists of an inner and outer steel cylinder and a movable steel piston. A steel head holds the two cylinders in position and piston movement is accomplished using two bearing surfaces, one attached to the top of the piston and the other at the base of the inner cylinder. A packing gland installed at the lower end of the outer cylinder prevents leakage of the fluid. The base of the inner cylinder has a hole or orifice through which a tapered metering pin mounted on the base of the piston operates to control flow of fluid. A snubber tube surmounted by a flap valve is integral with the base of the inner cylinder and both the cylinder and the snubber tube are equipped with ports to allow passage of fluid between the three chambers.

b. The cross section view (fig. 33 (1) shows the strut in deflated position. An air valve assembly mounted on the upper end of the strut permits compressed air to be pumped into the cylinders which partially extends the strut. The metering pin enters the orifice but never completely fills it. When the airplane takes off, compressed air and weight of the wheel fully extend the strut as shown in figure 33 (2) and the fluid flows down through the orifice to occupy the space in the hollow piston created by this extension. When the, airplane contacts the ground on landing, the strut is compressed and the fluid is forced back up through the orifice. Rate of flow of fluid through the orifice is restricted by the metering pin. Since the smaller end of the metering pin enters the orifice first, as the piston nears the end of its stroke, the flow of fluid becomes progressively more restricted. This slows the piston down to a gradual stop, thus absorbing the landing shock. During contraction of the strut, the flap valve permits

(1) Engaged.
FIGURE 32.-Tail wheel disengaging and antishimmy device.

(2) Disengaged.
FIGURE 32.-Tail wheel disengaging and antishimmy device-Continued.

the fluid to flow freely from the snubber tube, but during extension of the strut, the flap valve is held shut and the fluid must return to the orifice through the small return ports.

29. High-pressure pumps.-Since pressures required sometimes run as high as 1,000 pounds per square inch, special pumps are required for inflating air-oil struts. The two pumps now in general use are the high-pressure hand pump and the automatic pressure pump.

FIGURE 34.-High-pressure hand pump.

a. The high-pressure hand pump (fig. 34) is a hand-operated pump of small bore and long stroke. It is designed to boost a moderately high supply pressure ten or twelve times.

The air usually is fed to the booster pump by a motor-driven air compressor at about 100 pounds per square inch. In case this pressure is not available, any reasonably good automobile tire pump operated by a second man can be used as an emergency source of supply. The two methods are shown in figure 35. Instrument oil should be applied to the pump shaft when necessary, and cup leather oiled occasionally to keep it soft and pliable.

Caution.-The handle of the pump should be in the out or fully extended position when attaching air hose from the compressor to prevent any possible injury to personnel by a sudden extension of the piston.

b. The automatic booster pump shown in figure 36 is also operated by air pressure from any compressed air line and is capable of increasing

FIGURE 35-High-pressure hand pump connection.

this pressure a maximum of ten times. Thus, if air at 100 pounds per square inch is supplied to the pump, it will deliver air at 1,000 pounds per square inch to the strut. A few drops of light oil should be placed in the intake port occasionally to lubricate operating mechanism.

FIGURE 36.-Automatic booster pump connections.

30. Maintenance-a. Landing and tail gear.-The Technical Order Handbook of Instructions for the airplane must be studied carefully before performing maintenance operations as the following instructions are of a general nature only, and must be supplemented by specific information for each type of airplane.

(1) Periodically each airplane with retractable landing gear is placed on jacks as described in section XVI and all mechanisms operated through complete cycles with all controls. As the landing gear is retracted and extended, a careful check should be made to see that it operates freely throughout its total range and that no part binds on any part of the airplane structure.

(2) Cables, if used, are checked for broken wires and proper tension. In case bungees are used, the shock cord should be inspected for breaks, weakening, or fraying of the covering. Bungees should not be allowed to become soaked with grease or oil as this will cause rapid deterioration of the shock cord. Function of the locking device must be checked very carefully and nothing short of perfect is acceptable. The landing gear position indicator, especially in the extended and retracted position, is checked for proper indication and the landing gear position warning signal is checked to see that the signal is distinct and operates whenever the landing gear is in any position other than down and locked.

(3) Since action of the tail gear antishimmy device depends on friction, it is obvious that grease or oil between the friction disks will reduce its effectiveness. The disks may be separated, as shown in figure 32 (2) and washed with unleaded gasoline or alcohol.

b. Air-oil shock struts.-Periodically it is necessary to check level of fluid in shock struts and to replenish the supply due to leaks. It is necessary that all air pressure in the strut be relieved before this operation can be performed.

(1) Deflating a strut improperly is very dangerous, therefore care must be exercised when performing this operation. Two types of filler plugs are in use, one with a straight thread and another with a tapered pipe thread. When the former is used, the strut is deflated by unscrewing the plug slowly until the pressure relief vent is uncovered, which allows the air to escape. However, no attempt should be made to deflate a strut employing the pipe thread type filler plug in this manner, as this plug has no vent in the side and if unscrewed too far will be blown out with dangerous force by the air pressure in the strut. The strut in this case is deflated by carefully depressing the valve core and the plug is not unscrewed until all air pressure has been relieved. Even if carefully done, the core may be damaged, but this is the only method by which struts with this type of filler plugs can be deflated safely, and when the core is damaged it is replaced with a new one. The cores used in both types of plugs are quite similar to those used in automobile tires, but are especially designed for use in air-oil type shock absorbers and have a special composition seat material instead of rubber to resist deterioration by fluid. Under no circumstances will ordinary tire valves be used, as the rubber seat will deteriorate rapidly after contact with shock strut fluid.

(2) After all air is exhausted the filler plug is removed to check the fluid level, which should be even with the filler-plug opening. Fluid should be added at any time the level is below this point. During this operation the airplane must be in taxying position and the strut fully collapsed. It is advisable to rock the airplane slightly after deflating the strut to be sure that it is fully collapsed when filling a strut .Which has been entirely empty or very low in fluid, special effort must be made to work out any air which may be trapped in the strut. This is done by filling the strut as described above and inserting the plug loosely. The strut is then extended and collapsed several times, preferably by raising and lowering the airplane by means of a hoist and/or hydraulic jacks. The air will escape through the loosely screwed-in plug and fluid is then added to bring the level up to the filler plug hole and the strut again extended and collapsed. These operations are, repeated until the fluid level does not change, which indicates that all air has been eliminated. The filler plug is then screwed in tight, a copper gasket being used on the straight thread type plug.

(3) After the strut has been filled properly, it is inflated with a high-pressure pump. Care should be exercised to prevent damage to filler plug assembly by excessive tightening of the air-hose connection. When tightening this connection, if leakage cannot be stopped by hand tightening, the fitting gasket should be replaced. Under no circumstances will wrenches or pliers be used for tightening this connection. The amount of inflation is usually specified on the instruction plate attached to the strut or may be found in the handbook of instructions for the particular airplane. Amount of inflation of any air-oil strut is indicated by amount of extension of the strut with the airplane normally loaded and resting on a level surface in taxying position. It is specified in inches measured between two points as designated on the instruction plate, one on the cylinder and one on the piston. During inflation it is important that the airplane be rocked slightly to extend and compress alternately the strut to overcome packing friction. A plus or minus tolerance of 1/4 inch is per missible for the specified amount of extension. The airplane should be sheltered from the wind while struts are being inflated and checked.

(4) It is possible to release a small amount of air from the strut by depressing the valve core. However, extreme care should be exercised in doing this and the air should be allowed to escape slowly to avoid damage to the valve-core seat. It is best to support the tool used to depress the core against the lip of the valve stem. In this manner the core may be depressed slowly and the escape of air controlled. This method may be used when a strut is accidentally overinflated, but should not be used when completely deflating a strut, except when the pipe threaded type plug is used.

(5) After inflation, the valve assembly is carefully checked for leaks, using soapy water. Leaks around the filler plug, valve, and other parts will be evidenced by a seepage of fluid. A small seepage around the packing gland is desirable for lubrication of the piston and should not be confused with a leak at this point. In case of leakage at the packing gland as evidenced by excessive fluid seepage or by air bubbles when soapy water is applied, the packing nut should be tightened firmly but not excessively. However, before tightening air pressure must be released, weight of airplane removed from the strut, and all safeties removed from the packing nut. The packing cannot be tightened properly under pressure, and if normal tightening does not stop the leakage, the packing must be replaced. Replacement of packing is done normally by a repair depot or base engineering shop.

(6) Alcohol is the only fluid which may be used for cleaning or flushing out struts containing hydraulic fluid. Any mineral-oil compound or derivative will destroy the packing materials, and carbon tetrachloride, upon contact with the fluid, forms hydrochloric acid which is a very powerful corrosive agent.