Aircraft Landing Gear Systems 1

AIRCRAFT LANDING GEAR SYSTEMS

INTRODUCTION  The landing gear of the very first airplanes was not very complex. The Wright Flyer, for instance, took off from a rail and landed on skids,  However, soon after the basic problems of flight were solved, attention was turned to providing better control and stability of the aircraft while it was operated on the ground.  Bicycle and motorcycle wheels were first used, which in turn, gave way to specially designed landing gear and wheels that absorbed the extreme loads imparted during takeoffs and landings.

 In addition, braking systems were installed to provide safer and more efficient control for slowing an airplane after landing.  In later years, as aircraft designs improved to increase speed and efficiency, retraction systems were provided to allow the landing gear to be stowed during flight to reduce aerodynamic loads, or drag.

 With continued improvements in technology, landing gear systems on modern aircraft are highly reliable and capable of handling extreme conditions, enabling safe transitions between flight and ground mobility.

What is landing gear?  Landing gear is the undercarriage of an aircraft or spacecraft and may be used for either takeoff or landing.  For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage.  Wheels are typically used but skids, skis, floats or a combination of these and other elements can be deployed depending both on the surface and on whether the craft only operates vertically or is able to taxi along the surface.  Faster aircraft usually have retractable undercarriages, which fold away during flight to reduce air resistance or drag.

LANDING GEAR ARRANGEMENT  TRICYCLE TYPE LANDING GEAR  CONVENTIONAL LANDING OR TAIL WHEEL LANDING GEAR  FLOAT TYPE LANDING GEAR  TANDEEM TYPE LANDING GEAR  SKID TYPE LANDING GEAR

TRICYCLE TYPE LANDING GEAR  Tricycle gear is a type of aircraft undercarriage, or landing gear, arranged in a tricycle fashion.  The tricycle arrangement has a single nose wheel in the front, and two or more main wheels slightly aft of the center of gravity. Tricycle gear aircraft are the easiest to take-off, land and taxi, and consequently the configuration is the most widely used on aircraft

Advantage:  On the ground, tricycle aircraft have a visibility advantage for the pilot as the nose of the aircraft is level.  Tricycle gear aircraft are much less liable to 'nose over' as can happen if a taildragger hits a bump or has the brakes heavily applied. In a nose-over, the aircraft's tail rises and the propeller strikes the ground, causing damage.  The tricycle layout reduces the possibility of a ground loop, because the main gear lies behind the center of mass.  The nosewheel equipped aircraft also is easier to handle on the ground in high winds due to its wing negative angle of attack. Student pilots are able to safely master nosewheel equipped aircraft more quickly  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 jet-powered aircraft have been fitted with tricycle landing gear, to avoid the blast of hot, high-speed gases causing damage to the ground surface, in particular runways and taxiways.

Reason why tricycles is more stable than tailwheel.  1) the CG is forward of the main gear 2) the CG is closer to the mains than in a tailwheel aircraft

Nose over  'nose over' as can happen if a taildragger hits a bump or has the brakes heavily applied. In a nose-over, the aircraft's tail rises and the propeller strikes the ground, causing damage.

Landing flare  The landing flare, also referred to as the round out, is a maneuver or stage during the landing of an aircraft.  The flare follows the final approach phase and precedes the touchdown and roll-out phases of landing.  In the flare, the nose of the plane is raised, slowing the descent rate, and the proper attitude is set for touchdown.  In the case of tricycle gear-equipped aircraft, the attitude is set to touchdown on the main (rear) landing gear.

Piper PA-28 CHEROKEE FLARING FOR LANDING

GROUND LOOP  In aviation, a ground loop is a rapid rotation of a fixed-wing aircraft in the horizontal plane (yawing) while on the ground.  Aerodynamic forces may cause the advancing wing to rise, which may then cause the other wingtip to touch the ground. In severe cases (particularly if the ground surface is soft), the inside wing can dig in, causing the aircraft to swing violently or even cartwheel.  If the aircraft heading is different from the aircraft's direction of motion, a sideways force is exerted on the wheels. If this force is in front of the centre of gravity, the resulting moment rotates the aircraft's heading even further from its direction of motion.

 When you're landing in a tailwheel plane, if your plane starts turning right, two forces are created: 1) friction on your wheels turns the plane right, and 2) because of Newton's 3rd law, the CG is thrown in the opposite direction, which happens to be to the left. These two forces make your tailwheel plane want to spin in circles on the runway - which can cause a ground loop.

Ground loop occur when:  Moving on the ground such , taxiing , landing or during takeoff  When landing on muddy ground, wet pavement, or frozen surfaces  They may also occur when an aircraft depart a paved surface: for example, after an engine failure in multi-engine airplane produces asymmetric thrust  Failure of a tire or wheel brake, causing loss of directional control

 They may also occure without outside influence, due to pilot error.

Conventional landing gear  Conventional landing gear, or tail wheel-type landing gear, is an aircraft undercarriage consisting of two main wheels forward of the center of gravity and a small wheel or skid to support the tail

Advantages:  Due to its position much further from the center of gravity, a tailwheel supports a smaller part of the aircraft's weight allowing it to be made much smaller and lighter than a nosewheel. As a result, the smaller wheel weighs less and causes less parasitic drag.  Because of the way airframe loads are distributed while operating on rough ground, tailwheel aircraft are better able to sustain this type of use over a long period of time, without cumulative airframe damage occurring  If a tailwheel fails on landing, the damage to the aircraft will be minimal. This is not the case in the event of a nosewheel failure, which usually results in a prop strike.  Due to the increased propeller clearance on tailwheel aircraft less stone chip damage will result from operating a conventional geared aircraft on rough or gravel airstrips, making them well suited to bush flying.

 Tailwheel aircraft are more suitable for operation on skis.  Tailwheel aircraft are easier to fit into and maneuver inside some hangars.

Disadvantages  Tailwheel aircraft are more subject to "nose-over" accidents due to injudicious application of brakes by the pilot.  Conventional geared aircraft are much more susceptible to ground looping.

 Tailwheel aircraft generally suffer from poorer forward visibility on the ground, compared to nose wheel aircraft. Often this requires continuous "S" turns on the ground to allow the pilot to see where they are taxiing.  Tailwheel aircraft are more difficult to taxi during high wind conditions, due to the higher angle of attack on the wings which can then develop more lift on one side, making control difficult or impossible. They also suffer from lower crosswind capability and in some wind conditions may be unable to use crosswind runways or single-runway airports

Tandem type landing gear  The tandem wheel arrangement is seldom used on civilian aircraft other than gliders. Some of the heavy bombers of the past have used the tandem gear arrangement.

 The main wheels are located in line under the fuselage and outrigger wheels support the wings.

FIXED AND RETRACTABLE LANDING GEAR  All aircraft must contend with two types of aerodynamic drag, parasite drag, which is produced by the friction of the airflow over the structure and induced drag which is caused by the production of the lift.  while induced drag decreases as the speed increases because of the lower angle of attack required to produce the needed lift  Slower aircraft lose little efficiency by using the lighter-weight fixed landing gear.  . Faster aircraft retract the landing gear into the structure and thus gain efficiency even at the cost of slightly more weight.

FIXED LANDING GEAR  Fixed landing gear decreases parasitic drag markedly by enclosing the wheels in streamlined fairings, called wheel pants.

RETRACTABLE LANDING GEAR 

To decrease drag in flight some undercarriages retract into the wings and/or fuselage.

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If the wheels rest protruding and partially exposed to the airstream after being retracted, the system is called semi-retractable.

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Most retraction systems are hydraulically operated, though some are electrically operated or even manually operated.

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In retractable gear systems, the compartment where the wheels are stowed are called wheel wells, which may also diminish valuable cargo or fuel space.

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Multiple redundancies are usually provided to prevent a single failure from failing the entire landing gear extension process. Whether electrically or hydraulically operated, the landing gear can usually be powered from multiple sources.

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In case the power system fails, an emergency extension system is always available. This may take the form of a manually operated crank or pump, or a mechanical free-fall mechanism which disengages the uplocks and allows the landing gear to fall due to gravity. Some highperformance aircraft may even feature a pressurized-nitrogen back-up system.

Semi retractable gear

Fully retractable landing gear

RETRACTABLE LANDING GEAR  I. Minimizing the amount of parasitic drag by means of retracting the landing gear to its designated compartment known as “Landing Gear Compartment” or “Wheel” Well area.  Also contributed to more better and improved aircraft performance  NOTE: Extreme care is taken for the landing gear because it receive rough treatment throughout operation as such frequent landing shocks an irregular contact with ice, dirt and abrasive grit. By regularly washing, lubricate and servicing the landing gear, it guards against corrosion, seizure of mechanical parts, and failure of electrical components.

Wheel wells landing gear Wheel well compartment

Wheel well doors

RETRACTABLE LANDING GEAR

ARRANGEMENT OF LANDING GEAR  I.

Single type LG

 II. Double type LG  III. Tandem Type LG. ( Parallel to each other)  Iv. Multiple or Bogie Type LG

SINGLE TYPE LANDING GEAR

DOUBLE TYPE LANDING GEAR

TANDEM TYPE LANDING GEAR

MULTIPLE OR (BOGIE/TRUCK) LANDING GEAR  I. The number of wheels determined by the gross design weight of aircraft and the surfaces on which the loaded aircraft may require performing landing  II.

Advantages of Multiple wheel type landing gear. 1. spread the aircraft weight over a larger area of load distribution 2. providing safety margin in a case of one of the tire burst during take off or landing. 3.Extra braking effort or efficiency of braking is accomplish by the number of brake unit attach in each each wheels. Disadvantages of Multiple wheel type landing gear

1. More moving parts and therefore require greater amount of maintenance 2. Tire tends to scrubs during turning maneuver 3. Larger turning radius require to prevent or reduce tire wear and therefore need a larger space for movement.

SHOCK STRUT / OLEO STRUT I.

A type of hydro – pneumatic (oil and air )components.

II.

A device in an aircraft landing gear system that absorb the landing shock, which occur when an aircraft touches down when landing or during taxiing.

III. FUNCTION OF A SHOCK STRUT

SHOCK STRUT / OLEO STRUT There are 2 types of shocks struts

1. Non-absorbing shock strut 2. Absorbing shock strut Non-absorbing shock strut/oleo strut 1. rigid or skid

2. Spring steel 3. composite 4. bungee cord.

RIGID TYPE SHOCK STRUT (non-absorb). I.

Commonly found on helicopter (skid) and sail planes.

II.

The strut is rigidly attach to the aircraft without no specific components to cushion the ground contacts.

III. The only way of cushioning is no more than trough the flexing of the landing gear or airframe structure itself.

SPRING STEEL TYPE SHOCK STRUT (NONABSORBING) I.

Usually employed at the main landing gear position for the light aircraft

II. The Leg is consist of a tube, bar or a tapered spring

III. The upper wide end is being attach by bolts to a heavy structure of the aircraft under the cabin and the lower end terminating in axle on which the wheel and breaks are assemble IV. On ground contact during landing, the gear flexes and stores the impact energy

V. Periodically inspected for looseness of attachment bolts, damage, and corrosion by taking the load off the wheel By means of jacking the aircraft

LANDING GEAR SYSTEM COMPONENTS AND FUNCTIONS

LANDING GEAR MAIN COMPONENTS I.

Torque links/Torque Arms/ Torsion link(scissors Assembly)

* Maintain wheel and axle in a correct aligned position in relation to the strut (misalignment corrected by adding /installing a spacer or shim of different thickness)

* Prevent ram of a piston turning in the struts. * Restrict the extension of the piston during the extension of the struts. II.

Side brace link / side strut * Stabilizing the landing gear and support the aircraft laterally

III.

Over center link / Down lock strut / Jury strut

* Holds the drag link and the side brace in the DOWN AND LOCK position by applying pressure to the center pivot joint in a drag or side brace link. Operates hydraulically by bungee cylinder or mechanically by bungee spring.

Torque links/Torque Arms/ Torsion link(scissors Assembly -Maintain wheel and axle in a correct aligned position in relation to the strut.

LANDING GEAR MAIN COMPONENTS IV. Trunnion * supported at its by bearing / bushing assemblies, which allow the gear to pivot during the retraction and extension ( the struts from the cylinder for the oleo pneumatic shock absorber ) V. Shock strut * Vertical member of the landing gear assembly that contains the shock absorbing mechanism.

VI. Shimmy damper * hydraulic snubbing unit that reduces the tendency of the nose or tail wheel to oscillate from side to side ( rapid oscillation)

LANDING GEAR MAIN COMPONENTS VII. Actuator / Jack / Actuating cylinder

* Raise and lowering the landing gear. May also used as a downlock mechanism. VIII. UP lock cylinder / UP lock actuator * locking the landing gear in “UP” and “LOCK” position IX. DOWN lock cylinder / DOWN lock actuator * locking the landing gear in “DOWN” and “LOCK” position X. Indicator ( Microswitches/ Ground safety switch/ squat switch )

* Means of providing cockpit indication with regards to the landing gear position, either “UP” and “LOCK” “ DOWN” and “LOCK” AND “NOT LOCKED”. XII. Centering cam * Aligning the nose wheel before retracted to “UP” and back to its Wheel well compartment.

MULTIPLE TYPE OR BOGIE LG

LANDING GEAR MAIN COMPONENTS XIII. Landing gear door actuator * Allowing the opening or closing of landing gear door ( main and nose landing gear) XIV. Emergency Lowering Mechanism * Providing the means of lowering down the landing gear in the event of normal extension fails ( under FAR 23.729)

OLEO PNEUMATIC SHOCK STRUT I.

Uses a compressed dry air ( nitrogen) and hydraulic fluid (mineral base type) to absorb and dissipate shock loads.

II. Shock strut are essentially made up of two telescopic cylinder or tube with externally closed ends. III. The two cylinder known as cylinder and piston when assembled will form an upper and lower chamber for the movement of the fluid. IV. The lower chamber is always filled with fluid while upper chamber contains compressed air (nitrogen)

OLEO STRUT OPERATION I.

During compression of the shock strut at landing, the orifice provide a restriction of fluid flow and this reduce the of which (located in the inner cylinder) can move into the cylinder.

II. This provide a cushioning effect to reduced the shock of landing III. As the fluid flow through the orifice into the upper chamber, the air in the upper chamber is compressed to the point that the entire weight of the aircraft is supported by the air in the landing gear strut. IV. The extension stroke occurs at the end of the compression as the energy stored in the compressed air causes the aircraft to start moving upward in relation to the ground (the compressed air act as a spring to return the struts to its normal extension) V. The compressed air at rest then act as a shock absorber during the time that the aircraft shock of landing

Note: for efficiency operation of the shock strut, the proper fluid level and air pressure must be maintained.

RETRACTING LANDING GEAR  Improving aircraft performance by reducing parasite drag on the aircraft. The landing gear are being retracted inside the fuselage or wing and generally known as wheel well landing gear or landing gear compartment

RETRACTING LANDING GEAR  Method system for retraction and extension of retractable landing gear 1. Mechanical type 2. Electrical type

3. hydraulic type 4. pneumatic type

RETRACTION OPERATION I. To lower the landing gear, the pilot moves the landing gear handle to GEAR DOWN position and these events take place.

II. The landing gear handle actuates a switch that turn on the hydraulic pump motor in the power pack so that it turns in the direction shown by the arrow.

RETRACTION SYSTEM OPERATION Retractable landing gear also provided with: 1. Mechanical locks to ensure that each undercarriage is locked securely in the retracted and extended position 2. Devices to indicates to the crew about the position of each undercarriage 3. Means by which the landing gear can be extended in the event of hydraulic failure or electrical source failure 4. Means to prevent retraction when the aircraft is on the ground 5. Means to guard against landing with the landing gear retracted

4 TYPES OF RETRACTION DIRECTION 1. Rearward retraction 2. Sideward retraction

3. Forward retraction 4. Upward retraction

REARWARD RETRACTION  Could be used on nose landing gear and tail gear

 The disadvantage of this design is that the airflow could not be used as an aid or assisting in emergency extension  Require back up pressure system for emergency extension case

SIDEWARD RETRACTION  Landing gear retracting inboard or outboard is used only on main landing gear application and are not suitable to be used on nose landing gear  Fuselage does not allow any nose landing gear or tail wheel to be retracted in this direction

FORWARD RETRACTION  Used generally on both landing gear , nose landing gear and main landing gear  Considerable effort required to retract (raise)the landing gear with this method because its against the airflow during flight  Airflow act as an aid or assisting in emergency extension of the landing gear with forward retraction

UPWARD RETRACTION  The alighting is retracted upward into the fuselage known as pod , or blister  Mainly used on high wing plane aircraft such as Lockheed C-130

RETRACTION TEST  Replacement of faulty landing gear compartment such as valve, switches, actuator, etc.)  Adjustment of components related in the retraction system  After a hard landing or overweight landing occur  Whenever incorrect operation is reported or suspected by aircraft personnel ( pilot or engr.)

FUNCTIONAL TEST DURING RETRACTION TEST  Check the undercarriage/landing gear for proper operation  Check doors for correct operation and fits limit (flushed)

 Clearance in the wheel bay with the landing gear retracted and making due allowance for the effect of centrifugal force on the entire diameter (incase of damaging any hydraulic lines )  Permissible retraction and extension time (also time for free fall)