Landing gear

Landing gear is the undercarriage of an aircraft or spacecraft that is used for engaging the surface — typically land, but may also be the surface of a water body — when parking, taxiing, takeoff or landing. It was also formerly called alighting gear by some manufacturers, such as the Glenn L. Martin Company. For aircraft, Stinton makes the terminology distinction undercarriage = landing gear .
For aircraft, landing gear is the foundational part of airframe that supports the craft's weight when it is not in flight, keeping the fuselage at a clearance off the ground so it can avoid sustaining frictional/collisional damages. Wheeled landing gear is the most ubiquitous, used in almost all aircraft that perform conventional and short takeoff and landing, while skids or floats are used in aircraft that can take off and land vertically or operate from snow/ice/water. Landing gears from early aircraft are usually fixed, and remain protruded under the aircraft during flight, with no or only partial fairing coverage to reduce drag; while most modern aircraft have retractable undercarriages that fold into the fuselage during flight, which maximizes aerodynamic streamlining and allows for faster airspeeds and smoother flight control.
Landing gear must be strong and robust enough to handle the stress of both the aircraft's weight and the touchdown shock during landing, and its design is crucial to the aircraft's operational safety. Most landing gears constitute a tricycle layout with three sets of wheels, each with a single wheel or a wheelset/bogie, either in a "1-2" delta-shaped layout or a "2-1" nabla-shaped layout, while other atypical configurations such as bicycle, quadracycle and other multicycle arrangements. Some other anomalous landing gears have also been evaluated experimentally, including: no landing gear, made possible by operating from a catapult cradle and flexible landing deck: air cushion ; tracked.
For launch vehicles, spacecraft landers and rovers, the landing gear usually only supports the vehicle on landing and during subsequent surface movement, and is not used for takeoff.
Given their varied designs and applications, there exist dozens of specialized landing gear manufacturers. The three largest are Safran Landing Systems, Collins Aerospace and Héroux-Devtek.

Aircraft

The landing gear represents 2.5 to 5% of the maximum takeoff weight and 1.5 to 1.75% of the aircraft cost, but 20% of the airframe direct maintenance cost. A suitably designed wheel can support, tolerate a ground speed of 300 km/h and roll a distance of ; it has a 20,000 hours time between overhaul and a 60,000 hours or 20 year life time.

Gear arrangements

Wheeled undercarriages normally come in two types:

Conventional landing gear or "taildragger", where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear. The same helicopter arrangement is called tricycle tailwheel.
Tricycle landing gear, where there are two main wheels under the wings and a third smaller wheel in the nose. PZL.37 Łoś Was the first bomber aircraft with twin wheels on a single shock absorber. The same helicopter arrangement is called tricycle nosewheel.

The taildragger arrangement was common during the early propeller era, as it allows more room for propeller clearance. Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder to land and take off, and usually require special pilot training. A small tail wheel or skid/bumper may be added to a tricycle undercarriage to prevent damage to the underside of the fuselage if over-rotation occurs on take-off leading to a tail strike. Aircraft with tail-strike protection include the B-29 Superfortress, Boeing 727 trijet and Concorde. Some aircraft with retractable conventional landing gear have a fixed tailwheel. Hoerner estimated the drag of the Bf 109 fixed tailwheel and compared it with that of other protrusions such as the pilot's canopy.
A third arrangement has the main and nose gear located fore and aft of the center of gravity under the fuselage with outriggers on the wings. This is used when there is no convenient location on either side of the fuselage to attach the main undercarriage or to store it when retracted. Examples include the Lockheed U-2 spy plane and the Harrier jump jet. The Boeing B-52 uses a similar arrangement, except that the fore and aft gears each have two twin-wheel units side by side.
Quadricycle gear is similar to bicycle but with two sets of wheels displaced laterally in the fore and aft positions. Raymer classifies the B-52 gear as quadricycle. The experimental Fairchild XC-120 Packplane had quadricycle gear located in the engine nacelles to allow unrestricted access beneath the fuselage for attaching a large freight container.
Helicopters use skids, pontoons or wheels depending on their size and role.

Retractable gear

To decrease drag in flight, undercarriages retract into the wings and/or fuselage with wheels flush with the surrounding surface, or concealed behind flush-mounted doors; this is called retractable gear. If the wheels do not retract completely but protrude partially exposed to the airstream, it is called a semi-retractable gear.
Most retractable gear is hydraulically operated, though some is electrically operated or even manually operated on very light aircraft. The landing gear is stowed in a compartment called a wheel well.
Pilots confirming that their landing gear is down and locked refer to "three greens" or "three in the green.", a reference to the electrical indicator lights from the nosewheel/tailwheel and the two main gears. Blinking green lights or red lights indicate the gear is in transit and neither up and locked or down and locked. When the gear is fully stowed up with the up-locks secure, the lights often extinguish to follow the dark cockpit philosophy; some airplanes have gear up indicator lights.
Redundant systems are used to operate the landing gear and redundant main gear legs may also be provided so the aircraft can be landed in a satisfactory manner in a range of failure scenarios. The Boeing 747 was given four separate and independent hydraulic systems and four main landing gear posts. Safe landing would be possible if two main gear legs were torn off provided they were on opposite sides of the fuselage. In the case of power failure in a light aircraft, an emergency extension system is always available. This may be a manually operated crank or pump, or a mechanical free-fall mechanism which disengages the uplocks and allows the landing gear to fall under gravity.

Shock absorbers

Aircraft landing gear includes wheels equipped with solid shock absorbers on light planes, and air/oil oleo struts on larger aircraft.

Large aircraft

As aircraft weights have increased more wheels have been added and runway thickness has increased to keep within the runway [|loading limit]. The Zeppelin-Staaken R.VI, a large German World War I long-range bomber of 1916, used eighteen wheels for its undercarriage, split between two wheels on its nose gear struts, and sixteen wheels on its main gear units—split into four side-by-side quartets each, two quartets of wheels per side—under each tandem engine nacelle, to support its loaded weight of almost.
Multiple "tandem wheels" on an aircraft—particularly for cargo aircraft, mounted to the fuselage lower sides as retractable main gear units on modern designs—were first seen during World War II, on the experimental German Arado Ar 232 cargo aircraft, which used a row of eleven "twinned" fixed wheel sets directly under the fuselage centerline to handle heavier loads while on the ground. Many of today's large cargo aircraft use this arrangement for their retractable main gear setups, usually mounted on the lower corners of the central fuselage structure.
The prototype Convair XB-36 had most of its weight on two main wheels, which needed runways at least thick. Production aircraft used two four-wheel bogies, allowing the aircraft to use any airfield suitable for a B-29.
A relatively light Lockheed JetStar business jet, with four wheels supporting, needed a thick flexible asphalt pavement. The Boeing 727-200 with four tires on two legs main landing gears required a thick pavement. The thickness rose to for a McDonnell Douglas DC-10-10 with supported on eight wheels on two legs. The heavier,, DC-10-30/40 were able to operate from the same thickness pavements with a third main leg for ten wheels, like the first Boeing 747-100, weighing on four legs and 16 wheels. The similar-weight Lockheed C-5, with 24 wheels, needs an pavement.
The twin-wheel unit on the fuselage centerline of the McDonnell Douglas DC-10-30/40 was retained on the MD-11 airliner and the same configuration was used on the initial Airbus A340-200/300, which evolved in a complete four-wheel undercarriage bogie for the heavier Airbus A340-500/-600. The up to Boeing 777 has twelve main wheels on two three-axles bogies, like the later Airbus A350.
The Airbus A380 has a four-wheel bogie under each wing with two sets of six-wheel bogies under the fuselage. The Antonov An-225, the largest cargo aircraft, had 4 wheels on the twin-strut nose gear units like the smaller Antonov An-124, and 28 main gear wheels.
The A321neo has a twin-wheel main gear inflated to 15.7 bar, while the A350-900 has a four-wheel main gear inflated to 17.1 bar.

STOL aircraft

STOL aircraft have a higher sink-rate requirement if a carrier-type, no-flare landing technique has to be adopted to reduce touchdown scatter. For example, the Saab 37 Viggen, with landing gear designed for a 5m/sec impact, could use a carrier-type landing and HUD to reduce its scatter from 300 m to 100m.
The de Havilland Canada DHC-4 Caribou used long-stroke legs to land from a steep approach with no float.