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 (British) = landing gear (US). 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 (tricycle gears) or a "2-1" nabla-shaped layout ("conventional" gears, also called tail wheel-type gears or "taildragger"), while other atypical configurations such as bicycle (two gears in tandem layout), quadracycle (four gears in a "2-2" rectangular layout) and other multicycle arrangements (usually a single front gear with three or more rear gears, in a kite, dart or pentagonal layout). Some other anomalous landing gears have also been evaluated experimentally, including: no landing gear (e.g. on flying boats), made possible by operating from a catapult cradle and flexible landing deck: air cushion (to enable operation over a wide range of ground obstacles and water/snow/ice); tracked (to reduce runway loading). 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 (part of Raytheon Technologies) and Héroux-Devtek.
Aircraft
The landing gear represents 2.5 to 5% of the maximum takeoff weight (MTOW) and 1.5 to 1.75% of the aircraft cost, but 20% of the airframe direct maintenance cost. A suitably designed wheel can support 30 t (66,000 lb), tolerate a ground speed of 300 km/h and roll a distance of 500,000 km (310,000 mi) ; 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 (or wheel assemblies) under the wings and a third smaller wheel in the nose. 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 (because the arrangement is usually unstable, that is, a small deviation from straight-line travel will tend to increase rather than correct itself), 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 (known as tandem or bicycle) has the main and nose gear located fore and aft of the center of gravity (CG) 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
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