Landing Gear Of An Aircraft

Landing Gear Of An Aircraft

The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land. Typically wheels are used, but skids, skis, floats or a combination of these and other elements can be deployed, depending on the surface. Landing gear usually includes wheels equipped with shock absorbers for solid ground, but some aircraft are equipped with skis for snow or floats for water, and/or skids or pontoons (helicopters).The undercarriage is a relatively heavy part of the vehicle, it can be as much as 7% of the takeoff weight, but more typically is 4-5%.Wheeled undercarriages normally come in two types: conventional or "taildragger" undercarriage, where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear; or tricycle undercarriage where there are two main wheels (or wheel assemblies) under the wings and a third smaller wheel in the nose. 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 unstable, that is, a small deviation from straight-line travel is naturally amplified by the greater drag of the mainwheel which has moved farther away from the plane's centre of gravity due to the deviation), and usually require special pilot training. Sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case of tail strikes during take-off. The Concorde, for instance, had a retractable tail "bumper" wheel, as delta winged aircraft need a high angle when taking off. The Boeing 727 also has a retractable tail bumper. Some aircraft with retractable conventional landing gear have a fixed tailwheel, which generates minimal drag (since most of the airflow past the tailwheel has been blanketed by the fuselage) and even improves yaw stability in some cases.

Retractable gear
To decrease drag in flight some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors; this is called retractable gear.If the wheels rest protruding and partially exposed to the air stream after being retracted, the system is called semi-retractable.Most retraction systems are hydraulically operated, though some are electrically operated or even manually operated. This adds weight and complexity to the design. In retractable gear systems, the compartment where the wheels are stowed are called wheel wells, which may also diminish valuable cargo or fuel space.A design for retractable landing gear was first seen in 1876 in plans for an amphibious monoplane designed by Frenchmen Alphonse Pénaud and Paul Gauchot. Aircraft with at least partially retractable landing gear did not appear until 1917, and it was not until the late 1920s and early 1930s that such aircraft became common, with Grover Loening's military aircraft designs being among the first routinely using them for the main undercarriage members, in a system later licensed and used by his friend Leroy Grumman's aviation firm. By then, aircraft performance was improved to the point where the aerodynamic advantage of a retractable undercarriage justified the added complexity, weight and interior space penalties. An alternate method of reducing the aerodynamic penalty imposed by fixed undercarriage is to attach aerodynamic fairings (often called "spats" or "pants") on the undercarriage, with only the bottoms of the wheels exposed, as with the Junkers Ju 87 Stuka.

Pilots confirming that their landing gear is down and locked refer to "three green" or "three in the green.", a reference to the electrical indicator lights from the nosewheel and the two main gears. Red lights indicate the gear is in the up-locked position; amber lights indicate that the landing gear is in transit (neither down and locked nor fully retracted).
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. 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 high-performance aircraft may even feature a pressurized-nitrogen back-up system.