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 center 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 had a retractable tail bumper. Some aircraft with retractable conventional landing gear have a fixed tailwheel, which generate minimal drag (since most of the airflow past the tailwheel has been blanketed by the fuselage) and even improve yaw stability in some cases.
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.
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. By then, aircraft performance was improved to the point where the aerodynamic advantage of a retractable undercarriage justified the added complexity and weight. 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.
Pilots confirming that their landing gear is down and locked refer to "three green" or "three in the green.", a reference to electrical indicator lights from the nosewheel and the two main gears. Amber lights indicate the gears are in the up-locked position; red lights indicates that the landing gear is in transit (neither down and locked nor fully retracted).
As aircraft grow larger, they employ more wheels to cope with the increasing weights. The Airbus A340-500/-600 has an additional four-wheel undercarriage bogie on the fuselage centreline. The Boeing 747 has five sets of wheels: a nose-wheel assembly and four sets of four-wheel bogies. A set is located under each wing, and two inner sets are located in the fuselage, a little rearward of the outer bogies.
Unusual types of gear
Rarely, planes use wheels only for take off and drop them afterwards to gain the improved streamlining without the complexity, weight and space requirements of a retraction mechanism. In this case, landing is achieved on skids or similar simple devices. Historical examples include the Messerschmitt Me 163 and the Messerschmitt Me 321. A related contemporary example are the wingtip support wheels ("Pogos") on the U-2 reconnaissance aircraft, which fall away after take-off and drop to earth; the aircraft then relies on titanium skids on the wingtips for landing.
Some main gear struts on World War II aircraft, in order to allow a single-leg main gear to more efficiently store the wheel within either the wing or an engine nacelle, rotated the single gear strut through a 90º angle to allow the main wheel to rest "flat", or flush with the wing, when fully retracted. Examples are the Curtiss P-40, Vought F4U Corsair, Messerschmitt Me 210 and Junkers Ju 88,The Aero Commander family of twin-engined business aircraft also shares this feature on the main gears, which retract aft into the ends of the engine nacelles. The nosewheel on the Cessna Skymaster is similarly rotated 90 degrees as it retracts forward.
An unusual undercarriage configuration is found on the Hawker Siddeley Harrier, which has two mainwheels in line astern under the fuselage (called a bicycle or tandem layout) and a smaller wheel near the tip of each wing. On second generation Harriers, the wing is extended past the outrigger wheels to allow greater wing-mounted munition loads to be carried.
A multiple tandem layout was used on some military jet aircraft during the 1950s such as the Lockheed U-2, Myasishchev M-4, Yakovlev Yak-25, Yak-28 and the B-47 Stratojet because it allows room for a large internal bay between the main wheels. A variation of the multi tandem layout is also used on the B-52 Stratofortress which has four main wheel bogies (two forward and two aft) underneath the fuselage and a small outrigger wheel supporting each wing-tip. The B-52's landing gear is also unique in that all four pairs of main wheels can be steered. This allows the landing gear to line up with the runway and thus makes crosswind landings easier (using a technique called crab landing). The challenge of designing a tandem-gear layout is that the aircraft has to sit (on the ground) at the optimum flight angle for landing - when the plane is nearly in a stalled attitude just before touchdown, both fore and aft wheels must be ready to contact the runway. Otherwise there will be a vicious jolt as the higher wheel falls to the runway at the stall.
For light aircraft a type of landing gear which is economical to produce is a simple wooden arch laminated from ash, as used on some homebuilt aircraft. A similar arched gear is often formed from spring steel. The Cessna Airmaster was among the first aircraft to use spring steel landing gear. The main advantage of such gear is that no other shock-absorbing device is needed; the deflecting leaf provides the shock absorption.
There are several types of steering. Taildragger aircraft may be steered by rudder alone (depending upon the prop wash produced by the aircraft to turn it) with a freely-pivoting tail wheel, or by a steering linkage with the tail wheel, or by differential braking (the use of independent brakes on opposite sides of the aircraft to turn the aircraft by slowing one side more sharply than the other). Aircraft with tricycle landing gear usually have a steering linkage with the nose wheel (especially in large aircraft), but some allow the nose wheel to pivot freely and use differential braking and/or the rudder to steer the aircraft.
Some aircraft require that the pilot steer by using rudder pedals; others allow steering with the yoke or control stick. Some allow both. Still others have a separate control, called a tiller, used for steering on the ground exclusively.
When an aircraft is steered on the ground exclusively using the rudder, turning the plane requires that a substantial airflow be moving past the rudder, which can be generated either by the forward motion of the aircraft or by thrust provided by the engines. Rudder steering requires considerable practice to use effectively. Although it requires air movement, it has the advantage of being independent of the landing gear, which makes it useful for aircraft equipped with fixed floats or skis.
Some aircraft link the yoke, control stick, or rudder directly to the wheel used for steering. Manipulating these controls turns the steering wheel (the nose wheel for tricycle landing gear, and the tail wheel for taildraggers). The connection may be a firm one in which any movement of the controls turns the steering wheel (and vice versa), or it may be a soft one in which a spring-like mechanism twists the steering wheel but does not force it to turn. The former provide positive steering but make it easier to skid the steering wheel; the latter provide softer steering (making it easy to overcontrol) but reduce the probability of skidding the wheel used for steering. Aircraft with retractable gear may disable the steering mechanism wholly or partially when the gear is retracted.
Differential braking depends on asymmetric application of the brakes on the main gear wheels to turn the aircraft. For this, the aircraft must be equipped with separate controls for the right and left brakes (usually on the rudder pedals). The nose or tail wheel usually is not equipped with brakes. Differential braking requires considerable skill. In aircraft with several methods of steering that include differential braking, differential braking may be avoided because of the wear it puts on the braking mechanisms. Differential braking has the advantage of being largely independent of any movement or skidding of the nose or tail wheel.
A tiller in an aircraft is a small wheel or lever, sometimes accessible to one pilot and sometimes duplicated for both pilots, that controls the steering of the aircraft while it is on the ground. The tiller may be designed to work in combination with other controls such as the rudder or yoke. In large airliners, for example, the tiller is often used as the sole means of steering during taxi, and then the rudder is used to steer during take-off and landing, so that both aerodynamic control surfaces and the landing gear can be controlled simultaneously when the aircraft is moving at aerodynamic rates of speed.
Landing gear and accidents
Malfunctions or human errors related to retractable landing gear have been the cause of numerous accidents and incidents throughout aviation history. Distraction and preoccupation during the landing sequence played a prominent role in the approximately 100 gear-up landing incidents that occurred each year in the United States between 1998 and 2003. A gear-up landing incident is an accident that may result from the pilot simply forgetting, or failing, to lower the landing gear before landing or a mechanical malfunction that does not allow the landing gear to be lowered. Although rarely fatal, a gear-up landing is very expensive, as it causes massive airframe damage, and almost always requires a complete rebuild of engines, due to the propellers striking the ground and suffering a sudden stoppage if they were running on impact. Many aircraft between the wars - at the time when retractable gear was becoming commonplace - were deliberately designed to allow the bottom of the wheels to protrude below the fuselage even when retracted to reduce the damage caused if the pilot forgot to extend the landing gear or in case the plane was shot down and forced to crash-land. Examples include the Avro Anson and the Douglas DC-3. The contemporary Fairchild-Republic A-10 Thunderbolt II is similarly designed in an effort to avoid (further) damage during a gear-up landing, a possible consequence of battle damage.
Some aircraft have stiffened structure on the fuselage bottom, designed to prevent structural damage in a wheels-up landing. When the Cessna Skymaster was converted for a military spotting role (the O-2 Skymaster), fiberglass railings were added to the length of the fuselage; they were adequate to support the aircraft without damage if it was landed on a grassy surface.
On September 21, 2005, JetBlue Airways Flight 292 successfully landed with its nose gear turned 90 degrees sideways, resulting in a shower of sparks and flame after touchdown. This type of incident is very uncommon as the nose oleo struts are designed with centering cams to hold the nosewheels straight until the weight of the aircraft compresses it.
Automatic extension systems
The Piper Arrow was originally fitted with a system that automatically extended the landing gear when certain power and flap settings were selected. The manufacturer issued an Airworthiness Directive for owners to disable this system. Pilots were found to be relying on this system to extend the gear in routine flight operations, rather than just as an emergency backup. If the gear failed to extend then the manufacturer was exposed to liability for the resulting gear-up landing. There were also concerns over unintentional gear extension incidents where pilots placed the aircraft in "bad-weather" (low-power setting, flaps down) configuration and inadvertently activated the gear extension system.
Emergency extension systems
In the event of a failure of the aircraft's landing gear extension mechanism a back-up is provided. This may be an alternate hydraulic system, a hand-crank, compressed air (nitrogen), pyrotechnic or a free-fall system.
A free-fall or gravity drop system uses gravity to deploy the landing gear into the down and locked position. To accomplish this the pilot activates a switch or mechanical handle in the cockpit, which releases the up-lock. Gravity then pulls the landing gear down and deploys it. Once in position the landing gear is mechanically locked and safe to use and land on.
Unauthorized passengers have been known to stowaway on larger aircraft by climbing a landing gear strut and riding within the compartment. There are extreme dangers to this practice, including:
Published - July 2009
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