【正文】 This is also mon to large military transport aircraft like the C5 and C17(Figure 23), which use up every inch of interior space and need long flat floors inside so that tanks and other equipment can be rolled inside.[10]Figure 23 C17 transport aircraft Different types of gear Tricycle gearThis type of landing gear is the most mon type of landing gear. It often used on the mercial aircraft. We can see it’s structured like a tricycle(Figure 24), with one nose wheel in the front and two wheels located midway on the fuselage. The nose gear is steerable by using the rudder pedals easily than other types of landing gear. This type of landing gear is advantageous in many ways: it allows the pilot to apply the brakes more forcefully without making the plane nose over, offers better stability because the rear wheels are close to the center of gravity and permits better visibility for the pilot during takeoff, landing and taxiing. The majority of modern aircraft are fitted with tricycle landing gear. The following modeling of CATIA is this type of landing gear.Figure 24 Tricycle gear TaildraggerThis type of landing gear is often used on many small pistonengine planes(Figure 25). It consists of two main gear units located near the center of gravity and a tail unit located at the rear of the fuselage. Because the plane appears to drag its tail onto the runway upon landing, this type of landing gear is also known as 39。taildragger gear.39。s center of gravity is located behind the main gear, it may be more difficult to have directional control. For this reason, specific training is required for using aircraft equipped with tailwheel landing gear.Figure 25 “Taildragger” landing gearAnother conventional type named 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。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 takeoff. 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 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. Unusual types of gearSome aircraft use wheels for takeoff and then jettison them soon afterwards for improved aerodynamic streamlining without the plexity, weight and space requirements of a retraction mechanism. In these cases, the wheels to be jettisoned are sometimes mounted onto axles that are part of a separate dolly(Figure 26) (for main wheels only) or trolley (for a three wheel set with a nosewheel) chassis. Landing is then acplished on skids or similar other simple devices. Historical examples include the dollyusing Messerschmitt Me 163 rocket fighter, the Messerschmitt Me 321 Gigant troop glider, and the first eight trolleyusing prototypes of the Arado Ar 234 jet reconnaissance bomber. The main disadvantage to using the takeoff dolly/trolley and landing skid(s) system on German World War II aircraft, was that aircraft would likely be scattered all over a military airfield after they had landed from a mission, and would be unable to taxi on their own to an appropriately hidden dispersal location, which could easily leave them vulnerable to being shot up by attacking Allied fighters. A related contemporary example are the wingtip support wheels (Pogos) on the Lockheed U2(Figure 27) reconnaissance aircraft, which fall away after takeoff and drop to earth。 angle during the rearwardsretraction sequence to allow the main wheel to rest flat above the lower end of the main gear strut, or flush within the wing, when fully retracted. Examples are the Curtiss P40, Vought F4U Corsair, Grumman F6F Hellcat, Messerschmitt Me 210 and Junkers Ju 88. The Aero Commander family of twinengined business aircraft also shares this feature on the main gears, which retract aft into the ends of the engine nacelles. The rearwardretracting nosewheel strut on the Heinkel He 219 and the forwardretracting nose gear strut on the later Cessna Skymaster similarly rotated 90 degrees as they retracted.On most World War II singleengined fighter aircraft (and even one German heavy bomber design) with sideways retracting main gear, the main gear that retracted into the wings was meant to be raked forward, towards the aircraft39。s attachment point to the airframe this led to a plex geometry for setting up the angles for the retraction mechanism39。s famous Zero fighter, whose main gear stayed at a perpendicular angle to the centreline of the aircraft when extended, as seen from the side.Figure 28 A Royal Air Force P47A multiple tandem layout was used on some military jet aircraft during the 1950s, pioneered by the Martin XB51, and later used on such aircraft as the U2, Yakovlev Yak25, Yak28 and the B47 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 B52 Stratofortress which has four main wheel bogies (two forward and two aft) underneath the fuselage and a small outrigger wheel supporting each wingtip. The B5239。riot XI Channelcrossing aircraft of 1909 and also copied in the earliest examples of the Etrich Taube. In this arrangement the main landing gear39。s suspension fork, allowing the main gear to pivot on moderate crosswind landings. Landing gear and accidentsMalfunctions 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 prom