【正文】 s shock absorption was taken up by a vertically sliding bungee cordsprung upper member. The vertical post along which the upper member slid to take landing shocks also had its lower end as the rotation point for the forward end of the main wheel39。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 tandemgear 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. Figure 29 B36 PeacemakerOne very early undercarriage arrangement that passively allowed for castoring during crosswind landings, unlike the active arrangement on the B52, was pioneered on the Bleriot VIII design of 1908. It was later used in the much more famous Bl233。s axis of rotation, with some aircraft, like the P47(Figure 28) Thunderbolt, even mandating that the main gear struts lengthen as they were extended down from the wings to assure proper ground clearance for its large fourbladed propeller. One exception to the need for this plexity in many WW II fighter aircraft was Japan39。s nose in the down position for better ground handling, with a retracted position that placed the main wheels at some angle behind the main gear39。 the aircraft then relies on titanium skids on the wingtips for landingFigure 26 Takeoff dollyFigure 27 Aircraft U2Some main gear struts on World War II aircraft, in order to allow a singleleg main gear to more efficiently store the wheel within either the wing or an engine nacelle, rotated the single gear strut through a 90176。 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 mon 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 straightline travel is naturally amplified by the greater drag of the mainwheel which has moved farther away from the plane39。 Because the plane39。39。s Kitten, built in 1917, had retractable gear (but could not actually fly). The Dayton Wright RB1(Figure 21) of 1920 and the Verville Sperry R3 of 1922 also had retractable gear. But these aircraft were rarities and most airplanes had fixed landing gear at the end of metal struts because they were easy to design, strong, and relatively lightweight.Figure 21 Aircraft RB1When designing an aircraft, engineers have always had to address five conflicting requirements. These are: performance, weight, cost, reliability, and maintenance. The best solution to the performance requirement was to pull the landing gear pletely inside the fuselage and cover them over, presenting a smooth surface that produced no drag. But while ideal from a performance standpoint, this approach affected all of the other requirements at weighed more, cost more, was less reliable, and required more maintenance. The wind tunnel data from the PRT caused aircraft designers to look immediately for ways of developing landing gear that provided better performance and hopefully did not have too many other costs associated with them.The Boeing Monomail, which first appeared in 1930, and the Lockheed Orion are generally considered pioneers in the development of retractable landing gear, proving that it was practical. But airplane designer Jack Northrop, who was very interested in streamlining aircraft to improve performance, produced the Northrop Alpha, Beta, and Gamma with fixed landing gear during the 1930s. These aircraft had streamlined coverings that extended down from the fuselage, with the wheels sticking out at the bottom. These were generally referred to as trouser gear. (Figure 22) The Lockheed Sirius, which was built for Charles Lindbergh, had streamlined coverings over the wheels themselves that were referred to as pantstype landing gear. Although trouser and pants gear produced more drag than fully retracted gear, they were still a substantial improvement over uncovered landing gear. More importantly, they were still lighter, cheaper, more reliable, and easier to maintain than the retractable gear. But during the 1930s, numerous designers embraced retractable gear hey were willing to accept the other drawbacks of retractable landing gear simply to achieve the better performance.Figure 22 Pantstype landing gearAirplane designers explored numerous ways to retract aircraft landing gear into the fuselage. In some aircraft they were pulled straight up, usually into cowlings behind the engines, sometimes with part of the wheels protruding outside the airplane. In other cases, particularly with smaller aircraft, the struts folded inward so that the wheels fit into the bottom of the fuselage horizontally, sometimes covered with a door to further reduce drag. Some of the drive mechanisms were electric, whereas others were hydraulic. The designers had to ensure that the gear deployed and locked in place. Landing gear that collapsed when a plane landed could cause the plane to tumble out of control and kill the pilot, or at least seriously damage the airplane. For early retractable landing gear not even the performance improvements were clearly achievable, since a retractable landing gear with its motors and associated machinery weighed more than a fixed gear, thereby requiring greater lift from the aircraft and negating some of the benefits of the reduced drag of the retracted gear. While retracting the gear could improve the