881,718. Aircraft undercarriages. FAIREY CO. Ltd. Dec. 9, 1957 [Dec. 7, 1956]. No 37546/56. Class 4. [Also in Group XXXII] A retractable undercarriage for an aircraft includes a contractible telescopic leg which is arranged to contract under axial load in two successive stages, in each of which it exerts simultaneously two components of resistance to contraction, namely a hydraulic damping component produced by forcing a liquid through a restricted orifice and varying as a function of the velocity of contraction, and a resilient component dependent on the extent of contraction, and in which the rate of increase of the resilient component per unit increment of contraction is small throughout the first stage and is much greater throughout the second stage, the undercarriage, also including means for contracting the leg through the first stage only and for retracting the undercarriage with the leg so contracted. The leg 10 comprises two telescoping members 11 and 12, the inner 11 carrying wheel axle 13 and the outer 12 being retractably attached toaircraft structure. Member 11 is tubular and sleeved at 15 to provide larger and smaller cylinders 14 and 16 to co-operate respectively with a piston 20, secured to member 12 by a hollow rod 21, and a piston 28 secured to a hollow rod 29 which extends upwardly through a seal 30 in piston 20. Pistons 20 and 28 are separated by a partition 32, sealed at 34 for rod 29 and the cylinder 14, the space 16A in cylinder 16 between piston 28 and partition 32, the interior of piston rod 29 and the lower part of piston rod 21 are all filled with liquid, the upper part of rod 21 providing a sealed air space 23. The upper end of rod 29 carries a shoulder 36 resting on piston 20 to limit downward movement of piston 28 relative to piston 20 and above this shoulder the rod 29 has lateral openings 37 controlled by a ported sleeve valve 38 urged into contact with piston 20 by a spring 39. The piston rod 29 carries a cap 40 having a valve seating 41 which co-operates with a valve 42 normally closed by spring 43, but opened when the pressure in piston rod 29 exceeds that in chamber 23 by a predetermined value. The rod 29 has longitudinal calibrated grooves (not shown) providing orifices between cylinders 16 and 14 the size of the orifices being gradually reduced upwardly so that as the partition 32 rises relatively to piston 28 the size of the orifices is gradually reduced so as to increase the restriction and maintain the pressure difference as the rate of contraction slows down. The lower end of liner 15 carries a block 44 to engage piston 28 when the leg has contracted through a certain proportion of its total stroke necessary for shortening the undercarriage during retraction and the longitudinal grooves in rod 29 terminate in wider recesses 45 so positioned that partition 32 will reach them as stop 44 engages piston 28 to provide comparatively unrestricted communication between cylinders 14 and 16A, at the end of the first stage in the contraction of the leg and thereafter. Partition 32 is formed with a ring of axially-directed holes 46 closed by an annular flap valve 47 which permits flow of liquid when the leg is extending and a similar ring of holes 48 extend through piston 20 to co-operate with an annular valve 49, both valves 47 and 49 having small holes to provide a restricted bypassage. When the leg starts to be contracted liquid is forced from larger cylinder 14 to smaller cylinder 16A through the grooves in rod 29 and at the same time, due to difference in diameter between cylinders 14 and 16A a residual quantity of liquid is forced out through the hollow piston rod 29 and the lateral openings 37 in to chamber 23. At the rate of contraction that would occur during landing a majority of the energy would be dissipated in liquid friction whilst only a small proportion of energy will be stored in air chamber 23, sufficient to extend the leg when the undercarriage is lowered in flight. When stop 44 engages piston 28 recess 45 in piston rod 29 will be brought opposite partition 32, thus allowing comparatively free flow from cylinder 14 to cylinder 16A and through diagonal ports to interior of rod 29. Further, in this position piston rod 29 is raised relative to piston 20 and spring loaded valve 38 so that the valve closes openings 37. This means that the whole of the liquid ejected from cylinder 14 by continued contraction of the leg must now pass into chamber 23 through the interior of rod 29, and through valve 42 when the pressure within the rod is sufficient to overcome spring 43. During the first stage of contraction the resistance offered by liquid transfer by rapid compression, as in absorption of landing shocks will be high, though the resistance will be low due to the low rate of contraction necessary to shorten the leg in the air prior to retraction of the undercarriage. During the second state of contraction, i.e. when liquid is transferred to chamber 23 increasing pressure of the air in the chamber 23 will continue to provide increased resistance to landing shocks and will prevent the leg from bottoming.