Raisable landing gear having a shortenable leg

The invention relates to raisable landing gear having a shortenable leg, including a shock absorber fitted with a plunger rod, and a linkage connecting said plunger rod to the strut of the shock absorber, under the control of a resilient connecting rod having a threshold, serving to pull on the shock absorber when the leg is raised. According to the invention, the linkage includes two arms forming an alignment, with a first arm hinged on the plunger rod and having a lateral appendix whose free end is capable of co-operating with a stationary cam secured to the structure of the airplane, and a second arm which is hinged to the strut. The lateral appendix and the stationary cam are organized to operate in an emergency, in the event of the threshold connecting rod failing, thereby ensuring that the shock absorber is extended and that said lengthened shock absorber is locked in the undercarriage-down position.

The present invention relates to aircraft landing gear, and more 
particularly to landing gear of the type having a shortenable leg. 
BACKGROUND OF THE INVENTION 
Shortenable raisable landing gear is known that includes a leg hinged to 
the structure of an airplane, the leg being constituted by a strut housing 
a shock absorbing mainly composed by a sliding rod and a shock absorber 
plunger rod, together with a linkage connecting said plunger rod to the 
strut and under the control of a resilient connecting rod having a 
threshold and fixed to the airplane structure so as to pull the shock 
absorber while the leg is being raised and so as to push said shock 
absorber while said leg is being lowered. 
That disposition makes it possible to obtain landing gear which is long in 
the undercarriage-down position and short in the undercarriage-up position 
(thus making it easier to integrate the landing gear inside the associated 
housing in the airplane). 
Another possible approach for obtaining the same result consists in using 
extensible landing gear of a structure that is capable, while the airplane 
is stationary on the ground, of exerting a force equivalent to the static 
load thereof, thereby raising the strut of the shock absorber relative to 
the sliding rod. 
The first approach, which is the approach adopted by the present invention, 
consists in using landing gear of the above-specified type having a 
shortenable leg, in which the linkage is organized to lengthen the leg 
while the landing gear is being lowered, before the wheels touch the 
ground: lengthening is then intrinsically controlled by the structure 
without the pilot being required to trigger any kind of mechanism, in 
contrast to extensible landing gear. 
FIGS. 1 and 2 show prior art landing gear having a shortenable leg, and 
complying with the above definition. 
The axial section of FIG. 1 thus shows raisable landing gear 100 comprising 
a leg 101 hinged about an axis 102 to the structure of an airplane, and 
including a strut 103 and a sliding rod 104 fitted with a shock-absorbing 
plunger rod 105 disposed coaxially about the axis X of the strut. The 
bottom of the strut 103 has forks 106 hinged to two actuators 107 for 
steering the wheels, the rods of said actuators being connected to a 
rotary sleeve 109 coaxial with the strut. The bottom of the sliding rod 
104 carries a set of wheels (represented solely by an axle 109), and it is 
connected to the strut 103 by a scissors linkage having two arms 110 and 
111. The sliding rod 104 also includes a transverse wall constituting an 
intermediate partition that defines a bottom hydraulic fluid chamber 113 
which communicates in turn via a diaphragm with a top hydraulic chamber 
114 inside the plunger rod 105. The bottom of the plunger rod 105 has an 
end 115 fitted with holes 116 (to perform the above-mentioned diaphragm 
function), and has a rod 117 secured to the partition 112 of the sliding 
rod 104 passing through the center thereof. A system 118 of corresponding 
helical cams secured respectively to the sliding rod 104 and to the 
plunger rod 105 provide recentering and wedging in a particular relative 
angular disposition. 
A linkage 130 serves to pull the shock absorber without compressing it when 
the plunger rod 105 is pulled (towards the hinge axis of the leg) while 
the landing gear leg is being raised, or to lengthen the leg while the 
plunger rod 105 is pushed back by said linkage (while the landing gear leg 
is being lowered) as represented by the position shown in FIG. 1. The 
plunger rod 105 slides in a bearing 119 formed in the upper portion of the 
strut 103, and its top end 120 is connected to the linkage 130. 
The linkage 130 includes an arm 121 hinged at its bottom end to the plunger 
rod 105, and at its top end to one end of a lever 122 which is itself 
hinged at 123 to a pin carried by gusset plates 124 on the strut. The 
other end of the lever 122 is hinged to a resilient connecting rod having 
a threshold (a "spring rod") 125 which is in turn hinged to the structure 
126 of the airplane. The locked position of the linkage 130 is provided by 
the resilient connecting rod 125 having a threshold keeping the two 
alignment arms in an abutment position (by means of a lug 127 associated 
with the arm 121) which is slightly out of alignment, being slightly 
beyond dead center from the side that closes up when the undercarriage is 
raised. 
FIG. 2 is on a larger scale and shows the top portion of the 
above-mentioned landing gear 100, and in particular it shows said linkage, 
using continuous lines to show the position that corresponds to the leg 
being lengthened and the undercarriage down, and chain-dotted lines to 
show the position corresponding to the leg being shortened with the 
undercarriage up, with shortening being intrinsic to the linkage. The 
lengthening or shortening stroke obtained is referenced c. 
Such landing gear gives full satisfaction under normal operating 
conditions. However, if the resilient connecting rod having a threshold 
should fail, then normal operation is affected. 
Thus, if the connecting rod breaks while the landing gear is in its raised 
position or while it is being lowered, then the lowering of the landing 
gear is affected insofar as the leg does not lengthen normally. In 
addition, in the undercarriage-down position, there is no longer any 
positive locking of the alignment in the linkage, and said alignment can 
thus collapse on impact when the wheels touch the ground, which would 
cause the landing gear to collapse vertically. 
OBJECT AND SUMMARY OF THE INVENTION 
The invention seeks specifically to solve that problem by designing landing 
gear having a shortenable leg that is capable of operating safely even in 
the event of a failure in the resilient connecting rod having a threshold. 
The invention thus seeks to provide landing gear having a shortenable leg, 
in which the structure is such as to avoid the above-mentioned drawbacks, 
and in particular is capable of coping with the circumstance of the 
resilient connecting rod having a threshold being broken. 
More particularly, the present invention provides a raisable landing gear 
having a shortenable leg, including a leg hinged to the structure of an 
airplane, the leg being constituted by a strut housing a shock absorber 
mainly constituted by a sliding rod and by a shock absorber plunger rod, 
together with a linkage connecting said plunger rod to the strut and under 
the control of a resilient connecting rod having a threshold and fixed to 
the structure of the airplane, in such a manner as to pull the shock 
absorber while the leg is being raised and to push said shock absorber 
while said leg is being lowered, wherein the linkage includes two arms 
forming an alignment, with a first arm hinged on the plunger rod of the 
shock absorber and having a lateral appendix whose free end is capable of 
co-operating with a stationary cam integral with the structure of the 
airplane, and a second arm hinged to the strut, together with a lever 
constrained to rotate with said second arm and connected to the threshold 
connecting rod, said lateral appendix and said stationary cam being 
organized to act in an emergency to ensure that the shock absorber is 
extended and that said shock absorber is locked in the undercarriage-down 
position. 
In a particular embodiment, the lateral appendix of the first arm is 
disposed in the vicinity of the hinge between the two arms, and the 
lateral appendix extends in the alignment plane formed by the two arms, in 
a direction substantially perpendicular to the general direction of the 
first arm. 
Advantageously, the free end of the lateral appendix of the first arm is 
provided with a wheel that runs over the stationary cam during emergency 
operation. 
In another particular embodiment, the stationary cam includes, in 
succession, a first portion serving to ensure that the shock absorber 
begins to be extended, and a second portion forming a positive abutment 
for locking said shock absorber in the undercarriage-down position on 
contact being made with the ground. In particular, the first portion of 
the stationary cam may be generally concave in shape, while the second 
portion of said cam is generally convex. 
According to another possible feature, the second arm and the lever which 
is coupled thereto are disposed on opposite sides of a strut appendix 
associated with their hinge on said strut. In particular, the second arm 
and the associated lever may both extend from the same side of their hinge 
axis.

MORE DETAILED DESCRIPTION 
FIGS. 3 and 4 show shortenable-leg landing gear 200 of the invention in the 
undercarriage-down position, with the figures being restricted to the upper 
portion of said landing gear, it being understood that the lower portion 
thereof may be identical or analogous to the lower portion of the landing 
gear 100 described above with reference to FIGS. 1 and 2. 
FIGS. 3 and 4 show a leg 201 hinged about an axis 202 to the structure S of 
an airplane, and constituted by a strut 203 housing a shock absorber mainly 
comprising a sliding rod (not shown in these figures) and a shock absorbing 
plunger rod 205. The plunger rod 205 slides in a bearing 219 secured to the 
strut 203, and in this case said bearing is installed in the strut by means 
of fastening pins 219.1, as can be seen in FIG. 5. The shock absorbing 
strut 203 is upwardly extended by arms 203.1 which terminate at hinges 
associated with the hinge axis 202 of the landing gear leg, using 
conventional structures. 
In accordance with an essential aspect of the invention, the landing gear 
200 includes a linkage 230 constituted by two arms 240 and 250 that form 
an alignment, comprising a first arm 240 hinged on the plunger rod of the 
shock absorber 205 and having a lateral appendix 245 whose free end 246 is 
suitable for co-operating with a stationary cam 260 secured to the airplane 
structure S, and a second arm 250 hinged on the strut 203, together with a 
lever 251 constrained to rotate with the second arm 250 and connected to a 
threshold connecting rod 225, the lateral appendix 245 and the stationary 
cam 260 being organized to provide safety in an emergency by ensuring that 
the shock absorber extends and is locked in the extended position when the 
undercarriage is down, as explained below in greater detail with reference 
to FIGS. 5 to 8. 
The first arm 240 includes an alignment arm 241 which is hinged at its 
bottom end at 242 to the plunger rod 205, being held in this case between 
two appendices 243 on said plunger rod. This alignment arm 241 is hinged 
at 244 with the second arm 250, the top end of said alignment arm 241 
being received, in this case, between two bottom appendices 255 of said 
second arm 250. The second arm 250 of the alignment is hinged at 223 on 
two associated appendices 224 of the strut 203. 
The second arm 250 and the lever 251 coupled thereto are disposed, in this 
case, on either side of one of the appendices 224 of the strut 203, 
extending from the same side of their hinge 223, which configuration is 
naturally not essential, but nevertheless makes it possible to obtain a 
structure that is particularly compact (it would also be possible to use 
an organization analogous to that of the prior art landing gear shown in 
FIGS. 1 and 2, in which case the second arm 250 and the lever 251 would 
extend from opposite sides of the hinge 223). The free end of the lever 
251 is terminated by two branches 252 that receive one of the ends 253 of 
the threshold connecting rod 225. The other end of said resilient 
connecting rod 225 having a threshold is hinged at 226 to the airplane 
structure. It should be observed that the threshold rod 225 is represented 
merely by chain-dotted lines in FIG. 5, and that for reasons of clarity 
FIGS. 6 to 8 which show the three positions of interest of said 
shortenable leg landing gear of the invention do not show said threshold 
connecting rod and the associated lever. 
The lateral appendix 254 of the first arm 240 is disposed in this case in 
the vicinity of the hinge 244 between the two arms 240 and 250 of the 
alignment, but naturally, that is merely one particular embodiment. More 
precisely, the lateral appendix 245 extends in the plane of the alignment 
formed by the two arms 240 and 250 (which plane P is more clearly visible 
in FIG. 4), and it extends in a direction substantially perpendicular to 
the general direction of the first arm 240, as can be seen more clearly in 
FIGS. 5 to 8. The above-mentioned appendices 255 determine the direction of 
the lateral appendix in the plane P so that the end of said appendix is 
always properly positioned relative to the stationary cam 260. The first 
arm 240 is thus in the form of a crank lever, having one branch 
constituting a portion of the alignment of the leg-shortening system. The 
lateral appendix 245 is preferably terminated at its free end by two 
branches 245.1 between which a wheel 246 is disposed and is capable of 
running over the stationary cam 260 during an emergency operation, as 
described below. 
There follows a description of the way in which the landing gear 200 
operates, given with reference to FIGS. 5 to 8, initially how it operates 
normally, and subsequently how it operates in an emergency in the event of 
the resilient connecting rod having a threshold failing, which connecting 
rod is preferably designed to have two thresholds so as to operate both in 
traction and in compression. 
FIG. 5 shows the shortenable landing gear 200 in three positions of 
interest namely: a position referenced I which corresponds to the 
undercarriage being up (shown in continuous lines); a position referenced 
III corresponding to the undercarriage being down (also in continuous 
lines); and an intermediate position referenced II (in chain-dotted 
lines). The undercarriage-down position also includes a chain-dotted 
illustration of the first arm to show how locking is obtained in the event 
of the resilient connecting rod having a threshold being defective. 
FIGS. 6 to 8 show the three above-mentioned positions of interest I, II, 
and III separately, while omitting the lever coupled to the second arm and 
the threshold connecting rod. 
In position I, the landing gear is raised and housed in a housing L. The 
shock absorber then has maximum pull applied to its plunger rod 205, and 
the linkage formed by the alignment arm 241 and the second arm 250 is 
completely closed. In this position, the wheel 246 of the lateral appendix 
245 is disposed in the vicinity of the cam 260, but it does not come into 
contact therewith. 
During normal lowering of the landing gear, as represented by arrow 300 in 
FIG. 7, the landing gear moves to position II and the leg begins to 
lengthen under drive from the linkage 230 on the plunger rod 205. In 
normal operation, the resilient connecting rod having a threshold 225 thus 
makes it possible to deploy the linkage 230 correctly for the purpose of 
progressively lengthening the leg of the landing gear. It should also be 
observed that the wheel 246 of the lateral appendix 245 then continues to 
move past the surface of the cam 260 without coming into contact 
therewith. 
When the landing gear continues to move down as shown by arrow 301 in FIG. 
8, it finally reaches the undercarriage-down position referenced III. The 
arms 241 and 250 are then substantially in line with each other, forming 
an alignment (which may be slightly out of alignment, having gone slightly 
beyond dead center away from the raising side, as in the prior art landing 
gear, with an associated lug being provided, e.g. on alignment arm 241). 
The wheel 246 on the lateral appendix 245 then comes level with another 
portion of the surface of the cam 260 while still not coming into contact 
therewith. 
When the landing gear is raised, still under normal circumstances, it 
passes from position III to position I, passing through intermediate 
position II, with the linkage 230 deforming in the same manner as during 
lowering, thereby progressively shortening the leg until it is shortened 
as much as possible when the undercarriage is up. 
The above operation naturally assumes that the resilient connecting rod 225 
having a threshold performs its function normally. In the event of said 
threshold connecting rod failing, the landing gear equipment of the 
invention provides emergency operation serving firstly to enable the shock 
absorber to be extended and secondly to enable said shock absorber to be 
locked in the undercarriage-down position. 
In the event of the resilient connecting rod 225 having a threshold 
breaking in the undercarriage-up position, when the actuator associated 
with the landing gear begins to extend the landing gear, the wheel 246 is 
brought immediately into contact with the stationary cam 260, and more 
precisely into contact with a first portion 260.1 thereof. As a result, 
the landing gear passes in normal manner from position I to position II, 
with the wheel 246 running over said portion 260.1 of the cam, thereby 
making it possible to ensure that the proper trajectory is followed until 
the wheels have come out from the housing L (also known as the wheel-bay). 
The thrust between the wheel and the cam surface thus serves to develop the 
linkage correctly with very little departure from the normal trajectory. 
The corresponding angle occupies about 20.degree.. After this position, 
the wheel 246 runs over another portion 260.2 of the surface of the cam 
260, continuing to develop the linkage, i.e. continuing to cause the shock 
absorber to be extended. On reaching position II, which corresponds to an 
angle of about 45.degree., the mass of the shock absorber becomes a 
driving force and the wheel 246 leaves the cam 260, and this continues 
until the position III is reached, in which the hinges 242, 244, and 243 
are in alignment. 
The co-operation between the lateral appendix 245 and the portions 260.1 
and 260.2 of the stationary cam 260 thus make it possible to ensure that 
the shock absorber is extended normally even in the event of failure of 
the resilient connecting rod 225 having a threshold. 
When the landing gear is in the undercarriage-down position, and when the 
wheels make contact with the ground, a compression force is applied to the 
alignment 241, 250. The unstable equilibrium of this alignment could be 
destroyed by the hinge 244 moving towards the wall 203.1 of the strut 203 
(assuming that no abutment lug had been provided on one of the arms 
involved), in which case (with or without an abutment lug) locking is 
obtained naturally. In contrast, if the hinge 244 moves the other way, 
then the wheel 246 comes into contact with a portion 260.3 of the cam 
surface 260, thus forming a positive abutment that locks the shock 
absorber after it has been pushed in very slightly, thereby making it 
possible to retain substantially all of the available stroke for absorbing 
energy. It is thus possible in complete safety to lock the shock absorber 
of the lengthened leg in the undercarriage-down position. 
The stationary cam 260 thus comprises in succession: a first portion 260.1, 
260.2 that serves to ensure that the shock absorber begins to be extended, 
and a second portion 260.3 that forms a positive abutment to ensure that 
said shock absorber when compressed in the undercarriage-down position is 
locked when the landing gear comes into contact with the ground. In this 
case, the first portion is generally concave in shape, being constituted, 
for example, by two rectilinear lengths 260.1 and 260.2 that run into each 
other via a rounded portion, while the second portion 260.3 is generally 
convex in shape, e.g. being circular or angular. The exact shape of the 
surface of the cam 260 will be determined, in practice, as a function of 
the desired trajectory, so as to obtain a trajectory that is as close as 
possible to that of the normal trajectory, even in the event of a failure 
of the resilient connecting rod having a threshold. It should be observed 
that in the event of excessive friction preventing the driving mass of the 
shock absorber being capable of extending the wheels of the landing gear, 
the cam follower wheel remains in contact with the cam which makes it 
possible to achieve intermediate position II with full guidance. This thus 
ensures that the landing gear is nevertheless extended. 
The invention is not limited to the embodiment described above, but on the 
contrary extends to any variant that uses equivalent means to reproduce 
the essential characteristics specified above. 
In particular, the landing gear having a shortenable leg is applicable to 
all types of landing gear, and in particular both nose wheel landing gear 
and main landing gear.