Raisable undercarriage, in particular for a heavy transport aircraft

The invention relates to a raisable undercarriage comprising a hinged leg and associated bracing means. According to the invention, the undercarriage is of the leg-shortening type, and the bracing means are organized in the form of a foldable brace having its bottom arm hinged to bottom end of the sliding rod of the hinged leg, said brace further being organized so that, in the undercarriage down position, the lowered leg slopes towards the front of the aircraft.

FIELD OF THE INVENTION 
The invention relates to raisable undercarriages for heavy transport 
aircraft, such an undercarriage being of the type comprising a hinged leg 
and bracing means associated with the hinged leg, the hinged leg being 
composed of a shock-absorbing main strut having a hinge axis about which 
it is hinged to the structure of an aircraft, and of a shock-absorbing rod 
at the end of which a longitudinal beam is hinged that is equipped with a 
plurality of pairs of wheels. 
BACKGROUND OF THE INVENTION 
An undercarriage of this type includes at least four wheels, and in general 
includes six or eight wheels for a heavy transport aircraft, i.e. an 
aircraft weighing considerably more than 400 tons. The leg of such an 
undercarriage is naturally dimensioned accordingly, so that the recess for 
receiving the undercarriage when it is in the up position has to be long, 
and the structure of the aircraft must therefore be reinforced. 
Reference may be made for example to Documents EP-A-0 492 685, EP-A-0 492 
687, and EP-A-0 492 688. 
Furthermore, the undercarriage leg and the associated bracing means are 
disposed such that considerable cutting forces are exerted on the 
shock-absorbing rod by the set of wheels when they bear against the 
ground, so that the leg has to be dimensioned accordingly to enable the 
sliding rod to slide normally under any circumstances. 
Finally, the presence of a hinged longitudinal beam equipped with pairs of 
wheels (forming what may be referred to as a rocking bogie) makes it 
necessary to have a special linkage if, in the undercarriage up position, 
the beam is to be angularly positioned so as to be compatible with the 
desired compactness of the recess for receiving the undercarriage. To this 
end, in order to have a minimal recess height, it has been proposed to 
provide a linkage for turning the longitudinal beam over through 
180.degree. before retracting the main landing gear assembly in question. 
The technological background of the invention is also illustrated by 
Documents EP-A-0 361 549, U.S. Pat. No. 2,754,072, and FR-A-1 232 741. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the invention is to provide a raisable undercarriage that does 
not suffer from the above-mentioned drawbacks and/or limitations. 
An object of the invention is therefore to provide a raisable undercarriage 
having a structure whereby the recess for receiving it when it is in the 
up position has a minimal volume, and the longitudinal and lateral forces 
produced when contact is made with the ground are taken up optimally both 
in the undercarriage and in the aircraft structure. 
More particularly, the invention provides a raisable undercarriage 
comprising a hinged leg and bracing means associated with the hinged leg, 
the hinged leg being composed of a shock-absorbing main strut having a 
hinge about which it is hinged to the structure of an aircraft, and of a 
shock-absorbing rod at the end of which a longitudinal beam is hinged that 
is equipped with a plurality of pairs of wheels, wherein: 
the undercarriage is of the leg-shortening type, having a shock absorber 
composed mainly of a sliding rod and a plunging rod, and also of a hinged 
system connecting the plunging rod to the shock-absorbing main strut, the 
hinged system being controlled by a link fixed to the aircraft structure 
so as to pull the shock absorber up when the leg is being raised, and so 
as to push the shock absorber down when the leg is being lowered; and 
the bracing means are organized in the form of a foldable brace having a 
top arm hinged to the aircraft structure and a bottom arm hinged to bottom 
end of the sliding rod, said brace further being organized so that, in the 
undercarriage down position, the lowered leg slopes towards the front of 
the aircraft. 
Preferably, the hinge axis about which the shock-absorbing main strut is 
hinged to the aircraft structure is perpendicular to the longitudinal 
midplane of the aircraft, and the foldable brace is disposed in front of 
the leg so that the undercarriage is raised longitudinally and forwards. 
It is also advantageous for the leg to include a torque linkage having a 
top arm hinged to the shock-absorbing main strut, and a bottom arm hinged 
to the longitudinal beam which is hinged at the end of the sliding rod. 
Advantageously, a pitch damper is then interposed between the torque 
linkage and the shock-absorbing main strut, which pitch damper is hinged 
for example on a projection on the top arm of the torque linkage. Also 
preferably, the torque linkage and the pitch damper are disposed in front 
of the leg. 
It is further advantageous for the hinged system associated with shortening 
the leg to include a lever hinged at the top of the shock-absorbing main 
strut, which lever comprises a long arm forming an alignment with an arm 
hinged on the plunging shock-absorbing rod, and a short arm hinged at the 
end of a threshold resilient link which is fixed at its other end to the 
aircraft structure. 
Also preferably, the retraction stroke of the shock absorber and the two 
arms of the torque linkage are organized such that, in the undercarriage 
up position, the longitudinal beam is automatically in a substantially 
horizontal position. This then enables the recess for the undercarriage to 
be organized in a low-height space in the fuselage. 
Also advantageously, the two arms of the foldable brace are organized such 
that, in the undercarriage down position and with the shock absorber 
extended, the direction of the alignment formed by the brace is 
substantially orthogonal to the axis of the leg. Such a layout not only 
enables the cutting forces applied to the sliding rod to be minimized, 
thereby making it possible to lighten the structure of the sliding rod 
significantly because the moments are taken up by the brace and the torque 
linkage, but also enables the pivoting motion of the leg to be minimized. 
Finally, in a particular embodiment, the foldable brace is a collapsible 
brace provided with its own unlocking system, and the foldable brace is 
further organized such that the direction of the alignment that it defines 
in the undercarriage down position converges with the axis of the sliding 
rod as close as possible to the hinge axis of the longitudinal beam.

MORE DETAILED DESCRIPTION 
The figures show a raisable undercarriage 10 of the invention, the 
undercarriage comprising a hinged leg 100 and bracing means 140 associated 
with the hinged leg, the hinged leg being composed of a shock-absorbing 
main strut 101 having a hinge axis 102 about which it is hinged to the 
structure of an aircraft, and of a shock-absorbing rod 103 at the end of 
which a longitudinal beam 110 is hinged (the term "longitudinal" signifies 
that the long direction of the beam is parallel to the vertical 
longitudinal midplane of the aircraft), the longitudinal beam being 
equipped with a plurality of pairs of wheels 120. 
In accordance with a characteristic of the invention, the undercarriage 10 
is of the leg-shortening type, having a shock absorber composed mainly of 
a sliding rod 103 and of a plunging rod 104, and a hinged system 130 which 
connects the plunging rod to the shock-absorbing main strut 101, the 
system being controlled by a threshold resilient link 133 fixed at 126 to 
the aircraft structure, so as to pull the shock absorber up when the leg 
100 is being raised, and so as to push the shock absorber down when said 
leg is being lowered. 
In this example, the plunging rod 104 slides in a bearing 105 which is 
provided at the top of the main strut 101, (in a variant, the bearing 
could be provided on the plunging rod 104). The top end of the plunging 
rod is connected at 135 to a hinged system 130. In practice, the sliding 
rod has an intermediate partition delimiting a bottom hydraulic fluid 
chamber which communicates via a membrane with a top hydraulic fluid 
chamber inside the plunging rod. Furthermore, the plunging rod 104 has a 
bottom end provided with holes acting as a membrane, and a rod which is 
secured to the end of the sliding rod 103 passes through the center of 
said bottom end. 
The hinged system 130 makes it possible to pull the shock absorber up 
without compressing it when the plunging rod 104 is pulled up (towards the 
hinge axis 102 of the leg) during raising of the undercarriage leg 100, or 
to lengthen said leg when the plunging rod 104 is pushed down by the 
hinged system (during lowering of said undercarriage leg). In this case, 
the hinged system 130 for shortening the leg includes a hinged lever 132 
which is hinged at the top of the shock-absorbing main strut 101, and 
which comprises a long arm 132.1 which forms an alignment with an arm 131 
that is hinged at 135 on the plunging shock-absorbing rod 104, and a short 
arm 132.2 hinged at the end of a threshold resilient link 133 which is 
fixed via its other end 126 to the aircraft structure. 
If the undercarriage up position and the undercarriage down position shown 
in FIG. 1 are compared, it can be noted that, in the undercarriage up 
position, the hinged leg 100 is shortened, thereby making it easier to 
have a minimal-length recess L for receiving the raisable undercarriage 10 
when it is in the up position. A drive actuator 125 hinged to the aircraft 
structure about an axis 126, and hinged to the main strut 101 of the 
hinged leg 100 via a projection 108, raises and lowers the hinged leg, and 
as a result automatically shortens the leg on raising it, and lengthens 
the leg on lowering it. 
The longitudinal beam 110 is pivotally mounted to pivot about an axis 111 
at the end of the sliding rod 103 via a projection 112 provided at the 
center of the beam. End projections 113 are provided at the ends of the 
longitudinal beam 110, the various projections being associated with the 
axles 121 (best seen in FIG. 2) of the pairs of wheels 120 of the 
undercarriage. In this example, the longitudinal beam 110 is equipped with 
three pairs of wheels, but naturally the invention also applies to a leg 
equipped with two pairs of wheels (like conventional rocking bogies), or 
with four pairs of wheels. 
In accordance with another characteristic of the invention, the bracing 
means associated with the hinged leg 100 are in the form of a foldable 
brace 140 having a top arm 141 hinged at 142 on the aircraft structure, 
and a bottom arm 143 hinged at 144 (in this example via a projection 107) 
to the bottom end of the sliding rod 103, and the brace 140 is further 
organized such that, in the undercarriage down position, the lowered leg 
100 slopes towards the front of the aircraft (the direction of the front 
of the aircraft is represented by arrow 200 in FIG. 1). As described 
below, this layout enables the longitudinal and lateral forces to be taken 
up well on contact with the ground. 
In this example, the hinge axis 102 of the shock-absorbing main strut 101 
is perpendicular to the longitudinal midplane of the aircraft, and the 
foldable brace 140 is disposed in front of the leg 100, so that the 
undercarriage is raised longitudinally and forwards. In this way, by a 
combination of the pivoting motion of the shock-absorbing main strut 101 
and of the retraction of the leg shock-absorber, the set of wheels 120 is 
raised almost vertically into the associated recess L. Moreover, it is 
advantageous to provide forward raising insofar as the aerodynamic forces 
have a positive effect on the undercarriage leg when it is being lowered. 
The undercarriage leg 100 further includes a torque linkage 115 having a 
top arm 117 hinged (via a projection 106) to the shock-absorbing main 
strut 101, and a bottom arm 116 hinged to the longitudinal beam 110, via a 
projection 114 thereon. This layout of the torque linkage differs 
substantially from conventional layouts in which the bottom arm of the 
torque linkage is hinged on the shock-absorbing rod. 
Such a layout is particularly advantageous when the retraction stroke of 
the shock absorber and the geometrical shape of the torque linkage 115 are 
suitably chosen. By means of a suitable choice, it is possible to ensure 
that, in the undercarriage up position, the longitudinal beam 110 is 
automatically in a substantially horizontal position. In this way, without 
it being necessary to provide any additional linkage, raising the 
undercarriage leg 100 enables the rocking bogie to be brought back 
automatically into a horizontal position when said leg is fully raised. 
Such a layout naturally facilitates obtaining a recess L of minimal 
height. 
It is also advantageous to provide a pitch damper 119 interposed between 
the torque linkage 115 and the shock-absorbing main strut 101. In this 
example, the body of the pitch damper 119 is hinged at the above-mentioned 
projection 108 on the main strut 101, and the rod of said damper is hinged 
on a projection 118 on the top arm 117 of the torque linkage 115. In this 
example, the torque linkage 115 and the pitch damper 119 are disposed in 
front of the leg 100, so as to ensure that the undercarriage behaves well 
when the rear pair of wheels make contact with the ground S in the sloping 
landing position. Such a layout is described in detail in Document 
FR-A-2,616,410 filed by the Applicant, and that document can be referred 
to in order to gain a better understanding of the resulting advantages. 
Also with a view to optimizing the behavior of the undercarriage, it is 
advantageous to choose the dimensions of the two arms 141, 143 of the 
foldable brace 140 such that, when the undercarriage is in the down 
position and the shock absorber is extended, the direction of the 
alignment formed by the brace 140 is substantially orthogonal to the axis 
X of the undercarriage leg 100, as shown in FIG. 1, while ensuring that 
the direction of alignment converges with the axis X as close as possible 
to the hinge axis 111 of the longitudinal beam 110. 
Such a layout firstly enables the cutting forces exerted on the sliding rod 
103 by the set of wheels when the wheels bear against the ground to be 
eliminated, so that the structure of the sliding rod may be considerably 
lightened, insofar as the moments are taken up by the foldable brace 140. 
Moreover, this layout also minimizes the pivoting motion of the 
undercarriage leg when the sliding rod of the leg penetrates into the main 
strut. 
In this example, the foldable brace 140 is a collapsible brace which is 
provided with its own unlocking system 145. Such an unlocking system is 
well known, and it is therefore not necessary to describe the principle of 
it in detail. Suffice to say that its essential components comprise, in 
succession, a link 146 hinged at the end of the top arm 141 of the brace, 
and a lever 147 which is hinged on the link and at the end of the other 
arm 143, on which lever the rod of an unlocking actuator 148 is hinged, 
the body of the unlocking actuator being hinged on a projection 149 on the 
bottom arm 143. 
So as not to clutter the figure, the undercarriage up position shown in 
FIG. 1 only gives the essential members of the undercarriage leg, with 
dot-dash lines being used to represent the pitch damper 119, the arms 116 
and 117 of the torque linkage, the arms 141 and 143 of the foldable brace, 
as well as the link 146, the lever 147, and the actuator 148 of the 
unlocking system associated with said foldable brace. 
An undercarriage is thus provided having a structure enabling the recess 
for receiving the undercarriage when it is in the up position to be of 
optimum compactness, while also making it possible to take up the 
longitudinal and lateral forces produced when the undercarriage in the 
down position makes contact with the ground. 
The invention is not limited to the above-described embodiment. On the 
contrary, the invention covers any variant which uses equivalent means to 
reproduce the essential characteristics described above.