Aeronautics and astronautics – Aircraft power plants – Mounting
Reexamination Certificate
2000-04-20
2002-06-04
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft power plants
Mounting
C248S554000
Reexamination Certificate
active
06398161
ABSTRACT:
TECHNICAL FIELD
The invention relates to a fixing device by means of which an aircraft propulsion system comprising an engine and a pod is connected to a strut fixed to a structural element of the aircraft such as a fuselage or wing element.
The invention also relates to an attachment strut able to support an aircraft propulsion system through such a fixing device.
The fixing device and the strut according to the invention can be used on any type of aircraft. A preferred application relates to aircraft of modern design, whose engines are equipped with very large diameter fans.
PRIOR ART
On an aircraft, the strut constitutes the connecting interface between the propulsion system, including the engine and the pod, and the aircraft fuselage or wing. It permits the transmission to the aircraft structure of forces generated by the engine (structural function). It also ensures the passage of fuel, electricity (control and power), hydraulics and air between the propulsion system and the aircraft (system function). Apart from these two functions, the strut must respect different constraints such as the obtaining of maximum safety, with an aerodynamic drag, a weight and a cost which must be as low as possible.
To ensure the transmission of forces, the strut comprises a primary structure, having a frame, e.g. in box form. In this case, said frame comprises ribs and panels, as well as mounts through which the strut is connected on the one hand to the structure of the aircraft and on the other to the propulsion system.
The strut also comprises a secondary structure ensuring the segregation and retention of the systems, whilst supporting aerodynamic shrouds.
In order to be able to ensure the transmission of forces between the propulsion system and the aircraft structure, the mounts interposed between the strut and the propulsion system are at least partly anchored to the central casing. Therefore the strut penetrates the secondary flow duct formed between said central casing and the pod which surrounds it. To disturb to the very minimum the air flow in said secondary duct, the front part of the strut must be as narrow as possible.
As is very diagrammatically illustrated in
FIGS. 1A and 1B
of the attached drawings, there are at present two main types of devices for fixing a propulsion system
1
to a not shown strut fixed to a structural element of an aircraft.
A first type of known fixing device, illustrated in
FIG. 1A
, is generally known as a “core fixture”. This fixture is characterized by the use of a front mount
3
and a rear mount
4
, directly linking the strut to the central casing
5
. The front mount
3
links the strut to a front part of the central casing
5
, located just to the rear of the fan stator case
6
. Said front part of the central casing
5
mainly constitutes the casing of the high pressure compressor of the engine. The rear mount
4
is interposed between the strut and the rear of the central casing
5
.
To facilitate understanding, an orthonormal fix OXYZ is allocated to the propulsion system
1
and in it the longitudinal axis OX coincides with the longitudinal axis of the propulsion system
1
and is oriented towards the front. The lateral axis OY is perpendicular to the OX axis and to the median plane of the strut (said latter plane being vertical or perpendicular to the lower surface of the wing when the engine is suspended on said wing in the manner shown). Finally, the OZ axis is perpendicular to the OX and OY axes, i.e. vertical in the embodiment shown. The OZ axis is oriented from the engine to the strut, i.e. upwards. In the case of an engine attached laterally to the fuselage of an aircraft, the OY axis would be oriented downwards and the OZ axis in a substantially horizontal plane. However, the OY and OZ axes will be respectively called the “lateral axis” and “vertical axis” throughout the text.
In a fixture of the “core” type, as illustrated in
FIG. 1A
, the front mount
3
ensures the transmission of forces exerted between the central casing
5
of the engine and the strut in longitudinal X, lateral Y and vertical Z directions (in the case represented of a propulsion system beneath the wing) relative to the propulsion system
1
.
For its part, the rear mount
4
ensures the transmission of forces exerted between the central casing
5
of the engine and the strut in lateral Y and vertical Z directions, as well as the transmission of the moment M
X
in accordance with the longitudinal axis OX.
In the second type of conventional fixture illustrated in FIG.
1
B and generally known as the “hybrid fan fixture”, the link between the propulsion system
1
and the strut is also ensured by a front mount
3
′ and a rear mount
4
(cf. also EP-A-741 074 and EP-A-805 108).
The front mount
3
′ is interposed between the strut and the fan stator case
6
of the propulsion system
1
. It ensures the transmission of forces in the lateral direction Y and vertical direction Z with respect to the propulsion system
1
.
As in the core-type fixture, the rear mount
4
is interposed between the strut and the rear part of the central casing
5
. This rear mount
4
ensures the transmission of forces exerted between the central casing
5
of the engine and the strut in the lateral direction Y and vertical direction Z with respect to the propulsion system
1
, as well as the transmission of moments M
X
in the longitudinal axis OX. Moreover, two rods
7
linking the rear mount
4
to the front part of the central casing
5
enable the rear mount
4
to also transmit forces exerted between the central casing
5
of the engine and the strut in the longitudinal direction X.
To make aircraft engines more economic, aircraft manufacturers attempt to increase their bypass ratio. This leads to increasing the diameter of the fan, which is generally located at the front of the propulsion system. However, this engine size increase leads to numerous problems associated with existing fixture devices.
Thus, when using a core-type fixing device, as illustrated in
FIG. 1A
, the diameter difference between the fan stator case and the central casing of the engine increases engine bending phenomena, which are particularly sensitive with this fixture type. Particularly under certain flight conditions and in particular on take-off, the aerodynamic support on the air intake, transmitted on the front of the engine fan, gives rise to a significant engine bending between its two mounts
3
and
4
. To prevent the rubbing of the rotary fan blades on the fan stator case
6
and the rubbing of the rotary compressor and turbine blades on the central casing of the engine, it is consequently necessary to provide a clearance between the end of the different blades and the corresponding casings. These clearances increase in size with the rise in the bypass ratio of the engines. Under certain flight conditions and in particular in the cruising phase, the engine resumes its normal bending state. Thus, the clearance at the end of the blades increases with the bypass ratio, so that the overall engine efficiency is reduced.
When the link between the propulsion system and the strut is ensured by a hybrid fan-type fixing device, as illustrated in
FIG. 1B
, the increase in the diameter of the fan increases aircraft resonance problems, which are particularly sensitive with this fixture type. Thus, this fixture is characterized by the fact that the system constituted by the strut and the engine behaves like a pendulum including a weight (the engine) hung on a wing by a spring (the strut). Under certain flight conditions, the wing excites the thus designed pendulum. To solve this problem, it is not acceptable to increase the hung weight. It is therefore necessary to increase the stiffness of the strut by increasing the thickness of certain of the components forming it. This phenomenon also exists on using core-type fixing devices, but the hybrid fan-type fixture is more prejudicial because it requires a greater weight increase for obtaining the same strut stiffness increase. This problem is accentu
Jule Pascal
Levert Stephane
Porte Alain
Aerospatiale Airbus
Barefoot Galen L.
Burns Doane , Swecker, Mathis LLP
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