FLUID DISCHARGE VIBRATION DAMPING STRIPS FOR ACOUSTIC PftOTFCTlON OF
AIRCRAFT TURBOMACHINE FAN CASING
DESCRIPTION
5
TECHNICAL FIELD
The invention relates to the mounting of an acoustic protection inside
an aircraft turbomachine fan casing. More particularly, the invention is concerned with
the vibration damping strips provided between the fan casing and the acoustic panels.
10 The invention is applicable to any type of aircraft equipped with a fan
casing, in particular turbojet engines.
STATE OF PRIOR ART
On current turbojet'engines, acoustic protection panels are generally
15 assembled on the internal surface of the fan casing, about the engine axis, The assembly
is usually performed by screwing panels to the casing.
However, risks of failure of some fixing screws of the acoustic panels
have been identified. In case of failure, the screw heads released in the jet can produce
high damage to the fan vanes, after ingestion. Among the main causes identified, there
20 are the strong vibratory stresses induced by the coincidence between a fundamental
"mode of the acoustic panel and an excitation of the fan vanes. These strong stresses
h
transmitted to the screws cause the loosening thereof, and are likely to cause strong
wear, even possibly up to failure, as discussed above.
To cope with this problem, it has been proposed to introduce vibration
25 damping strips in order to decrease the vibratory levels. These strips, usually made of
elastomeric material, have the advantage of significantly damping the vibratory levels
measured in panel testing, when these strips ar« compressed.
However, the selected material may be sensitive to the contact with
liquids such as oil and kerosene, when dipped for a Eong period. This prolonged contact
30 with such liquids can indeed cause a risk of degradation of the materia! and/or an
S51404AP-P
2
efficiency loss of the damping achieved. But, in the upstream zone of the fan casing
where there are the acoustic panels, oil or kerosene Hows arc actually possible, in case of
leak.
Normally, these fluids shouid flow down to the low part of the fan
5 casing, to be then discharged by a drain located at 6 o'clock at the casing,
3n view of the orientation of the strips in the direction of the
longitudinal axis of the engine, a retention of these liquids is then possible on some edges
of the strips, which is likely to induce a detrimental prolonged contact,
To cope with this drawback, different alternatives have been proposed,
10 such as making transverse grooves in the strips to discharge liquids.
However, the presence of these grooves can embrittle the strips, in
addition to decrease the contact area with the casing, which results in a reduced
b
efficiency. On the other hand, the presence of these grooves can also decrease the
stiffness of the strips when they are compressed, which is also likely to impact their
15 * damping ability. Finally, with this solution, liquid retention zones remain on the edges,
between the grooves. Consequently, this solution is not fully satisfactory.
DISCLOSURE OF THE INVENTION
The purpose of the invention is thus to overcome at least partially the
20 above mentioned drawbacks, relative to the systems of prior art,
To do so, first, one object of the invention is to provide art acoustic
protection device for a fan casing of an aircraft turbo ma chine, comprising an acoustic
protection panel forming a ferrule sector centered on a center axis intended to be the
same as the longitudinal axis of the fan casing, said device also comprising one or more
25 vibration damping strips pressed on one side to the external surface of the acoustic
protection panel and for being pressed on the other side to an internal surface of the fan
casing, each damping strip having two opposltes edges each having an upstream end and
a downstream end spaced from each other along the direction of the center axis.
S51404AP-P
3
According to the invention, at least one of the two opposite edges of at
[east one of the damping strips is shaped such that liquid present in this edge can flow by
gravity towards either or both of its upstream and downstream ends.
The invention thus provides a simple and high performance solution
5 capable of discharging liquids towards either or both ends of the edges likely to be wetted
by such liquids, as oil or kerosene.
- After this discharge off the edges, allowed by a simple gravity
phenomenon, the Fluids can fEow onto the fan casing down to the low part of the same, or
at which they are preferentially-discharged by a drain located at 6 or clock on the casing.
10 The damping strips are therefore non longer likely to be degraded by
"the prolonged retention of liquids on their edges, which enables a high performance
damping function to be preserved for a long time,
Preferably, the arrangement specific to the present invention is
provided on each edge of each strip likely to collect fluid dripping by gravity on the lower
15 surface of the casing, that is generally a single edge per band, this being oriented
upwards,
Preferably, at Eeast one of both opposite edges of at least one of the
damping strips has at least one portion tilted with respect to the center axis, such that
liquid present on this portion can flow by gravity, towards either or both of its upstream
20 and downstream ends. The tilted portion is preferentially planar, but can alternatively be
curved, Each" edge concerned can include here only a single tilted portion, preferably
making up the entire edge.
Preferentially, at Eeast one of the two opposed edges of at least one of
the damping strips consists of two successive portions in the direction of the center axis,
25 each of both portions being tilted along the center axis such that liquid present on each of
both portions can flow by gravity towards the upstream end and the downstream end
respectively. This enables the liquid to be distributed towards both upstream and
downstream ends of the edge concerned, and a possible liquid over How at one of its
ends, and a spilling into the j e t !n order to evenly distribute the f l ow onto both ends of
30 the strip, the two tilted portions, arranged as downwardly open V, can be symmetrical,
S51404AP-P
4
in this regard, the angle formed between both edge portions is
preferably between 1 and 15°, and more preferably between 1 and 3", With these ranges
of values, a compromise was reached, enabling both a good liquid f i ow by gravity, and a
negligible decrease in the contact area of the strip with-its adjacent e!emerits.
5 As discussed above, both edge portions are at the same length, and
symmetrically arranged.
Preferably, at least one of the damping strips is shaped so as to be
mountabJe onto the acoustic panel at two reversed mounting positions, wherein the
positions of said two opposite edges are respectively reversed, and in that in eacn of both
10 mounting positions, at feast one the two opposite edges of said damping strip is shaped
such that liquid present on this edge can flow by gravity towards either a( both of its
upstream and downstream ends.
ThEs solution avoids the detrimental consequences on the strips in case
of a mounting mistake, since in both mounting directions, the edge Sikeiy to collect
15 liquids, that is the one oriented upwards, will be able to lead the liquids by gravity up to
either or both ends of this edge, to provide for discharge thereof.
In the same way, and still for the purpose of avoiding detrimental
consequences on Ihe bands in the case of a possible mounting mistake, at least one of
the damping strips has a first plan of symmetry arranged between the two opposite
20 edges. Preferably, all the strips have an identical shape, which still further simplifies the
mounting.
Still preferentially, at least one of the damping strips has a second plan
of symmetry passing through both opposite edges, said first and second plans of
symmetry being preferentially orthogonal between them, and also orthogonal to the strip
25 plan when the same has a substantially planar surface.
Finally, as discussed above, each damping strip Is preferably made of an
elasto merit material.
An other object of the invention is also to provide an aircraft
turbomachine front part comprising a fan casing the internal surface of which is coated
30 with at least one acoustic protection device such as described above.
S51404AP-P
5
Further advantages and characteristics of the invention will be more
apparent in the non limiting detaiEed description below.
BRIEF DESCRIPTION OF THE DRAWINGS
5 This description will be made with regard to the appended drawings
wherein:
- figure 1 shows a perspective view of an aircraft turbojet engine fan
casing, equipped with an acoustic protection device according to a preferred embodiment
of the invention;
10 - figure 2 represents a perspective view of the acoustic protection
device shown in the preceding figure, at an angle enabling its internal surface to be seen;
- figures 3 and 4 respectively represent cross section views taken aiong
lines Ett-lJ! and IV-IV of figure 2, respectively;
- figure 5 represents a perspective view of one of the vibration damping
15 strips equipping the acoustic'protection device shown in the preceding figures;
- figure 6 represents a side view of an acoustic protection device part
shown in the preceding figures; and
- figure 7 represents a view similar to that of the preceding view,
according to an alternative embodiment
20
DETAILED DESCRIPTION OF THE PREFERRED FMBOD3MENTS
First in reference to figure 1, is represented a fan casing 1 intended to
be an integral part of a front part of an aircraft turbojet engine, preferably of the two fJow
twin-spool type.
25 The casing assumes a conventional ferrule shape, with a longitudinal
. i
axis 2 corresponding to the longitudinal axis of the turbojet engine assembly.
At its upstream end, the internal surface la of the casing la is equipped
with a plurality of acoustic protection devices 4, only one of which has been represented
in figure 1.
S51404AP-P
6
This device 4 includes an acoustic protection panel 6 forming a ferrule
sector cornered on a center axis the same as the longitudinal axis 4. When at! the
protection devices are installed on the casing, their panels 6 located in the continuity of
each other along the circumferential direction form an internal acoustic protection ring
5 together,
The panel 6 is preferentially assembled by screwing on the casing, the
fixing screws a being mounted hidden in the panel so as not to disturb the streamline
flow of the jet.
The device 2 also comprises one or more vibration damping strips 10
10 pressed on one side lo the external surface 6a of the panel 6, and for being pressed on
the other side to the internal surface la of the casing 1.
The strips 10, spaced apart f r om each other along the circumferential
direction, are made of elastomoric material, They are generally oriented parallel to the
axis 2, that is they have their length parallel to this^axis, and their width orthogonal to the
15 same axis 2. in the preferred embodiment represented, each strip 10 has a tow width,
enabling It to preserve a substantially planar shape, but it could alternatively be curved,
without departing from the scope of the invention.
Figure 3 shows the pane! 6 assembled on the casing 1 by the screws S,
whereas figure 4 shows the interposition of one of the damping strips 10 between both
20 surfaces l a , 6a between which this strip is compressed in order to provide its vibration
damping function, " "
Figures 5 and 6 more precisely show one of the strips 10, as well as its
arrangement on the casing. In this regard, it is noted that to make the mounting easier, all
the strips 10 of the device 4 are provided to be identical.
25 Here, the strip 10 has a first plan of symmetry 12 orthogonal to the axis
2, and corresponding to a median plan passing between the two longest opposite edges,
called longitudinal edges 4. On the other hand, the strip 10 has aiso a second plan of
symmetry 16 orthogonal to the first plan 12, and corresponding to a median plan passing
between the shortest two opposite edges, called transverse edges 18. Both plans 12, 16
30 are orthogonal to the plan of the strip 10, the transverse edges IS of which are parallel to
S514G4AP-P
7
the first plan 12, Consequently, in a side view as shown in figure 6, the edges IB are
considered as vertical edges.
One of the features of the present invention resides in the shape of the
longitudinal edges 14, which are not oriented along ths direction of the longitudinal axis
5 2, itself also oriented horizontally in use, indeed, each longitudinal edge 14 extends
between an upstream end and a downstream end spaced from each other along the
direction of the center axis 2, by consisting of two successive portions 14a, 14b in the
direction of the same axis 2, tilted with respect to each other, and each substantially
planar. Both portions of the longitudinal edge 14a, 14b, of a substantially identical length,
10 form an angle preferably between 1 and 3°.
It is noted that the damping strip 10 is shaped so as to be mountahle to
the panel 6 at two reversed mounting positions, wherein the positions of both opposite
longitudinal edges 14 are respectively reversed, that is that in one of both positions, one
of the edges is usually oriented upwards, whereas it is oriented downwards in the other
15 position. Because of the symmetry of the strip 10 along ihe first plan 14, regardless of the
mounting direction adapted, the strip has a same configuration on the panel &. Therefore,
a mounting mistake can not happen, since in both mounting directions, the longitudinal
edge 14 likely to collect liquid ieaks, that is the one oriented upwards, will be able to lead
liquids by gravity up to both ends of this edge, to provide discharge thereof.
20 Indeed, by being tilted along the tenter axis 2 horizontally oriented in
'use, each of the two edge portions 14a", 14b can lead by gravity a collected liquid, for
i
example an oil and/or kerosene leak, to the edge end it integrates, Jn other words, once
the liquid flowing on the internal surface la of the casing is deposited onto the upwardly
oriented longitudinal edge 14, it flows by gravity to the upstream or downstream end of
25 this edge by being confined between the same edge, the external surface 6a of t he panel,
and the internal surface la of the casing. After reaching the transverse upstream or
downstream edge IB, the liquid can then f l ow between both surfaces l a , 6a down to the
low part of the blower casing, to be then discharged through a drain (not shown) located
at 6 o' clock at the casing 1,
p
s
•Each upwardly oriented edge 14 is related to a facet/rim enabling, by
virtue of its tilting(s) with respect to the horizontal direction, discharge of liquid leaks
remote from the damping strips 10.
Figure 7 shows an alternative to make the strips 10, each herein taking a
generally paraiielogram shape with each longitudinal edge 14 consisting of a single
portion tilted with respect tn the center axis 1, so as to form a slope dedicated to the
pouring of the liquid to one of the edge ends, herein the downstream end.
Of course, various alterations can be made by those skilled in the art to
the invention just described, only by way of non limiting examples,
5514G4AP-P
9
CLAIMS
1. An acoustic protection device (4} for a fan casing (1) of an aircraft
turbomachine, comprising an acoustic protection panel (6) forming a ferrule sector
5 centered on a center axis (2) for being the same as the longitudinal axis of the fan casing,
said device also comprising one or more vibration damping strips (10) pressed on one side
to the externa! surface (6a) of the acousltc protection panel and for being pressed on the
other side l o a n internal surface { l a ) , o r t h e f an casing, each damping strip (10) having two
opposites edges {14) each having an upstream end and a downstream end spaced from
10 each other along the direction of the center axis (2),
characterised in that at least one of the two opposite edges (14) of at
least one of the damping strips (10) is shaped such that liquid present in this edge can
flow by gravity toward cither of both of its upstream and downstream ends.
' 15 2. The device according to claim 1, characterised in that at ieast one of
these two opposite edges {14) of at least one of the damping strips (10) has at least one
portion {14,14a, 14b) tilted with respect to the center axis {2}, such that liquid present on
this portion can flow by gravity toward either or both of its upstream and downstream
ends,
20
3, The device according to claim 2, characterised in thaTat least one of
the two opposite edges {14) of at ieast one of the damping strips {10} consists of two
successive portions (14a, 14b) in the direction of the center axis {2), each of both portions
being tilted along the center axis (2) such that liquid present on each of both portions can
25 flow by gravity toward the upstream end and the downstream end respectively,
4, The device according to claim 3, characterised in that the angle
formed between the two edge portions {14a, 14b) is between 1 and 15°, and more
preferentially between 1 and 3°,
S51404AP-P -" • -'ifi.>
10
5. The device according to claim 3 or cEaim 4, characterised in that the
two edge portions (14a, 14b) are the same length.
6. The device according to any of the preceding claims, characterised in
5 that at least one of the damping strips (10) is shaped so as to be mountable on the
acoustic panel (10) at two reversed mounting positions, wherein the positions of said two
opposite 'edges (14) are respectively reversed, and in that in each of both mounting
positions, at least one of both opposite edges {14) of said damping strip (10) is shaped
such that liquid present on this edge can flow by gravity toward either of .both of its
10 upstream and downstream ends.
7. The device according to any of the preceding claims, characterised in
that at least one of the damping strips {10} has a first plan of symmetry (12) arranged
between the two opposite edges [14).
. - 1 5
8. The device according to any of the preceding-claims, characterised in
that at Eeast one of the damping strips (10) has a second plan of symmetry (16) passing
through both opposite edges {14),
20 9, The device according to any of the preceding claims, characterised in
that e~a"ch"darnping strip (10) is made of an efastomerlc material
10. An aircraft turbomachine front part comprising a fan casing[l) the
_ internal surface (la) of which is coated with at least one acoustic protection device (4)
25 according to any of the preceding claims.