Patent Description:
There is a desire to isolate sounds in one section of aircraft from other sections of the aircraft and/or the outside of the aircraft. This is particularly important for an aircraft nacelle, where noise generated by the engine and by airflow at the intake to the engine should be contained or attenuated in order to avoid excessive noise pollution.

Existing acoustic panels often involve a sound attenuating sheet positioned between two sheets, an inner and outer sheet, with one of these sheets having apertures to allow sound to enter the closed panel, but this arrangement often leads to difficulty in sealing the acoustic panel to adjacent panels, due to the abrasive nature of the sound attenuating sheets. Further, if a sound attenuating sheet having more than one layer is to be used, it may be beneficial to hold the multiple layers of the sound attenuating sheet together strongly.

<CIT> discloses a metal sandwich structure having a small bend radius.

The present invention may improve manufactuability and robustness.

A first aspect of the invention provides an acoustic panel for an aircraft according to claim <NUM>.

With such an arrangement, the arrangement of the connection of the first and second sheets allows the sound attenuating sheet to be held in compression and so the chance of separation of the sound attenuating sheet or of other damage to the sound attenuating sheet due to tensile stresses being generated therein is reduced. The first and second sheets may prevent expansion of the sound attenuating sheet in a thickness direction of the sound attenuating sheet. The first and second surfaces of the sound attenuating sheet may thus be prevented from moving further apart.

Further, by having a portion of a sheet extending over the edge of the sound attenuating sheet, there is the prospect of improved sealing between the acoustic panel and an adjacent panel, feature or component.

The sound attenuating sheet comprises two layers of sound attenuating sheets, each comprising a plurality of cavities, the two sound attenuating sheets being separated by a septum. By providing two separate layers of sound attenuating sheets, the acoustic panel can attenuate sound over a greater range of frequencies and may thereby provide improved sound insulation.

The sound attenuating sheets may be cellular sheets, such as honeycomb sheets, or in the case of a single sheet, the single sound attenuating sheet may be a cellular sheet such as a honeycomb sheet. By using a cellular sheet, the separate cells may act as Helmholtz resonators and thereby attenuate sound at particular frequencies.

The first sheet and the second sheet form a single continuous sheet. By using a single sheet to cover the portions of the first and second surfaces of the sound attenuating sheet and the end surface of the sound attenuating sheet, a uniform tensile force can be applied to the sheet so that the sound attenuating sheet has a uniform compressive stress. There may also be provided a simpler overall arrangement, improving use of manufacture.

The acoustic panel further comprises a third sheet, separate or discrete from the first and second sheets, extending over and abutting at least a portion of the first surface of the sound attenuating sheet and a portion of the first sheet, the portion of the first sheet extending over the at least a portion of the first surface of the sound attenuating sheet. By using a third sheet, the acoustic panel may be more easily manufactured and the single continuous sheet may have a C shape, into which the sound attenuating sheet may be slid and the third sheet can be overlaid on the sound attenuating sheet and the portion of the single continuous sheet extending over the first surface of the sound attenuating sheet. Overall, this may provide a more easily manufactured acoustic panel since the sound attenuating sheet may be more easily slid into place.

In an example not forming part of the invention, first sheet and the second sheet may overlap on the end surface of the sound attenuating sheet. This arrangement provides an alternative possible manufacturing method, in which the first and second sheets may be applied to the sound attenuating sheet at the same time, meaning that the sound attenuating sheet does not need to be slid over a sheet.

The first and/or second sheets may comprise an inspection hole through which the end surface of the sound attenuating sheet is visible. The provision of an inspection hole can allow a person manufacturing the acoustic panel to ensure that the sound attenuating sheet is correctly placed and may allow inspection of the sound attenuating sheet for detecting damage.

The acoustic panel may further comprise a seal or other close-out feature attached to the first and/or second sheet where the respective sheet extends over the end surface of the sound attenuating sheet, the seal being arranged to abut an adjacent panel. By attaching the seal directly to the sheet extending over the end surface, the convenient and non-abrasive qualities of the sheet at an end surface may be fully utilized, as opposed to attaching a seal to an adjacent panel.

The panel is curved, such that the first surface is a radially outer surface and the second surface is a radially inner surface, the end surface having an annular or arcuate shape. This may improve the suitability of an acoustic panel for use on an aircraft nacelle or fuselage, by following the natural shape of those parts.

The second sheet may be air permeable at a location where it extends over the second surface. This may improve the function of the sound attenuating sheet, in particular when there are open cells within the sound attenuating sheet arranged to act as Helmholtz resonators.

The end surface of the sheet may be substantially orthogonal to the first and second surfaces. This may allow easier fitting of the sheet to existing adjacent panels. The panel may be suitable for use in an air inlet cowling of an aircraft nacelle or a different aircraft structure where noise attenuation is required, such as a fuselage. The first, second and third sheets may be composite sheets or may be metallic, for example aluminium.

According to a second aspect of the invention, there is provided an air inlet cowling for an aircraft nacelle, comprising an acoustic panel according to the first aspect of the invention.

Such an air inlet cowling may have improved sound insulation and may have acoustic panels less susceptible to wear or damage.

The end surface of the sound attenuating sheet may be arranged at an aft end of the inlet cowling. This can allow the acoustic panel to be fitted within existing shapes of aircraft nacelles.

The air inlet cowling may further comprise a seal arranged to abut the acoustic panel at an aft end of the panel.

<FIG> shows an aircraft nacelle <NUM>, the nacelle exterior being formed of three primary parts: an air intake <NUM>, a fan casing <NUM> and a thrust reverser <NUM>. The purpose of the air intake <NUM>, which may be referred to as an inlet cowling, is to direct airflow into the fan and into the engine and to create an aerosmooth surface for airflow over the exterior of the aircraft nacelle <NUM>. The airflow into the inlet cowling <NUM> generates a significant amount of noise, and so the inlet cowling <NUM> should function to attenuate the noise and prevent excessive noise pollution outside of the aircraft.

<FIG> shows a cross section of an embodiment of an acoustic panel <NUM> for use within an inlet cowling. The acoustic panel <NUM> has a sound attenuating sheet <NUM>, which may be a honeycomb sheet or a different low density material, such as a cellular foam or sponge. The sound attenuating sheet <NUM> has three surfaces shown in <FIG>, a top, first surface 102a, a bottom, second surface 102b, and an end surface 102c.

A composite material layer <NUM> (see <FIG>), which may be referred to also as a sheet, extends over a portion of the first surface 102a, a portion of the second surface 102b, and over the end surface 102c. The sheet <NUM> therefore forms a C-shape, and covers at least a portion of all of the three above listed surfaces of the sound attenuating sheet <NUM>.

The sheet <NUM> can be considered as comprising three portions, a first portion 104a extending over at least a portion of the first surface 102a, a second portion 104b extending over at least a portion of the second surface 102b, and a third portion 104c extending over the end surface 102c. Each respective portion 104a, 104b, 104c of the sheet <NUM> may lie over and abut the respective surface 102a, 102b, 102c of the sound attenuating sheet <NUM>. The sheet <NUM> may be attached to the sound attenuating sheet <NUM> along the entirety of its length by an adhesive or may be attached over only a portion of its length.

A further sheet <NUM> extends over the top, first surface of the sound attenuating sheet <NUM> and overlaps at least a portion of the first portion 104a of the sheet <NUM>. The further sheet <NUM> may be referred to as a stepped or joggled sheet. Within the context of a nacelle, the surface of the acoustic panel <NUM> formed of the first portion 104a of sheet <NUM> and of the further sheet <NUM> is a radially outer surface and a second surface of the acoustic panel <NUM> opposite the first surface, the second surface of the acoustic panel <NUM> being formed of the second portion 104b of the sheet <NUM>, is a radially inner surface.

The second portion 104b of the sheet <NUM>, or at least a portion of the second portion 104b, may comprise perforations or other air holes <NUM> for allowing fluid communication between the inside of the nacelle and the sound attenuating sheet <NUM>. In the case where the sound attenuating sheet is formed from a honeycomb material, the honeycomb cells may be aligned such that they are open to the second surface of the acoustic panel <NUM>. The air holes <NUM> may allow the honeycomb cells to function as Helmholtz resonators.

The acoustic panel <NUM> also comprises a seal <NUM> adhered to the sheet <NUM> where the sheet overlaps the end surface 102c of the sound attenuating sheet <NUM>. The seal may optionally be a P-shaped seal but may be a different shape or a different type of close-out feature.

The sheet <NUM> is considered as a first and a second sheet, which form a continuous sheet, and the further, joggled, sheet <NUM> is considered as a third sheet.

Both the first and the second sheet are extending over a portion of the end surface of the sound attenuating sheet, and the single continuous sheet may be considered as two sheets that are connected so that a tensile force can be transferred between the two sheets. Alternatively, not forming part of the present invention, two sheets may be adhered together or have interleaved plies which are connected so that a tensile force can be transferred between them.

<FIG> shows an acoustic panel <NUM> fitted to a fan casing <NUM> via a flange <NUM>, having a bolt hole <NUM>, for bolting the acoustic panel <NUM> to a corresponding flange <NUM> of the fan casing <NUM>. The fan casing <NUM> may have a corresponding fan casing panel <NUM>.

The sound attenuating sheet <NUM> of <FIG> has two honeycomb portions: a first honeycomb portion <NUM> and a second honeycomb portion <NUM>, separated by a septum <NUM>. The septum <NUM> may be air-permeable. A sound attenuating sheet <NUM> having two honeycomb portions separated by a septum may be referred to as a <NUM> degrees of freedom sheet, since the two different honeycomb sections may have the same or different sizes of honeycomb cells and thereby may attenuate sounds at different frequencies.

<NUM> degrees of freedom sound attenuating sheets benefits from the provision of a sheet extending around the sound attenuating sheet as this may prevent the sound attenuating sheet from sufferring from tensile forces in a through-thickness direction, i.e. from the first surface 102a to the second surface 102b. This can improve the robustness of the acoustic panel <NUM> by holding the separate layers of the sound attenuating sheet together strongly.

In <FIG>, it can also be seen that the seal <NUM> substantially seals a gap between the acoustic panel <NUM> and the fan casing panel <NUM>.

<FIG> shows an alternative example, not forming part of the invention, for an acoustic panel <NUM>. In this example there are two overlapping sheets at the end surface 102c of the sound attenuating sheet <NUM>. This example has a first sheet <NUM>, which extends over the first surface 102a and over at least a portion of the end surface 102c, and a second sheet <NUM>, which extends over the second surface 102b and over at least a portion of the end surface 102c, overlapping the first sheet <NUM> in a region where the first sheet <NUM> extends over the end surface 102c. The first and second sheets <NUM>, <NUM> may be co-cured or may be adhered together so as to carry a tensile load between them.

Alternatively, not forming part of the invention, the first sheet <NUM> may overlap the second sheet <NUM> such that the second sheet <NUM> is between the first sheet <NUM> and the end surface 102c and the acoustic panel may still be manufactured and function in substantially the same way.

<FIG> shows a further alternative example, not forming part of the invention, in which individual plies <NUM> of the first and second sheets <NUM>, <NUM>, which are composite sheets, may be interleaved at the end surface 102c of the acoustic sheet <NUM>. The first and second sheets <NUM>, <NUM> may thereby be joined in order to carry a tensile load between them.

<FIG> shows sound attenuating sheet <NUM> being slid into position, in a direction indicated by arrow A, such that the end surface 102c may abut and mate with the third portion 104c of the sheet <NUM>. The C-shape formed by the first, second, and third portions 104a, 104b, 104c of the sheet <NUM> is shown and is shaped to mate closely with the corresponding first, second, and end surfaces 102a, 102b, 102c of the sound attenuation sheet <NUM> respectively.

The sheet <NUM> has inspection holes <NUM>, arranged such that a manufacturer can observe whether the sound attenuating sheet <NUM> is correctly seated within the sheet <NUM> and/or a maintenance person may inspect whether the sound attenuating sheet <NUM> has suffered damage during use. The inspection holes <NUM> may be aligned with the septum <NUM> so that any deterioration or damage to the septum <NUM> or between the layers of the sound attenuating sheet <NUM>, such as between the first and the second honeycomb portion <NUM>, <NUM> can be seen via the inspection hole <NUM>.

While the inspection holes <NUM> are shown only in conjunction with a single continuous sheet <NUM> extending over the end surface 102c of the sound attenuating sheet <NUM>, it will be understood that the embodiments of <FIG> and <FIG> may also comprise inspection holes, and that inspection holes may be formed through overlapping or interleaved sheets also.

<FIG> shows the sound attenuating sheet <NUM> fitted in position and abutted upto face 102c. Inspection holes <NUM> may be aligned in this position so that an engineer can check the abutment of the sound attenuating sheet <NUM> against sheet <NUM>.

<FIG> shows the sound attenuating sheet <NUM> fitted within the sheet <NUM> after the joggled sheet <NUM> has been applied. The sheet <NUM> and the joggled sheet <NUM> may be attached to the sound attenuating sheet <NUM> by bolts or by adhesive.

Claim 1:
An acoustic panel for an aircraft comprising:
a sound attenuating sheet (<NUM>) comprising two layers of sound attenuating sheets (<NUM>, <NUM>), each comprising a plurality of cavities, the two sound attenuating sheets being separated by a septum (<NUM>), the sound attenuating sheet having a first surface (102a), a second surface (102b) opposite to the first surface and an end surface (102c) connecting the first and second surfaces,
a first sheet portion (104a) extending over and abutting both at least a portion of the first surface and at least a portion of the end surface,
a second sheet portion (104b) extending over and abutting both at least a portion of the second surface and at least a portion of the end surface, and
a third sheet (<NUM>), separate from the first and second sheet portions,
wherein the first and the second sheet portions form a single continuous sheet such that a tensile force can be transferred there between, and
wherein the panel is curved, such that the first surface is a radially outer surface and the second surface is a radially inner surface, the end surface having an annular or arcuate shape,
characterised in that
the third sheet (<NUM>) includes a stepped portion extending over and directly abutting a part of the first sheet portion (104a) that extends over at least a portion of the first surface (102a) of the sound attenuating sheet (<NUM>), the third sheet (<NUM>) further including a planar portion extending over and directly abutting the first surface (102a) of the sound attenuating sheet (<NUM>), the planar portion of the third sheet being coplanar with said part of the first sheet portion.