Acoustics – Sound-modifying means – Sound absorbing panels
Reexamination Certificate
1999-02-08
2001-01-30
Nappi, Robert E. (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
C181S290000, C181S285000, C181S198000
Reexamination Certificate
active
06179086
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a sandwich composite panel, especially for the fuselage or cabin shell of a helicopter, including a hollow cell core sandwiched between two fiber composite cover skins. The sandwich composite panel has a noise attenuating characteristic.
BACKGROUND INFORMATION
It has been a longstanding problem in the design and construction of helicopters, that the rotor drive train and auxiliary devices as well as the main rotor and the tail rotor generate a substantial noise load in the interior of the cabin of the helicopter. In order to reduce this noise loading, it has become known to cover or panel the interior walls of the helicopter cabin with noise damping panels or liners. However, the use of such noise damping panels or the like entails a very substantial effort and expense in terms of the installation and construction thereof, and also causes a substantial weight penalty in the helicopter. This is especially true if the noise damping elements are to be effective over the various noise frequency ranges of noise generated by the various above mentioned helicopter noise sources.
Another approach to noise attenuation is known from the publication “AGARD CONFERENCE PROCEEDINGS 549” “Impact of Acoustic Loads on Aircraft Structures” (September 1994) in an article by G. Niesl et al., entitled “Helicopter Internal noise ”, which describes a helicopter wall structure having a sandwich construction type with an integrated noise insulation. In the known sandwich structure, the fiber composite cover layer that is arranged facing toward the source of noise is provided with a plurality of through-going holes that lead to the individual honeycomb cells of the sandwich core. By providing such holes through the noise-loaded cover layer, the honeycomb cells are thereby embodied to act as noise absorbing cavities in a discrete frequency range in the manner of a Helmholtz-resonator.
However, such a known construction suffers disadvantages, for example each individual through-going hole causes fiber breaks or interruptions in the fiber composite material of the cover skin. As a result, the specific strength of the sandwich structure relative to the surface or area weight thereof, becomes significantly reduced compared to a comparable sandwich structure without such through-going holes, especially if the density of holes is relatively large. Moreover, boring the individual holes through the cover layer requires an increased effort and expense in fabricating the composite structural panel. Furthermore, the open holes provide an undesirable access path for various environmental influences, such as moisture and dust deposits and the like, to penetrate into the interior of the sandwich structure, which increases the weight of the structure, reduces the noise absorbing performance over time, and leads to the accelerated degradation of the structure.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a sandwich composite panel of the above described general type that achieves an effective noise protection characteristic in combination with a low weight and a low manufacturing effort and expense, in comparison to the prior art. It is a further object of the invention to provide such a composite panel that achieves noise damping or noise absorption over a broad noise frequency band including several sub-ranges. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved in a sandwich composite panel according to the invention, including first and second fiber composite cover skins and a hollow cell core made up of hollow cell bodies extending transversely, or especially substantially perpendicularly, between the two cover skins. Further according to the invention, especially for achieving noise absorption, at least one of the cover skins of the panel, and particularly the cover skin that is to be arranged facing toward the main noise source, comprises an open mesh fiber composite net, and a flexible cover film covering the open mesh net. The open mesh net has a smaller mesh size than the internal crosssectional size of the hollow cell bodies of the panel core, whereby there are preferably a plurality of mesh openings of the net arranged over and opening into the hollow cell chamber of each cell body of the core while the fibers remain continuous and uninterrupted. For reasons of cost reduction, the cover skins preferably comprise a glass fiber composite material, which may be a solid composite layer or an open mesh composite net as described above.
The inventive embodiment of the composite panel cover skin as a fiber composite net covered by a flexible cover film, in combination with the inner core of the composite panel formed of hollow cell bodies, achieves a highly effective noise absorption in the sandwich composite panel by means of a plurality of individual resonators respectively formed by the hollow cell bodies of the inner core of the panel. Moreover, the area density or surface area weight of the finished panel, relative to the necessary strength and stiffness of the panel, as well as the manufacturing effort and expense for fabricating the sandwich composite panel are held to a minimum. This is achieved especially through the use of the fiber composite net in at least one of the cover skins, because such a net has a low weight or density, and because such a fiber composite net can be easily produced having the required mesh size, for example by a wrapping or winding method, so as to provide the necessary acoustic access openings into the interior of the hollow cell bodies with a minimum of manufacturing effort, e.g. avoiding the need for boring individual holes through a cover skin. In this manner, the inventive composite panel completely meets the requirements pertaining especially in the field of helicopter construction, namely a lightweight construction, with excellent noise protection characteristics and a high load strength.
In order to provide the most effective shielding against narrow band frequency ranges, it has been proved to be especially effective and advantageous to use a closed or solid cover film as the covering over the fiber composite mesh net of the cover skin. In this manner it is possible to achieve a high degree of noise absorption, e.g. at least 95% and nearly up to 100%, while simultaneously achieving the additional advantage that the solid or closed film prevents the penetration of dust and moisture into the hollow cells of the sandwich composite core. On the other hand, a broad band absorption characteristic of the sandwich composite panel is achieved by using a perforated or porous film as the cover film of the cover skin. Any known porous or perforated film can be used, as long as it is durable under the expected operating conditions of the panel, e.g. with regard to temperature, moisture, etc.
As a simple manner of integrating different noise resonators having different noise absorption characteristics into the sandwich composite panel, an embodiment of the invention provides that the hollow cell bodies have different hollow cell heights in a direction substantially perpendicular to the cover skins of the panel. This can be achieved simply by providing cell closing end walls at different heights in different cells, or by the provision of a separating wall as will be described below.
In order to achieve a further reduction in the weight, while simultaneously embodying the sandwich composite panel in such a manner that it is noise absorbing for noises incident both from the interior side as well as from the exterior side, the panel may be constructed with both the inner and outer cover skins comprising the above described arrangement of an open mesh fiber composite net covered with a flexible cover film. In this context, an especially effective noise absorption with respect to noise incident from both sides is preferably achieved in that a closed
Bansemir Horst
Gembler Walter
Haider Ottmar
Ritzer Christian
Eurocopter Deutschland GmbH
Fasse W. F.
Fasse W. G.
Martin Edgardo San
Nappi Robert E.
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