Aeronautics and astronautics – Aircraft structure – Fuselage and body construction
Reexamination Certificate
2002-08-07
2004-07-27
Carone, Michael J. (Department: 3641)
Aeronautics and astronautics
Aircraft structure
Fuselage and body construction
C244S11700R, C244S129100
Reexamination Certificate
active
06766985
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a structural assembly of part of an aircraft, particularly a rotary-wing aircraft and especially a helicopter, and to an aircraft equipped with such a structural assembly.
The part of the aircraft taken into consideration in this invention is preferably a part intended to be occupied by people, such as the cockpit or the passenger cabin for example. It is known that the structural assembly of such a part of an aircraft comprises, in particular, part of the structure of the fuselage of the aircraft, and cladding panels which are fixed to the interior of this part of the fuselage structure.
It is also known that present inside such a structural assembly is a phenomenon which is particularly troublesome to the occupants, particularly in the case of a rotary-wing aircraft, and that is, noise.
More particularly, with a rotary-wing aircraft it is known that the acoustic spectra defined in the domain between 20 Hz and 20 kHz originate from the superposition of noises of differing origins, which can be grouped into two different groups according to their spectral characteristics, namely pure sounds or spectral-line noise and broadband noise.
In the known way, pure sounds or spectral-line noise occur particularly, as appropriate:
at the characteristic frequencies of the aircraft driveline;
at the sweep frequencies of the (main and tail) rotor blades and at the harmonics of these frequencies;
at the sweep frequencies of the blades of the compressors of the turbine engine units; and/or
at the sweep frequencies of the blades of the fans that cool the main transmission gearbox and/or electrical equipment and at the harmonics of these frequencies,
whereas broadband noise comprises, in particular, as appropriate:
the noise of the boundary layer which grows along the fuselage;
the noise generated by the rotors;
the air inlet and nozzle flow noise;
the engine noise; and/or
the noise of the cockpit or passenger cabin climate-control or heating circuits.
Although all these noises can pose problems, the acoustic annoyance felt by the passengers and crew is caused essentially by the spectral-line noise originating from the main transmission gearbox [arranged between the engine(s) and the (main and tail) rotors] and/or from the engine(s) situated over the cabin, and from noise of aerodynamic origin coming from the main rotor and the air intakes. As a result, the present invention which intends to limit this acoustic annoyance has the objective mainly of reducing said spectral-line noise.
There are various known solutions for reducing such noise inside a rotary-wing aircraft, particularly a helicopter.
It is known that the technique generally employed for reducing, on an industrial scale, the internal noise of a helicopter consists in reducing the level of vibration or the radiation of sources of noise and/or of the fuselage. The design of the appropriate treatments calls upon the following physical mechanisms:
reducing the vibration response by adding highly damping material to the panels that make up the structure;
reducing the acoustic transmission and/or the acoustic radiation of these same panels by modifying the stiffness (use of localized stiffeners or optimization of the stack of layers in the case of a sandwich structure);
introducing acoustic absorption by bonding sound-deadening foams onto one side of the structural panel or cladding or using sachets of glass wool, rockwool, etc.;
having a double-partition effect between a structural panel and its cladding panel; and
using a Helmholtz resonators effect by adding perforated fabrics to the panels.
The first three solutions do actually make it possible to reduce the overall noise level in a cabin over a large range of frequencies but incur a high penalty in terms of mass. In addition, they entail direct treatment of the structure and have therefore to meet numerous requirements: fire resistance, maintenance, etc. Furthermore, the attenuation of the emergence of the spectral-line noise is not sufficient to cause the acoustic annoyance specific to pure sounds to disappear. As to the fifth solution, it does actually allow noise to be reduced in a narrow frequency band, but only for a frequency chosen at the time of design of the panels. The fourth solution on the other hand is far more attractive because it leads to an increase in the acoustic attenuation by virtue of the double partition effect.
Furthermore, it is known that the cladding panels are generally fixed to the structure of the fuselage of a helicopter using blocks. This solution has the disadvantage of not sufficiently attenuating the noise level, because of the numerous blocks that have to be used for this purpose and because of the proximity of some of the blocks to the main lift and forward travel rotor of the helicopter, which is the main source of noise. Furthermore, such a known solution often leaves gaps and discontinuities between the cladding panels, and this of course leads to reduced acoustic insulation.
DESCRIPTION OF THE PRIOR ART
To at least partially overcome this noise problem, document U.S. Pat. No. 6,158,690, envisages a structural assembly of part of an aircraft, which comprises:
an external structure which corresponds to part of the structure of the fuselage of the aircraft;
a cladding assembly which comprises a rigid framework formed of arcs which are held together by crossmembers, and cladding panels attached to this framework; and
main connecting means, in this instance insulators, for connecting, through an elastic fixing or semi-rigid connection, the framework of said cladding assembly to the side walls of said external structure.
These insulators comprise in particular elements, particularly rings, made of elastomer, that is to say of an elastic material able to reduce vibration, and are arranged toward the bottom of the walls, so as to be distant from the roof where, in the case of the cabin of a rotary-wing aircraft, the lift and forward travel rotor, which is an important noise source, is situated. These various characteristics of the insulators make it possible to reduce the noise inside the cladding assembly.
Furthermore, said cladding assembly is also held by auxiliary means, with respect to the external structure, namely:
at the top of the walls, by elastic buffers which are arranged in such a way as always to be simultaneously in contact with the cladding assembly and with the external structure, but which are fixed only to the external structure and are simply in contact with the cladding assembly; and
at the bottom, by elastic attachments (equipped with elastomer insulating elements) connecting the cladding assembly to the floor of the external structure.
However, this known structural assembly, which thus makes it possible to bring about a reduction in noise, does exhibit several disadvantages:
the reduction in noise remains limited, particularly:
because of the elastic buffers which transmit vibration from the external structure to the framework of the cladding assembly because they are always in contact with these two elements, which vibration is great in the upper part where these buffers are located because of the proximity to the main rotor in the case of a rotary-wing aircraft; and
because of the insulators which, although distant from the main rotor, are nonetheless situated a certain height off the floor, and which above all act directly on the framework which, as is known, because of its construction (arcs connected together by cross members to form a rigid assembly) encourages the transmission of the various vibrations which are sources of noise;
the various connecting means (insulators) and holding means (buffers and attachments) have a short life, because of the presence of the elastic material (elastomer); and
in the event of a crash or violent impact, the retention of the cladding assembly in the upper part is at the very least uncertain because there is no fixing present in the upper part of the cladding assembly, the effectiveness of the buffers (which are fix
Dussac Marc
Vincent Henri-Charles
Carone Michael J.
Eurocopter
Palabrica Rick
Stevens Davis Miller & Mosher LLP
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