Passive functions are not used for load transfer alone, as in classic lightweight design, but fulfil other requirements for the overall product, such as minimising aerodynamic drag, providing electrical conductivity and thermal or acoustic insulation.

3.1 Structures for the Natural Laminar Flow

Laminar flow reduces frictional drag and, according to Brequet, contributes directly proportional to the reduction of an aircraft’s energy consumption, Sect. 1.2.

The reduction in frictional drag due to laminar flow on the upper surface of the wing is estimated to be up to 8% [51]. Designing structures in such a way that the flow remains laminar for as long as possible therefore has a direct influence on the energy efficiency of an aircraft. Natural laminar flow (NLF) can be significantly supported by suitable shaping of the structure, in particular by avoiding gaps or discontinuities at joining edges.

Even today’s CFRP wing shells are still manufactured with rivets. These cause unevenness and protrusions, which promote vortex formation and premature flow reversal. An integral laminar upper wing shell ‒ taking into account PID and ensuring a high FVC ‒ places special demands on design and manufacturing. A solution approach for a fully integral laminar CFRP wing upper shell is described using the example of a 2.5 m × 1.5 m demonstrator in [52] (Fig. 3.1).

Fig. 3.1
A photograph of a grid-shaped structure of the laminar upper wing shell. Each grid is surrounded by walls.

Demonstrator panel of an integral laminar wing upper shell

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The manufacturing-related waviness of a CFRP wing upper shell influences the run length of the laminar flow over the wing depth. An ideally laminar wing should have laminar flow up to a relative wing depth of 60%. PID leads to concave waviness in the skin and load-induced deformation (LID) leads to a convex “pillow” formation. Net, a disturbing ripple remains, which increases the flow resistance of the laminar wing. The laminar run length of the flow over a wing surface can be reduced by up to 4% due to influences from PID and LID [46].

Essential for NLF is the avoidance of gaps or steps between adjacent components on the flowed- around side of a structure, cf. Fig. 3.2 left. Rivet heads of the connection of the leading edge to the wing shell also have a negative effect. In addition, there is the maintenance requirement for rapid replacement of the leading edge in the event of damage while maintaining the smallest tolerances. Therefore, a laminar CFRP leading edge with metallic covering foils was developed with a maximum step height < 0.15 mm [99], whose rapid replacement is ensured by a tolerance-compensating connection with eccentric bushings [107].

Fig. 3.2
3 illustrations. Two diagrams depict the disturbance pattern in the laminar flow. The diagram at the top depicts the maximum value at 0.5 m m and that at the bottom depicts the maximum value at 0.12 m m. A photograph and a 3-D model present the tearing off of the metallic cover foil.

Laminar wing leading edge ‒ smoothing out unevenness through metallic cover foil

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3.2 Electrical Conductivity of CFRP

CFRP structures have low electrical conductivity due to the matrix material. In order to realize the required electrical conductivity (Electric Structure Network: ESN), currently CFRP fuselage structures (A350 fuselage, [10]) are equipped with additional non-load-bearing metal. This results in additional weights for the aircraft and additional costs in production.

At 6000 S/m, carbon fibres exhibit about 30% of the metallic conductivity, but the surrounding matrix material at about 10−8 S/m acts as an insulator to prevent both lightning protection and the ground connection of electrical loads. Electrical conductivity in CFRP laminate thickness can be increased to 600 S/m by using silver-coated polyamide filaments in combination with conductive non-woven layers [95] and reduces the required basis weight of lightning protection on a CFRP surface from 175 g/m2 to 25 g/m2 [94] (Fig. 3.3).

Fig. 3.3
An illustration of a thin layer of fleece in the middle of two thick layers of conductors. The threads in the punctures depict an N C F bonding. Conductive thread is labeled in the image.

Increasing the electrical conductivity of NCF bonding between textiles in the contact plane [94]

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Electrical cables with a total weight of 3 tons are installed in the A380 for data communications [61]. FML hybrids are redundant with respect to contacting and damage; local defects cannot affect electrical conductivity. Together with mechanical advantages, low frequency electrical signals can thus be transported through the laminate on several levels. The dielectric strength of the individual layers is decisive for the permissible electrical voltage that can be applied. For metal foils electrically separated with three 0.1 mm thick glass-fibre layers and epoxy resin matrix, 250 V to 600 V are transmissible [91].

The integration of electrically conductive layers in an FC also enables targeted heating, for example, for deicing leading edges of wings without feeding system lines. With a suitable design implementation (Fig. 3.4), electrical deicing is possible with one third of the heat output of a conventional anti-icing system, i.e., about 3.6 kW/m2 [89].

Fig. 3.4
An illustration of an integrated model of the electrically conductive layers in a deicing system. It presents alternate layers of dark and bright sheets.

Wing leading edge with deicing system and metallic abrasion protection

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Cabin elements are usually made of non-conductive sandwich FC and are characterised by a large number of electrical consumers. In addition to grounding, the power supply must be installed separately with resulting additional weight and costs.

By integrating the conductors into the rear wall of an A330 galley, weight savings of 30% could be demonstrated compared to the current state of the art [90] (Fig. 3.5).

Fig. 3.5
2 photographs present an integrated network of wires and rectangular boards on the walls. The photograph on the left depicts the side view and the one on the right exhibits the front view.

Trace integration on the rear wall of an A330 galley

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The integration of electrical traces into an FC structure offers a lot of potential if reliable contacting can be ensured. A multifunctional load insert for SW structures with integrated electrical signal transmission and thermal load transfer has recently been designed and successfully tested for a satellite wall panel [86].

3.3 Noise Transmission into the Cabin

CFRP fuselage structures increase sound propagation and radiation into a cabin because of the high material stiffness. Insulation material is currently used to reduce turbine and flow noise to the cabin. A lighter option is the use of a passive damping layer (Passive Constrained Layer Damping: PCLD), whose weight per unit area is lower at approx. 0.83 kg/m2. The effect of PCLD in a rigid grid panel (comp. Sect. 2.3) on the radiated sound power was found to be −2 dB for frequencies above 300 Hz [108].

Rigid sandwich panels of an aircraft cabin are only suitable for sound insulation to a limited extent. However, the sound reduction index of these secondary structures can be increased by a suitable, mass-constant design of the honeycomb core and used specifically for acoustically adapted transmission properties.

In a simulation, it can be shown that for an SW panel with GFRP face sheets and printed plastic honeycomb core, the sound reduction index can be changed selectively [93]. Experiments show that the sound reduction index becomes effective from 500 Hz upwards, depending on the honeycomb core geometry and its cover layer support [92] (Fig. 3.6).

Fig. 3.6
A photograph of 4 different patterns of plastic honeycomb core. It presents a web of M-shaped pattern in the first set, irregular polygons in the second set, hexagonal structures in the third set, and rhombus in the fourth set.

Different plastic honeycomb structures with different sound reduction indexes

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Conclusion

The fully integral CFRP design and modern bonding technologies allow for steps and irregularities on the aerodynamic surface of structures to be largely avoided. This enables laminar flow and effective reduction of aerodynamic drag.

Selectively increasing or decreasing the electrical conductivity of structures allows for different functions to be realised with minimum weight.

Design possibilities of the FC structure are effective up to the range of acoustic radiation.