Lightweight System Design with Integration of Active Functions

Abstract

FC-compatible designs, structural bonding and the use of smart materials from adaptronics characterise the integration of active functions. In interaction with aerodynamics, lightweight system design can have a special effect by enabling hybrid laminar flow control. Furthermore, active reduction of sound transmission into the cabin or structural monitoring can be realised in an integrated way.

FC-compatible designs, structural bonding and the use of smart materials from adaptronics characterise the integration of active functions. In interaction with aerodynamics, lightweight system design can have a special effect by enabling hybrid laminar flow control. Furthermore, active reduction of sound transmission into the cabin or structural monitoring can be realised in an integrated way.

4.1 Structures for Hybrid Laminar Flow Control

The importance of laminar flow for the energy consumption of an aircraft has already been discussed in Sect. 3.1. The particular challenge is to synthesise the different requirements of a support structure with the extended requirements of, for example, active hybrid laminar flow control (HLFC). HLFC with active suction of the boundary layer flow theoretically allows for a drag reduction of 30% [17].

Already at the leading edges of the wing, turbulence starts to form under certain inflow conditions. An active suction system in the structure and a micro-perforated airfoil surface are required to actively prevent this. The supporting structure behind an aerodynamic surface must be suitably designed with chambers through which—adapted to the flow conditions around the leading edge—suction is applied with different pressure gradients, Fig. 4.1. Systematic and multidisciplinary development of an HLFC system was prototyped for the A350 horizontal tailplane (HTP) in the “Clean Sky II” ECHO project [105]. Based on a new HLFC leading edge design, a potential of 5% fuel savings was identified [39].

Fig. 4.1

Model of an HLFC leading edge of a tailplane

4.2 Shape Variability

Contour changes of airfoils allow for adaptation to different flight conditions. However, active shape variability must count with the inherent stiffness of the structure into which it is integrated. Thus, there are limits to the resulting shape changes. Within these limits, a contour change can have an strong effect and replace otherwise necessary additional aggregates or functional elements.

Laminar airflow is not possible with conventional moving leading edges due to unavoidable steps to the wing box. In order to ensure a step- and gap-free change of the leading edge angle of attack in take-off and landing configurations, an inherently shape-variable leading edge of the wing is desirable. In the 7th EU Framework Programme, the SARISTU (Smart Intelligent Aircraft Structures) project investigated possible applications of morphing (shape variability) on leading and trailing edges. Together with Airbus and partners, a shape-variable wing leading edge was developed, built and experimentally investigated. The particular challenge is the simultaneous consideration of the other requirements for this component such as lightning protection, de-icing, abrasion, and bird strike protection. In an aircraft configuration with rear mounted engines, the new leading edge would save 1% fuel [59]. Newer designs, thanks to a special material hybrid, can realise a leading edge lowering of up to 20° with a 4‒9% increase in lift coefficient [115], Fig. 4.2.

Fig. 4.2

“Droop Nose” demonstrator with elastomer-GFRP hybrid skin

In order to reduce the wave resistance of the transonic flow over the wing, so-called shock control bumps (SCBs) can be used, which cause controlled turbulence condition from a defined wing depth and with a defined height.

Thus, a shape-variable adaptive spoiler was prototypically developed, which allows to adjust the required deformability in wing depth direction and height [68], Fig. 4.3.

Fig. 4.3

Design of a flap with actuable shock control bump

One way of increasing the lift of an airfoil is to exploit the Coanda effect: the flow on a flap is controlled for a longer period of time if it is fitted with an active blow-out lip. This requires rapidly actuable structural elements for targeted flow control, which are integrated into the flap in a small installation space.

A structurally compliant dynamic piezoelectric actuation of a blow-out lip shows a lift increase of ΔC_a = 0.57 in wind tunnel measurements [119] (Fig. 4.4).

Fig. 4.4

Active blow-out lips for utilising the Coanda effect in the wind tunnel

Gapless flaps or movable winglets avoid flow losses and can be used for load reduction. One way of actuating a structure with built-in joints is to apply internal pressure. Pressure-actuated cell structures (PACS) and hydraulically actuated compact unit structures (fluid actuated morphing unit structures: FAMoUS) allow for the realisation of large deformations, Fig. 4.5. However, the fatigue strength of the solid-state joints and the pressure tightness of the cells pose challenges. Here, continuous fibre-reinforced 3D printing opens up new perspectives (cf. Sect. 2.5).

Fig. 4.5

Pressure-actuated cell structures for variable shape wing structures

For a wing airfoil, a trailing edge lowering of 15° and thus a theoretical increase in lift by a factor of 3 was demonstrated by a PACS design [36]. A shape-variable winglet with structurally conformal actuation was developed and tested in the wind tunnel as part of the EU project NOVEMORE [114].

4.3 Vibration Influence

Vibrations emanating from propellers and turbines cause material fatigue, wear and comfort restrictions. Active vibration reduction is a major application area of lightweight system design, in that structurally integrated actuators counteract the resulting deformations at the same frequency. Two CFRP rods with an integrated piezo stack actuator in a truss structure, similar to an engine suspension, controlled by an adaptive controller, reduce amplitudes by 40 dB [101]. In a trusswork, Fig. 4.6, the vibration transmission of a propeller to the support structure is reduced by 80% to 90% [102].

Fig. 4.6

Vibration reduction in a truss with active beam element [103]

Ice buildup on the leading edges of aerodynamic surfaces increases flow resistance. Beyond a certain ice adhesion, this becomes critical to safety and must be detected and dissolved. By integrating suitable active elements into the supporting structure of the leading edge of the wing, both safe and fast ice detection and mechanical deicing—instead of thermal deicing, cf. Sect. 3.2—can be realised.

Integrated piezo actuators allow for detection of local ice adhesions on leading edges of wings from 2 mm thickness by means of ultrasonic signals [79]. Investigations with integrated electro-mechanical systems show a potential for active deicing by means of local, high-frequency skin deformation [33], Fig. 4.7.

Fig. 4.7

Structure-integrated deicing in ice wind tunnel; ice buildup (a) before and (b) after deicing after 4 min at −10 °C (1) and after 2 min at −20 °C (2); [33]

Structural vibrations from engines and flow turbulence on the fuselage are transmitted to the cabin, where they are perceived as noise. In addition to heavy insulating material, counter-sound techniques (Active Noise Control: ANC) now minimize sound radiation.

A lighter and more efficient system takes advantage of the fact that sound waves can no longer be radiated by a structure if the structure vibrates below the so-called coincidence frequency in wavelengths that are smaller than the wavelengths of the radiated sound waves (Active Structure Acoustic Control: ASAC), the so-called acoustic short circuit. More information on the ASAC method in [82].

With the aid of an ASAC system integrated into the cabin lining, multitone, low-frequency interference excitations can be reduced by up to 20 dB [81]. A ready-to-install active cabin panel that integrates an ASAC system reduces the sound pressure level for a turboprop cabin by 6.8 dB [80], Fig. 4.8.

Fig. 4.8

Cabin panel with ASAC system in transmission test rig

The wireless use of sensors for flight condition monitoring saves manufacturing costs and weight if energy is locally available. For a wireless sensor network (WSN), the necessary operating energy can also be generated by energy harvesting, e.g., from operationally induced vibrations of the structure. The energy demand of autonomous sensor elements can be met by structure-integrated piezoceramics [37].

4.4 Structural Health Monitoring—SHM

Continuous monitoring of the structural integrity of lightweight structures has a major influence on structural properties and resulting design weight when using FC components, Sect. 2.2. In recent years, the Lamb wave method has become established for thin-walled shell structures typical of aircraft design. Structurally integrated (or applied) piezoelectric elements are used to establish an actuator-sensor network that is used to transmit high-frequency longitudinal and transverse waves through the shells. These are reflected and transmitted at stiffness discontinuities in certain proportions. This produces a signal pattern for the current state of a shell, which, compared to a stored pattern for the intact state, can be used to infer locations of damage and damage magnitudes [85].

On a CFRP door frame shell (Fig. 4.9), a network of 584 piezoelectric elements is able to localise visually barely detectable delaminations (Barely Visible Impact Damages: BVID) from 310 mm² to 2311 mm² with an accuracy of 5 mm to 85 mm [83]. Lamb-wave SHM can also be applied for damage detection over a temperature range of −42 °C to 85 °C [84].

Fig. 4.9

CFRP door frame shell with SHM network for automated damage detection in the EU “Clean Sky II” project SARISTU

For the evaluation of a detected structural damage with respect to load-carrying capacity and a possible need for repair, an automated comparison with a simulation is required, which analyses the residual bearing behaviour considering the detected damage. Linking of Lamb-wave-based damage detection and damage assessment becomes possible by using fast surrogate models [35]. Lamb-wave SHM can also be used in aircraft maintenance in combination with simulation and augmented reality [120].

One possibility for damage detection in joints is offered by film sensors made of polyvinylidene fluoride (PVDF), which also have piezoelectric properties and allow for strains to be measured very accurately with suitable pretreatment. Since PVDF can be used simultaneously in a bond as a crack stopper because of its toughness, Fig. 4.10, such sensors also lend themselves to monitoring bonded joints. A 100 μm thick PVDF film as a crack stopper with applied metal measuring grid of 200 nm allows sensing of strains in an adhesive seam [41].

Fig. 4.10

Sensor inlay for crack detection in glued seams

4.5 Structural Batteries

In the context of increasing electrification of on-board functions and drives, the need for batteries for intermediate storage is growing. Battery storage can also be designed as load-bearing structures and used for load transfer. The more mechanical load it carries as part of the structure, the more its weight share as a separate battery decreases. Decisive for the structural load-bearing capacity is the use of solid-state electrolytes as storage medium and suitable electrical contacting with maximum surface area for the best possible capacitive energy storage. While solid-state electrolytes (e.g., Li_1+xAl_xTi_2−x(PO₄)₃: LATP) are required, carbon nanotubes (CNT) are suitable for electrical contacting. Thus, parts of the secondary structure of an aircraft could be used to store a few kWh of electrical energy, Fig. 4.11.

Fig. 4.11

Comparison of power and energy density for different energy storage devices

By combining an LATP with CNTs and a suitable structural design, a storage capability of 11.59 mF/cm³ can be demonstrated [69].

A structure-integrated capacitive element in an aerospace application for energy storage from the deceleration of rotating masses could be built 73% lighter and 78% smaller than the classic comparative structure with pure batteries. The mechanical load capacity was retained by 80% in the process [87].

Conclusion

The integration of active functions into the load-bearing structure offers further and diverse potential for weight and drag reduction. However, developments in this field require a paradigm shift in aircraft certification, as systems that were previously approved separately must be considered integrally. In this field of lightweight system design, a new quality of interdisciplinary cooperation is required for success.