Concept and design of extended hybrid laminar �ow control suction panels

Fully laminar aircraft are one step towards reaching eco-efﬁcient aviation. However, high system complexity and signiﬁcant manufacturing effort prevent the wide usage of existing laminarisation concepts such as laminar ﬂow control, which are rarely found in commercial aircraft. Hybrid laminar ﬂow control concepts reduce the manufacturing effort signiﬁcantly at the cost of only achieving partial laminar ﬂow. This paper presents extended hybrid laminar ﬂow control concepts for fully laminar wings, with reduced system complexity. A detailed study of structural and aerodynamic requirements provides the foundation for partial design solutions of active suction structures. The authors derive two concepts for active suction panels from the structural design space. While the ﬁrst concept relies on state of the art manufacturing techniques, the focus of the second concept is on additive manufacturing technologies. Based on these concepts, it is feasible to design fully laminar wings with structurally integrated active suction systems. The authors propose an aerodynamic test strategy for further developing extended hybrid laminar ﬂow control.


Introduction
Ever since the 1930s considerable efforts have been made to investigate laminarisation technology for aircraft of all types and sizes 1 .However, despite the numerous wind tunnel tests and flight test campaigns that have taken place over the past few decades, only few concepts are ready for commercial aviation.The most promising concept for large transport aircraft is Hybrid Laminar Flow Control (HLFC), a combination of Laminar Flow Control (LFC) by means of active suction on the aerofoil nose followed by Natural Laminar Flow (NLF).The first investigation of HLFC for transport aircraft was carried out by Pfenninger 2 in the 1980s.As flight tests show, the HLFC concept allows to establish laminar flow up to 36% of the chord length 3 .A passive HLFC system can be found on the horizontal and vertical tail planes of the Boeing 787 commercial aircraft 4 .
Yet, the advantages of a fully laminar aircraft are undeniable.Beck et al. 5 quantify the total drag reduction for a fully laminar medium range jet to approximately 50%.The present paper complements the existing wing laminarisation concepts summarised by Krishnan et al. 4 with two extended Hybrid Laminar Flow Control (xHLFC) concepts.The xHLFC family allows laminar flow up to the trailing edge devices using an effective combination of LFC and NLF regions.The effective combination allows reducing the drawbacks of high system complexity and the interference of the suction systems with the load carrying wingbox, while still providing a fully laminar wing.The xHLFC concepts are developed in the cluster of excellence for sustainable and energy-efficient aviation (SE 2 A) 6 .
Even though the effectiveness of the aerodynamic concepts have been shown in wind tunnel experiments and flight tests, issues arise during manufacturing and operation of the laminarisation technology 7 .Even today, the high aerodynamic requirements for laminar aircraft surfaces are a major challenge for the manufacturing process.
This paper compares different laminarisation concepts for aircraft wings and evaluates them from a structural and aerodynamic perspective.On the basis of the structural and aerodynamic requirements specific to the xHLFC concept, a generic design approach for the active suction structure is developed and used to classify existing design solutions.Next to classical manufacturing techniques, the design space includes Additive Manufacturing (AM).
To capitalise on the benefits of AM, the paper proposes an integral design approach for a suction panel.The integral manufacturing of the perforated suction skin and the support structure below avoids the issue of blocked suction skin at the interface.As a completely new approach, this paper suggests 3D printed Triply Periodic Minimal Surfaces (TPMS) as flow conducting core structure in the suction panel.Although TPMS structures are promising as flow conducting core structures for sandwich applications, extended investigations are necessary for the assessment of their aerodynamic and mechanical performance.

Wing laminarisation concepts
Three major wing laminarisation concepts are reported in the literature.The concepts include passive, active and hybrid systems.The most common passive system is NLF, where laminar flow is established through favourable aerofoil design in combination with high manufacturing requirements towards surface roughness and waviness.The most common active system is LFC 1 .In this concept, boundary layer suction prevents the boundary layer flow from deceleration.The removal of the decelerated boundary layers by active suction stabilises the boundary layer flow velocity and ultimately maintains a laminar boundary layer.Figure 1 shows the working principle of boundary layer suction.The combination of LFC and NLF results in a hybrid system.The most common hybrid system is HLFC 4 , where LFC is applied at the aerofoil nose followed by NLF. Figure 2 (a-d) compares the three major laminarisation concepts to a conventional aerofoil.Out of the three major laminarisation concepts, only LFC enables full wing laminarisation.Wind tunnel studies conducted by Wortmann and Althaus in the 1960's investigate a hybrid concept, where NLF is established on the forward portion of the aerofoil and LFC is used to keep the flow laminar up to the trailing edge 8 .This concept was intentionally developed for gliders and is shown in figure 2 (e).From this point forward the concept will be referred to as extended Hybrid Laminar Flow Control (xHLFC ).It combines the advantages of a fully laminar wing of LFC and the reduced system complexity of HLFC.Boermans 9 investigates the same concept in a subsequently published paper.For the purpose of completeness, the xHLFC-2 concept is shown in figure 2 (f).This concept may be used when natural laminar flow cannot be established at the aerofoil nose.
All active and hybrid laminarisation concepts require partially a wing surface facilitating air withdrawal from the boundary layer flow.Next to the global wing laminarisation concepts shown in figure 2, a local concept for the suction structure is necessary.Figure 3 shows a local suction structure concept for xHLFC.This suction structure consists of five functional units A-E.The major components are a porous suction skin (A), a supporting core structure (C) and a load carrying wing skin (E).The major components are joint by interfaces (B) and (D).The majority of existing LFC and HLFC design solutions include these functional units [10][11][12] .However, a generic suction structure concept allowing comparison and assessment of the single design solutions could not be found.
The suction structure concept implements suction rate control by modelling pressure losses in the suction skin (A) and in the conductive core structure (C).The design of the suction skin is fairly restricted by aerodynamic requirements.However, pressure drop control in the conductive core structure seems to be a promising approach for suction rate control.It is assumed, modelling of the core structure porosity allows precise control over the pressure drop.
The core structure (C) implements two functionalities.Next to suction rate control, it serves as stiffener concept for the load carrying wing skin (E).State of the art transport aircraft use stringers as stiffening elements 13 .Stringers increase the An open research question in this concept is where to remove the drawn-in air.A central blow out system requires a relatively large piping system due to the spanwise accumulation of air.A spanwise distributed pump and blow out system needs a comparatively small piping system which can be integrated in the suction panel.The redundancies of the distributed system also support an increased system robustness.
The suction structure concept is the basis for the collection of the structural and aerodynamic requirements.The next section summarises all requirements for each functional unit.The structural and aerodynamic requirements allow justifications of design decisions while deriving a detailed design from the generic design in section "structural design space".

Structural and aerodynamic requirements
The overall objective of the xHLFC concept is to keep the boundary layer flow laminar.The suction structure has to meet concept specific aerodynamic requirements, such as skin porosity or suction rate control in order to ensure laminar flow.Additionally, structural requirements such as manufacturability and mechanical properties have to be met.Many of these requirements have been specified in previous investigations 1 .This section summarises the key structural and aerodynamic requirements specific to the xHLFC suction structure.This is intended to provide a fundamental understanding of the requirements.
LFC depends on a precise control of the volume flow rate (suction rate).While "undersuction" does not prevent the boundary layer flow from deceleration, "oversuction" causes itself a disturbance in the boundary layer flow and ultimately triggers transition 14 .Within the boundaries of under-and oversuction, suction stabilises the laminar boundary layer.The required volume flow rate for laminarisation increases with the aerofoil pressure gradient and demands for a chordwise suction rate control 5 .Figure 4 shows schematically the volume flow rate required for laminarisation and its boundaries.
The suction pressure of the pump(s) and the pressure losses in the suction system determine the volume flow rate of the suction system.Assuming a constant suction skin porosity (constant hole size, pitch and pattern), figure 5 shows the required pressure profile just below the suction skin.An increasing suction rate along the chord length translates into terms of pressure as increasing pressure drop at the wing skin ∆C P,skin .At a given pump pressure level, the necessary pressure distribution below the wing skin determines the required pressure drop in the core structure ∆C P,core .Using multiple pressure pumps with different pressure levels has the potential of a significant reduction of required pressure losses.
The integration of the suction panel into the wing structure is not possible without considering the general aircraft configuration.The main purpose of developing a suction panel for xHLFC lies in the need of significant drag reduction.A hybrid laminar aircraft only makes sense in combination with other means of drag reduction, e.g.reducing induced drag with a high Aspect Ratio (AR).Consequently, many aircraft subject to current low drag research come with an AR of 16 or higher 15 .In comparison, the transport aircraft with the highest AR is Boeing's 787-10 featuring an AR of approximately 11 16 .Additionally, a Forward Sweep (FS) is beneficial for laminar aircraft, as it has a reduced sweep angle at the wing leading edge.The angle reduction induces less cross flow and ultimately reduces the disturbance of the laminar boundary layer flow 5 .
In contrast to xHLFC , the HLFC concept permits a separation of the loaded wing structure and the suction system.Although the xHLFC concept has a reduced suction area compared to LFC, a separation of loaded structure and suction system seems not Figure 5.The suction rate depends on the pressure level of the pump, the pressure drop in the core structure ∆C P,core and the pressure drop at the perforated skin ∆C P,skin feasible.The high ARs of low drag aircraft lead to large Root Bending Moments (RBM).Large RBMs have to be compensated for by constructive measures that increase load-bearing capacity and buckling strength.High AR and FS combined demand a high torsional stiffness of the wingbox in order to avoid static torsional divergence.According to Bredt's formula for closed thin-walled profiles, the cross sectional area strongly affects the torsional stiffness 17 .The distance of the front-and rear spar consequently determine the wing's torsional stiffness.Typical spar positions for two-spar wingboxes are at 15% of the relative chord length for the front spar and 60% for the rear spar 13 .This distance should rather increase than decrease in low drag aircraft configurations.Avoiding interference of the load carrying wingbox and the suction structure is not feasible in the xHLFC concept, even under the assumption of 40 to 50% NLF.Therefore, the xHLFC suction panel has to contribute to load transfer and buckling stiffness to some extend.
Turbulent flow is desirable on control surfaces for improved handling qualities.Spoilers, flaps and ailerons are therefore the rearwards limit of an active suction system.On an A320, spoilers begin at relative chord lengths between approximately 65 and 75%, depending on the spanwise position 18 .For smaller aircraft, it is possible to replace spoilers with non-wing-located speed brakes and therefore allow for an extended suction length up to the movables.Speed brakes are for example mounted at the rear fuselage of Fokker F-28 and BAe146 19 .Movables are usually mounted at the rear spar.Consequently, the rear spar represents a natural end of the suction system.
Next to the primary aerodynamic and structural requirements, secondary requirements have been investigated in other laminarisation research.These include operational and environmental requirements such as maintainability, de-and anti-icing capabilities or decontamination.Maintenance considerations are an important aspect in the design of an xHLFC suction panel due to its complexity and the average in service time of transport aircraft.Also exposure to rain and radiation needs to be considered in the design of the wing skin.However, icing and contamination severity is estimated to be secondary due to the rearward position of the suction panels on the wing.

Suction skin requirements
The suction skin properties have a significant impact on the flow behaviour and the transition region.Important properties are the surface quality determined by its roughness and waviness as well as the size, pattern and quality of the perforations.The size and the distance (pitch) of the suction holes determine the porosity of the suction skin.Equation 1 gives the porosity Φ for the reference area (A), the hole diameter (d) and the pitch (a) defined in figure 6.The hole pattern and the surface quality influence the disturbance in the boundary layer flow caused by suction.
A precise control of the pressure drop at the suction skin is essential for the suction rate control.The pressure drop depends on the porosity and the quality of the suction holes (shape and size) 4 .Irregularities inside the holes, e.g.surface unevenness and roughness, increase the wall friction and potentially lead to laminar turbulent transition.Such irregularities exist in all manufactured parts.Smaller perforations are more difficult to manufacture and geometric imperfections increase with decrease in hole diameter and pitch.Consequently, larger hole diameters are beneficial when manufacturing suction skins.In contrast to a continuously porous suction skin, a micro-perforated suction skin causes a disturbance in the boundary layer flow 20 .Figure 7 shows the influence of suction on the flow velocity distribution in the boundary layer.The flow velocity in the proximity of suction holes remains constant, whereas it decelerates in between holes.A well balanced suction hole pattern allows to reduce the effect of discrete holes, e.g. the triangular pattern shown in figure 7. Minimising the disturbance caused by discrete holes is essential.Reducing the hole size and pitch at constant porosity minimises the flow disturbance and is beneficial for laminarisation.The disturbance in the boundary layer flow diminishes for a continuously porous suction skin, which is exactly the lowest limit of hole pitch and size.
Discrete suction holes cause eddies in the boundary layer 20 .The eddies are caused by the shear in the boundary layer flow induced by the suction.Consequently, the suction rate determines the size of the eddies.As described before, a favourable hole pattern prevents the eddies from growing.MacManus and Eaton 20 investigate cylindrical and conical holes between 50 µm and 80 µm and also take into account inclinations of the holes.They come to the conclusion that the hole shape, cylindrical or conical, has no significant influence on the boundary layer.In contrast, inclinations of the holes reduce the pressure drop at the wing skin but cause increased vorticity in the boundary layer flow.Hole geometries and patterns as shown in figure 8 have not been investigated so far.Patterns and geometries reducing the shear in the boundary layer flow by keeping the flow velocity constant may be an option to use larger suction holes.It seems promising to include inclined holes in such an investigation for their potential to reduce the deflection of the boundary layer flow during suction.

Core structure requirements
A smooth surface with continuous suction is essential for laminar boundary layer flow.A thin suction skin allowing manufacturing of micro-perforations requires a dense support structure in order to avoid waviness.However, hole blockage at the interface between suction skin and core structure should be reduced to a minimum.Consequently, the suction skin requires a continuous core structure providing a cavity underneath each suction hole.Pressure drop control in the core structure allows for suction rate control in the xHLFC concept.A high porosity of the core structure leads to low internal flow velocities and results in a small pressure drop.In contrast, low porosity of the core structure leads to high internal flow velocities and a large pressure drop.An adaptive porosity of the core structure allows modelling of the internal pressure losses and consequently modelling of the suction rate.The xHLFC suction structure requires a distributed support structure, as for example in a sandwich panel, including an adaptive porosity for suction rate control.

Wing skin requirements
The main task of the wing skin is to carry compression and shear loads.Consequently, stability constraints size the upper wing cover.The stability of the wing structure is usually increased by stiffening concepts.State of the art stiffening elements are Ω-, I-, T-and J-stringers.However, the installation of the core structure onto the load carrying wing skin allows its usage as stiffening concept.This has the advantage that no additional space for stiffeners is required where the suction structure is integrated.As mentioned before, the interface between core structure and wing skin is preferably realised with temporary joints for maintenance reasons.
Integrating suction panels into the wing requires lowering the load carrying skin.The created depression provides the space for the suction panel integration.The thickness of the suction skin and the core structure therefore determine the depth of the depression.The depression reduces the planar moment of inertia of the profile and thus results in a weight penalty.Also, stress concentrations might occur in the transition area between NLF and LFC, in which the load carrying skin is lowered.If composites are used for the wing skin, delaminations have to be prevented there.

Interface requirements
Structural and aerodynamic requirements determine the design of the two structural interfaces joining the suction panel.While interface B in figure 3 is mainly driven by aerodynamic requirements, interface D is driven by structural requirements.In order to allow maintainability of the suction system, either of the interfaces should be designed as temporary joint.If interface B is a temporary joint, removing the suction skin gives access to the core structure.In contrast, if interface D is a temporary joint, the entire suction panel is exchangeable.
Continuous suction of the boundary layer is essential for laminar flow.Avoiding blockage of holes in the suction skin is therefore a key requirement for the interface between the suction skin and the core structure (interface B).If hole blockage cannot be avoided, blocked holes on lines perpendicular to the flow have the smallest negative impact on its turbulence intensity.The width of these lines should be kept to a minimum in order to avoid deceleration of the boundary layer flow.
Next to its impact on the skin porosity, the interface's impact on the surface quality has to be considered.Bolts or rivets accessible from the outside generate a step in the suction skin and disturb it's continuous porosity.Steps larger than 25 µm (0.001 Inch) possibly cause transition 1 .Therefore, perforations of the suction skin, for the purpose of the interface, should be avoided.Surface waviness caused by residual stresses, e.g.resulting from welding or tempering, should consequently be avoided as well.
While the skin-core interface (interface B) is dominated by aerodynamic requirements, the interface between core structure and load carrying wing skin (interface D) is dominated by structural requirements.In the xHLFC concept, the core structure simultaneously supports the suction skin and the wing skin.Interface D therefore needs to transfer significant loads from the wing skin to the core structure.However, the loads in the core structure cannot be mistaken with the far higher compression and tensile loads in the wing's upper and lower cover.

System considerations
An open question of the xHLFC concept is the ducting and pumping system.This paper does not intend to propose a detailed concept for ducting and pumping systems.However, the authors suggest two principal approaches for the system design.In the first approach, the drawn-out air is discharged through holes in the load carrying wing skin.In the second approach, the suction panel is thickened and divided into two sections.While the upper section allows pressure drop control, the lower section can be used as piping system.
The first approach places all piping and pumping systems inside the wingbox.This requires open holes in the load carrying skin.The holes cause peaks in the stress distribution and lead to an increased weight.Current aircraft wingboxes accommodate the fuel tanks, whereas in hydrogen driven or battery powered aircraft the energy storage is likely to be located in the fuselage, leaving free space in the wingbox.For installing and maintaining the ducting system, the inside of the wingbox needs to be accessible.This can be realised via access holes in the lower wing cover of each rib bay.Access holes also exist in state of the art aircraft structures.
The second approach requires a higher core structure, which has to consist of two different functional layers.The upper layer, underneath the suction skin, is identical to the core in the first scenario.In the lower layer, the flow stream needs to be guided rearwards.The pumping system has to be located behind the wingbox.In this approach, open holes in the primary structure can be avoided.Nevertheless, the increased thickness of the core structure further reduces the planar moment of inertia of the load carrying wingbox.

Structural design space
Low complexity and high robustness are key requirements for a viable xHLFC system.The authors chose a generic design approach in order to find robust and low complex design solutions fulfilling the aerodynamic and structural requirements defined in section "structural and aerodynamic requirements".The generic design approach collects feasible solutions for each functional unit, defined in figure 3. Partial design solutions include elements from existing concepts such as: (1) ALTTA (Application of hybrid laminar flow technology on transport aircraft) 10 , (2) TSSD (Tailored Skin Single Duct) 11 and (3) ECHO (Evaluation of a Certified HLFC Elevator Operation) 21 .Additionally, the generic design approach includes partial design solutions specific to additive manufacturing.The assessment of the partial design solutions with respect to the structural and aerodynamic requirements allows design decisions and ultimately leads to an overall design solution.This design approach may as well be applicable for HLFC and LFC concepts.The approach may be used for fuselages or other surfaces with the need of laminarisation, even though the focus here is on wing laminarisation.
Figure 9 shows the design space for the subsystems (A-E) collected in a "morphological box".The partial solutions include structural design, materials and manufacturing techniques.A global suction structure design is a set (tuple) of partial design solutions.The morphological box allows a comparison of different suction structure concepts regarding complexity, robustness, manufacturability and fulfilment of the aerodynamic and structural requirements.While the morphological box is an adequate tool to identify and assess global design solutions, it has its limitations regarding design details.

Potential of additive technologies
Additive Manufacturing (AM) technologies rapidly spread in the aerospace industry 22 .The design space of suction structures includes design solutions explicitly optimised for AM.In contrast to traditional manufacturing techniques, the manufacturing efforts and cost in AM do not necessarily depend on the geometrical complexity.That allows complex geometries and highly integrated designs.One example for a partial solution where traditional manufacturing methods reach their limits are TPMS.The high specific material properties 23 and the possibility of designing adaptive porosities make TPMS an ideal core structure candidate.While having significant advantages, AM also has its challenges.In the case of xHLFC suction panels these challenges are the limited total part size and the accuracy of printed parts.
Potential AM technologies are Fused Deposition Modelling (FDM), Stereolithography (SLA), Selective Laser Sintering (SLS) and Selective Laser Melting (SLM).FDM printers manufacture parts by adding lines of molten thermoplastics on a build platform.Available FDM printers produce rough surfaces which are not necessarily watertight and have a relative low accuracy.These characteristics rule out the FDM process for manufacturing suction skins.However, the cost and time efficient FDM technology may be an option for core structures.In the SLA process, printed parts are pulled out of a liquid resin tank where the layers are added by curing UV-sensitive resin with a light source.The characteristics of SLA printed parts are smooth, watertight surfaces and a high printing accuracy.Due to the printing process, the part size is limited and tiny holes tend to clog due to the capillary effect.While FDM and SLA only work with plastics, SLS and SLM can produce metal parts.In the manufacturing process, plastic or metallic powder is sintered or melted together by a laser.Both processes have a similar precision as SLA but produce parts with an increased surface roughness and increased mechanical anisotropy.

Existing HLFC structural concepts
The analysis of existing suction structures allows identification of the initial design space.This study examines the structural design of three well investigated HLFC approaches.These are the ALTTA concept 10 , the TSSD concept 11 and the ECHO concept 21 .Figure 10 compares the structural design of the three concepts.In the generic design space (figure 9), the concepts can be identified as tuples of partial solutions, e.g. the ALTTA concept has the tuple (A2, B2, C4, D1/5, E2).
The ALTTA design (A2, B2, C4, D1/5, E2) 10 is the most common approach for LFC and HLFC suction panels.In this concept, the suction skin is a laser drilled titanium sheet supported by discrete metal stringers.The stringers form chambers where the pressure in each chamber can be regulated by throttle holes.This allows a chamber-wise control of the suction rate.The stringers are connected to a load carrying substructure which includes the throttle holes.Examples of the ALTTA concept can be found in 3,4,10 .
The TSSD concept (A4, B2, C5, D5, E2) 11 is a relatively new approach where the suction rate is regulated by meshes of different porosities.While the suction skin is an etched stainless steel foil, the support structure is realised with meshes.The combination of different meshes allows regulating the pressure drop and therefore the suction rate.Instead of the chamber-wise suction rate control, here the suction rate can be controlled in discrete sections by varying the mesh layup.The meshes are welded to the suction skin and to bolts, which serve as connection to the attachment ribs.The TSSD concept has the advantage 8/20 that the full cavity below the support structure can be used as a duct and no additional second skin or throttle holes are required.However, the mesh-structure has only low specific mechanical properties.
In the ECHO concept (A2, B1, C2, D1, E1) 12 , CFRP Ω-stringers support a laser drilled titanium suction skin.The bonding of the suction skin to the stringers causes significant blocking of the suction holes.Reducing the blocking area consequently reduces the bonding area.In order to realise small bonding areas, Schollerer et al. 12 use a surface toughening concept for its crack arresting characteristics.As in the ALTTA concept, the Ω-stringers allow a chamber-wise regulation of the suction rate.The advantage of the ECHO concept is its high structural integrity.However, the conflict of bonding area and suction hole blockage is a real challenge.

Porous suction skins
The suction skin is the most vital component of the suction structure.Row A of figure 9 summarises different types of suction skins.Suction skins investigated so far are composite 8,24 and metallic 10,25 micro-perforated skins.Continuously porous suction skins are susceptible to hole blockage and have low material properties.Due to these disadvantages they are not subject to current laminarisation research.While metallic suction skins with a thickness t≥0.1 mm are classified here as sheets, skins with a thickness t<0.1 mm are classified as foils.Additional to metallic and composite suction skins, this study introduces additively manufactured suction skins.
The most common perforations are triangular patterned cylindrical holes in the submillimetre scale (≤ 250 µm) 1,20,24 .The skin porosity, determined by aerodynamic requirements, is usually given as 1%, resulting in a hole distance (pitch) in the range of 0.5 mm -2.5 mm.The hole pitch determines the maximum wall thickness of the supporting core structure that is possible without hole blockage.
The geometry of the hole plays an important role in keeping the boundary layer flow laminar 20 .Investigating the aerodynamic and structural properties of different hole shapes is therefore the basis in selecting a suitable geometry.So far, circular holes and slots have been investigated 1 .As mentioned in section "suction skin requirements", other hole geometries are feasible for boundary layer suction.However, the manufacturability of submillimetre holes with shapes such as ellipses, half-circles or triangles has not been shown yet.
Perforation manufacturing techniques depend on the skin type.Metallic sheets allow manufacturing perforations by mechanical and laser drilling 26,27 .Laser drilled holes generally have conical shapes with the bigger diameter where the laser beam enters the sheets.The same applies for electron beam drilling 28,29 .
Etching the micro perforations is possible in thin metallic foils 11 .Etching allows manufacturing cylindrical holes.Young et al. 24 investigated laser drilled CFRP composites.Drilled composites have two major disadvantages.The cutting of fibres reduces the load transfer capability and initialises crack propagation.Secondly, the protecting matrix is removed in the Wortmann and Althaus 8 include the perforation in the lamination process.They perforate the composite sheet with a needle bed during the curing process, therefore avoiding cut or unprotected fibres.All skin types reviewed here were tested in simulations, wind tunnel or flight tests which make them feasible solutions for suction skins in the xHLFC concept.
The additive manufacturing of suction skins has not been investigated so far.However, an assessment of its potential is possible, based on the characteristics of the manufacturing technique.Additive manufacturing allows almost arbitrary suction hole geometries including an inclination of the hole with respect to the surface.While manufacturing a thin skin without support structure is challenging, an integral concept for wing skin and support structure is feasible.Whether or not the surface roughness and waviness is in the bounds of the aerodynamic requirements is a subject of an ongoing test campaign.Section "test strategies" introduces a test strategy for investigating the aerodynamic properties of suction skins for active laminarisation.
The quality of suction holes impacts their aerodynamic behaviour.Only a high suction hole quality guarantees laminar boundary layer flow.Figure 11 compares the hole quality of laser drilled titanium sheets (a), etched stainless steel foils (b), SLA printed plastic sheets (c) and SLM printed stainless steel sheets (d).The CT scans show that each manufacturing technique results in a characteristic hole geometry.While the laser drilled sheet shows typical conical holes, the etched foil shows cylindrical holes with a slight inclination.SLA printed cylindrical holes tend to clog because viscous resin remains in the holes.SLM printed cylindrical holes mostly have uniform cross sections but rough surfaces due to partially melted metallic powder particles.Therefore, optimum hole geometries for printed holes are expected to differ from holes created with traditional manufacturing techniques.
Metallic skins investigated so far are made of aluminium, titanium and stainless steel.The material influences the feasible perforation manufacturing techniques.While laser drilling works with all materials, etching depends on the skin material and acid combination.While etching stainless steel foils results in a high hole quality, etching titanium foils results in a poor hole quality.
Next to the manufacturing of perforations, the material selection depends on environmental considerations.The suction skin has to withstand all environmental exposure because the application of protecting coat results in significant hole blockage.
Highly corrosive materials as aluminium are therefore not suitable for suction skins.Due to the lack of coating options, plastic surfaces as CFRP are not feasible in operation either.

Core structures
Supporting the suction skin is the main objective of the suction panel's core structure.Thin suction skins need dense supports in order to avoid surface waviness.For the removal of air below the suction skin, the support structure should allow significant internal flow.In the xHLFC concept, the core structure regulates the suction rate by pressure drop control.Therefore, the core structure needs a control mechanism such as throttling holes or variable porosity.Next to suction skin support and pressure drop control, the core structure functions as stiffening element for the wing skin.Consequently, it has to withstand significant mechanical loading.This section presents core structure concepts in agreement with these requirements.However, few concepts fulfil all requirements simultaneously.
Row C in figure 9 shows the core structure design space.Here traditional stringers are compared to corrugated structures, meshes and TPMS.Boermans 9 uses paper-made perforated fold cores designed by Klett and Drechsler 30 in an xHLFC concept for gliders.The ECHO concept uses laminated corrugated Ω-stringers 12 for HLFC in horizontal tail planes of transport aircraft.Horn and Seitz use meshes as core structure 11 for HLFC in vertical tail planes of transport aircraft.TPMS structures have not been used as support structure in active laminarisation concepts.In this study TPMS are included as an additive manufacturing specific solution.
While discrete stringers provide significant load bearing capacity, corrugated structures, meshes and TPMS provide almost continuous support.Meshes and TPMS allow significant internal flow without modification.Discrete stringers and corrugated structures can be perforated with throttle holes to incorporate internal flow and pressure drop control.Pressure drop control can be achieved in meshes by combining meshes of different porosities.TPMS are expected to allow pressure drop control by continuous modification of their porosity.
Perforations of discrete stringers and corrugated structures reduce their mechanical strength and stiffness.Paper-made perforated fold cores are therefore no feasible stiffening concept for transport aircraft.The same is true for meshes, despite of their good pressure regulation capacity.Laminated and metallic corrugated structures can be sized including the required perforations.Including perforations in sizing is also possible with metal stringers.Additively manufactured TPMS can be printed from engineering plastics and metals.Stringers, corrugated structures and TPMS are therefore feasible as stiffening concept for transport aircraft.
Aerodynamic requirements demand minimum hole blockage in the interface between core structure and suction skin.Corrugated structures however need to be bonded to the suction skin and require a minimum bonding area.Hole blockage therefore cannot be avoided with corrugated structures.In contrast, discrete plastic and metallic stringers can be micro welded to the suction skin, causing negligible hole blockage.Horn and Seitz 11 present a spot welding concept for meshes avoiding hole blockage.Integral concepts, where stringers and suction skin are milled from solid or printed as one part, also avoid hole blockage.An integral concept including the suction skin is feasible for TPMS structures as well.
Honeycombs are one of the most lightweight sandwich core structures used in aircraft design.They guarantee high stiffness and strength of sandwich materials with distributed core structures.However, for the application in suction panels they have two major disadvantages.The first disadvantage is that it is challenging to warp honeycombs around curved surfaces, such as aircraft wings.The second disadvantage is that they allow air transport only perpendicular to the panel's surface.In the case of xHLFC suction panels, air transport is required parallel to the panel's surface.
Structures with a similarly high mechanical capacity as Honeycombs are TPMS 23 .Next to continuous support and lightweight design, TPMS structures allow significant internal flow in all dimensions.The stiffness and strength of TPMS can be modified by controlling the relative density (solid volume per total volume).The relative density is determined by the wall thickness and the unit length of the unit cell.Figure 12 shows perforated stringers and TPMS structures for aerodynamic pressure drop investigations.For thin walled TPMS structures, the pressure drop is expected to depend mainly on their unit cell length, while the mechanical properties depend on the relative density.Consequently, the internal pressure drop can be modelled independent of the structure's stiffness and strength.Therefore, TPMS are an ideal candidate for a suction panel's core structure.

Load carrying wing skin
Row E in fig. 9 shows the design space for the load-carrying wing skin.Aluminium and CFRP are the most commonly used materials in commercial aircraft's primary structures.The extraordinary strength and stiffness characteristics of CFRP in fibre direction offer a large lightweight potential over metal alloys.On the other hand, the use of CFRP comes along with a variety of problems, e.g.complex design, high manufacturing costs and difficulties in detecting local damage.
The integration of holes in composites, which are needed e.g. for bolted joints and ducts, is challenging.Drilling holes damages the fibres and reduces the strength of the laminate in the affected area.Nevertheless, bolted joints are standard in joining composite parts 31 .A measure to reduce disadvantages from interrupted fibres is to make use of moulded-in holes 32 .Larger cutouts can be included in the layup.This however, increases the manufacturing complexity.Different reinforcements for open cut-outs in composite panels have been investigated e.g. in 33 , where bonded reinforcing rings at both sides of the cut-out were found to be most effective in order to reduce stress concentrations and improve buckling stability.The discharging holes at the suction panel attachment need to be of significant size in order to prevent pressure losses.Therefore, including cutouts in the laminate layup is a preferable solution in the xHLFC concept.
Integration of the suction panel into the wing structure requires a sink in the wing skin.In the transition area, the sink causes a kink in the wing skin.Here three-dimensional stress states increase the risk of delamination in composite structures.Thin ply laminates offer advantages in strength and fatigue behaviour and are therefore subject of ongoing investigation for this region.A review on the characteristics of thin ply laminates can be found e.g. in 34 .

Structural interfaces
The structural interfaces between porous skin, core structure and loaded wing skin hold together the suction panel and fix it to the wing skin.Row B in figure 9 shows the design space of the skin-core interface, while row D shows the design space of the panel-wing interface.Both design spaces are identical, but, with respect to the the different requirements, feasible design solutions are different.Next to traditional joining techniques, such as bonding, welding, clamping and bolting, the design space includes interface solutions specific to AM.
Traditional joining techniques for stiffeners are bonding, welding, bolting and riveting.While bonding, bolting and riveting require an areal connectivity, welding is an option for point and line connections.Of the suggested interface concepts, bolting is the only concept allowing for a temporary joint, as required for the maintenance of the suction panel.However, bonded and welded interfaces are the only joining options avoiding irregularities on the skin's surface.
Surface smoothness and minimum hole blockage are major requirements for the suction skin.Consequently, both requirements also apply for interface B, between suction skin and support structure.Welding potentially reduces the suction skin blocking area to a minimum and avoids surface irregularities.The ALTTA concept uses laser-welded joints between suction skin and core structure to reduce the blocking length below 1 mm 10 .In contrast, in the ECHO concept the suction skin is bonded to the support structure.Only by implementing surface toughening 12 the blocking length of the bonding area can be reduced to below 10 mm.The TSSD concept achieves negligible blockage of the suction holes by spot welded connections of the suction skin and the underlying meshes.None of the existing concepts use bolted connections for the skin-core interface (interface B).
Additive manufacturing allows two additional joining solutions for interface B. Solution B3 combines additively manufactured support structures with conventionally manufactured suction skins by directly printing the support structure on the suction skin.The success of the direct print depends on the material combination, e.g. the combination of polished stainless steel and FDM printed thermoplastics fails immediately after the print.Also direct print with SLM using metal powder is challenging due to the immense thermal loads during the printing process.
Another option for an additively manufactured interface between suction skin and support structure is integral manufacturing.Here, the suction panel is printed as one part combining suction skin and core structure.The integral concept requires only a minimal interface area between suction skin and support structure and causes no stress concentrations at the interface.In the integral concept, the suction holes can either be included in the printing process or added later applying conventional techniques.It is worth mentioning the integral approach is also possible with conventional manufacturing techniques, e.g. using high speed cutting and laser drilling from solid stainless steel or titanium.
Interface D connects the support structure to the load carrying wing skin.Structural support of the wing skin requires the interface to transfer significant loads.Additionally, the interface has to be realised with a temporary joint, because temporary joints are no feasible option for the skin-core interface.The only option for a temporary joint with significant load bearing capacity is a bolted joint connection.Bolted joints can be found in the TSSD concept 11 .In the ALTTA concept the support structure is bonded to the load carrying structure which prevents maintenance of the suction panel 35 .However, in the ALTTA concept the load carrying structure is not part of the wingbox (HLFC concept) and therefore the whole suction panel including load carrying structure can be replaced.

Structural design solutions
The objective of this research is a structural design for xHLFC suction panels.This section identifies feasible design solutions based on laminarisation concepts (section "wing laminarisation concepts"), structural and aerodynamic requirements (section "structural and aerodynamic requirements") and a generic design space (section "structural design space".Their level of complexity, functionality and robustness determines the quality of the solutions.While the design complexity is a result of this study, functionality and robustness of the design solutions have to be verified in aerodynamic and mechanic investigations.
In this section the authors suggest two design solutions for xHLFC suction panels.The "micro-stringer concept" is the design solution developed for traditional manufacturing techniques.The "TPMS concept" is the design solution developed 14/20 for additive manufacturing.While the micro-stringer concept has the advantage of well known and available manufacturing techniques, the TPMS concept combines high geometric complexity with low manufacturing effort.It is subject to future research in the cluster of excellency for Sustainable and Energy-Efficient Aviation SE 2 A 6 to characterise both concepts in a predefined set of tests.Section "test strategies" describes the test levels from the unit tests to the functional demonstrator.

Micro-stringer concept
The micro-stringer concept is a suction structure design solution which can be realised with traditional manufacturing techniques.It consists of a laser drilled porous titanium skin supported by perforated titanium stringers.For a minimum blocking length, the stringers are laser welded to the suction skin.The resulting suction panel is mounted on the loaded wing skin with bolts, which create a temporary joint and allow maintenance of the suction structure.In figure 9 the micro-stringer concept is defined by the tuple (A2, B2, C4, D5, E1/2).From a structural view the concept is very similar to the ALTTA concept, with the difference of perforated stringers allowing for internal air flow and pressure drop control.
The micro-stringer concept fulfils the structural and aerodynamic requirements defined in section "structural and aerodynamic requirements".While the porous titanium skin enables continuous suction over the full surface, titanium stringers provide sufficient support for the load carrying wing skin.Titanium is proven resistant to environmental exposure and can be combined with aluminium and CFRP wings.The stringers are bolted to the wing skin, which is the state of the art interface for stiffeners on metal wings.The perforation of the load carrying wing skin is reduced to a minimum by collecting the withdrawn air in the suction structure.All materials and manufacturing techniques are state of the art in aircraft industry.The baseline approach is therefore a feasible design solution for the xHLFC laminarisation concept.
Although manufacturing the micro-stringer concept is feasible, implementation of prototypes results in high manufacturing effort.In the process of testing and developing the suction structure, suction skin perforations, stringer spacing and stringer perforations are likely to change.Therefore, testing the concept requires rapid manufacturing of prototypes.Rapid prototyping using additive methods is consequently used to manufacture the prototypes for testing the micro-stringer concept.
Integral manufactured prototypes for aerodynamic testing raise concerns regarding surface and perforation quality.One solution for the problem of imprecise printed skin perforations and a roughly printed surface is the application of an etched perforated stainless steel foil.Bonding a stainless steel foil on top of SLA printed suction panel prototypes allows printing larger holes which are ultimately covered by the smaller holes of the steel foil.However, an areal bonding of the steel foil and the printed panel with negligible hole blockage requires precise alignment of the perforations.The suggested prototype concept combines rapid, economical manufacturing with high surface quality.This concept may be applicable to gliders and small drones as well.

TPMS concept
The additive manufacturing design concept takes advantage of the specific design opportunities of AM.This allows a highly integral design and complex geometries without a significant increase in manufacturing effort.From the perspective of AM, the suction structure can be thought of as one component where surface porosity, channel systems and connectors for the installation on the wing are all included.
The TPMS concept combines integral and geometrical complex solutions from the generic design approach.The concept is consequently defined by the tuple (A6, B6, C6, D5, E1/2) in figure 9.The integral design allows manufacturing of suction skin and support structure as one component.The core structure is realised with TPMS sheet networks, which combine a high load bearing capacity with an integrated channel system.Bolt sockets are integrated into the core structure, in order to realise a temporary joint at the panel-skin interface.Figure 13 shows an SLA printed demonstrator of an additively designed suction panel.
Compared to traditionally manufactured suction structures, the TPMS concept has three major advantages.Due to its integral design approach, suction holes can be placed where no support structure is attached.Therefore, hole blockage can be completely avoided.Since welding or bonding is not required at the skin-core interface, stress peaks are avoided in the structure.This prevents potential structural deformations and material weakening in the manufacturing process.The third advantage is the dense and robust core structure in combination with an integrated channel system realised by TPMS structures.These three conceptual advantages underline the necessity of further investigations of additive suction structures.
The TPMS concept raises two research questions.The first question is the printability of the suction skin, including perforations in the sub-millimetre scale and a high surface quality.The second question is the suitability of TPMS structures.Next to the load bearing capacity under compression, especially the load bearing capacity under shear loads is of interest.Additionally, the pressure drop control capability of TPMS needs to be investigated in detail.Ultimately the combined capability of pressure drop control and mechanical robustness determines a suitable type of TPMS structures, such as Gyroid or Schwarz Primitive.The questions of a printable suction skin and of the suitability of TPMS core structures are investigated in a series of tests (section "test strategies").One outstanding characteristic of TPMS structures is their energy absorbance in impact scenarios 37,38 .On aircraft leading edges bird strike and hail impact scenarios are usually the governing load cases.Although the TPMS concept suction structure presented here is intentionally developed for the xHLFC concept, its application on the leading edge seems promising as well.While in the xHLFC concept TPMS structures support the wing skin against buckling, in the HLFC concept larger TPMS structures protect the front spar against bird strike and hail.

Test strategies
The suction panel designs are tested in four test levels.In level I, the suction skin is investigated regarding the hole geometry, porosity and pressure drop characteristics.Level II investigates the mechanical properties and pressure drop characteristics of different core structures.Based on the results of level I and II tests, wind tunnel tests of 2D aerofoils (level III) and 3D wings (level IV) will be conducted in order to evaluate the design concepts.Figure 14 shows schematically the main idea of test level I-IV.
Test level I investigates the aerodynamic pressure drop characteristics of suction skins.The investigations include laser drilled, etched, SLA printed and SLM printed suction holes.The characterisation of suction holes allows ideal suction hole designs unique to each manufacturing technique.Samples of the suction skins will be tested in the DLR Large Flow Metre (LFM) as described in 25 .The pressure drop characteristic of the perforated skin allows to determine the correct pressure required beneath the suction skin.
The purpose of the Level II tests is the aerodynamic characterisation of core structures.Additive manufactured core structure test specimens including perforated stringers, Schwarz Primitive structures and Gyroid structures.Specimens as shown in figure 12 allow investigating pressure drop characteristics at defined volume flow rates.While the flow rate is controlled by a flow metre behind the test specimen, pressure tabs are integrated into the test specimens.The pressure drop characteristics of the core structures depend on the volume flow rate and the core geometry e.g.hole diameter or TPMS unit length.The relationship between pressure drop and core geometry allows tailoring of the core structure design in order to fit a pressure drop profile required for laminarisation.
While level I and II tests provide the fundamental knowledge required to design a suction panel, level III and IV tests aim at validating and improving the concept.Level III tests investigate the xHLFC laminarisation concept at a 2D wing.The wind tunnel tests results are used to validate and adjust the design prediction tools.Subsequently, the final design for a 3D xHLFC wing is tested in a wind tunnel as well.

Conclusion and outlook
This paper introduces an extended HLFC concept for full laminarisation of aircraft wings.The xHLFC concept combines the reduced system complexity of HLFC and the fully laminar boundary layer of LFC systems.The major requirements towards an active suction structure on the rearwards wing is almost continuous suction and significant load bearing capacity.While the continuous boundary layer suction prevents flow deceleration, the stiffness of the suction panel supports the wing skin against stability failure.
The authors propose two suction structure design solutions developed with a generic design approach.In the generic design approach partial solutions for suction skins, support structures and interfaces are collected and compared.Existing  solutions, such as ALTTA, TSSD or ECHO, are integrated in the design space and therefore considered in the design solutions.The first solution is the "micro-stringer concept".The micro-stringer concept is designed for traditional manufacturing techniques and consequently a robust concept fulfilling the structural and aerodynamic key requirements.The second design solution is the "TPMS concept".The TPMS concept takes advantage of the geometric complexity offered by additive manufacturing technologies.The TPMS concept is a lightweight design solution in agreement with the structural and aerodynamic requirements.
Both design concepts for xHLFC suction panels require further aerodynamic and mechanic testing.This paper suggests a 4 level test strategy for the aerodynamic investigation of suction panels.Parallel to the aerodynamic investigations, the mechanical properties of suction panels are essential.While the mechanical properties of the micro-stringer concept can be determined with classical engineering models, the characterisation of the TPMS concept is far more challenging.Especially the material characteristics of TPMS structures in combination with their pressure drop control capability are of great interest for the realisation of the TPMS concept.
The focus of this paper is the structural design of suction panels.Connections between adjacent suction panels are not part of this investigation.Also, the transition between the NLF zone on the front wing and the LFC zone on the rear wing is not included in the investigation.However both interfaces are essential when laminarisation is to be achieved on a full scale wing.Therefore such interfaces need to be subject to future investigations.

Figure 1 .
Figure 1.Boundary layer suction prevents deceleration of the lowest laminar layers and consequently delays the transition to turbulent flow

Figure 6 .Figure 7 .
Figure 6.Definition of the skin porosity for a triangular hole pattern with a uniform hole distance (a) and a hole diameter (d)

Figure 8 .
Figure 8. Cylindrical suction holes investigated by MacManus and Eaton 20 compared to suction holes not investigated so far

Figure 9 . 20 Figure 10 .
Figure 9. Generic design space for an active suction panel including partial solutions for AM

Figure 12 .
Figure 12.SLA printed core structure specimens for aerodynamic pressure drop investigations of perforated stringers and TPMS structures

Figure 13 .
Figure13.Sample of an integral manufactured suction panel with a Gyroid core structure and 1% skin porosity at 300 µm holes.The suction panel is manufactured on a Formlabs Form 2 SLA printer36