Structural Analysis of a Large-scale Model for the Wind Tunnel Test of a Multiadaptive Flap

Innovative studies on morphing wing structures with the highest potential to improve aerodynamic performance on large aircraft are running under the CleanSky2 platform to validate the morphing architectures on true-scale demonstrator through ground and wind tunnel tests. Research activities have been conducted to develop a revolutionary multi-modal camber morphing flap to enhance the aerodynamic behaviour of a new generation of regional aircraft within this challenging framework. The design and validation of the intelligent architecture capable of different morphing modes required for low-speed (take-off/landing) and high-speed (cruise)conditions. To enhance the significance and applicability of the wind tunnel test campaign, a significant scale factor of 1:3 was selected for the test article. In this study, general layout of the mechanical model and FE analysis performed both inner and outer flap will be presented. A comprehensive structural analysis of the flap test article was conducted to ensure the safety and effectiveness of the conceived mechanical solutions. Linear static analyses were performed using the finite element (FE) method within the Ansys Workbench® environment. These analyses aimed to assess the adequacy of the mechanical solutions and validate the test article’s structural integrity.


Introduction
Aircraft wings and wind turbine blades are typically designed for specific operational conditions.Integrating adaptive geometry adjustments that facilitate active load management and gust load mitigation has the potential to greatly enhance the overall system performance.Up to this point, numerous research projects have been conducted to explore active morphing systems.A variety of concepts have been investigated over the past few decades, and their findings have been consolidated and discussed in several review articles [1], [2], [3].In the Adaptive Wing project at the German Aerospace Centre (DLR), Belt-Rib concept was characterized by a sealed belt structure reinforced by in-plane spokes.The strategy involved the judicious allocation of structural flexibility to achieve the desired geometry modifications, offering the flexibility to optimize material and structural stiffness distribution according to specific design and application needs.[4] Through experimental investigations, it has been conclusively demonstrated that the Belt-Rib concept is a viable solution for achieving aerofoil shape control, fulfilling the criteria of geometric adaptability, load-carrying capacity, and lightweight design.Additionally, a subsequent investigation into this concept has suggested that through leveraging the aerodynamic and aeroelastic amplification effects of the aerofoil, it is possible to substantially diminish the actuation demands placed on the Belt-Rib system.[5] A flexible trailing-edge (TE) control surface was designed and subjected to testing as part of the Smart Wing program, which is a collaborative effort involving DARPA (Defense Advanced Research Projects Agency), AFRL (Air Force Research Laboratory), and NASA (National Aeronautics and Space Administration).[6] In their innovative design approach, the control surface was intricately partitioned into ten segments, with each segment equipped with an eccentuator mechanism that enabled out-ofplane shape modifications.The eccentuator, an innovation credited to Vought [7] during the late 1970s for variable-camber control surfaces envisioned for supersonic transport applications, involves a curved beam mechanism engineered to convert rotary input motion into simultaneous vertical and lateral translations.The TE (trailing edge) configuration under consideration consists of a trio of key elements: a highly deformable outer skin crafted from elastomeric silicone with significant strain tolerance, a flexible honeycomb core that reinforces structural stiffness in the through-thickness direction, and a central composite leaf spring designed to provide stabilization for both the core and the skin.Wind tunnel experiments revealed that TE segment exhibited the capacity to execute significant trailing-edge deflections, reaching an impressive angle of up to 20 degrees.[6] Airbus has conducted extensive research and analysis concerning to the variable camber concept for civil transport aircraft.[8] The chance in chordwise camber plays a central role in enhancing aerodynamic efficiency, by optimizing the lift-to-drag (L/D) ratio to match the current flight conditions.It is directly related to decrease fuel consumption.They formulated a concept to replace inflexible, fixed ribs with a flexible alternative characterized by substantial stiffness.The basic kinematics of the design can be seen in Figure 1.

Figure 1. Camber variation flap
In 2004, Poonsong [9] created and put into practice a multi-section wing model employing the NACA 0012 airfoil which had the capability for variable curvature.Each rib was subdivided into six sections, with the ability to rotate up to 5 degrees relative to the preceding one, all without causing notable interruptions on the wing's surface.Wind tunnel testing revealed that the lift generated by an analogous wing closely matched that of a one-piece airfoil.Nevertheless, there was a noticeable increase in surface drag attributed to the elevated flexibility demands placed on the wing's skin.
Within the framework of the JTI-Clean Sky project [10], specifically within the initial phase of the GRA (Green Regional Aircraft) low noise domain, efforts were directed towards the design and technological validation of an innovative architecture.This architecture was developed to facilitate camber adjustments in a flap segment, with the specific objective of augmenting the high lift capabilities of the next-generation, environmentally friendly regional aircraft in the 90-seat category.To provide an initial demonstration of the viability of the morphing solutions, the investigations were confined to a segment of the flap element.This segment was derived by slicing the actual flap geometry with a chord length of 0.62 meters using two cutting planes positioned 0.8 meters apart along the wing's span.[11] The conceptual arrangement for an articulated rib structure (finger-like), was conducted to enable the physical transformation from the initial airfoil configuration to the desired target configuration.The rib structure as seen in Figure 2 was conceptualized as a mechanical system consisting of four plates interconnected by hinges: B0, B1, B2, and B3.Each plate is linked to its adjacent plate via a hinge positioned along the rib's camber line, denoted as points A, B, and C.

Figure 2. Morphing rib layout
The actuated rib arrangement, as detailed previously, was replicated three times along the span of the flap segment.Stiffening elements, including spars and stringers, were used to link corresponding plates of adjacent ribs, as illustrated in Figure 3. Lightweight aluminum alloys were selected for all structural components.A physical prototype was fabricated to experimentally confirm the feasibility and robustness of the envisioned architectural concept.The functionality and static tests were conducted to demonstrate the device's capability to transform into the desired aerodynamic configurations and observe the aeroelastic behavior of the structure under the limit loads.The entire shape transition was captured using a high-resolution camera.Comprehensive photographic documentation of the morphing process was performed, resulting in a precise alignment between the realized section shapes and their intended counterparts.Throughout the static testing procedures, the strain gauges positioned within the device's most highly stressed region did not register any lasting deformations.With the validation of the wing camber morphing design, the investigation area for the advanced design of the technological demonstrator was expanded to half of the outer wing flap segment of the reference aircraft.
With the successful demonstration that flap camber morphing could be achieved through an intelligent architecture that closely resembled conventional aeronautical devices, thereby holding promise for certification, the project advanced into a more complex phase focused on comprehensive technology validation.Within the innovative design framework, the morphable flap section boasts the following dimensions: a wingspan extending over 3.60 meters, a root chord measuring 1.20 meters, a tip chord span 0.9 meters, and a maximum thickness of 0.24 meters (Fig 4).

General Layout of Wind Tunnel Test Article
Following these encouraging outcomes, additional investigations were conducted as part of the Clean Sky 2 program.The goal was to improve the aerodynamic performance of the 90-Seat Turboprop regional aircraft, which has a wing spanning 5.15 meters and a mean chord equal to 0.6 meters.These studies aimed to assess the practicality of a full-scale morphing prototype and address the complexities associated with tapered geometrical configurations, significant limit loads, and the incorporation of multiple morphing modes.Moreover, an added functional requirement was established, specifying that the flap must feature three separate morphing modes based on the particular flight condition.These modes were outlined as follows: • Morphing Mode 1: overall camber morphing to optimize high-lift performance during take-off and landing, particularly when the flap is deployed,  To enhance the significance of the wind tunnel testing, a highly challenging scale ratio of 1:3 was adopted for the test model.This decision was coupled with the replication of the exact Mach numbers anticipated during actual flight.However, when scaling the model, it was not possible to directly scale the actuators and mechanical structures used for shape transformation.Therefore, it was necessary to design an entirely new scaled model with the same characteristics as the full-scale model.Scaled model designs were created for both the inner and outer flaps using a similar design philosophy.The (outer) morphing flap's wind tunnel model, as shown in Figure 5, is notable for its segmented structure.

Figure 6. Exploded View of Outer Flap
Each block slots into its designated position, similar to the well-known Tetris puzzle, and subsequently interlocks with its counterpart through the use of positive elements (pins).Finally, the mechanical connection is firmly established using screws.The physical rotation between adjacent flap segments is achieved through the utilization of lubricated hinge connections positioned between each pair of companion blocks (light blue elements with item number 9 in Figure 6).

Hex Elements
The entire structure of the flap is fabricated from Al-alloys 7075 and 2024, except for specific components within the rib's internal mechanisms, which are constructed from steel.Table 1, there is a summary of the chosen materials, mesh details including mesh type, and the number of elements for each part.
Figure 7 shows the deformation of flap under the aerodynamic loads.The maximum displacement equals to 0,014 meter and occurs tab section of the flap.The small deformation and rotation observed in the analysis results serves as evidence that we have accurately tested the correct form in the wind tunnel.The most significant stress concentrations occur in the lower covers, with the maximum Von Mises stress is measured at 863,67 MPa near the rear hole (Figure 8).Although, it is higher than Al7076 -T6 yield stress (503 MPa), the main reason of the high stress concentration is singularity.

Conclusion
When dealing with morphing surfaces, their value lies primarily in the potential aerodynamic enhancements they can offer at the aircraft level.Therefore, wind tunnel tests play a critical role in quantifying the magnitude of these improvements, essentially determining whether these gains are substantial enough to offset the technology's drawbacks, such as increased complexity, added weight, and effects on aircraft maintenance plans and procedures.
Creating wind tunnel test models that include morphological capabilities is often a challenging endeavor, primarily due to the scaling of mechanisms responsible for enabling morphological changes, which poses significant difficulties, and these structures cannot be directly scaled.
Within the framework of the CleanSky program, a multi-modal camber morphing flap model was created for the purpose of assessing a novel technology in a wind tunnel setting.Despite the model's substantial dimensions, with a wingspan of 1.7 meters and an mean geometric chord of 0.25 meters, a new design featuring similar functional characteristics to the full-scale device was necessary, as direct scaling of the morphological mechanisms from the full-scale device was not feasible.The suitability of the chosen mechanical and structural solutions was verified through the utilization of simplified sizing methods, complemented by a finite element analysis.

Figure 4 .
Figure 4. Investigation domain and tab deflections[13] Morphing Mode 2: was characterized by tab-like morphing, which involved the ability to rotate the flap tab both upwards and downwards within the range of [-10° to +10°].This function helps to maximize the aerodynamic efficiency of the wing during the cruising phase.• Morphing Mode 3: Twisting the tip by ±5 degrees along the outer span of the flap.

Table 1 .
Material and Mesh Details of Parts Indicated with Item Numbers