PATHWAYS TOWARDS THE ULTRA-EFFICIENT AIRCRAFT #2

The pursuit of higher efficiency in aviation has led to aircraft designs featuring higher aspect ratios and increasingly lightweight structures. While these advancements offer benefits—such as improved aerodynamic efficiency, which leads to lower carbon emissions and reduced fuel consumption—they also introduce undesirable aeroelastic phenomena, such as the interaction between flight and structural dynamics, flutter, deterioration of flying and handling qualities, limit cycle oscillations, reduction of the flight envelope, and increased sensitivity to atmospheric disturbances. Active control systems can mitigate these challenges by dynamically influencing the aerodynamic flow around structures or directly altering structural shapes. This can result in increased flutter margins, suppression of limit cycle oscillations, decoupling of structural and flight dynamics modes and mitigation of maneuver- and gust-induced loads. Achieving these outcomes requires carefully designed active control algorithms, which are typically grounded in accurate reduced-order dynamic models of the system.

In the first part of this session, researchers from the Technical University of Berlin will present their work on developing a free-flying flight dynamic model of an aeroelastic aircraft. Their approach combines first-principles modeling with high-complexity simulations and real-world flight test data. They will also discuss the potential of using artificial intelligence tools to optimize model parameters. Following this, the team will demonstrate how to design a multi-objective flight control system that addresses various performance metrics, including stability margins, handling qualities, structural load attenuation, and even drag reduction.

In the second part of the session, researchers from Delft University of Technology will explore a data-driven approach to aeroelastic control design. This paradigm emphasizes real-time adaptability based on incoming data, rather than relying solely on offline, finely tuned models. Several application examples will be presented, including a data-driven smart vortex generator for load alleviation, a data-enabled policy optimization strategy for unified gust load alleviation and active flutter suppression, and an online aerodynamic performance optimization framework using event-triggered adaptive dynamic programming for managing limit cycle oscillations.

The growing interest in artificial intelligence and data-driven methodologies raises important questions about their reliability and certifiability. In the final part of the session, researchers from the University of Stuttgart will share their perspectives on this issue. They will argue that assurance is key to ensuring AI systems function as intended across the entire lifecycle—from specification and development to verification and operation. In the safety-critical domain of aerospace, it is essential to understand how and when AI behaves reliably, and how assurance methods can support compliance with rigorous certification standards.

Despite being promoted as a crucial component in the advancement of future aircraft, multidisciplinary design optimization (MDO) tools based on high-fidelity prediction capabilities have not yet achieved widespread industrial implementation in the aeronautics sector, even after two decades of rigorous research. In this context, several obstacles have been encountered by aircraft designers throughout the years. Yet, addressing those obstacles unveiled several others. Much of the MDO academic research culture, for example, emphasizes on achieving substantial improvements in the design performance at the expense of neglecting significant design constraints. In contrast, the primary responsibility of aircraft designers is to adhere to these constraints. This exposes two realms with similar interests, but few intersections.

In response, a joint research-industry MDO benchmark based on the DLR-F25 aircraft configuration will be proposed to address the necessity of redirecting MDO research to better align with industry requirements, while challenging the current capabilities of MDO available today to automatically support the design process. The next-generation DLR F25 configuration features a high aspect ratio wing, an ultra-high bypass-ratio turbofan engine and a landing gear integrated into the extended belly and was developed collaboratively by DLR and Airbus. This benchmark will offer the necessary geometry and computational models as well as a comprehensive outline of the design task.

To align the developments of MDO in the research field with the needs of the aircraft industry, a set of MDO tasks at different levels of complexity will be defined around the DLR-F25 configuration. The tasks consider industry-specific requirements and design constraints, which drive the development of methods to be aligned with industry demands. This includes global and local design constraints that are typically neglected in MDO research studies, yet crucial in the daily work of aircraft designers. Additionally, this approach ensures a thorough evaluation of the feasibility of current MDO capabilities and processes.

To achieve that goal, the workshop aims at engaging experts from industry, research centers and universities to refine the definition of the design tasks and to motivate them to participate in the benchmark by contributing models and by exercising their MDO capabilities.

In the first part of this session, the motivation for establishing a new and comprehensive MDO Benchmark will be discussed, followed by a description of the DLR F25 aircraft configuration including top-level aircraft requirements, the disciplines engaged in the MDO task as well as the available disciplinary models. Then different levels of geometrical complexity and the different MDO tasks will be presented.

In the second part of the session, we will shift focus from presentation to collaboration. This interactive segment will engage participants in open discussion and guided brainstorming to collectively refine and prioritize the proposed MDO benchmark tasks. Experts from academia, industry, and research institutions will be invited to share their perspectives on the applicability, realism, and potential impact of the benchmark, especially in terms of bridging current research methodologies with industrial design practices. Key topics will include:

• The integration of real-world design constraints into MDO tasks

• The balance between model fidelity, computational feasibility, and design relevance

• Opportunities for modular contributions of disciplinary models

• Identifying critical gaps in current MDO tools and processes

Participants will also be encouraged to provide feedback on the proposed structure of the benchmark and explore possible modes of collaboration, including contributing models, data, or optimization workflows. The ultimate goal of this segment is to foster a shared sense of ownership and initiate a community-driven effort that not only challenges existing MDO capabilities but also advances them in ways that are directly applicable to industrial aircraft design.

By the end of the session, we aim to establish a preliminary network of contributors and set the stage for a sustained, collaborative benchmark campaign that will serve as a catalyst for more industry-aligned MDO research.

The Clean Aviation Joint Undertaking is at the forefront of European research, driving the development of disruptive technologies crucial for achieving a climate-neutral aviation sector by 2050. A core pillar of this effort, reflected in the overall workshop theme "PATHWAYS TOWARDS THE ULTRA-EFFICIENT AIRCRAFT," is the pursuit of enhanced aerodynamic efficiency. A key enabler for this is the adoption of High Aspect Ratio (HAR) wings, which significantly reduce induced drag, thereby lowering fuel burn and emissions.

This session will provide a critical deep dive into the most promising structural and aerodynamic configurations being explored to unlock this efficiency gain. While the ultimate goal is ultra-efficiency, achieving it requires overcoming significant technical hurdles, especially concerning wing structural weight, aeroelastic stability, and integration challenges.

The session will open with a series of expert presentations offering state-of-the-art insights on:

• The exploration on novel approaches to structural load management and fuel storage for LH2 configurations (Dry-wing concept).

• The need for innovative materials and design methods necessary for realizing these slender structures.

• A broad comparative analysis of potential cantilever and strut-braced wings concepts.

• The crucial modelling and simulation issues associated with the aeroelastic and aerodynamic analyses of these structures (stability, performance).

And more….

Following these presentations, the session will transition into a focused open discussion between the experts and attendees, structured around a few pivotal questions shaping the next generation of aircraft design, including but not limited to :

• Current State and Way Forward for Strut-Braced Wings (SBW): We will evaluate the current technological readiness of SBW concepts, identifying key recent successes and mapping the necessary steps to transition from demonstrator to operational viability.

• "Showstoppers" for SBW: A candid assessment will be made of the major technical and certification challenges preventing the immediate adoption of SBW, and the specific research efforts required to lift these issues, encompassing areas like impact protection, load alleviation, and fail-safe design.

• The Cantilever vs. Strut-Braced Trade-off: The discussion will address why the conventional cantilever wing remains the dominant configuration in many current Clean Aviation projects, exploring the trade-offs in structural complexity, integration risk, and maturity level. We will seek to define if there exists a definitive and well-defined trade-off methodology, or if the optimal wing design is highly sensitive to the specific mission profile and aircraft size, thereby necessitating a more nuanced, case-by-case evaluation.

This session is essential for researchers, engineers, and program managers aiming to understand the critical design choices and technological pathways that will define the ultra-efficient aircraft of the future. Join us to contribute to this vital discussion on the future of aerodynamic efficiency.

The maturation of High Aspect Ratio (HAR) wing technologies, critical for next-generation fuel-efficient and environmentally sustainable aircraft, demands a robust validation and proof-of-concept framework to advance key enablers across Technology Readiness Levels (TRLs). HAR wing configurations introduce complex interactions among aerodynamics, aeroelasticity, and advanced systems such as aeroelastic stability and load control. To expedite TRL advancement, traditional validation techniques such as wind tunnel (WT) testing and full-scale flight prototypes must be complemented with novel testing and validation techniques that bridge existing gaps and enable a more synergetic development process.

Conventional WT testing, while offering tests in a highly controlled environment supported by vast experience and track record, faces significant limitations for HAR wing validation such as Reynolds number mismatch, limited capability to conduct dynamic aerodynamic/aeroelastic tests integrating crucial advanced systems for aeroelastic stability and loads control. Likewise, full-scale prototypes, though essential for certification-level demonstrations, are prohibitively expensive for early-stage testing and offer limited flexibility for iterative design exploration due to operational and regulatory constraints.

A complementary validation strategy that integrates increased-fidelity wind tunnel tests, scaled flight demonstrators, and full-scale flying testbeds has the potential to address these challenges. Increased-fidelity WT tests are crucial for isolated aerodynamic/aeroelastic validation and basic proof of concept to progress TRL up to 3–5 for individual technologies. Scaled flight demonstrators serve as a critical intermediate step (TRL 4–6), allowing for evaluation of HAR wing aeroelastic behaviour, system interactions in flight-like conditions. These platforms enable iterative development while being more cost-effective and agile than full-scale prototypes. Finally, full-scale flight testbeds (TRL 6–8) provide the necessary environment for end-to-end system validation, integration with realistic operational loads, and demonstration of certifiable performance.

The aim of the session is to explore and define a validation strategy and technological roadmap to accelerate the maturation of key enablers for HAR wings. Emphasis will be placed on identifying and addressing limitations in conventional validation methods and establishing a synergetic approach across multiple testing platforms. Specifically, the workshop will aim to:

1. identify gaps in current validation capabilities of conventional wind tunnel testing and full-scale prototypes;

2. define the roles of increased-fidelity wind tunnel testing, scaled-flight testing, and full-scale flying testbeds in a complementary and integrated framework;

3. explore possibilities for data exchange and methodological interaction between these techniques;

4. assess the potential of the proposed new validation methods to support HAR wing development in terms of risk reduction, cost savings, and performance enhancement;