This paper introduces an approach for parameterizing airfoil geometries using a Variational Auto encoder (VAE)with a focus on achieving a low-dimensional and interpretable model. The primary focus is to facilitate efficient use in design optimization environments by capturing essential airfoil features in a minimal number of latent dimensions. To address the black-box nature of VAEs and enhance interpretability, a correlation analysis is performed to uncover the relationships between the airfoil properties and these inferred latent dimensions. Key to this research is the incorporation of both geometric and aerodynamic properties in this analysis, enabling the generation of air foils with desired aerodynamic characteristics through manual tuning of the latent vector by a designer. The method is evaluated using the extensive UIUC airfoil database, which includes a diverse range of airfoil categories. The VAE is trained on airfoil surface coordinate points, and the generated output geometries are refined using a composite Bezier curve to smooth out local imperfections. Results demonstrate that theVAE can successfully extract and parameterize key airfoil features using a limited number of interpretable latent parameters. These parameters show clear correlations with geometric and aerodynamic airfoil properties, providing a practical and understandable parameterization model that facilitates the intuitive generation of new airfoil designs through smooth interpolation of the training data.

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A design can only be as good as its mathematical representation. In engineering design optimization, the chosen method of parameterization can have significant impact on the outcomes. This paper introduces a novel methodology for airfoil design parameterization utilizing variational autoencoders (VAEs), a class of neural networks known for their proficiency in reducing dimensionality. However, a significant challenge with VAEs is the interpretability of the encoded latent space. This work aims to address this issue by creating a network with an interpretable latent space, yielding parameters that are understandable to humans. The effectiveness of this approach is evaluated using the comprehensive UIUC airfoil database, which offers a diverse range of airfoil shapes for analysis. We show that a VAE can successfully extract key features of airfoil geometries and parameterize them using six parameters, which show a clear correlation with airfoil properties in a way that remains understandable by the designer. Additionally, it smoothly interpolates the data points, allowing the generation of new airfoils and thus offering a practical and interpretable airfoil parameterization.@en

Advanced Air Mobility (AAM) vehicles are usually capable to operate in both forward and vertical flight thanks to their design configurations featuring multiple (tilting) rotors. Such maneuvering agility can be leveraged to minimize their acoustic footprint during operations close to the ground. The present work compares the acoustic footprint of optimal trajectories of AAM aircraft using low-order aero-acoustic models. The methodology is applied to the case of an AAM quad-rotor aircraft, which is modelled as a rotating point-mass with three Degrees of Freedom and two input controls. The trajectories are globally optimal in the sense of standard mission objectives, such as maneuver time or traveled horizontal distance, and are subject to realistic performance constraints. Results show that minimum-time trajectories generate higher and more concentrated noise footprints compared to minimum-distance trajectories, which distribute noise levels more evenly and result in overall lower noise footprints. Departure trajectories exhibit lower noise levels than approach trajectories. Adopting minimum-distance trajectories can significantly reduce noise impacts for both approach and departure maneuvers. @en

This paper introduces a methodology for predicting the noise footprint of urban air mobility (UAM) vehicles in vertiport environments during approach and departure maneuvers. The methodology integrates a flight mechanics model, an aerodynamic model, aeroacoustic models, and a noise propagation model. The flight mechanics model employs a point-mass dynamic model to determine optimal trajectories based on prescribed criteria. The aerodynamic model utilizes blade element momentum theory, while aeroacoustic models include frequency-domain acoustic formulation and a noise propagation model based on Gaussian beam propagation method, which accounts for 3D variations in terrain and atmospheric profiles. Noise footprints are computed for several waypoints, featuring significant variations in vehicle flight speed and pitch angle, and are subsequently compared. It is observed that variations in vehicle pitch angle significantly influence noise radiation directivity. Specifically, when the vehicle pitches up, on-ground noise levels beneath the source increase, while those at receivers farther away decrease. Conversely, when the vehicle pitches down, on-ground noise levels beneath the source decrease, while those at receivers farther away increase. Additionally, as flight speed increases, on-ground noise levels rise accordingly regardless of whether the vehicle pitches up or down. This trend suggests that lower flight speeds during approach and departure maneuvers are desirable to reduce the noise footprint. Furthermore, it is noted that building blocks further shield incoming noise and decrease noise levels at receivers distributed behind them. These findings underscore the necessity of the proposed approach in evaluating the noise footprint of UAM flight trajectories in vertiport environments, providing valuable insights for early design stages.@en

This paper presents a System of Systems Engineering approach to aircraft design. For this purpose, conventional design disciplines are coupled with Agent-Based Modeling and Simulation (ABMS) defining a unique optimization problem. The proposed methodology is applied to design seaplanes for an on-demand transportation system connecting the Greek islands. Within this network, diverse scenarios are analyzed by varying parameters of the model such as fleet size and travel demands at each seaport. The objective is to show the impact of including ABMS in the design workflow on the optimized seaplane design parameters. The optimum designs are evaluated on the basis of a number of performance metrics, to assess to what extent they can aid (or substitute) existent maritime means of transportation. The results reveal optimal fleet performance for seaplanes characterized by lower cruise speeds and passenger capacities, as compared to those derived from conventional methodologies and to existing designs. @en

This paper introduces an approach for parameterizing airfoil geometries using a Variational Autoencoder (VAE) with a focus on achieving a low-dimensional and interpretable model. The primary focus is to facilitate efficient use in design optimization environments by capturing essential airfoil features in a minimal number of latent dimensions. To address the black-box nature of VAEs and enhance interpretability, a correlation analysis is performed to uncover the relationships between the airfoil properties and these inferred latent dimensions. Key to this research is the incorporation of both geometric and aerodynamic properties in this analysis, enabling the generation of airfoils with desired aerodynamic characteristics through manual tuning of the latent vector by a designer. The method is evaluated using the extensive UIUC airfoil database, which includes a diverse range of airfoil categories. The VAE is trained on airfoil surface coordinate points, and the generated output geometries are refined using a composite Bezier curve to smooth out local imperfections. Results demonstrate that the VAE can successfully extract and parameterize key airfoil features using a limited number of interpretable latent parameters. These parameters show clear correlations with geometric and aerodynamic airfoil properties, providing a practical and understandable parameterization model that facilitates the intuitive generation of new airfoil designs through smooth interpolation of the training data. @en

This paper compares optimum control surface layouts designed and sized to obtain the same Flying Qualities (FQs) performance with different Control Allocation (CA) methods, and proposes novel layouts for staggered box-wing aircraft aimed at transonic commercial flight. Box-wings allow the installation of redundant control surfaces for which no explicit role can be defined a priori, but present challenges related to aerodynamic interaction and interference effects. To evaluate the impact of different CA methods on top-level layout parameters, the cumulative control surface span and the properties of the Attainable Moment Set (AMS) corresponding to each control surface layout are used. A physics-based multi-disciplinary optimization framework is developed to size the control surface layout. FQs are evaluated through non-linear flight dynamics simulation, using a variable-architecture flight control system that allows their assessment as a function of different CA methods. The most traditional Mechanical Gearing and Ganging (MGG) approach, the Constrained Pseudo-Inverse (CPI) method and the Direct Control Allocation (DCA) method are compared. Results show that different optimum layouts exist with comparable cumulative span, for a given CA method and same FQs requirements. The traditional MGG approach requires the largest cumulative control surface span, but retains the best ability to generate coupled roll-pitch moments. DCA requires the smallest cumulative control surface span, with the largest AMS volume. By using this method, a novel layout featuring a mid-wing rear elevon has been discovered, which reduces the total required control surface span by about 13%, results in a 3.7% increase of span available for flaps on the front wing, and avoids detrimental aerodynamic interaction effects near the wing-tail intersection region.@en

This study evaluates the flight performance of a Flying-V aircraft designed for transonic passenger transport. The Flying-V is a disruptive aircraft configuration that has shown to possess promising aerodynamic performance during preliminary design. It is compared to a competitor aircraft reminiscent of the Airbus A350-1000, for the same thrust-to-weight ratio and a similar number of passengers. The most common performance metrics for the take-off, landing, climbing and cruise phases have been assessed using a modular flight mechanics model. Take-off and landing performance are evaluated through flight simulation using a simple Euler method, while climb and cruise performance are evaluated in trimmed, steady-state conditions. Only instantaneous performance is available for the latter two phases. The Flying-V outperforms its competitor for basically all investigated metrics. Take-off length is shorter, mainly due to a larger tail strike attitude that reduces the minimum unstick speed. Service and absolute ceiling are higher, and its superior lift-over-drag ratio results in a 21% increase in the cruise range parameter. Landing field lengths are similar for both aircraft, but the Flying-V has a significantly larger pitch angle during approach. This causes longer de-rotation length, and a large obscured segment of the pilot’s vision which could be problematic during operations.@en

The objective of the present dissertation is to show how redundant control surfaces can be exploited to shape an aircraft dynamic behavior and obtain desired flight mechanics performance. This is achieved by introducing novel approaches and methods for flight mechanics and control, mainly revolving around original implementations of traditional formulations of the Control Allocation (CA) problem. Control surfaces and, more in general, control effectors are defined as redundant if they are capable to independently control the same motion axis of the aircraft. Redundant effectors can be linked together, and to the pilot input, in many ways according to different optimality criteria and/or performance objectives. In particular, the research presented in this dissertation focuses on the possibility to achieve Direct Lift Control (DLC). The latter is intended as the ability to use control effectors to alter the aircraft lift "without, or largely without, significant change in the aircraft incidence, and ideally is meant not to generate pitching moment." The ability to do so is essentially dependent on the position of the Control Center of Pressure (CCoP), which is the center of pressure of aerodynamic forces solely due to control surface deflections. In case of a single control surface dedicated to DLC, the CCoP coincides with the control surface itself. In case of redundant control surfaces, their deflections can be coordinated to induce the position of the CCoP towards some preferred location, as allowed by the architecture of the aircraft and the available control effectiveness. The first three chapters of the dissertation are dedicated to establishing the societal, scientific, and technical background underlying the subsequent research studies, including an overview of the CA problem for redundant control effectors. The following four chapters present, in this order: an evaluation of the mission performance of a staggered box-wing aircraft model designed for commercial transonic operations; a comparison of different CA methods on the design of an optimum control surface layout for a box-wing aircraft, with control surface both fore and aft the aircraft center of gravity; a trim problem formulation which employs forces and moments due to the aircraft control surfaces as decision variables, to maximize control authority, minimize aerodynamic drag or obtain a prescribed pitch angle; a CA-based formulation aimed at altering the characteristics of the transient response of an aircraft by exploiting the properties of the CCoP. The conclusive chapter presents a comprehensive, top-level recap of the main aspects and topics covered within the dissertation. It reflects on the classic meaning of DLC, and what it means to achieve it with redundant control surfaces that are not expressly dedicated to it. With some considerations on the needs of aviation market, it speculates on the practical role of unconventional aircraft configurations in the near future. Lastly, it provides suggestions for improvements and future research studies. @en

This paper presents a generic trim problem formulation, in the form of a constrained optimization problem, which employs forces and moments due to the aircraft control surfaces as decision variables. The geometry of the Attainable Moment Set (AMS), i.e. the set of all control forces and moments attainable by the control surfaces, is used to define linear equality and inequality constraints for the control forces decision variables. Trim control forces and moments are mapped to control surface deflections at every solver iteration through a linear programming formulation of the direct Control Allocation algorithm. The methodology is applied to an innovative box-wing aircraft configuration with redundant control surfaces, which can partially decouple lift and pitch control, and allow direct lift control. Novel trim applications are presented to maximize control authority about the lift and pitch axes, and a “balanced” control authority. The latter can be intended as equivalent to the classic concept of minimum control effort. Control authority is defined on the basis of control forces and moments, and interpreted geometrically as a distance within the AMS. Results show that the method is able to capitalize on the angle of attack or the throttle setting to obtain the control surfaces deflections which maximize control authority in the assigned direction. More conventional trim applications for minimum total drag and for assigned angle of elevation are also explored.

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This paper presents a Control Allocation formulation aimed at altering the dynamic transient response of an aircraft by exclusive means of the aerodynamic effectiveness of its control effectors. This is done, for a given Flight Control System architecture and, optionally, closed-loop performance, by exploiting the concept of Control Center of Pressure, i.e. the center of pressure due to only aerodynamic control forces. Two formulations are proposed, and their advantages and disadvantages presented. The first is based on the straightforward augmentation of the control effectiveness matrix, the second on a weighting matrix to prioritize control effectors. The latter is implemented in three application studies on a box-wing aircraft configuration with redundant control surfaces: a simple pull-up maneuver, a trajectory tracking task, and an altitude holding task in turbulent atmosphere. Results show that the proposed formulation can significantly impact performance metrics that are closely related to the aircraft transient response. In the best case scenario, the aircraft is able to completely cancel the non-minimum phase behavior typical of pitch dynamics, hence achieving a sharp initial response to longitudinal commands. If compared to a standard Control Allocation algorithm, the proposed formulation results in improved tracking precision, better disturbance rejection, and a measurably improved feeling of comfort on board.

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A unified approach to aircraft mission performance assessment is presented in this work. It provides a detailed and flexible formulation to simulate a complete commercial aviation mission. Based on optimal control theory, with consistent injection of rules and procedures typical of aeronautical operations, it relies on generalized mathematical and flight mechanics models, thereby being applicable to aircraft with very distinct configurations. It is employed for an extensive evaluation of the performance of a conventional commercial aircraft, and of an unconventional box-wing aircraft, referred to as the PrandtlPlane. The PrandtlPlane features redundant control surfaces, and it is able to employ Direct Lift Control. To demonstrate the versatility of the performance evaluation approach, the mission-level benefits of using Direct Lift Control as an unconventional control technique are assessed. The PrandtlPlane is seen to be competitive in terms of its fuel consumption per passenger per kilometer. However, this beneficial fuel performance comes at the price of slower flight. The benefits of using Direct Lift are present but marginal, both in terms of fuel consumption and flight time. Nonetheless, enabling Direct Lift Control results in a broader range of viable trajectories, such that the aircraft no longer requires cruise-climb for maximum fuel economy.

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This paper presents a method to find trimmed flight conditions while maximizing the available control authority about one or more motion axes. Maximum pitch-up, or lift-up, control authority could find interesting application in aborted landing situations, while maximum balanced control authority about all motion axes is a reformulation of the classic concept of minimum control effort. The trim problem is formulated in the form of a constrained optimization problem. The constraints and the objective function are obtained by exploiting the geometric properties of the Attainable Moment Set, a convex polytope containing the forces and moments attainable by the aircraft control effectors. The method is applied to an innovative box-wing aircraft configuration called PrandtlPlane, whose double wing system can accommodate a large number of control surfaces, and hence allow Pure Torque and Direct Lift Control possibilities. Control surface deflections are compared for trim conditions with maximum control authority in the pitch axis, in the lift axis, and maximum balanced control authority, for symmetric and asymmetric flight. Results show that the method is able to capitalize on the angle of attack or the throttle setting to obtain the control surfaces deflections which maximize control authority in the assigned direction.@en

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Sub-scale Flight Testing (SFT) is potentially useful in predicting aircraft flight behaviour, especially in the case of unconventional designs for which legacy information is unavailable and wind tunnel tests are unable to predict aircraft dynamics. A necessary condition for SFT is the design of properly scaled models. However, even in case of perfect scaling, the sub-scale model needs adequate flight performance and handling qualities to enable the execution of flight tests. Thus, the (static and dynamic) stability and control (S&C) and handling qualities (HQ) of sub-scale designs should be evaluated accurately as well as quickly, to allow conceptual design iterations. To this purpose, we propose the use of a 3D panel method (3DPM) for the generation of the non-linear aerodynamic database, in combination with a non-linear flight dynamics analysis. Two main challenges affect the proposed approach. The first concerns the validity of the low-fidelity 3DPM data for the assessment of the sub-scale design S&C and HQ. The second is about the time consuming and error-prone pre/post-processing activity demanded by the hundreds of analysis cases for the aerodynamic database generation. The first issue is investigated by predicting the longitudinal S&C performance and HQ of a sub-scale design using 3DPM analysis and comparing them with the prediction from wind-tunnel test (static) data supplemented by (dynamic) data from 3DPM. Both models appear trimmable and stable and the difference in their HQ are quantified, thus verifying the suitability of 3DPM analysis for sub-scale design assessment. The pre/post-processing challenge is tackled by the development of a knowledge-based engineering application to automate the aerodynamics database generation, reducing the time needed for geometry modeling, discretization and postprocessing of hundreds of cases from weeks to hours. The proposed methodology and its flexibility are demonstrated in this paper, where a commercial 3DPM code and an in-house developed non-linear flight dynamics analysis tool have been used to assess two sub-scale designs, one conventional and one based on the box-wing configuration. @en

The aim of this research is to investigate the combined use of throttle and aerodynamic control vanes for aircraft optimal control. A new disruptive aircraft configuration concept is presented, featuring control vanes downstream of two rear-mounted ducted propellers. The aerodynamic interaction between the horizontal vane and the throttle is analyzed in the scope of a longitudinal control study. A static criterion is proposed to discern the efficiency of the interaction, with respect to a generic pitch command. A traditional control allocation logic is used to exploit the throttle as a secondary pitch effector, and a modified version based on the interaction criterion is proposed; its behavior is tested through an open-loop design space exploration of actuator time constants and effectors prioritization weights. A flexible control system architecture is designed to compare the aircraft closed-loop response in conjunction with a phugoid damper loop. Results show that the best tracking performance is obtained with pilot commands allocated to the elevator, and phugoid damper commands to both elevator and throttle. The traditional allocation method achieves the best tracking performance at the expense of the largest control effort. The modified allocation alleviates the effort while still achieving better performance than the non-coupled control. @en

This work focuses on the wake encounter problem occurring when a light, or very light, aircraft flies through or nearby a wind turbine wake. The dependency of the aircraft normal load factor on the distance from the turbine rotor in various flight and environmental conditions is quantified. For this research, a framework of software applications has been developed for generating and controlling a population of flight simulation scenarios in presence of assigned wind and turbulence fields. The JSBSim flight dynamics model makes use of several autopilot systems for simulating a realistic pilot behavior during navigation. The wind distribution, calculated with OpenFOAM, is a separate input for the dynamic model and is considered frozen during each flight simulation. The aircraft normal load factor during wake encounters is monitored at different distances from the rotor, aircraft speeds, rates of descent and crossing angles. Based on these figures, some preliminary guidelines and recommendations on safe encounter distances are provided for general aviation aircraft, with considerations on pilot comfort and flight safety. These are needed, for instance, when an accident risk assessment study is required for flight in proximity of aeolic parks. A link to the GitHub code repository is provided.

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