Unconventional Aircraft Configurations Research Articles

A discussion on benchmarking unconventional configurations with conventional aircraft: the box-wing study case

This article is intended to introduce an alternative approach to comparative analyses between innovative aeronautical technologies and established state-of-the-art references. Commonly, the tendency is to use a ‘like-for-like’ comparative approach with respect to current technologies -defined as reference benchmarks- that is, to evaluate the possible incremental improvements that can be achieved by introducing a specific technological innovation. However, when innovations that potentially introduce step improvements or new functions compared to the state of the art are evaluated, typically referred to as ‘breakthrough’, this approach may not be the most formally sound one, and it may introduce bias and misjudgements. In the field of aircraft design, using the same top-level requirements and figures of merit as those used for conventional aircraft to initialise and steer the design of unconventional configurations, could undermine the exploitation of their operating and functional potential. The soundness of the comparative approach is of paramount importance, especially in the very early stages of the development of disruptive technologies and unconventional aircraft configurations. In this paper, with the supporting example of the application of the box-wing configuration to medium-range transport aircraft, a general discussion is offered on the necessity of leaving aside the ‘like-for-like’ benchmark approach when investigating the potential of disruptive aircraft innovations. This argumentation does not only refer to the case study proposed as an example, but is generally extendable to aeronautical innovations that may introduce operating and functional novelties compared to current technologies.

Review of Sonic Boom Prediction and Reduction Methods for Next Generation of Supersonic Aircraft

The reduction of sonic boom levels is the main challenge but also the key factor to start a new era of supersonic commercial flights. Since 1970, a FAA regulation has banned supersonic flights overland for unacceptable sonic booms at the ground, and many research studies have been carried out from that date to understand sonic boom generation, propagation and effects, both on the environment and communities. Minimization techniques have also been developed with the attempt to reduce sonic boom annoyance to acceptable levels. In the last 20 years, the advances in both knowledge and technologies, and companies and institutions’ significant investments have again raised the interest in the development of new methods and tools for the design of low boom supersonic aircraft. The exploration of unconventional configurations and exotic solutions and systems seems to be needed to effectively reduce sonic boom and allow supersonic flight everywhere. This review provides a description of all aspects of the sonic boom phenomenon related to the design of the next generation of supersonic aircraft. In particular, a critical review of the prediction and minimization methods found in the literature, aimed at identifying their strengths, limitations and gaps, is made, along with a complete overview of disruptive unconventional aircraft configurations and exotic active/passive solutions to boom level reduction. The aim of the work is to give a clear statement of state-of-the-art sonic boom prediction methods and possible reduction solutions to be explored for the design of next low-boom supersonic aircraft.

Preliminary Analysis of the Stability and Controllability of a Box-Wing Aircraft Configuration

This paper presents a study on the aeromechanical characteristics of a box-wing aircraft configuration with a focus on stability, controllability, and the impact of aeromechanical constraints on the lifting system conceptual design. In the last decade, the box-wing concept has been the subject of several investigations in the aeronautical scientific community, as it has the potential to improve classic aerodynamic performance, aiming at reducing fuel consumption per unit of payload transported, and thus contributing to a reduction in aviation greenhouse emissions. This study characterises the aeromechanical features of a box-wing aircraft, with a specific focus on the correlations between the aeromechanical constraints and the (main) aircraft design parameters. The proposed approach provides specific insights into the aeromechanical characteristics of the box-wing concept, both in the longitudinal and lateral plane, which are useful to define some overall design criteria generally applicable when dealing with the conceptual design of such an unconventional aircraft configuration.

Conceptual design and comparative study of strut-braced wing and twin-fuselage aircraft configurations with ultra-high aspect ratio wings

Sustainable and fuel-efficient next-generation air transportation demands a step change in aircraft performance. The ultra-high aspect ratio wings (UHARW) configuration is one key enabling strategy for improving aircraft aerodynamic efficiency and reducing fuel consumption and emissions. Unconventional aircraft configurations and advanced airframe technologies are required to address the large bending moment and shear stresses in the UHARW structure. This paper considers two promising unconventional configurations for adopting UHARW design, including strut-braced-wing (SBW) and twin-fuselage (TF), with advanced airframe technologies, i.e., active flow control, active load alleviation, and advanced airframe structures and materials. Three typical missions, including short-range (SR), medium-range (MR), and long-range (LR), are considered for aircraft design. A conceptual design and performance analysis framework for SBW and TF configurations is developed in this paper. According to the mission profile and top-level requirements proposed for each mission, an SBW and a TF configuration are designed, respectively. A comparative study is carried out to determine the best-in-class configuration of the corresponding mission to evaluate the potential of SBW and TF configurations for next-generation sustainable aviation applications. The results showed that the TF configuration has a better wing weight reduction effect than the SBW configuration, and the MR-TF and LR-TF aircraft have lower takeoff weight and fuel weight than those of the SBW aircraft for the same mission. However, due to the adjustment of the cabin dimensions for the SR-TF aircraft, the SBW configuration outperforms the TF configuration in this mission.

On the dynamic fluid–structure stability response of an innovative airplane configuration

This paper provides, for the first time, a physical explanation of the dynamic fluid–structure stability response of an unconventional aircraft configuration called PrandtlPlane, which has been recently acknowledged as a possible candidate to foster the ambition of a greener aviation. As observed in previous literature efforts on this configuration, flutter onset is significantly different when considering the aircraft being free in the air or fixed in space. Thanks to the formulation adopted in this effort, it is shown how the aerodynamic coupling of elastic and rigid modes has a beneficial effect on dynamic aeroelastic instability (flutter) onset. However, the different modal properties, consequence of the diverse boundary conditions, when switching from fixed-in-space to free-flying aircraft, also play a relevant role in determining the flutter occurrence. Whereas for the longitudinal case both effects are synergistic, contributing to increase flutter speed, for the lateral-directional case the variation in modal properties has a detrimental and dominating effect, leading to a flutter speed well within the flight envelope. Not only effects of rigid and elastic modes interaction have been addressed with respect to the aeroelastic side but the consequent effect on the flexible flight dynamics in terms of deterioration of the flying qualities has been quantified. Within the adopted formulation, unsteady aerodynamic forces are modeled by means of an enhanced Doublet Lattice Method able to take into account terms typically neglected by classic formulations. The work also discusses the relevance of such extra contributions to the dynamic response of the aircraft.

Linear Fractional Transformation co-modeling of high-order aeroelastic systems for robust flutter analysis

This article presents a new paradigm for robust flutter modeling and analysis of high-order uncertain and linear aeroelastic systems. The fundamental idea is to couple the state-of-art in robust worst-case analysis (Linear Fractional Transformation modeling and µ analysis) with the state-of-practice in aeroelasticity (fluid-structure-interaction solvers). The issue with the latter is that, although capable of providing different levels of fidelity, they are less efficient in coping with the analysis of systems subject to uncertainties. In fact, while they have the advantage of capturing directly the physical uncertainty, the analyses can only be applied to a defined parameter combination, and due to their computational cost, it is usually only possible to consider a limited set of cases. To tackle this lack of robustness, in recent works the application of analytic worst-case methods has been proposed, but the intimately related problem of constructing accurate uncertain models has not been fully addressed. In this article, a co-modeling framework is presented that leverages the main features of both fluid-structure interaction solvers and robust control-based methods. The key idea is to combine these two typically distinct steps in a single one, enabling in this way to obtain an uncertainty description which is flexible and reconciles the physical sources of uncertainty with the uncertain parameters used in the LFT model. An exemplification of the developed framework on an unconventional aircraft configuration is provided. Results show its potential to provide valuable physical insights into the problem when analyzing complex systems.

Evaluation of ultra-high bypass ratio engines for an over-wing aircraft configuration

Today, main hub airports are already at their capacity limit and hence, smaller airports have become more interesting for providing point-to-point connections. Unfortunately, the use of regional airports induces an increased environmental footprint for the population living around it. In an attempt to solve the related problems, the research project Coordinated Research Centre 880 aims to examine the fundamentals of a single-aisle aircraft with active high-lift configuration powered by two geared ultra-high bypass turbofan engines mounted over the wing. Low direct operating costs, noise shielding due to the over-wing configuration, and short runway lengths are the main advantages. Highlighting the performance, economical and noise benefits of a geared ultra-high bypass engine is the key aim of this paper. This assessment includes a correspondingly adjusted aircraft. Open literature values are applied to design the two investigated bypass ratios; a reference engine with a bypass ratio of 5 and 17 respectively. This study shows that a careful selection of engine mass flow, turbine entry temperature and overall pressure ratio determines the desirable bypass ratio. The aircraft direct operating costs drop by 5.7% when comparing the designed conventional with a future ultra-high bypass ratio engine. Furthermore, the sound at source for a selected mission and operating condition can be reduced by 7 dB. A variable bypass nozzle area for the ultra-high bypass ratio engine is analysed in terms of performance and operability. An increase of safety margin is shown for the turbofan engine with a variable bypass nozzle. It is concluded that this unconventional aircraft configuration with ultra-high bypass ratio engines mounted over the wing has the potential to relieve main hub airports and reduce the environmental impact.

Reduced basis methods for structurally nonlinear Joined Wings

There is currently a large interest towards unconventional aircraft configurations. According to many experts, only revolutionary concepts can meet the demands of a sustainable aviation. Within this scenario, studies and investigations on innovative aircraft layouts have been flourished: Blended Wing Body, Joined Wings and Box Wings are relevant examples of configurations departing from the traditional monoplane. Joined Wings are characterized by strong geometric nonlinear effects, whose complexity is enhanced by the over-constrained layout and the intrinsic relevant bending-torsion coupling. Thus, the application of standard linear analysis tools of routine use in the aerospace industry is neither acceptable nor practical: the preliminary design phase needs a large number of simulations that have to be computationally fast and at the same time preserve an acceptable level of accuracy/fidelity. For this reason, reduced order models are particularly relevant for the design of these systems. However, several reduced order methods applied in the past to Joined Wings did not provide satisfactory accuracy and reliability. The poor performance is mainly due to the very early onset of nonlinear effects, even at very moderate deflections. This work provides an assessment, and an algorithm enhancement when needed, of several existing techniques aimed to build an effective reduction basis for model order reduction. In particular, Vibration Modes, Modal Derivatives, Ritz Vectors, Static Modes, Trial Solutions and Corrections Vectors are critically examined when applied to a representative JW configuration. The results indicate that Ritz Vectors and Modal Derivatives provide excellent accuracy of the reduced solution when compared with the full order solution. Specifically, a drastic increase in the performance of the reduced order model is noticed when ab-initio information regarding Modal Derivatives are included. This highlights and confirms the relevance of nonlinearities even at the early deformation stages. Hence, taking into account nonlinearities since the early design phases seems to be an unavoidable necessity for Joined Wings. As a consequence, efficient and reliable reduced order modeling might play an essential role in the design of such innovative configurations.

HTS Electrical System for a Distributed Propulsion Aircraft

As the commercial aircraft industry aims to meet the challenge of 2050 emissions targets, it becomes apparent that unconventional aircraft configurations may be necessary to achieve them. Distributed electrical propulsion offers opportunities for improving aircraft efficiency. However, the conversion and transfer of power could prove prohibitive when using conventional machines and conductors due to weight and inefficiency. An aircraft with distributed propulsion and boundary layer ingestion is considered in this paper, and the current view of the future of high-temperature superconductor technology as applied to aircraft is explained.

Surrogate models and mixtures of experts in aerodynamic performance prediction for aircraft mission analysis

The accurate evaluation of aircraft fuel burn over a complete mission is computationally expensive and may require millions of aerodynamic performance evaluations. Thus, it is advantageous to use surrogate models as approximations of high-fidelity aerodynamic or aerostructural models. Conventional surrogate models, such as the radial basis function and kriging, cannot model these functions accurately, especially in the transonic regime. To address this issue, we explore several ways to improve the accuracy of surrogate models. First, we employ an adaptive sampling algorithm to complement a traditional space-filling algorithm. Second, we improve the kriging surrogate performance by including gradient information in the interpolation (a form of gradient-enhanced kriging—GEK) and by introducing a known trend in the global model component (kriging with a trend). Lastly, we propose a mixture of experts (ME) approach, which is based on the divide-and-conquer principle. We validate our surrogate models using aerodynamic data for conventional and unconventional aircraft configurations, and we assess their performance in predicting the mission ranges by analyzing ten mission profiles. Our results show that the proposed ME approach is superior to the traditional models. Using a mixture of GEK models to approximate the drag coefficients gives approximation errors of less than 5% with fewer than 150 samples, whereas the adaptive sampling fails to converge when training a global model. However, when we have a simple function profile, such as the lift and moment coefficients, using a conventional surrogate model is more efficient than an ME model, because of the added computational complexity in the latter. The range estimation errors associated with the ME models are less than 2% for all the benchmark mission profiles considered, whereas some traditional models yield errors as high as 20–80%. We thus conclude that the ME technique is both necessary and sufficient for modeling the aerodynamic coefficients for surrogate-based mission analysis.

Weight Prediction of the Lifting System for an Unconventional Aircraft Configuration

In this paper a tool, previously developed and validated for the prediction of wing box structural weight in very early design stages, is improved to better match effects on panel sizing of stability constraints. It is applied to an unconventional configuration aircraft based on the Best Wing System concept, introduced by Ludwig Prandtl in 1924, to achieve minimum induced drag: such lifting system is composed of two swept wings (fore and aft) connected by vertical wings at their tips and two fins connecting the rear wing to the fuselage. The system is over-constrained to the fuselage and, thus, the structural design, as well as the static aeroelasticity and flutter characteristics, totally differs from a conventional aircraft. An optimization method finds out a suitable prediction of the structural weight by defining the wing span stiffness behavior compatible with a mix of global and local design constraints, but without any claim about a structural design. The link among stiffness properties and structural weight is made by means of simplified models of the box cross-sections, suitable also to supply responses for the approximated evaluation of local constraints.

Fan Noise Sources and Passive Reduction Methodologies in High Bypass Turbofan Engines

Increasing air traffic and more stringent aircraft noise regulations continue to increase requirements on community noise prediction capabilities for conventional and unconventional aircraft configurations. For engines with a high bypass ratio, fan noise is a major noise source. The main established technologies for fan reduction and those currently under evaluation are scarf inlets, forward swept fans, swept and leaned stators, fan trailing edge blowing, acoustic treatment placed over the fan and acoustic liners. Acoustic liners have desirable acoustic attenuation properties and thus are commonly used to reduce noise in jet engines. The liners typically are placed upstream and downstream of the rotors (fans) to absorb sound before it propagates out of the inlet and exhaust ducts. This paper focusses on about the main fan noise sources in new modern turbofan engines.

Multidisciplinary Integrated Preliminary Design Applied to Unconventional Aircraft Configurations

A preliminary aircraft design tool is presented as a means of performing multidisciplinary, integrated preliminary design of unconventional aircraft configurations. Higher fidelity numerical methods for some of the involved disciplines are discussed as key elements of this process. A noise propagation module is described as a first step toward introducing aircraft noise analysis into the preliminary design process. Application of the tool to a specific unconventional aircraft concept is shown. The results of this study provide an understanding of why multidisciplinary design analysis and optimization, as well as higher fidelity methods, are required for the design of unconventional aircraft configurations.

Structural sizing for an unconventional, environment-friendly aircraft configuration within integrated conceptual design

This paper discusses the application of an integrated multidisciplinary design process to an unconventional Low Noise Aircraft (LNA) configuration, with a focus on the topic of structural sizing. Through the use of higher fidelity numerical methods, structural sizing can be performed and analysed at a much higher level than in classic conceptual design processes. The effects governing structural sizing for this specific configuration are illustrated and discussed. It is shown that considerable differences exist between the LNA and a conventional reference configuration. This analysis provides a basic understanding of how an unconventional geometry can impact the global parameters of an aircraft design.

On the use of small scale aircraft models for dynamic wind tunnel investigation of stability and control

An experimental facility for the dynamic investigation of the stability and control characteristics of small scale model aircraft in a wind tunnel is described. Some typical results for a conventional and unconventional aircraft configuration are presented.