Energy-based Aeroelastic Analysis and Optimisation of Morphing Wings

Morphing aircraft can change their shape radically when confronted with a variety of conflicting flight conditions throughout their mission. For instance the F-14 Tomcat fighter aircraft, known from the movie Top Gun, was able to sweep its wings from a straight wing configuration to a highly swept version. Such concepts, which are basically rigid body morphing concepts, have been developed further into aircraft which are able to have a distribution of morphing deformations over their wing, exhibiting large elastic straining of the skin. Most popular examples are the Lockheed-Martin Folding Wing concept, and the NextGen Aeronautics Batwing concept. A problem that became apparent from the literature is that there exists no dedicated conceptual design tools for morphing wings. The main focus in the literature has been on design and optimisation of predefined morphing mechanisms. Therefore the research presented in this dissertation focuses on the development of a low-fidelity aeroelastic design tool including a particular method of discretising the morphing deformation distribution over the entire wing. A distribution of local wing fold, shear, and twist is considered to create a model which can morph into any arbitrary wing shape. The model has been successfully applied to a design study of a morphing winglet of a regional airliner and a morphing outboard section of a transport aircraft wing.Aerospace Structures and Computational MechanicsAerospace Engineerin

On the Importance of Morphing Deformation Scheduling for Actuation Force and Energy

Morphing aircraft offer superior properties as compared to non-morphing aircraft. They can achieve this by adapting their shape depending on the requirements of various conflicting flight conditions. These shape changes are often associated with large deformations and strains, and hence dedicated morphing concepts are developed to carry out the required changes in shape. Such intricate mechanisms are often heavy, which reduces, or even completely cancels, the performance increase of the morphing aircraft. Part of this weight penalty is determined by the required actuators and associated batteries, which are mainly driven by the required actuation force and energy. Two underexposed influences on the actuation force and energy are the flight condition at which morphing should take place and the order of the morphing manoeuvres, also called morphing scheduling. This paper aims at highlighting the importance of both influences by using a small Unmanned Aerial Vehicle (UAV) with different morphing mechanisms as an example. The results in this paper are generated using a morphing aircraft analysis and design code that was developed at the Delft University of Technology. The importance of the flight condition and a proper morphing schedule is demonstrated by investigating the required actuation forces for various flight conditions and morphing sequences. More importantly, the results show that there is not necessarily one optimal flight condition or morphing schedule and a tradeoff needs to be made.Aerospace Structures & Computational Mechanic

Unsteady Non-linear Control Surface Modelling for Aeroservoelastic Applications

In this paper, we present a data-driven method to model the unsteady non-linear response of aircraft control surfaces. This method relies on aerodynamic reduced-order models (ROM) derived from computational fluid dynamics with Reynolds averaged Navier-Stokes (CFD-RANS) analysis in the transonic domain. The ROM consists of a combination of look-up tables and transfer functions, with which we can capture the incremental unsteady loads from aileron and spoiler large deflections. The ROM can replicate transient CFD results with a 5% margin of error in most scenarios using a realistic 3D wing model. We also investigate a hybrid approach to calculate aeroelastic wing deformations. To do so, we simulate the control loads with our the aforementioned ROM, while we rely on a fast but robust low-fidelity method to model the wing aeroelastic response. We compared this method against high-fidelity analysis and estimated an average error of 5% to 10% in most of the cases with a three orders of magnitude decrease in simulation time. The rapidity of such load estimation technique makes it suitable for wing sizing and flight control optimisation problems.Aerospace Structures & Computational Mechanic

Preliminary aeroelastic design framework for composite wings subjected to gust loads

Including a gust analysis in an optimization framework is computationally inefficient as the critical load cases are not known a priori and hence a large number of points within the flight envelope have to be analyzed. Model order reduction techniques can provide significant improvement in computational efficiency of an aeroelastic analysis. In this paper a reduced order aeroelastic model is formulated by reducing the aerodynamic system with a balancedproper orthogonal decomposition and coupling it to a structural solver. It is demonstrated that the dominant modes of the aerodynamic model can be assumed to be constant for varying equivalent airspeed and Mach number, enabling the use of a single reduced model for the entire flight envelope. Comparison of the results from the full and reduced order aeroelastic model shows a high accuracy of the latter and a large saving in computational cost. A dynamic aeroelastic optimization framework is then formulated using the reduced order aeroelastic model. Results show that both dynamic and static loads play a role in optimization of the wing structure. Furthermore, the worst case gust loads change during the optimization process and henceit is important to identify the critical loads at every iteration in the optimization.Aerospace Structures & Computational Mechanic

Assessment of an Increased-Fidelity Aeroelastic Experiment for Free Flying Wing Response to Gust Excitation

The paper proposes a methodology for increased-fidelity aeroelastic testing in a wind tunnel environment to improve the correlation between the aeroelastic response measured in a wind tunnel experiment and the aeroelastic response observed on an aircraft in flight. The focus of the current study is to assess the potential of the proposed methodology to improve load and response predictions by emulating the motion of a free flying aircraft at the root of the wing. For this purpose a numerical aeroelastic model of a free flying aircraft is used to obtain a reference aeroelastic response to gust excitation. The model is reduced to obtain an aeroelastic model comprising only the main wing of the aircraft which is clamped at the root as if it would be mounted in a wind tunnel. The wing is then subjected to five different motion profiles emulating the free flight to a various degree. The considered motion profiles are clamped boundary condition, heave-pitch motion of a free flying aircraft, motion profile following the angle of attack of the aircraft, and two modified heave-pitch motion profiles which match the angle of attack and the aerodynamic loads in the wind tunnel with those in free flight. The study shows that the considered motion profiles can significantly improve the correlation between the wind tunnel experiment and free flight. However, the effectiveness of each motion profile strongly depends on the gust length which indicates that the optimum motion profile depends on the gust length. Finally, the paper presents a conceptual design of a wind tunnel demonstrator to serve as a proof-of-concept for the proposed methodology.Aerospace Structures & Computational Mechanic

Low-fidelity 2D isogeometric aeroelastic optimization with application to a morphing airfoil

Low-fidelity isogeometric aeroelastic analysis has not received much attention since the introduction of the isogeometric analysis (IGA) concept, while the combination of IGA and the boundary element method in the form of the potential flow theory shows great potential. This paper presents a two-dimensional low-fidelity aeroelastic optimization framework consisting of a loosely coupled isogeometric potential flow model and isogeometric curved Timoshenko beam model. Application of the framework to the optimization of the landing performance for an active morphing airfoil shows its potential, although the absence of viscosity in the aerodynamic model has a detrimental effect.Aerospace Structures and MaterialsAerospace Engineerin

Dynamic aeroelastic tailoring of a strut braced wing including fatigue loads

High aspect ratio strut braced aircraft can significantly reduce the induced drag. The inherent anisotropic behaviour of the composite material along with their weight saving potential can improve the performance of the aircraft during the flight. Thus, a composite strut braced aircraft is one of the promising candidates to achieve the targets set by the European Commission in Flightpath 2050 report. In their previous works, authors have developed methodologies to include gust loads using a reduced order model and account for fatigue loads through an analytical model. In this paper, previously developed methodologies are used, to carry out a stiffness and thickness optimization of a composite strut braced wing which includes critical gust loads as well as fatigue loads. The results show that a composite strut braced wing is sized by both dynamic as well as static load cases. Additionally, by accounting for fatigue through analytical model instead of a knockdown factor, a lighter wing can be obtained.Aerospace Structures & Computational Mechanic

Low-Fidelity 2D Isogeometric Aeroelastic Analysis and Optimization Method with Application to a Morphing Airfoil

Low-fidelity isogeometric aeroelastic analysis has not received much attention since the introduction of the isogeometric analysis (IGA) concept, while the combination of IGA and the boundary element method in the form of the potential flow theory shows great potential. This paper presents a two-dimensional low-fidelity aeroelastic analysis and optimization framework consisting of a closely coupled isogeometric potential flow model and isogeometric curved Timoshenko beam model combined with a boundary layer model. Application of the framework to the optimization of the landing performance for an active morphing airfoil demonstrates the potential of the isogeometric aeroelastic framework.Aerospace Structures & Computational Mechanic

Post-buckled precompressed (PBP) subsonic micro flight control actuators and surfaces

This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP) piezoelectric elements to provide much improved actuator performance. These PBP actuator elements are modeled using basic large deflection Euler-beam estimations accounting for laminated plate effects. The deflection estimations are then coupled to a high rotation kinematic model which translates PBP beam bending to stabilator deflections. A test article using PZT-5H piezoceramic sheets built into an active bender element was fitted with an elastic band which induced much improved deflection levels. Statically the bender element was capable of producing unloaded end rotations on the order of ±2.6°. With axial compression, the end deflections were shown to increase nearly 4-fold. The PBP element was then fitted with a graphite-epoxy aeroshell which was designed to pitch around a tubular stainless steel main spar. Quasi-static bench testing showed excellent correlation between theory and experiment through ±25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up to 120kts (62m/s, 202ft/s). Testing showed that deflections up through ±20° could be maintained at even the highest flight speed. The stabilator showed no flutter or divergence tendencies at all flight speeds. At higher deflection levels, it was shown that a slight degradation deflection was induced by nose-down pitching moments generated by separated flow conditions induced by extremely high angles of attack.Aerospace StructuresAerospace Engineerin

Passively actuated spoiler for gust load alleviation

This paper summarises a conceptual study regarding a passively actuated spoiler for gust load alleviation. The design of such system is intended to limit the use of computers, sensors and actuators to operate the device. The study mainly relies on using Theodorsen’s unsteady flow theory for a typical airfoil section. To cope with the limitations of this type of model for spoiler aerodynamic, corrections are brought from unsteady high fidelity flow simulations by means of transfer functions. The outcome is a 2D aeroelastic model with three degrees of freedom. It describes the spoiler contribution to the overall aeroelastic behaviour of the airfoil in the event of a gust encounter. Results show that the spoiler can help to reduce loads passively, but requires to be retracted with an active system to work properly.Aerospace Structures & Computational Mechanic