This paper focuses on the characterization of the response of a very ∞exible aircraft in ∞ight. The 6-DOF equations of motion of a reference point on the aircraft are coupled with the aeroelastic equations that govern the geometrically nonlinear structural response of the vehicle. A low-order strain-based nonlinear structural analysis coupled with unsteady flnite-state potential ∞ow aerodynamics form the basis for the aeroelastic model. The nonlinear beam structural model assumes constant strain over an element in extension, twist, and in/out of plane bending. The geometrically nonlinear structural formulation, the flnite state aerodynamic model, and the nonlinear rigid body equations together provide a low-order complete nonlinear aircraft analysis tool. The equations of motion are integrated using an implicit modifled generalized-alpha method. The method incorporates both flrst and second order nonlinear equations without the necessity of transforming the equations to flrst order and incorporates a Newton-Raphson sub-iteration scheme at each time step. Using the developed tool, analyses and simulations can be conducted which encompass nonlinear rigid body, nonlinear rigid body coupled with linearized structural solutions, and full nonlinear rigid body and structural solutions. Simulations are presented which highlight the importance of nonlinear structural modeling as compared to rigid body and linearized structural analyses in a representative High Altitude Long Endurance (HALE) vehicle. Results show signiflcant difierences in the three reference point axes (pitch, roll, and yaw) not previously captured by linearized or rigid body approaches. The simulations using both full and empty fuel states include level gliding descent, low-pass flltered square aileron input rolling/gliding descent, and low-pass square elevator input gliding descent. Results are compared for rigid body, linearized structural, and nonlinear structural response.

/pdf/nonlinear-flight-dynamics-of-very-flexible-aircraft-1btiknl88r.pdf

Nonlinear Flight Dynamics of Very Flexible Aircraft

Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics

A review on non-linear aeroelasticity of high aspect-ratio wings

A recent consideration in aircraft design is the use of folding wing tips with the aim of enabling higher aspect ratio aircraft with less induced drag, but also meeting airport gate limitations. This study builds on previous work investigating the effect of exploiting folding wing tips in-flight as a device to reduce dynamic gust loads, but now with the introduction of a passive nonlinear hinge spring to allow wing-tip deflections only for larger load cases. A representative civil jet aircraft aeroelastic model is used in a multibody simulation code to explore the effect of introducing such a hinged wing-tip device on the loads behavior. It was found that significant reductions in the dynamic loads were possible.

/pdf/nonlinear-folding-wing-tips-for-gust-loads-alleviation-lzr7a3hoeg.pdf

Nonlinear Folding Wing-Tips for Gust Loads Alleviation

This paper investigates the model reduction, using balanced realizations, of the unsteady aerodynamics of maneuvering flexible aircraft. The aeroelastic response of the vehicle, which may be subject to large wing deformations at trimmed flight, is captured by coupling a displacement-based flexible-body dynamics formulation with an aerodynamic model based on the unsteady vortex lattice method. Consistent linearization of the aeroelastic problem allows the projection of the structural degrees of freedom on a few vibration modes of the unconstrained vehicle but preserves all couplings between the rigid and elastic motions and permits the vehicle flight dynamics to have arbitrarily large angular velocities. The high-order aerodynamic system, which defines the mapping between the small number of generalized coordinates and unsteady aerodynamic loads, is then reduced using the balanced truncation method. Numerical studies on a representative high-altitude, long-endurance aircraft show a very substantial reducti...

https://spiral.imperial.ac.uk/bitstream/10044/1/11566/4/Hesse%202013.%20Reduced-Order%20Aeroelastic%20Models%20for%20the%20Dynamics%20of%20Manoeuvring%20Flexible%20Aircraft.pdf

Reduced-Order Aeroelastic Models for Dynamics of Maneuvering Flexible Aircraft

This paper addresses the development of aircraft models for flight loads analysis in the pre-design stage. The underlying model structure consists of the nonlinear equations of motion of a free flying, flexible aircraft, as well as a model, which calculates the distributed aerodynamics over the entire airframe. Different possibilities in modelling the unsteady arodynamic interactions for pre-design purposes are explored and the effects on the loads are compared in order to assess the tradeoffs between accuracy and speed.

/pdf/comparison-of-unsteady-aerodynamic-modelling-methodologies-2icvoxkaxb.pdf

Comparison of Unsteady Aerodynamic Modelling Methodologies with Respect to Flight Loads Analysis

This article gives an overview of reduced-order modeling work performed in the DLR project Digital-X. Parametric aerodynamic reduced-order models (ROMs) are used to predict surface pressure distributions based on high-fidelity computational fluid dynamics (CFD), but at lower evaluation time and storage than the original CFD model. ROMs for steady aerodynamic applications are built using proper orthogonal decomposition and Isomap, a manifold learning method. Approximate solutions in the so-obtained low-dimensional representations of the data are found with interpolation techniques, or by minimizing the corresponding steady flow-solver residual. The latter approach produces physics-based ROMs driven by the governing equations. The steady ROMs are used to predict the static aeroelastic loads in a multidisciplinary design and optimization context, where the structural model is to be sized for the (aerodynamic) loads. They are also used in a process where an a priori identification of the critical load cases is of interest and the sheer number of load cases to be considered does not lend itself to high-fidelity CFD. An approach to correct a linear loads analysis model using steady CFD solutions at various Mach numbers and angles of attack and a ROM of the corrected aerodynamic influence coefficients is also shown. This results in a complete loads analysis model preserving aerodynamic nonlinearities while allowing fast evaluation across all model parameters. The different ROM methods are applied to a 3D test case of a transonic wing-body transport aircraft configuration.

/pdf/reduced-order-models-for-aerodynamic-applications-loads-and-4apnmfo9f8.pdf

Reduced-order models for aerodynamic applications, loads and MDO

This paper proposes an approach for developing integrated aircraft ight loads models that may be used for used for both manoeuvre as well as gust loads analysis. These types of analysis are traditionally based on models of very dierent nature. Mod- els for manoeuvre loads analysis are based on nonlinear rigid body equations of motion to account for large amplitude responses as resulting from prescribed, aggressive pilot in- puts. Airframe exibility is accounted for only quasi-statically by correction factors in the

https://elib.dlr.de/97798/1/IFASD-2009-106.pdf

Unifying manoeuvre and gust loads analysis models

Multi-disciplinary flight dynamics models for rigid and flexible aircraft are
used in various areas and stages of the aircraft design process, like for example flight loads
analysis and flight control law design. These models must be sufficiently accurate, but also
allow for thousands of simulations in a reasonable amount of time (i.e. ’loop-capable’).
Depending on the application, flight dynamics models need to be available in various
forms, like ODEs for nonlinear simulation, linear state space models for control analysis,
etc. A major challenge in development of multi-disciplinary flight dynamics models is
the handling and integration of data from various sources, removing overlaps, and filling
in gaps in case specific data is missing. During the last decade, the DLR Institute of
Robotics and Mechatronics has developed methods and tools for these tasks, as well as
for model implementation in various forms suitable for aircraft design analysis. Recently,
these aspects have been integrated in a standardized efficient model integration process,
called DAMIP (DLR Aircraft Model Integration Process). DAMIP uses the data structure
of VarLoads, which is developed in cooperation with Airbus, and is backed by appropriate
methods and tools. In this paper an overview of DAMIP will be given, as well as some
recent application examples.

Process, methods and tools for flexible aircraft flight dynamics model integration

The paper presents the control design approaches for the European research project FLEXOP. The ultimate goal is to develop and apply active flutter suppression and load alleviation techniques on an unmanned flying aircraft demonstrator. Due to the flexible wing of the aircraft new challenges rise for the control design: the traditional rigid body (baseline) control loops have to be augmented with flutter control laws. In our approach, the controllers are designed based on a dynamicalmodel, which is briefly discussed first. Details of the baseline control design, as well as the two different flutter suppression algorithms are discussed in the paper. Hardware-in-the-Loop testing of the controllers are reported before the first test flights of the aircraft.

Thiemo Kier

Papers

Comparison of Unsteady Aerodynamic Modelling Methodologies with Respect to Flight Loads Analysis

Reduced-order models for aerodynamic applications, loads and MDO

Unifying manoeuvre and gust loads analysis models

Process, methods and tools for flexible aircraft flight dynamics model integration

Flight control design for a highly flexible flutter demonstrator