Search

Within the DLR project VicToria various high fidelity-based MDO processes were set-up as applicalble methods for aircraft design. Apart from aerodynamic optimization using high fidelity-based CFD analysis, the sub-processes overall aircraft design synthesis, loads analysis, and structural optimization were part of the MDO processes. The presented paper expounds such MDO sub-processes in order to exhibit their contributions and capabilities for the respected MDO process.

This paper presents a modelling scheme suitable for loads analysis of manoeuvres and gusts of a flexible controlled aircraft. In contrast to most ongoing research, the component to be investigated is not the wing but the vertical tail plane (VTP). Critical load conditions for vertical tail plane include yawing maneoeuvre conditions as well as discrete lateral gusts. The resulting design loads are heavily influenced by the flight control system and the associated parameters for yaw damping functions or travel limiter. This poses an interesting problem regarding trade-offs between handling qualities and the structural loads acting on the VTP. This paper investigates the influence of lateral flight control laws on the loads of the different gust and manoeuvre load conditions for certification as specified by the authorities. Sparked by previous incidents, a new load condition was recently introduced: the rudder reversal condition. This new condition features three full reversals of the rudder pedal input instead of just one step input and a mere return to neutral. The loads resulting from the new load condition are considered to be ultimate and not limit loads. The new load condition is assessed and compared to loads envelopes according to the old certification specifications. The introduction of the new rudder control reversal load condition was mainly motivated by wake vortex encounters during which the pilots made excessive or inappropriate use of the rudder. The conjecture is that the loads are primarily caused by pilot action and not the external windfield. Since the present modelling scheme is also capable of simulating position and attitude dependent windfields such as wake vortex encounters, the resulting loads are compared to the new rudder control reversal condition as well.

An integrated modelling approach for gust and manoeuvre loads analysis is presented. Typically, the unsteady aerodynamics for gust loads are computed by the Doublet Lattice Method. However, this method does not account for effects, which are important from a flight dynamics and manoeuvre loads perspective. E.g., the roll-yaw coupling, also responsible for the flight mechanical eigenmodes like the dutch roll mode, are unaccounted for by the DLM. Since, the dutch roll mode might also be excited by lateral gusts, this might have an influence on the loads response. In this paper, the unsteady aerodynamics are computed by a 3D panel method, capable of capturing the flight dynamics accurately and provide complex Aerodynamics Influence Coefficients for gust loads analysis as a function of reduced frequency similar to the DLM. The aerodynamics of the panel method are validate by comparison with CFD results. Differences in the flight mechanics between the DLM and the panel methods are assessed for a parametric variation of the wing dihedral of an aircraft model. Finally, results from a frequency domain gust loads analysis of a lateral gust for the DLM and the 3D panel method are compared.

This work gives an overview of Reduced Order Model (ROM) applications employed within the context of the DLR Digital-X project. The ROM methodology has found widespread application in fluid dynamics. In its direct application to CFD it seeks to reduce the computational complexity of a problem by reducing the number of degrees of freedom rather than simplifying the physical model. Here, parametric aerodynamic ROMs are used to provide pressure distributions based on high-fidelity CFD, but at lower evaluation time and storage than the original CFD model. ROMs for steady aerodynamic applications are presented. We consider ROMs combining proper orthogonal decomposition (POD) and Isomap, which is a manifold learning method, with interpolation methods as well as physics-based ROMs, where an approximate solution is found in the POD-subspace or non-linear manifold by minimizing the corresponding steady or unsteady flow-solver residual. The issue of how to train the ROM with high-fidelity CFD data is also addressed. The steady ROMs are used to predict the static aeroelastic loads in an MDO 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. We also show an approach combining correction of a linear loads analysis model using steady, rigid CFD solutions at various Mach numbers and angles of attack with a ROM of the corrected Aerodynamic Influence Coefficients (AICs). This integrates the results into a complete loads analysis model preserving aerodynamic nonlinearities while allowing fast evaluation across all model parameters. Thus, correction for the major nonlinearities, e.g. depending on Mach number and angle of attack combines with the linearity of the baseline model to yield a large domain of validity across all flow parameters at the expense of a relatively small number of CFD solutions. The different ROM methods are applied to 3D test cases and in particular to a transonic wing-body transport aircraft configuration.

This paper demonstrates how the robust performance analysis framework can be used to determine worst case loads of flexible aircraft due to 1-cos gusts. This framework is a part of control theory and is used to characterize a system by input-output norms: e.g. the induced L2-L∞ norm represents the worst case energy-to-peak gain. Two very efficient ways to compute this norm are presented. Based on the physical interpretation of this norm, an aircraft model is weighted such that the induced L2-L∞ norm of the weighted system represents a close but guaranteed upper bound for 1-cos gusts and similar excitations. The analysis results are compared to simulations. Since the energy-to-peak gain can be computed in a very efficient way for LTI models, the norm is used to quickly obtain an upper bound for loads and to identify critical flight conditions. In a second example, the analysis is additionally performed on an uncertain mode