13:30
Loads 2
Chair: Mohammadreza Amoozgar
13:30
30 mins

Influence of Cruise Altitude Variation on the Turbulence Loads of a MediumRange Transport Configuration
Vega Handojo, Sunpeth Cumnuantip
Abstract: The climate impact of aviation not only lies in the CO2 emission, but also in the thermodynamic effects of condensation trails [1]. One possible method to mitigate the latter is by reducing the cruise altitude (from 35000 ft to e.g. 27000 ft) to have warmer ambient air, thus a higher water vapor containing capacity before condensation occurs. At lower altitudes, however, the expected turbulence amplitudes are higher. Hence, the research question emerges: How do the turbulence loads change if the cruise altitude is reduced?
For that aim, a midrange configuration DLRF25 is taken as reference. The configuration originates from the LuFo project AirTiMe. The aircraft main characteristics are its high aspect ratio wing (wing aspect ratio higher than 15), its novel wing movables configuration and its sustainable aviation fuel high bypass ratio engine. The aeroelastic simulation models for MSC.Nastran are created using the DLR inhouse cpacsMONA process.
Before the load calculations are carried out, parameter changes for the flight mission with the lower cruise altitude are to be defined first. For the lower cruise altitude, the dynamic pressure is kept constant to enable the aircraft flying with the optimal lift coefficient. Hence, the Mach number and the true airspeed will decrease. The latter in turn leads to a longer flight time. By assuming that the glide ratio in the lower cruise stays constant, the required block fuel and the takeoff mass will increase. A more detailed derivation of those parameters will be included in the final paper.
The turbulence load calculation is carried out in the frequency domain based on Handojo [2]. For that aim, a vonKármán turbulence spectrum is generated, and the root mean square (rms) values of the turbulence are adjusted to the desired altitude and probability of exceedance. For a comparison between two different flight conditions, turbulence intensities with the same probability of exceedance are assumed. On the aircraft side, transfer functions of interesting quantities (e.g. wing root bending moment or fuselage vertical acceleration) are extracted using MSC.Nastran. The aircraft responses in the frequency domain are then calculated with a multiplication of the turbulence spectrum and the transfer functions. Subsequently, the responses are transformed into the time domain, from which load spectra are derived using the frainflow counting method and rms values are calculated.
With the different cruise conditions, comparative conclusions regarding the loads and e.g. passenger ride comfort can be drawn. The term “comparative” implies that all aircraft responses depend on the selected turbulence intensities, which in turn depend on the selected probability of exceedance. As an outlook, a comparative analysis of the fatigue damage due to turbulence can be conducted.
[1] M. Bickel, Climate Impact of Contrail Cirrus, Dissertation, LudwigMaximiliansUniversität, 2023.
[2] V. Handojo, Contribution to Load Alleviation in Aircraft Predesign and Its Influence on Structural Mass and Fatigue, Technische Universität Berlin, Berlin, 2021. https://elib.dlr.de/139558/.

14:00
30 mins

Robust design through identification of main components from multivariate random loads
Cyrille Vidy, Carlo Aquilini
Abstract: Important progresses in terms of methods and computer technique made possible to numerically analyse the dynamic response of the aircraft and loads with high level of accuracy. Also, modern testing techniques can capture highly dynamic pressure fluctuations and accelerations for full wetter surfaces. All of this leads to important amounts of data that, in case of turbulence or buffeting analyses, produce complex and fluctuating loading conditions that need to be considered in structural design adequately.
Aircraft structural design traditionally relies on a downselection of critical loading conditions, using adequate monitoring stations in order to derive nodal load cases. This process is especially complex for stochastic phenomena such as buffeting, where unsteady patterns need to be robustly captured with a large number of monitoring stations. Consequently, many unneeded similar load cases are derived for airframe sizing due to the high correlation of some results.
This problem is addressed in the current paper. Accepting that one cannot predict the unsteady patterns and that these need to be captured, a finer monitoring grid is applied. But instead of deriving directly nodal load cases associated to each monitoring station, a principal component analysis (also called proper orthogonal decomposition) is applied to the results either at full aircraft or at component level, extracting the main loading components to be applied for airframe sizing. Therefore, the accuracy provided through the huge input data is kept and the number of sizing cases is drastically reduced for structural design, still covering the most critical load cases.
This method is applied to a full aircraft configuration, using high accuracy buffeting loading and response data. The main loading directions are identified and structural sizing cases are derived. A comparison in terms of quality, robustness and efficiency is presented in order to underline the advantage of applying this method for structural design loads compared to the traditional process.
To conclude, measurement and analysis of very complex multivariate random loads has become possible, and the presented method allows to adequately take benefit of this unprecedented level of quality for structural design.

14:30
30 mins

Doublet lattice modelling and analysis of unsteady aerodynamic effects for a flexible unmanned aircraft during maneuvers and gust encounters
Leif Rieck, Benjamin Herrmann, Frank Thielecke
Abstract: This paper presents the modeling and analysis of unsteady aerodynamics for a slightly flexible 25kg unmanned aircraft during maneuvers and gust encounters. The unsteady model is based on a doublet lattice method (DLM) implemented in Matlab. It is combined with an existing highfidelity quasisteady aerodynamics model derived from system identification using flight test data. Utilizing the physical rational function approximation, it is possible to differentiate between the steady and unsteady components of the DLM. Consequently, the steady component of the DLM can be replaced by the highfidelity model such that quasisteady and unsteady DLM forces and moments are superimposed. The combined unsteady aerodynamics are integrated with linear structural dynamics identified from ground vibration tests and nonlinear equations of motion based on the practical meanaxes formulation. Simulation studies are conducted to analyze the impact of unsteady aerodynamic effects on the flexible aircraft. The results indicate that unsteady effects, while noticeable during rapid maneuvers in the aeroelastic frequency range, are especially significant when considering highfrequency control surface deflections and encounters with short gusts. The proposed modeling approach successfully combines highfidelity quasisteady aerodynamics with unsteady DLM aerodynamics, demonstrating validity across a wide range of reduced frequencies.

15:00
30 mins

Lidarbased gust load alleviation – increasing the load reduction potential through a twodegreeoffreedom controller architecture
Christian Wallace, Nicolas Fezans
Abstract: In this paper a novel load alleviation control approach is discussed. It consists of two controller functions, a feedforwardcontrol function which uses wind estimates on the basis of lidar measurements, and a feedbackcontrol function which uses rates and accelerations measured on the airframe.
The paper concentrates on discussing a strategy for combining the two controllers in such a way that negative mutual interference can be almost completely avoided and the potentials of the individual controllers with regard to load reduction even add up.

