Influence of Cruise Altitude Variation on the Turbulence Loads of a Medium-Range Transport Configuration

ifasd2024 Tracking Number 4

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 mid-range configuration DLR-F25 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 in-house cpacs-MONA 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 take-off 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 von-Kármán 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, Ludwig-Maximilians-Universität, 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/.