One concept to reduce an aircraft's fuel consumption is to increase wing aspect ratio, i.e. the ratio of wing span to wing chord. Although slender (i. e. high aspect ratio) wings have low induced drag, a number of aeroelastic issues arise. As an example, high aspect ratio wings are relatively soft in bending and torsion when compared to conventional designs. These structural dynamic properties can lead to a reduction of the flutter speed, the airspeed where the aircraft becomes aeroelastically unstable. Flutter is an aeroelastic phenomena in which the interactions between the oscillations of an aircraft structure and its surrounding airflow can become unstable, which may finally lead to dynamic structural failure.
As flutter can be an important factor limiting the operational envelope of the aircraft, analyzing the flutter stability early in the design is thus important in order to avoid major design modifications at a late design phase.
The Institute of Aeroelasticity currently participates in the UP Wing (Ultra Performant Wing) project under the EU's Clean Aviation funding program. The article "New wing concepts for transport aircraft - less CO2 through higher aspect" has given an overview of the project. The initial aircraft configuration in the UP Wing project is the DLR F25 configuration, which was originally developed in the Lufo VI project VirEnfREI. Within UP Wing´s preliminary design stage, wings with different aspect ratios are designed. The designs are then compared with respect to their characteristics, such as wing mass and aerodynamic drag. The Institute of Aeroelasticity is responsible for the preliminary aeroelastic evaluation. However, a civil transport aircraft with high aspect ratio wing is an unconventional configuration for which limited information and empirical knowledge to assess flutter during preliminary design are available. In the project, the in-house automated aeroelastic structural design and analysis tool cpacs-MONA [1] is used for a preliminary flutter assessment. In a first step, cpacs-MONA extracts the information about the aircraft, such as the wing structural topology and engine pylon information from a CPACS-dataset. CPACS is the Common Parametric Aircraft Configuration Schema developed by DLR [2] to enable large collaborative design workflows. In a second step, a parametric finite element model is set up using the in-house model generator ModGen [3], which is part of the MONA process, followed by an extensive loads analysis campaign of the flexible aircraft and a structural optimization of the wing structures. Finally, a flutter check in the relevant flight envelope is performed at the end of the process for the defined aircraft mass cases and flight altitudes.
The VirEnfREI configuration and the initial UP Wing configuration are nearly identical with the same wing aspect ratio of more than 15, the same twist distribution of the main wing and a comparable relative profile thickness distribution along the span. However, they differ in wing airfoils. This results in different internal structures of the wings, visible for example in the height of the spars. Figure 1 shows the different spar heights of these two aircraft over the relative spanwise coordinate eta. Due to the different airfoils, the front spar of the UP Wing configuration is higher in the inner wing and slightly lower in the outer wing than the initial VirEnfREI configuration, while the rear spar is higher along the full wing span, up to 17%. This difference, among others, has a significant influence on global wing structural stiffness.
Both wing designs are still being further developed in the respective projects. However, an initial look at the flutter characteristics of both configurations and a comparison of the relevant structural properties is valuable. As an example, the flutter velocity of the UP Wing configuration is higher compared to that of the VirEnfREI aircraft. The dominant structural eigenforms for both configurations are the engine pitch mode (see Figure 2), in coupling with the eigenmodes of wing bending and wing torsion.
One of the possible reasons for the higher flutter speed of the UP Wing configuration is the difference of their internal main wing structures as discussed above. The higher rear spar results in a stiffer wing in bending and torsion for the UP Wing configuration. Consequently, the torsion and bending eigenmodes of the softer VirEnfREI wing couple at a lower speed with the engine pitching eigenmode, thus resulting in a lower flutter velocity of the VirEnfREI configuration.
As demonstrated above, the simulation-based tool cpacs-MONA of the Institute of Aeroelasticity enables the aircraft designer to gain knowledge on the effect of the aerodynamic shape and the internal structure of the main wing on the aircraft´s aeroelastic behavior early in the design process. Specifically, for the evaluation of the aeroelastic characteristic of high aspect ratio wing aircraft, the analyses have demonstrated the importance of the coupling of the engine/pylon eigenmode with the wing eigenmodes. This awareness allows modification of major wing components to prevent critical flutter behaviour even in the early concept phase.
The project Ultra Performance Wing (UP Wing, project number: 101101974) is supported by the Clean Aviation Joint Undertaking and its members.
Co-Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or Clean Aviation Joint Undertaking. Neither the European Union nor the granting authority can be held responsible for them.
EU
Clean Aviation
Sunpeth Cumnuantip and Matthias Schulze, Department Loads Analysis and Design, DLR Institute of Aeroelasticity
