The concept of distributed electric propulsion units has great potential on the track to zero-emission aircraft. Investigations raise expectations that distributed propulsion engines improve the aerodynamic performance, flight mechanical aspects as well as the aircraft loads. Reduced loads will lead to a reduction of the structural weight. Besides environmental aspects, the physics and technical based aircraft characteristics are of great importance.
Finite-element model of the aircraft structure – dynamic analysis of the wing
Thus, the following article adresses the structural design, including the estimation of the mass of the primary structure, the loads of the elastic structure and the aeroelastic characteristics of such a configuration. Distributed propulsion units lead to unusual structural dynamic characteristics compared to conventional aircraft with two or four engines and influence therefore the flutter characteristics.
In the project SynergIE (2018-2021, 5th German national Luftfahrtforschungsprogramm) the design of a regional aircraft for about 70 passengers (see Figure 1) using a distributed propulsion architecture was the main focus. In the course of the project several variants for such aircraft have been set-up using statistic-based conceptual design methods (using the conceptual design tool openAD from the DLR Institute for System Architectures in Aeronautics). The variants comprise changes in the number of the electrically powered propeller engines, ranging from two to twelve, two different tail concepts (T-tail, conventional tail) and wing aspect ratio variants, 14.3 and 17.
In order to improve the statistic-based estimation of the structural mass estimated by openAD, especially for the wing, the DLR-AE’s physics-based parametric aeroelastic design process cpacs-MONA (see Figure 2) was applied. Besides performing a comprehensive loads analysis and sizing of the load carrying primary wing structure, using structural optimization methods, the aeroelastic characteristics of the aircraft configuration with distributed propulsion were investigated. Aeroelastic characteristics include the loads of the flexible structure, the control surface efficiency and the flutter behavior. A more even distribution of the propulsion units over the wingspan promises a better utilization of the mass forces. These counteract the aerodynamic forces, resulting in a reduction of the loads. This leads to an expected reduction of the wing structure mass. The approach of parametric modeling in cpacs-MONA makes it possible to set up the simulation models for the different configuration variants at the preliminary design level and to carry out the aeroelastic design process.
The executed parameter study shows a significant influence of the distributed propulsion units on the structural weight of the wing. Furthermore, the structural dynamic characteristics of the evenly distributed propulsion units with more than two propeller engines per wing exhibit a larger variety and number of mode shapes within a fixed frequency range, and furthermore a drastic decrease of the frequencies of bending and in-plane modes and a frequency increase of the torsional modes (see Figure 3).
For the configuration with six electrically powered propeller engines it was found that the positioning of the propulsion units plays a significant role even with respect to the structural design (see Figure 4). The more evenly distributed engines along the wing lead to a lower structural wing weight compared to more narrow propeller engine positioning. The final configuration with ten electrically powered propeller engines exhibited the lowest wing mass.
The final flutter analysis of the converged cpacs-MONA aeroelastic design process resulted in classical flutter characteristics for all investigated configurations (wing bending torsion flutter). Though outside the aeroelastic stability envelope, the configuration with the two propeller engines exhibits the lowest flutter speed. Regarding the aileron efficiency no structural measures had to be undertaken (see Figure 5).
Further investigations of configurations with distributed propulsion units should expand the scope of the loads analysis, for example by considering gust analysis and include load alleviation methods, as well as more sophisticated structural design using carbon fiber material. Especially the propeller effects on the loads (e.g. p-loads, overblowing of the wing for the aero loads) should be considered. Finally, the flutter analysis should include whirl flutter, the typical rotorcraft flutter phenomena.
Author: Thomas Klimmek, DLR-Institute of Aeroelasticity, Department: Loads Analysis and Aeroelastic Design