Investigation of Nonlinear Structural Dynamics and Aeroelastic Properties of Strut-braced Wings at a Conceptual Design Stage

Kurzfassung

The strut is an additional load-carrying element that alleviates the bending moment on the wing. It allows for the integration of higher aspect ratio wings, which produce less induced drag and thus reduce fuel consumption for a given mission. Strut-Braced Wings (SBW) are used on a number of small aircraft configurations. However, this concept has not yet been implemented on larger transport aircraft that operate in the transonic speed range. Onera has investigated the SBW design with the Albatros project in the framework of the European Clean Aviation joint undertaking. The follow-up AWATAR (Advanced Wing mATuration And integRation) project aims to mature this wing design with highfidelity simulations, wind tunnel tests, and ultimately the development of a ground based demonstrator. In the first part of this work, the AWATAR SBW configuration is modeled in a Conceptual Load Analysis (CLA) tool to conduct several parametric studies in search of a minimum structural mass configuration. The aileron efficiency has been found to be a limiting constraint, which, if included, increases the SBW mass by 42%. The optimal strut position is at 45% of the semi-span at the front spar, if aeroelastic constraints are not included in the conceptual design study. The inclusion of the aileron efficiency constraint shifts the optimal strut attachment to the rear spar. The engine should be placed as forward as possible. The optimal AWATAR configurations from the CLA tool are exported as beam models with an aerodynamic mesh. In the second part of this work, the derived beam models are analyzed in MSC Nastran. An L-shaped junction is included to reduce aerodynamic interference effects between the strut and the wing. The vertical element forming the 'elbow' is chosen of moderate length to retain the desired load alleviation in the wing and avoid large reductions in natural eigenfrequencies of the system associated with this element. A variation of the constraints in the beam model shows that the statically determined system with pinned joints at the strut ends and at the wing root produces the lowest wing bending moment, but also the lowest eigenfrequencies of the system. Fixed joints produce larger internal loads, but generally increase the eigenfrequencies. Seven maneuver load cases are derived from the flight envelope. A prestressed modal analysis shows that the eigenfrequencies tend to increase for positive load factors and decrease for negative load factors when compared to the unloaded results. The eigenvectors change considerably if the differential stiffness due to nonlinear prestress is included. All optimal AWATAR configurations show flutter instability within the flight envelope. The inclusion of nonlinear prestress in the flutter equation may stabilize the system or change the flutter speed and the critical flutter mode depending on the load case.