Design Methodology for Unconventional Engine Mounting Structures, Including Crashworthiness Assessment

Abstract

The need for better fuel efficiency within the growing aviation sector is increasing every day. Meanwhile conventional aircraft are reaching an asymptote regarding fuel efficiency and therefore the focus switches to unconventional aircraft. These aircraft have unique challenges whereof one is the structural connection between engine and aircraft. In this research an automated design methodology is developed which allows for optimization of such a structure. This design methodology uses a parameterised computer-aided design (CAD) connected to Python, which can automatically alter the parameters and make a finite element (FE) model of the design and subsequently run the desired simulations. The
results of the simulations are extracted and post-processed into margins of safety (MoS) using the direct results or (semi-)analytical equations for each individual component. In total five failure modes are assessed, which are Von Mises stress, displacement of engine, crippling, panel buckling and column buckling. The most critical MoS and the relation to the parameters is used in combination with an adaptive damping parameter to optimize each individual part. This design methodology is tested using the Flying-V, which is a novel aircraft configuration in the shape of a V, as a reference aircraft. Therefore, first the requirements, critical loadcases, loads and initial design have been developed based on regulations and previous research regarding the Flying-V and have been used as input for the CAD model and simulations. The design methodology has proven not be perfect as regions with stress concentration cause the minimum MoS to be below zero for all iterations and could not be solved within the time frame of this research. However, the design methodology has proven to be significantly faster compared to a design of experiments used in previous research. Overall it should be noted that there are still many other limitations regarding this method. These limitations are related to the limited amount of failure modes used and the fact that it still requires some kind of Design of Experiment, which is time consuming. The final mass of an optimized engine mounting structure is estimated to be approximately 40% higher than the design methodology used in previous research, which is due to the higher amount of details in the design as well as the significant higher estimated mass for the engine. This means that the current estimated mass for the full engine mounting structure including landing gear and engine is approximately 22150 kg compared to the estimated 17360 kg before.

Furthermore, an assessment of the influence of the developed design on the crashworthiness of the Flying-V has been done. This assessment focuses on one of the four main criteria regarding occupant protection, namely the guarantee of acceptable acceleration and loads sustained by the occupants. This is done by connecting the engine mounting structure to the developed wing-fuselage structure of the Flying-V with a tie connection. In order to compare different cases a new variable has been created, which is based on empirical equations developed by the U.S. Air Force. In addition, a new method based on moments of inertia has been developed to take the rest of the aircraft into account without increasing the size and thus computational effort of the simulation. To validate this method, the wing-fuselage structure has first been extended to fit the structure and results are compared to previous research. This has shown large, unexpected, differences which could not be explained even with thorough investigation. Applying the inertia method to the extended fuselage has shown small but explainable and expected differences, validating that the method has no unforeseen effects on further results. Simulations using this inertia method and the wing-fuselage with full engine mounting structure shows an improved performance in crashworthiness as the engine mount also absorbs some of the energy. Similar simulation without engine shows an even more improved performance, meaning that engine separation is of interest for the Flying-V. Initial further-reaching investigation into engine separation, using a point-mass model, shows that engine separation might be possible without impacting the Flying-V for crashes without any initial yaw, pitch or roll angle.