APPU Aircraft Empennage Design

Abstract

The APPU project aims to lower the threshold of installing hydrogen-driven and boundary-layer-ingesting propulsion systems in the short term. This is done by replacing the Auxiliary Power Unit with an Auxiliary Power & Propulsion Unit (APPU). To study these concepts, the Airbus A321neo is taken as a reference baseline. The implementation of the APPU system on the A321neo aircraft necessitates a redesign of the existing empennage. The research objective of this thesis project is to investigate how an optimal empennage design for an aircraft equipped with an APPU system differs from an optimal empennage design for an aircraft without such a system. Multidisciplinary design optimization is used to minimize fuel weight for different aircraft configurations by optimizing the empennage geometry. Four different disciplines are identified. A weight discipline estimates the empennage weight based on the empirical Raymer equations. Two separate aerodynamic disciplines are implemented with different fidelity levels. The low-fidelity version is based on AVL's vortex lattice method, expanded with a constant friction coefficient drag model to account for viscous effects. The high-fidelity version is based on FlightStream's panel method. FlightStream proved to be infeasible for use in this study due to long run times and limited mesh robustness. Therefore, only the low-fidelity version is used to generate the final results. The static stability and control of the aircraft are ensured through a set of constraints that require specific stability and control derivatives to remain within defined limits. These derivatives are provided by the stability and control discipline that is based on AVL as well. The performance discipline applies the Breguet equation to convert the aircraft weight and aerodynamic performance into an estimate of the fuel load required to complete the design range. ParaPy is used as a multi-model-generator to provide the required input geometries for the disciplines. The study reveals that optimal empennage designs for aircraft equipped with an APPU system differ notably from optimal empennage designs for conventional aircraft. Both configurations benefit from high aspect ratios and reduced tailplane areas. However, APPU-equipped aircraft require a low cruciform tail to accommodate the hydrogen tank and reduced sweep angles to position the aerodynamic center aft without intersecting the propulsor plane. The low-fidelity aerodynamics discipline is unable to optimize the airfoils. AVL’s modeling of only the camber line makes it unsuitable for optimizing symmetric profiles, such as those on vertical tailplanes. Furthermore, the horizontal tailplane airfoils showed limited variation from the initial design. This is likely due to the dual-parameter definition of the camber line, as the class-shape transformation parameterization method is applied independently to both the upper and lower surfaces of the airfoils.