OVERALL DESIGN AND ASSESSMENT OF AIRCRAFT CONCEPTS WITH BOUNDARY LAYER INGESTING ENGINES D. Silberhorn German Aerospace Center (DLR), Institute of System Architectures in Aeronautics, 21129 Hamburg, Germany C. Hollmann, M. Mennicken, F. Wolters German Aerospace Center (DLR), Institute of Propulsion Technology, 51147 Cologne, Germany F. Eichner German Aerospace Center (DLR), Institute of Aeroelasticity, 37073 Göttingen, Germany M. Staggat German Aerospace Center (DLR), Institute of Propulsion Technology, 10623 Berlin, Germany Abstract The assessment of a kerosene driven aircraft concept with a boundary layer ingesting propulsion (BLI) system is content of this paper. The objective is to determine the potential of this BLI technology for short/medium range single aisle aircraft. For this an overall aircraft design (OAD) interpretation of a current Airbus A320neo, called D165, is chosen to be the reference to calibrate the design and assessment methodology. However, the final technology integration results are compared to the Baseline D165-2035 assuming technology level maturation for 2035. This D165-2035 aircraft is equipped with two geared turbofan engines with an increased bypass-ratio of 15.3 at take-off condition. To evaluate the various phenomena driving the aircraft performance and to isolate the impact of the integration of the BLI propulsion system, two intermediate aircraft with rear-mounted engines have been designed. The first one features structurally and flight performance driven positioned rear engines at the location of the rear pressure bulkhead and the second one takes additional constraints into account considering the possibility of burying the engines into the fuselage at the next step without any interaction with the passenger cabin and hence the bulkhead. Finally the BLI configuration D165-2035-TB including two partially buried engines at the rear of the fuselage and a T-tail empennage is investigated and assessed. Besides the investigation of this specific configuration a general overview and further performance improvement potentials of aircraft with boundary layer ingesting propulsion systems are demonstrated and their feasibility is discussed in this paper. Keywords Boundary Layer Ingestion (BLI); Overall Aircraft Design; Technology Integration; Technology Assessment; Rear Engine Integration 1. INTRODUCTION part of the project “Hybrid Electric Propulsion for Emission Reduction in Flight” (HYPER-F) within the framework of The expected growth in air travel demand in the coming Clean Sky 2. decades drives the requirements of future aircraft The theory behind a BLI propulsion system is that the regarding their environmental impact and fuel reduced averaged intake velocity results in a lower intake consumption. These requirements become increasingly momentum. For the same thrust and mass flow, this challenging nevertheless the level of environmental impact results in a reduced exhaust velocity for the same delta in acceptable by our society is not known today. This is why intake and exhaust velocities which increases the alternative technologies and concepts reducing the efficiency. environmental impact have to be examined and evaluated. BLI concepts in general have been investigated in various One potential concept is a boundary layer ingesting publications [1] [2] [3] [4] [5] [6]. The results at overall propulsion system (BLI). This specific engine integration aircraft level sometimes vary significantly which is why a affects the aircraft in various ways and hence it is study at this level with the investigation at high fidelity level assessed at overall aircraft level in the scope of the first of the important disciplines is presented subsequently. 2. DESCRIPTION OF METHODOLOGY Masses: The description of the overall aircraft design methodology Wing masses are calculated with a reduced order model together with important additional disciplines which have “CLA” [11] developed at the DLR-Institute of Aeroelasticity to be considered with higher fidelity approaches in order to Conceptual Sizing tool and Synthesis evaluate the current boundary layer ingestion concept, is content of the following section. At the end of the level one loop, the conceptual sizing tool openAD can again be applied for synthetization. This 2.1. Overall Aircraft Design Process means that all results from higher fidelity methods are set The applied aircraft design methodology is a model based as an input and the remaining parameters together with system engineering (MBSE) approach in which the effect at the geometry is calculated. disciplinary tools are communicating via the Common 2.2. Additional Disciplines Parametric Aircraft Configuration Schema [7]. It is a multidisciplinary, iterative sizing process with semi 2.2.1. Power Saving Coefficient Calculation empirical and physics based models set up in the Remote Control Environment (RCE) [8]. The pure benefit of Boundary Layer Ingestion (BLI) The main part of the workflow is the level zero and level considering the difference in intake momentum is one toolchain which consists of two convergence-loops expressed in terms of the Power Saving Coefficient (PSC) with which the aircraft is iteratively designed. first defined by Smith [12] and shown in the following The Level 0 convergence loop is based on the conceptual equation (1). sizing tool openAD using semi empirical models. The 푃 − 푃퐵퐿퐼 Level 1 loop consists of semi empirical and physics based (1) 푃푆퐶 = 푃 models. The different disciplines are described below: It describes the difference in propulsive power between an Aerodynamic undisturbed inflow condition and an inflow condition with For the aerodynamic performance calculation, the tools an ingested boundary layer. It is notable that within this “LIFTING_LINE” [9] and “HANDBOOK_AERO” developed investigation, the benefit expressed in terms of the PSC at the DLR-Institute of Aerodynamics and Flow only takes the pure momentum deficit difference into Technology are applied. They consist of Vortex-Lattice account. Any other effects are considered separately. together with empirical and semi-empirical methods. Otherwise a clean bookkeeping of the different effects becomes difficult and error-prone. High Speed Performance The general theory of the BLI propulsion system benefit is explained below. The thrust is described as the difference The stepwise mission performance estimation “AMC” with in momentum flow in the following equation (2) without in flight trim drag calculation is applied, developed at the taking the pressure difference in front and at the rear of DLR-Institute of System Architectures in Aeronautics the engine into account. Additionally it is assumed that the Propulsion: mass flow 푚̇ increase due to the added fuel flow is negligible small. The gas turbine performance is integrated via a reduced order model from “GTlab” (Gas Turbine Laboratory) [10] (2) 푇 = 푚̇ ∙ (푣9 − 푣0) developed at the DLR-Institute of Propulsion Technology It is assumed that the mass flow as well as the thrust Reference Baseline Intermediate Baseline BLI D165 D165-2035 D165-2035-T D165-2035-T-Back D165-2035-TB • Similar to A320 NEO • Similar aircraft • Similar aircraft • Slightly further back • Similar aircraft • Well known Aircraft technology technology mounted Engines to technology • Serves as basic assumptions as the assumptions as the work out the structural assumptions as the reference D165 D165 and configurational D165 • Current geared • Same Design Mission • Same Design Mission effects • Same Design Mission turbofan technology • Redesigned Wing and • Redesigned Wing and • Advanced geared • Redesigned Wing and • BPR 12.5 Tail Tail turbofans Tail • Advanced geared • Rear mounted • BPR 15.5 • Rear mounted partially turbofans Engines buried Engines • BPR 15.5 • Advanced geared • Advanced geared turbofans turbofans • BPR 15.5 • BPR 15.5 • BLI effect taken into account Figure 1. Overview of aircraft concepts remains constant for the clean and the distorted inflow The thrust of the boundary layer ingesting engines is the condition. This leads to a constant difference in intake and total mid-cruise thrust of the current BLI concept. exhaust speed and a varying fan face area. The resulting PSC’s for a concept, where 360 degree of The propulsive power which has to be added to the jet to the rear fuselage circumference is covered by the engine generate thrust is shown in the next equation (3). It is intake, is displayed in Figure 2. An intake which covers derived by the difference in kinetic Energy. 100 degree, as it is approximately the case for the current BLI concept, is shown in Figure 3. 푚̇ (3) 푃 = (푣2 − 푣2) 푗푒푡 2 9 0 Thus it is recognizable that the power reduces for a decreased intake velocity 풗ퟎ and a constant velocity difference 풗ퟗ − 풗ퟎ due to the squared dependence. To get these two velocities for the undisturbed and disturbed case, the boundary layer has to be calculated for a certain flight condition together with the required thrust. The boundary layer height and hence a reduced averaged intake velocity at the position of the propulsion system is achieved by converting the total drag in front of the intake via a given boundary layer profile into an momentum deficit flow. In other words, the total drag represents a momentum deficit flow through a specified area and velocity distribution. This results iteratively, together with additional geometrical dimensions, into a boundary layer height. Nevertheless, if the inlet height is higher than the boundary layer, the boundary layer velocity profile