Influence of Novel Airframe Technologies on the Feasibility of Fully-Electric Regional Aviation
Influence of Novel Airframe Technologies on the Feasibility of Fully-Electric Regional Aviation
Karpuk, Stanislav;Elham, Ali
2021-06-10 00:00:00
aerospace Article Influence of Novel Airframe Technologies on the Feasibility of Fully-Electric Regional Aviation Stanislav Karpuk * and Ali Elham * Institute of Aircraft Design and Lightweight Structures, Technical University Braunschweig, 38106 Braunschweig, Germany * Correspondence: s.karpuk@tu-braunschweig.de (S.K.); a.elham@tu-braunschweig.de (A.E.) Abstract: The feasibility of regional electric aviation to reduce environmental impact highly depends on technological advancements of energy storage techniques, available battery energy density, and high-power electric motor technologies. However, novel airframe technologies also strongly affect the feasibility of a regional electric aircraft. In this paper, the influence of novel technologies on the feasibility of regional electric aviation was investigated. Three game-changing technologies were applied to a novel all-electric regional aircraft: active flow control, active load alleviation, and novel materials and structure concepts. Initial conceptual design and mission analysis of the aircraft was performed using the aircraft design framework SUAVE, and the sensitivity of the most important technologies on the aircraft characteristics and performance were studied. Obtained results were compared against a reference ATR-72 aircraft. Results showed that an all-electric aircraft with airframe technologies might be designed with the maximum take-off weight increase of 50% starting from the battery pack energy density of 700 Wh/kg. The overall emission level of an all-electric aircraft with novel technologies is reduced by 81% compared to the ATR-72. On the other hand, novel technologies do not contribute to the reduction in Direct Operating Costs (DOC) starting from 700 Wh/kg if compared to an all-electric aircraft without technologies. An increase in DOC ranges from 43% to 30% depending on the battery energy density which creates a significant market obstacle Citation: Karpuk, S.; Elham, A. for such type of airplanes. In addition, the aircraft shows high levels of energy consumption which Influence of Novel Airframe Technologies on the Feasibility of concerns its energy efficiency. Finally, the sensitivity of DOC to novel technologies and sensitivities Fully-Electric Regional Aviation. of aircraft characteristics to each technology were assessed. Aerospace 2021, 8, 163. https:// doi.org/10.3390/aerospace8060163 Keywords: aircraft design; airframe technologies; aircraft sizing; all-electric aircraft; multi-disciplinary design optimization Academic Editor: Dieter Scholz Received: 3 May 2021 Accepted: 4 June 2021 1. Introduction Published: 10 June 2021 Significant climate changes and potential environmental impact due to increased transportation in the near future have motivated many industries to focus on reducing CO Publisher’s Note: MDPI stays neutral and NOx emissions. As a major transportation method, the aviation industry also follows with regard to jurisdictional claims in the trend to reduce the emission of new generations of aircraft. Improvements in airframe published maps and institutional affil- and engine technologies increase aircraft efficiency and reduce their emission. However, iations. a potential increase in air transportation may still lead to an increase in overall CO and NOx emissions. Under Flightpath 2050 [1], The European Commission has set a future challenge for the new generation of aircraft to reduce their total emission. Figure 1 shows three schematic trends: if no advancements in aircraft technologies are present, if currently Copyright: © 2021 by the authors. feasible technology advancements are achieved, and if novel technologies are introduced. Licensee MDPI, Basel, Switzerland. This challenge leads to developing alternative environmentally-friendly energy sources This article is an open access article as one of the main solutions for environmental impact reduction. Aircraft electrification distributed under the terms and has become one of the most popular approaches to reduce aircraft emissions. Today, many conditions of the Creative Commons companies work in various directions to make electric flights available: improve battery Attribution (CC BY) license (https:// energy capacity, modify and develop new propulsion systems, and introduce new aircraft creativecommons.org/licenses/by/ configurations more applicable for future electrification. 4.0/). Aerospace 2021, 8, 163. https://doi.org/10.3390/aerospace8060163 https://www.mdpi.com/journal/aerospace Aerospace 2021, 8, 163 2 of 29 Figure 1. Forecast of CO emission impact due to increased transportation and emission reduction goals [1]. At the moment, research is highly focused on hybrid- and all-electric propulsion systems and aircraft configurations to reduce the environmental impact produced by aircraft. Friedrich introduced a simulation technique for a single-seat hybrid-electric demonstrator, designed the aircraft, and performed a scaling analysis to determine a large fuel savings impact for the small- and mid-scale sector of aircraft [2]. Hamilton investigated the effect of hybrid-electric aircraft from the operational perspective to reach an optimal aircraft operation with battery energy density constraints [3]. A lot of work is also dedicated to the introduction of distributed electric propulsion (DEP) energy networks and aircraft concepts featuring DEP. Kim summarized major contributions towards the development of DEP aircraft and relative technologies [4]. Finger focused on sizing methodologies, and aircraft design of future general aviation aircraft that could be not only hybrid-electric but also fully electric [5–7]. De Vries also developed initial sizing methodologies for hybrid-electric aircraft for conceptual aircraft design but introduced effects of DEP at early conceptual design stages [8,9]. Pornet also introduced a sizing methodology for hybrid- electric aircraft and compared performance between the conventional reference aircraft and its hybrid version [10]. Sgueglia introduced an MDO framework for hybrid-electric aircraft with coupled derivatives and performed various optimization analyses to demonstrate the capabilities of the framework [11]. The work of Hepperle [12] addressed the potentials and limitations of all-electric aircraft specifically and introduced aircraft modifications that could potentially improve the aircraft performance at given battery energy densities. The ultimate goal of aircraft electrification is to achieve a fully-electric flight at adequate aircraft weight characteristics to maximize the emission reduction at the aircraft level. However, the availability of all-electric aircraft is limited to the General Aviation sector due to low battery energy density compared to the Jet-A fuel and relatively low maximum power capabilities of modern electric motors. Multiple all-electric concepts have already been introduced or are being developed at the moment. From existing airplanes, Pipistrel has already certified a twin-seat all-electric aircraft that has a maximum speed of 185 km/h and a maximum range of 139 km [13]. Bye Aerospace is currently performing flight tests on a one-seat general aviation aircraft that will be certified under the FAR Part 23 category. The eFlyer has a maximum cruise speed of 250 km/h and endurance of three hours [14]. In 2016, Siemens tested an all-electric energy network by retrofitting the Extra 300 aerobatic aircraft and set two world records [15]. The Equation Aircraft has also demonstrated an all-electric twin-seat amphibious aircraft with the empennage-mounted engine that can fly up to 240 km/h and up to 200 km [16] Airbus has also developed several versions of an experimental all-electric E-fan aircraft to test the capabilities of an all-electric aircraft [17]. The E-Fan concept had multiple versions, from a technology demonstrator to a production variant. MagniX has introduced an all-electric propulsion system and first retrofitted De Havilland Beaver and then 208B Cessna Grand Caravan. The electrified Caravan became the largest all-electric airplane until today [18]. It must be noted that the list of existing all-electric aircraft companies is incomplete and more companies exist and are at different design stages. Aerospace 2021, 8, 163 3 of 29 To achieve all-electric regional aviation faster, not only the energy source and propul- sion system technologies must be improved, but also advanced airframe technologies need to be considered. Such novel technologies may significantly increase the aircraft performance characteristics and overall efficiency that will enable earlier integration of all- electric commercial aircraft into the market before. Liu introduced initial estimations of the impact of novel technologies for a range of aircraft from the short-range to the long-range [19]. Results showed that novel airframe technologies may significantly improve aircraft energy efficiency. Under the Excellence Cluster Se A (Sustainable and Energy Efficient Aviation), three energy-efficient aircraft are to be designed to cover the majority of commercial aircraft operations. Figure 2 shows three sample new energy-efficient airplanes: a short-range propeller aircraft that shall be applicable to the ATR-72, the medium-range aircraft having similar mission requirements to the Airbus A320, and the long-range aircraft to cover ranges similar to the Boeing B777. Figure 2. A family of energy-efficient aircraft under the Se A cluster. The initial cluster requirement for the short-range aircraft was to investigate the feasibility and availability of an all-electric short-range aircraft if novel airframe and propulsion technologies are introduced. A few novel technologies have been considered for the design of future regional aircraft. Hybrid laminar flow control (HLFC), load alleviation, and advanced materials and structure concepts. In addition, high-temperature superconducting motors and high-energy capacity batteries have been considered for the present research. An aircraft with similar top-level requirements as ATR-72 is designed considering the mentioned technologies and a battery-based full electric propulsion system. The present work is divided into multiple sections. Section 2 describes novel tech- nologies implemented in the new short-range aircraft. Section 3 describes methodologies, models, assumptions, and software used to perform novel technology assessments to de- sign the aircraft. Section 4 describes the aircraft’s top-level requirements (TLRs). Section 5 describes the aircraft concept selection and its initial sizing. Section 6 presents multidis- ciplinary design optimization studies of the SE A aircraft to refine the design and make important design decisions. Finally, Section 7 assesses the influence of novel technologies on aircraft availability and the strength of each technology impact. 2. Novel Airframe and Propulsion Technologies 2.1. Hybrid Laminar Flow Control The generation and extension of laminar flow over the aircraft surface significantly affect the overall aircraft drag, reducing aircraft weights, fuel burn, and operating costs. Preliminary estimations of an aircraft that features extended laminar flow along the wing, empennage, and fuselage demonstrated significant reduction (up to 50%) in overall drag and proved the importance of laminar flow control (LFC) [20]. Figure 3 shows the effects of the new wing design featuring the LFC and compares overall drag to the reference aircraft. However, natural laminar flow (NLF) is limited along the shape, so to extend it even more, hybrid laminar flow control (HLFC) is required: the laminar boundary layer is not only extended using NLF design but also using the active suction technology. In this technology, the air is sucked from the aircraft’s outer surface to delay the transition of the boundary layer and enable substantially higher percent laminarization compared to conventional surfaces. The skin of each aircraft component is split into two segments: Aerospace 2021, 8, 163 4 of 29 a porous sheet and an inner sheet that supports the outer sheet. The inner sheet has orifices that suck the air from the boundary layer and delay transition. In each chamber, an individual pressure is adjusted by the throttle orifices, so that the pressure difference between the outside and the chamber delivers the locally desired amount of mass flow through the surface. Figure 4 shows a schematic image of the skin layout and the HLFC system for a wing section. The applied technology in this project is based on [20,21], which describes numerical approaches with active laminar flow control and also describes current progress in this technology. Figure 3. Effect of laminar flow control on overall aircraft drag [20]. Figure 4. Schematic views of the active suction system [20]. 2.2. Load Alleviation Load alleviation introduces various techniques to reduce the bending moment expe- rienced by the aircraft and that have a passive or active nature. Reduction of maximum bending moment enables the design of lighter wings for lower limit load factors, which will improve aircraft fuel efficiency. Passive load alleviation solutions consider nonlinear stiffness material design [22], viscoelastic damping design [23], new structural concepts [24,25], and local morphing structures [22]. Nonlinear stiffness materials may improve the load distribution on the wing under low load cases (0.5 g to 1.5 g) and improve performance efficiency at those load cases. Finally, both aeroelastic tailoring and local morphing structures aim to extend the aeroelastic design space. Morphing is considered into two scenarios: deliberate structural non-linearity that affects the wing deformation and reduces effective angle-of-attack of the wing section under the load and change in airfoil shape to achieve load reduction [22]. The wing active load alleviation uses different types of flow control over the wing to achieve a more favorable wing load distribution and reduce the wing bending moment. Previously, researchers have approached the design of active load alleviation systems in different ways. Rossow et al. [26] investigated the feasibility of an aircraft featuring load alleviation technologies; Sun et al. [27] looked at active load alleviation from the Aerospace 2021, 8, 163 5 of 29 control system perspective; and Zing et al. [28] demonstrated the experimental results of the high aspect-ratio wing wind-tunnel test with active load alleviation while using piezoelectric control. The work of Fezans et al. focused on novel sensors and control systems technologies to enable rapid and robust active load alleviation [29–32]. Finally, active load alleviation could also be reached by fluidic or micro-mechanical flow actuators that change the load distribution along the wing and reduce the bending moment [33]. 2.3. New Materials and Structure Concepts Novel structural concepts and materials are being developed to improve the aircraft structure in terms of stiffness and weight. Bishara et al. [34] describe advanced structural design with the integration of active flow control, which is directly applicable to the presented research. Under the excellence cluster, the reduction in the airframe weight is assumed to be reached by application of CFRP thin ply laminates. In addition, new structural concepts must be applied to satisfy HLFC requirements [22]. 2.4. High-Power Superconducting Motors One of the major problems in hybrid- and all-electric commercial aircraft deals with limitations of maximum available electric motor power. The motor resistance increases rapidly with the increase of its power, which rapidly diminishes the engine efficiency. One solution to preserve the motor efficiency and even increase it is to use superconducting motors. Such motors can be synchronous, trapped flux, and fully superconducting. They may also operate at low and high temperatures. To maintain required temperatures inside the motor, a cryogenic cooling system is required. High-temperature superconducting (HTS) motors can achieve high power-to-weight ratios with high power densities without major weight losses compared to low-temperature superconducting motors. Ref. [35] provides a descriptive survey of HTS motors and their future trends. For the present research, a high-temperature, fully superconducting motor was considered. 2.5. High Energy Capacity Batteries Currently, the gravimetric energy density of modern batteries generally does not exceed 300 Wh/kg [36]. Such energy densities may be suitable for ultra-light or limited general aviation all-electric aircraft. However, they are insufficient to enable not only all-electric aircraft of the size of ATR-42 and larger, but even the hybrid-electric version of such aircraft is infeasible at the moment since the energy capacities are around 50 times less compared to kerosene-based fuels. On the other hand, the research of high-capacity batteries makes progress. The latest laboratory research demonstrated battery energy capacities of 800 Wh/kg on the cell level and 500 Wh/kg on the pack level for military applications [36]. In addition, the National Academy of Sciences predicts battery pack energy density to achieve 400–600 Wh/kg at Technology Readiness Level (TRL) 6 in the next 20 years (roughly, by 2035) and be commercially available in 30 years [37]. If the prediction becomes real and the linear trend remains, then batteries may reach TRL 6 with energy densities of 550–850 Wh/kg and be commercially available by 2060. To investigate potential scenarios for the far future and examine battery energy impact on the aircraft feasibility, it is assumed that the pack energy densities may range between 500 and 1100 Wh/kg. 3. Implementation of Novel Technologies in Aircraft Analysis and Design 3.1. Initial Aircraft Sizing The conceptual design was performed using various tools. OpenVSP [38] and CATIA were used for the aircraft geometric modeling, and SUAVE [39] was used for more defined aircraft sizing, performance, and mission analysis. To have more capabilities from the air- craft performance analysis standpoint, SUAVE was extended to have classical performance analysis methods used in [40–43]. The aircraft sizing within SUAVE is performed iteratively. First, initial geometric specifications based on TRLs and any available information are input into the SUAVE. Aerospace 2021, 8, 163 6 of 29 The information includes the initial wing planform, its section properties, flaps charac- teristics, empennage and fuselage geometries, the definition of the propulsion system, and all required components such as batteries, gearboxes, power management systems (PMAD), cables, etc. Finally, initial guesses of aircraft weight are input. Then, the first iteration of constraint analysis using equations provided by Gudmundsson [41] is per- formed. The constraint diagram block includes rapid estimation methods for gas-turbine, piston, and electric aircraft power lapse with altitude and has options to either estimate required aerodynamic properties for the constraint diagram or have them as fixed in- puts based on historical trends. Based on the design point selection, the wing loading and power-to-weight ratio are used to run the SUAVE mission analyses to estimate the aircraft performance and its required weight. Then, obtained maximum take-off mass (MTOM) is compared to the initial guess and updated if the tolerance is not reached. In addition, parameters such as the minimum drag coefficient (C ), Oswald efficiency Dmin (e), and maximum lift coefficient (C ) for clean and flapped configurations are input Lmax into the constraint diagram again to update all constraint curves and run the loop again. When the tolerance is reached, the program moves to the aircraft performance block to obtain performance plots for the given aircraft. Particular care was taken to size the wing flaps to meet take-off and landing field length requirements. Methods of Torenbeek [42] and Roskam [43] were implemented within SUAVE to analyze various types of leading- and trailing-edge devices. The empennage sizing within SUAVE is based on the fixed tail volume ratio and then updated and corrected separately based on the desired CG envelope. Performance analyses within SUAVE included take-off, all engines operative (AEO), and one engine inoperative (OEI) climb, cruise, descent, and landing. Figure 5 shows the sizing process within SUAVE. Figure 5. Initial aircraft sizing framework using SUAVE. 3.2. Energy Network In the SUAVE analysis framework, a propeller electric energy network with HTS motors was implemented. Figure 6 shows the all-electric energy network layout. Aerospace 2021, 8, 163 7 of 29 Figure 6. Battery-electric energy network layout. Propeller modeling was performed using the cubic spline method described by Gud- mundsson [41]. Propeller thrust can be described with a cubic spline based on design operating conditions. The thrust is defined by: 3 2 T = AV + BV + CV + D (1) where A, B, C, D are the coefficients required to fit the curve accurately. To obtain those coefficients, a system of four equations is established. The first equation is the equation for the static thrust. The second equation is derived for the cruise speed and the desired propeller efficiency. The third equation is the derivative of the curve at the cruise speed, which is equal to zero. Finally, the last equation is derived for the desired maximum cruise speed. The final system of equations for the thrust as a function of cruise speed becomes: 2 38 98 9 0 0 0 1 A T > >> static > > >> > < =< = 3 2 6 7 V V V 1 B T C C C 6 7 (2) 2 2 4 5 3V 2V 1 0 > C>>