Assessment on critical technologies for conceptual design of blended-wing-body civil aircraft

Zhenli CHEN, Minghui ZHANG, Yingchun CHEN,, Weimin SANG,Zhogung TAN, Dong LI, Binqin ZHANG

a School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China

b Shanghai Aircraft Design and Research Institute, Commercial Aircraft Corporation of China Ltd, Shanghai 201210, China

KEYWORDS

Abstract Civil aviation faces great challenges because of its robust projected future growth and potential adverse environmental effects.The classical Tube-And-Wing(TAW)configuration following the Cayley’s design principles has been optimized to the architecture’s limit, which can hardly satisfy the further requirements on green aviation. By past decades’ investigations the Blended-Wing-Body (BWB) concept has emerged as a potential solution, which can simultaneously fulfill metrics of noise, emission and fuel burn. The purpose of the present work is to analyze the developments of critical technologies for BWB conceptual design from a historical perspective of technology progress. It was found that the high aerodynamic efficiency of BWB aircraft can be well scaled by the mean aerodynamic chord and wetted aspect ratio, and should be realized with the trade-offs among stability and control and low-speed performance. The structure concepts of non-cylinder pressurized cabin are of high risks on weight prediction and weight penalty. A static stability criterion is recommended and further clear and adequate criteria are required by the evaluations of flying and handling qualities. The difficulties of propulsion and airframe integration are analyzed.The energy to revenue work ratios of well-developed BWB configurations are compared,which are 31.5%and 40%better than that of TAW,using state-of-art engine technology and future engine technology, respectively. Finally, further study aspects are advocated.

1. Introduction

Rapid increasing demands of civil aviation are driven by the high-speed transport requirements of growing middle class,global economic growth and urbanization.1With growing urbanization,there is a greater demand to connect the world’s cities. Since 1990, air transportation has grown at a fast around 5%Compound Annual Growth Rate(CAGR),and is resilient to external shocks.

The global commercial market forecasts of different companies (Airbus, Boeing and Commercial Aircraft Corporation of China(COMAC)indicate robust growth of civil aviation at a rate around 4.5%in the next 20 years,as shown in Table 1.2-4As a result,the passenger number and global fleet will be doubled to 8 billion and 48000 aircrafts,respectively,till 2037.The market will be dominated by the Asia Pacific region.Although the number of wide-body aircraft is not large,the value is more than 40%.

The expanded and rising demand poses the long-term challenges in efficiency, safety and environmental sustainability of aviation.5In terms of adverse environmental impact, aviation transport produces noise, particles, chemical product emissions and contrail formation. The predicted robust growth of civil aviation will bring much more pressure on the global warming. The current consensus is that the commercial aviation currently contributes about 2-3% of the carbon dioxide(CO2) produced by human (anthropogenic CO2emissions).However, the total effect on global warming is probably more like 3-5% when taking into account the effects of oxides of nitrogen from combustion and of contrail cirrus by cloud formation.6,7If the business is as usual, the impact of aviation emission on the climate change will grow fast as the commercial air travel grows.

Therefore, it is urgent to limit and to reduce environment impact of aviation by legislation,new infrastructure and flight management,introduction of new highly effective aircrafts and adoptions of advanced low-emission technologies. To increase the environmental adaptability of conventional aircraft by technical evolution, it would become eventually prohibitively expensive and lead to high risk on sustainable aviation. A major step forward cannot be achieved without vast investigations and a radical change of the aircraft concept. To reduce the impact of aviation on the climate, an alternative new generation of airliners is highly expected.

To limit the adverse environmental impact of aviation, the International Air Transport Association (IATA) expressed a strongly expected resolution. The International Civil Aviation Organization (ICAO) updated more stringent regulations on aircraft noise and oxides of nitrogen(NOx)emission and introduced two completely new regulations on carbon dioxide(CO2)and non-volatile Particulate Matter(nvPM)emissions.8To enable sustainable growth and address climate change, in the IATA’s resolution, it calls for a 1.5% average annual fuel efficiency improvement between 2010 and 2020,carbon neutral growth from 2020 onward, and a reduction of 50% in net emissions by 2050 compared to 2005 levels.9The new standard on aircraft CO2certification is to reduce greenhouse gas emissions from the air transport system. It is the first global technology standard for CO2emissions for any sector with the aim of encouraging more fuel efficient technologies into aircraft designs. The recommended CO2standard has been developed at the aircraft level, and therefore has considered all technologies associated with the aircraft design including propulsion, aerodynamics and structures.10However, the regulations themselves are not driving technology advancements.

The environmental regulations continue to be tightened but do so in a way that keeps up with technology advancement,which still can stimulate the investments of industries and governments. To satisfy the regulations and gradually realize the green aviation, the National Aeronautics and Space Administration (NASA) of United States of America (USA) and the Advisory Council for Aeronautics Research in Europe(ACARE) had promoted systematic and adaptive metrics on noise, NOxemissions of landing and takeoff (LTO) and fuel burn, as shown in Table 2.11-15

To realize these objectives, the regulations, operations and new technology adoptions are required and expected. USA and European Union(EU)have implemented continuing large scale projects on new key enabling technologies. NASA has fundamental aviation research program,16Subsonic Fixed Wing (SFW) research program, Environmentally Responsible Aviation (ERA) and subsequent New Aviation Horizons(NAH).In EU the joint adventure of clean sky I has been finished and the clean sky II program will be implemented until 2026.17As developing new technologies for new generation Highly effective aircraft requires substantial time and resources, it takes a long time for new technologies to propagate into and through the aviation fleet. Whereas fuel efficiency and noise reduction can come from technologies of both airframe and propulsion systems, LTO NOxemission reduction can only be directly achieved through propulsion technology advancements, which is aimed at the reduction of harmful productions per unit weight of the fuel burnt.

It is hard to realize the objectives set by ICAO on CO2emission of net zero increasing on 2020 and of half-reduction on 2050. In recent researches, through different scenarios, it was shown that current technologies and evolutionary improvements will not keep pace with many of these growing challenges, nor will new technologies and concepts satisfy the expectations of IATA’s resolution.18,19Even so, new concepts and game-changing technologies will be needed to capture the opportunities of the future, and this is an alternative way. To ultimately realize industry innovation, long-term stable aeronautics researches are required.

Table 1 Market prediction of civil aviation in next 20 years (2018-2037).

The classical Tube-And-Wing(TAW)configuration follows the Cayley’s design principles that the forms follow the functions, i.e., a cylinder fuselage providing volume, a wing generating lift,the horizontal and vertical tails realizing stability,the control surfaces acting for control, and the engines providing thrust. The swept-wing TAW has been well optimized to the architecture’s limit. It is built from materials that provide a superior balance between weight,strength,durability and cost.It is powered by the turbofan engines which also achieve high efficiency using the simple Joule cycle.

Table 2 Metrics of NASA and ACARE.

From 1990s different alternatives were promoted to improve the aerodynamic efficiency.20,21The key is to reduce aerodynamic drag,structural weight and to improve the engine efficiency. The application of composite materials towards main structures, like fuselage and wing, and the adoption of high-bypass ratio turbofans make the TAW outstandingly efficient. A truss-braced wing concept adopts a high aspect ratio to reduce the lift-induced drag. The ‘C’ wing and ‘Box’ wing concepts have high spanwise efficiency to reduce the liftinduced drag. Laminar flow control technologies are used to reduce the friction drag.Bended-Wing-Body(BWB)concept22was promoted to reduce form drag by increasing wetted aspect ratio, to increase spanwise efficiency by using lifting fuselage,and to reduce the interference drag by smoothly blended the wing and body. By the studies of the past decades, the BWB configuration emerges as an unconventional but promising concept, which has the potential to simultaneously fulfill all relevant metrics. However, there are still enormous challenges that need to be resolved.22,23

The aim of the present work is to analyze the development of several critical technologies of BWB concept from a historical perspective of technology progress. First, the projects of BWB development are summarized and analyzed.Second, several key-enabling and enhancing technologies are scrutinized. Third, a non-dimensional parameter, Energy To Revenue Work ratio (ETRW) indicating aircraft efficiency,24is adopted to evaluate the efficiency of well-developed BWB configurations. Finally, the recommendations and conclusions are drawn.

2. Characteristics of BWB

Table 3 Aircraft concept defined by forms-following-functions.

Different concepts including conventional TAW, Flying Wing(FW), BWB, Integrated Wing-Body (IWB) and Hybrid Wing Body(HWB)appear in the literature,which can be categorized by the relationship between forms and functions, as shown in Table 3. For TAW the required function is corresponding to the form almost individually following the Cayley’s design principles. For flying wing concept all functions are provided by the wing and are inherently coupled. Some functions are contrary to others,like high aerodynamic efficiency versus stability and control.To ease these conflicts and design difficulties of FW, an individual body using extended chord length was adopted to provide volume,lift and hosts of the stabilizer/control surfaces, landing gear and embedded engines, and further to reduce wetted area. Meanwhile, a smooth transition between the body and wing is adopted to further reduce the interferences and the consequent drag.All these features result in the BWB concept with a multi-functional body without vertical and horizontal stabilizers.

Comparing with FW, BWB has a more obvious centerbody, highly aerodynamic efficient outer wing providing part of the lift, and a smoothly blended (integrated) region in between. The length of the centerbody is normally less than the span width. There is no distinct horizontal and vertical stabilizer as on conventional TAW configuration. The longitudinal stability is always relaxed or realized by using downloading of rear center body, outer wing, highly swept angle of the outer wing or their combinations. The directional stability is provided by the winglet or vertical tail, which is usually insufficient. There are a series of control surfaces along the trailing edge of the centerbody to the outer wing, functioning simultaneously as elevator and aileron for longitudinal and lateral controls, therefore, named as elevons. The directional control is mainly provided by rudders on the winglets and is assisted by the outboard splitting drag rudders. Due to the positions of these control surfaces, the moment/lever arms for pitch and directional control are much shorter than that of conventional TAW configuration. Therefore, to satisfy the requirements on the control authority, the areas of the elevons are large, which leads to lift loss and high hinge moments, when further longitudinal trim and stability augments are required.

Although high aerodynamic efficiency and preferable structural weight of the wing can be obtained due to a wetted-area reduction and a preferred spanwise lift distribution, respectively, this concept still brings design challenges on noncircular pressurized cabin and Stability and Control (SC) for civil applications. Therefore, the podded engines mounted on the aft-upper centerbody and additional vertical stabilizers are adopted to eliminate difficulties of engine design and to satisfy the SC requirements. To alleviate the disadvantages of short moment arms, the centerbody is usually lengthened.All these features can be found on TAW that motivate the BWB to be named as HWB. The IWB concept is also of this kind.

2.1. General characteristics of BWB

The centerbody of BWB is multi-functional. Its non-circular cross section leads to specific difficulties for pressurized cabin design.The centerbody provides about 30%lift at cruise when an elliptical lifting distribution is assumed in spanwise direction. Its integrations with engines and stabilizer/control surfaces introduce much more difficulties. The smooth integration of the centerbody,a transition region and the outer wing determines the attached flow over the BWB, and brings distinct aerodynamic efficiency at cruise. Meanwhile due to the specific geometrical outer mold line of the BWB and the inherent integration, it is not obvious how to define the reference area(Sref)and Mean Aerodynamic Chord(MAC).At the earlier development of BWB, Liebeck22used the area of the trapezoidal wing as reference area, but the definition of MAC is not clear. During the development, it is normally to use gross planform area and the corresponding MAC as the reference values. Although the specific reference values do not change the lift-to-drag ratio,but they do affect the magnitude of the force and moment coefficients and the consequent stability margins.Therefore, they are very important for flight control system design.

Several system level assessments having gradually improved fidelities were performed to know the capability of BWB comparing with TAW under the same advanced technology scenario.12,23-27It was found that only the BWB configuration with advanced turbofans can achieve the green-aviation goals simultaneously.The predicted fuel burn benefit over advanced TAW designs is much less than early results as Boeing studies suggested, which is mostly of single digit gains. However, the noise margin improvement is great because of the noise shielding provided by the centerbody.

2.2. Advantages and challenges

Table 4 Advantages and disadvantages of BWB configuration.

For BWB concept aerodynamic design, structures, SC, noise features, Propulsion/Airframe Integration (PAI), and internal layout are a few areas that differ significantly from the traditional TAW design. The advantages and disadvantages of BWB can be summarized as in Table 4. Despite potentially high aerodynamic and possible structural efficiencies, the BWB concept has not yet been embraced by aircraft manufacturers. One argument is that BWB have some inherent disadvantages/challenges that can partially offset its advantages.29

Large leaps in aircraft efficiency,coupled with reductions in noise and harmful emissions, are critical to the aviation community’s resolutions of achieving environmental sustainability.It cannot be excluded that the arguments in favor of or against the BWB are often based on the superficial suspicion against the introduction of new technology. Instead, a satisfactory decision must be based on a rational investigation of a class of vehicles with various degrees of integration of configurations using multi-disciplines.

3. Projects on BWB developments

The early development of BWB was reviewed by Liebeck on the promotion of this concept at McDonnell Douglas company and its subsequent development under the support of NASA and Boeing.22Recently, an evolving trend of BWB design was reviewed by Okonkwo and Smith with specific emphasize on multidisciplinary design synthesis and optimization.30A historical perspective on BWB was given by Torenbeek29with the conclusions that BWB has the potential to satisfy the expectations of real eco-friendly airliners. Here the technologies of developing BWB are reviewed from the perspective of different development phases and in different nations. The developing trends are summarized.

3.1. BWB development in USA

A series of projects were carried out in the USA on the development of BWB,as shown in Table 5.22,31-41The promotion of BWB was in response to the Bushnell’s query ‘‘Is there naissance for the long haul transport?” in 1988.22A ‘Batwing’concept was conceptualized with the constraints on cylindrical pressure vessel and engines buried in the wing root. This concept has large aerodynamic advantages over conventional TAW configuration.

Then a one-year project from 1993 to 1994 was funded by NASA to McDonnell Douglas company on the topic of Concepts for Advanced Subsonic Transports (CAST), which resulted in the first generation BWB (BWB-800-I) releasing the earlier constraints on the pressure vessel and the installation of engines.23,42After that path-finding project a threeyear project conducted by NASA in 1994 was given to a NASA/industry/university team led by McDonnell Douglas/Boeing company on BWB technology study, which resulted in the second generation BWB (BWB-800-II). This study demonstrated the feasibility and performance potential of the BWB.42,43In these two projects the design mission of 800 passengers (PAX) and a 13,000 km range at a Mach number of 0.85 was set by NASA, which was deemed beyond market forecast data for the further development at that time. Then the design mission was adapted to a BWB-450 concept as shown in Table 5. New flattened transonic airfoils and a new planform of a longer centerbody were obtained by optimization using an inverse design code CDISC combined with Reynolds Averaged Navier-Stokes (RANS) solver CFL3D.22,44

After the three-year project, NASA and Boeing did lots of joint researches on a converged configuration (BWB-450-1L),including system studies,22low-speed and transonic wind tunnel experiments,45boundary-layer ingesting inlet with active flow control,46,47noise assessment,48-51structure design of non-cylindrical fuselage52,53and flight test.32,40

Table 5 Projects of USA on BWB development.

At the same time period,a collaborative,multi-disciplinary project named Silent Aircraft Initiative (SAI) was funded by Cambridge-MIT Institute (CMI) from 2003 to 2006, aiming at design an aircraft inaudible outside the airport boundary within a well populated, urban environment.33The BWB configuration was selected as a technology collector due to its potentials on aerodynamic efficiency and noise reduction.Low-noise technologies of propulsion system, airframe and their integration were studied. The resulting SAX-40 concept for an Entry Into Service (EIS) year 2025 realized a predicted noise level at the airport perimeter of 62 dBA and had the potential for a fuel efficiency 28% improvement when compared with existing commercial aircraft. Although the priority was the noise reduction,SAX-40 achieved remarkable fuel efficiency due to the aerodynamic advantages of the BWB configuration.The SAX-40 incorporated a novel concept employing a cambered forward centerbody that increased the forward lift so that the outer wing airfoils could be more highly aft-loaded(supercritical type loading)and still be trimmed at cruise without undue drag from pitch trimming with elevons. This, in turn, allowed a lower sweep with a thicker but lighter outer wing.

In 2008, Nickol54performed a risk assessment of SAX-40 concept.The technologies including structures and weight prediction, Boundary-Layer Ingestion (BLI) and inlet design,variable-area exhaust and thrust vectoring, displacedthreshold and Continuous Descent Approach (CDA)operational concepts, cost, human factors and overall noise performance were considered of high risk. By comparison, it was found that the SAX-40 would have significantly greater research, development, test, and engineering and production costs than a conventional aircraft with similar technology levels. It was recommended that further design should strive to achieve an appropriate balance among a variety of metrics,such as maintenance costs, fuel burn, emissions, and noise. In 2009 Hall et al.55also assessed the benefits and penalties of each of the proposed technologies of SAX-40. A method had been developed to estimate the overall change in fuel consumption and engine noise caused by modifications to an aircraft design. A low-risk configuration SAX-L/R1 of the silent aircraft with podded engine was designed by tradeoffs between noise and fuel consumption for various technologies. The low-risk configuration can achieve both lower fuel burn and lower engine noise.

From 2007 to 2011 a large scale project titled ‘‘Acoustic Prediction Methodology and Test Validation for an Efficient Low-Noise Hybrid Wing Body Subsonic Transport” was funded by NASA in the framework of SFW project under the fundamental aeronautics program.56The investigation was a team effort led by Boeing with major contributions from participants.The primary objective was to design a HWB configuration to meet the initial N+2 goals in 2009(see Table 2)of NASA SFW project with an EIS year 2020.

Based on the previous assessments of SAX-40,54,55a preliminary cargo version of SAX-40 was developed by MIT,named SAX-40F, which was identified as N2 for N+2 noise goal. Two concepts were derived from the SAX-40 by using Boeing Integrated Vehicle and Design System (BIVDS) tool suite: a low risk version designated as the N2A with podded engines, and the other retaining the SAX-40 type embedded engines designated as the N2B. The cambered forward centerbody concept was adopted from the SAX-40 concept.Conventional propulsion and flight control characteristics were assumed to determine configuration design methods for low noise as opposed to employing and evaluating operational procedures used in SAX-40 concept.

In 2009 under the projected growth of the air transportation and the consequence of increasing stringent certification levels for noise and emissions, and further requirement on vehicle fuel efficiency improvements, NASA initialized the Environmentally Responsible Aviation(ERA)project to identify advanced integrated vehicle, system and component technologies. The integration of these technologies will enable transport aircraft to simultaneously achieve N+2 vehiclelevel goals of reduction of noise, emissions and fuel burn in the 2025 timeframe. The enabling technologies needed to be at a Technical Readiness Level (TRL) of 6 by 2020.11

The ERA project had been organized into two distinct phases and had three sub-projects on airframe technology,engine technology and PAI.Both the cargo and passenger aircraft concepts were required. In phase I, driven by the three vehicle-level goals of noise, emission and fuel burn reduction,the investigations on noise assessment of HWB, integration of open rotors, and low speed flight experiments of X-48B/C were carried out.23Three advanced vehicle concepts and an Ultra-High Bypass-ratio (UHB) engine concept were introduced. Five critical technologies on flow-control concepts,advanced composite, advanced UHB-engine design, advanced combustor and PAI concept were identified. In phase II, eight Integrated Technology Demonstrations (ITD) were implemented. They were active flow control of vertical tail and advanced wing design for drag reduction, Pultruded Rod Stitched Efficient Unitized Structure(PRSEUS)of pressurized cabin design for weight reduction, front block compressor design for reduction of thrust Specific Fuel Consumption(TSFC), UHB engine design for TSFC and noise reduction,fuel-flexible combustor design for LTO NOxreduction, airframe component design for noise reduction and UHB engine integration on HWB for noise and fuel reduction. To reduce risk on achieving the NASA’s N+2 noise goal, a one-year post-ERA project was also rewarded to Boeing to research on the noise of a preferred system concept (PSC).

Historically, NASA funded most of the projects on BWB development and accumulated knowledge and technologies on BWB design. Several different BWB configurations were developed with increasing fidelity. In 2004, Burg et al.56and Hill et al.49designed a 300-PAX BWB transport powered by two General Electric (GE) GE-90-like engines using NASA standard toolset (FLOPS, NPSS and WATE) aiming at down-select and assessment of candidate Propulsion Airframe Aeroacoustics (PAA) technologies under NASA’s quiet aircraft technology project. In 2009, Nickol and McCullers11designed a 305-PAX HWB configuration (HWB300-2009) for system study using the same toolset based on the Boeing BWB-450. In 2010 Collier et al.57presented two 300-PAX HWB configurations, a HWB300A with two podded engines and a HWB300B with two embedded engines to set fuelburn goals and to identify the key technology areas of ERA project. In the same year, Thomas et al.58promoted eleven HWB configurations based on HWB300-2009 to assess different noise-reduction technologies.

In 2012, Nickol59studied the scaling effects of HWB with improved FLOPS, a higher order centerbody weight estimation methodology(HCDStruct)60and technology assumptions on hybrid laminar flow control, riblets, variable trailing edge camber, stitched composites of centerbody, several advanced subsystems and advanced propulsion system.It was concluded that given a well-suited set of requirements, the HWB configuration has the potential to simultaneously reduce fuel burn and noise compared to an equivalent technology TAW configuration.

In 2016, Nickol and Haller12assessed the performance potentials of advanced subsonic concepts including the conventional TAW and unconventional configurations.Three categories of HWB were designed using the technology advancements of ITDs of ERA and updated weight estimation method. The airframe technologies of the ERA vehicles for 2025 included a lighter weight structure enabled by damage arresting composites, natural laminar flow wings (enabled by a Krueger leading-edge high lift system) and nacelles, and smaller vertical tails implementing active flow control enhancements. The engine technologies included a low-fan pressure ratio with short inlet, swept and leaned fan exit stators, a highly loaded high-pressure compressor enabling higher Overall Pressure Ratios (OPR), and a low NOxcombustor. By examining the noise, fuel burn, and emissions results, the HWB concepts adopting GTF engines provide the best overall performance.

In 2018, both regional and single-aisle BWB aircrafts were shown superior to the corresponding TAWs under the NASA NAH project. And a subsonic BWB X-plane was designed.41

3.2. BWB development in European Union

Soon after the BWB research of the USA were published, the anticipated benefits of the BWB concept spurred a series of systematic projects on the investigations of BWB in Europe,as shown in Table 6. The early BWB concept similar to the Boeing concept was studied to objectively evaluate the inherent advantages and challenges of BWB in College of Aeronautics,Cranfield.The resulting configuration is named BW-98.A subscale flying demonstrator was also developed. However, the details are not available.61

The BWB concept was studied progressively in detail in the 5th,6th and 7th EU framework programs.A Multidisciplinary Optimization of BWB based on BW-98 (MOB)62and a Very Efficient Large Aircraft (VELA) project63were carried out in the 5th program, with the objective to develop design tools and methods,and to design and optimize a very efficient large BWB aircraft concept (resulting VELA-3), respectively. The VELA BWB resembles the earlier Russia concept IWB-750.The New Aircraft Concept REsearch (NACRE) project64-66started in 2005 and was completed in the 6th framework program. A Passenger-drive Flying Wing (PFW) configuration was advanced. An early version NACRE-PFW1 was derived by modifying the centerbody airfoil and applying outer-wing twist to the VELA-3 aircraft,and had satisfactory aerodynamics, stability and control characteristics. A later version NACRE-PFW2 derived from NACRE-PFW1 by changing the location of the engines, kinematics and position of the main landing gear. The engines were moved from under to over-the-wing to minimize forward radiated fan noise. The kinematics of the main landing gears, which were retracted sideways, were made to retract longitudinally and positioned beside instead of behind the cargo bay. The cabin layout was improved by varying aisle widths, positions, alignments and shapes regarding emergency evacuation time. The average evacuation time was improved from initial 89.3 s65to 84 s for the new baseline cabin layout.66Split aileron was also introduced to enhance the handling quality as well as the stability and control.66

The Active Flight Control for Flexible Aircraft 2020(ACFA2020) project67funded under the 7th framework program focused on the development of innovative active control concepts for advanced configurations from 2008 to 2011. The design goals were derived from the metrics of ACARE vision 2020, as shown in Table 2, which resulted in ACFA-2020 aircraft.

Besides the projects under the EU framework, there were investigations on BWB in different nations in Europe. A business-jet class BWB having 6 PAX was performed at Von Karman Institute for fluid dynamics (VKI).68In France several projects were performed.69-74And in Germany, the study was performed by German Aerospace Center (DLR)75and Universities.76A collaborative framework to solve complex aeronautics problem was created during AGILE EU project from 2015 to 2018. In this project, a BWB concept was also studied as an example of multi-disciplinary design practice.73

3.3. BWB development in Russia

Table 6 Projects of European countries on BWB development.

There is a long continuous development effort in Russia.Several BWB concepts appeared in the open literature,as shown in Table 7.77-79As early as in 1989, the Central Aerohydrodynamic Institute (TsAGI) investigated the ultra-high capacity aircraft using flying-wing layout.80In 1997 a Russia project was supported by International Scientific and Technical Center(ISTC)to investigate the technologies critical for implementing a flying-wing layout aircraft with super-high capacity.77The project was undertaken in conjunction with Airbus and Boeing. Four large aircraft configurations were compared in the conceptual level, which identified three candidate concepts,an integrated wing body,a lifting body and a pure flying wing,for analysis. These concepts were compared with a similarly designed conventional configuration in terms of the aerodynamics, weight and fuel efficiency. Based on the experimental investigations of different wing planforms and conceptual level comparisons, the IWB concept was preferred, as shown in Table 7. This concept is resembling but different with BWB.Additionally, the most critical design issue on airworthiness requirement for emergency egress was identified.

Table 7 Projects of Russia on BWB development.

In recent years,there were continuous efforts on the critical issues of BWB aircraft. Specifically, smaller BWB concepts were studied. The aerodynamic design, PAI,78noise shielding effect81,82and structure design of pressurized cabin were performed.79

3.4. BWB development in China

The investigations on BWB configurations were started late in China,83but were progressively funded and continuously studied, as shown in Table 8.84-86Most of the BWB studies were performed in the academia. Zhu et al. introduced the developments of BWB-800-II, BWB-450 concepts87and SAX project.88Zhang et al.investigated the cabin layout,89stability and control90and effects of design parameters.91Jiang et al.studied the effects of design parameters on cruise efficiency.92

In 2008 COMAC started the conceptual design of C919.As an option a concept named NPU-150 was systematically investigated by Northwestern Polytechnical University in corporation with COMAC from 2008 to 2010. An aircraftlevel benefit of 15% less fuel consumption was obtained compared with conventional TAW under the same technology assumptions. To resolve the constraints on cabin height, the cargo payloads were arranged between the outboard wing and the center pressurized cabin. Based on the requirements of green aviation at timeframe 2020 to 2025, a subsequent project was funded on BWB investigation to resolve several challenges under the cruise performance constraints from 2011 to 2014. A new configuration without vertical stabilizer named NPU-300 featured with an innovative ship-shaped body was conceived to resolve the requirement of evacuation and to improve passenger experience by providing windows on both sides. Meanwhile reconciling the low-speed and high-speed performances was concentrated on. Low-speed wind-tunnel experiments were performed to validate the lowspeed aerodynamic design,whereas the high-speed characteristics were studied by using CFD method.84

To further improve the concept for potential applications,it was decided to increase payload and cruise Mach number and to keep several experienced design principles on the blending of wing and body.85,86The centerbody was enlarged for longitudinal control authority and twin vertical tails were adopted for providing directional static stability and control.The final configuration for experiments was NPU-330. The low-speed and high-speed tests were conducted in the industrial wind-tunnels, and the low-speed and high-speed designs were thoroughly scrutinized and validated.

3.5. Categorization and development trends

The BWB concepts can be categorized into three kinds,BWB,HWB and IWB, as shown in Fig. 1. The first kind is represented by BWB-450, which has double swept planform and short aft-body. Several concepts of this kind are still under investigation in EU countries. The second kind is represented by N2A-EXTE, which has leading-edge carving over the centerbody to reduce aft-body reflex camber and incorporates high-efficiency supercritical airfoils on outboard wing. Additionally, the aft-body is extended to improve noise shielding,PAI and SC.The third kind is represented by IWB-750,which has a more conventional body and an extended aft-body. The wing position is much more forward relative to the body.Most of the concepts have under-wing mounted engines, which can also be categorized as HWB. However, Boeing’s ERA-0009A is a hybrid of BWB-450 and N2A-EXTE, and is very similar to X-48C with the aft-body extended of X-48B.It is obvious that NPU-330 has a distinct feature comparing with the main three kinds, showing a forward wing integrated with a slender body.

Table 8 Projects of China on BWB development.

Usually the gross area and the corresponding MAC are used as the reference area and length scale.Both are quite larger than that of TAW as a function of the maximum take-off mass (MTO), as shown in Fig. 2(a) and (b), respectively. Both increase linearly with aircraft size as that of TAW.The wetted area to reference area ratio (Swet/Sref) of an advanced TAW(B787-8)is 4.89,which is quite larger than that of BWB.They are 2.57, 2.32 and 2.14 for IWB-750, BWB-OREIO and N2A-EXTE, respectively.

Fig. 1 Evolution of BWB concepts.

Some developing trends can also be observed and are summarized as in the following:

(1) The noise shielding effects of the centerbody are gradually realized and are enhanced by providing further shielding of jet noise.

(2) It is obvious that the aft-body is extended to alleviate the challenges on noise shielding, PAI and SC.

(3) To alleviate PAI and directional stability and control challenges and to reduce risks, the podded engines and the vertical stabilizer are incorporated.

(4) The size of BWB concepts in USA and EU decreases from original large value to recent concepts of 200-PAX to accommodate to the projected market. However,the size of concept in China increases from a small number to around 300-PAX which was established in design requirement.

(5) There is a lack of depth on investigations of a specified concept projected for future applications, except BWB-450 and ERA-0009A.

(6) There are no detailed investigations to resolve several challenges related to safety including uncontained engine blade burst, thrust reverser and evacuation.

4. Critical technologies of conceptual design

The fundamental characteristics of BWB as an advanced technology collector are high aerodynamic efficiency, low operational empty weight and potential noise shielding when the engines are mounted on the upper rear centerbody,which are the consequences of entire integration of lifting body, wing, control surfaces and engines. Therefore, BWB has a feature of inherent-multidisciplinary integration and appears as a Multidisciplinary Design Optimization (MDO)problem. Trade-offs between different disciplines cannot be avoided.

The critical challenges are how to reconcile the low-speed aerodynamics requirement on take-off and landing with highspeed cruise efficiency, how to balance the adverse requirements on stability and cruise efficiency, and how to realize efficient structures for pressurized cabin and to accurately predict the structure weight. For the unconventional BWB aircraft geometry, a solution towards ultra-high efficiency commercial transport systems requires precise knowledge of all aspects of aerodynamics, propulsion, SC, and component flow physics, including some nonlinear effects.

Fig. 2 Comparisons of reference area and mean aerodynamic chord length of TAW93,94 and BWB as function of MTO.

The enabling technologies of BWB are listed in Table 9 corresponding to its disadvantages listed in Table 4, which are based on the Boeing’s study23and our experiences. They collectively enable large benefits of BWB, although each item may just be of little benefit individually. Some technologies including laminar flow control of wing, PRSEUS structure concept, noise shielding, and advanced UHB engines can enhance the capability of BWB to achieve specified fuel burn and noise goals.In the following subsections the enabling technologies are scrutinized and the enhancing technologies are also analyzed.

4.1. Aerodynamic design and efficiency

The lift-to-drag ratio (L/D) is well known as the aerodynamic efficiency, which can be determined from the drag polar of an aircraft.For straight and level flight,the maximum L/D can be easily obtained as

where lref=10m.

Table 9 BWB enabling technologies.

Fig. 3 Lift-to-drag as a function of mean chord length and wetted area (The data of TAW from literature,93,94 TAW predicted by Eq. (2), BWB predicted and trend line by Eq. (1)).

The maximum L/D of TAW and BWB as the function of MAC and wetted aspect ratio Awet=b2/Swetare compared in Fig. 3. Both MAC and Awetcontribute to the high aerodynamic efficiency of BWB. The theoretical formula Eq. (1)can predicted the trend well. Taking the effect of cruise Mach number into account,the cruise aerodynamic efficiency is then proportional to multiplication of Mach number(Ma)and L/D for aircraft with similar engines in a small range of Mach numbers.The cruise aerodynamic efficiency parameters MaL/D for turbofan-powered TAW and BWB aircrafts as a function of aircraft size are compared in Fig. 4. The efficiency of BWB is obviously higher than that of TAW cross a large range of aircraft size.However,only high cruise aerodynamic efficiency is not enough, because the aircraft must be trimmed, easily controlled and of balanced low-speed aerodynamic characteristics.

Fig. 4 Comparisons of cruise efficiency for TAW and BWB.

4.1.1. Trade-off between stability and cruise performance

The aerodynamic design of BWB transport is primarily driven by cruise performance under constraints on stability and low speed aerodynamic requirements. High cruise efficiency of BWB is rooted in the integration of the centerbody, outer wing, control surfaces and engines to reduce wetted area,and structurally efficient use of large wing span, resulting in increased wetted aspect ratio and L/D.

The cruise aerodynamic design should be under stability and trim constraints. A high-speed aerodynamic optimization without considerations on stability, pitching up or down moment, and the lift coefficient at maximum L/D will lead to practically degraded configuration, because a practical aircraft should be trimmed at design lift coefficient. Due to the development of multidisciplinary optimization, an aerodynamic-design principle gradually forms to realize the coincidence of zero pitching moment, cruise lifting coefficient and the maximum lift-to-drag ratio.83Therefore, it is better to have positive zero-lifting moment coefficient(Cm0)with positive longitudinal stability margin and negative one vice versa.When the longitudinal stability margin is more relaxed, a higher L/D can be achieved. At the earlier development of BWB, the longitudinal static stability margin was vitally relaxed to -15% for the first generation Boeing BWB-800-I,therefore, a fly-by-wire (FBW) flight control system was heavily depended on. For the second generation Boeing BWB-800-II it was still assumed statically unstable to achieve high L/D, hence the flight-critical stability augmentation and flight-envelope protection were required. Boeing BWB-450 was trimmed at the static stability margin of 5%, which was realized by a Boeing proprietary code WingMOD using the planform,new transonic airfoil for centerbody and twist distribution.44The new airfoils were featured with reduced relative thickness and after-body closure angle, and were designed through a careful contouring of the upper and lower surfaces.

Technically, there are three different ways to achieve static stable design. Historically, the flying wings were trimmed by sweeping the wing and downloading the wingtips using washout, which allows the wingtips to functionally serve as a horizontal tail. However, this method leads to lift loss near the wing tip and introduces a significant induced-drag penalty.Due to high swept angle and unideal lift distribution,the effective aerodynamic wingspan is less than the physical span. Second, the centerbody downloading with rear reflex-cambered airfoils can realize the static stability,which make the rear centerbody function as horizontal tail. This method was adopted in Boeing BWB-450 design.44As a large portion of lift is required on the centerbody for the elliptical lift distribution,the rear reflex-cambered after-body will lead to higher cruise angle of attack, which is contrary to the requirement on the small cruise deck angle.In the project of SAI,the third method was promoted to design SAX-40 using leading-edge carving of the centerbody, which makes the neutral point of the centerbody after the center of pressure with leading-edge uploading.This method can use supercritical airfoil for the outer-board wing design without trim penalty.96Although significant improvements have been made recently thanks to the advancements of aerodynamic reverse design and optimization tools,a suitable stability margin for a high cruise efficiency design is still an open question. Since there is no historical data for BWB and its MAC is about two times to that of traditional TAW configuration, as shown in Fig. 2(b). Many recently designed BWB configurations are still having relaxed stability.In our experience, the recommended stability margin can be half of the traditional TAW aircraft due to its large MAC and requirements of the dynamical stability. The lateral static stability for BWB is not much different with the conventional configuration,in consideration of that the effect of sweepback can increase the effective dihedral with angle of attack.97-99However,it is very difficult to realize directional stability without vertical stabilizer. Additional stabilizer will increase the wetted area and weight, but it is still adopted in almost recent designs.22,36,66,67

4.1.2. Trade-off between low and high speed aerodynamic performances

Since the robust projected growth of civil aircraft in next twenty years, system capacity improvements are required at hub airports to resolve the airport congestion and delays,which has strong implications on the low-speed performance.Therefore, in the SFW project, the goals on performance of field length were anticipated to decrease 33%and 50%at timeframe N+1 and N+2, respectively, as shown in Table 2.However, wing sizing for conventional transonic transports represents trade-offs between efficient high-speed cruise and effective high-lift characteristics, which also includes tradeoffs among weight, cost, complexity, reliability and airframe noise.It is difficult to achieve the field-length performance with a system level trade balance.100

The trade-offs between low-speed and cruise performances result in the high-lift system normally combining leading-edge slotted slats and trailing-edge slotted flaps on conventional TAW aircrafts. BWB aircrafts have larger wing area and low wing loading than that of conventional TAW aircrafts, which implies simplified or eliminated high-lift devices.22However,clean BWBs without high-lift devices have exacerbated lowspeed stall characteristics due to its planform shape.22,26Therefore,it cannot provide high enough maximum lift coefficient to enhance take-off and landing performance required by the regulations.

High lift system have strong implications with safety and allow compliance with several different certification requirements and procedural necessities, such as approach speeds.Typical approach speeds for the conventional large twin-aisle aircrafts are 130-160 knots, which is still difficult for BWB to achieve with traditional high-lift devices.101High lift systems are required on BWB configurations despite its low wing loading,which introduces challenges due to its inherent limited longitudinal control authority.Traditional single slotted flap is hard to be used as a high-lift device,because BWB aircraft cannot trim the resulting high pitching moments.

BWB-800-II and BWB-450 use leading edge slat as high-lift devices to prevent outboard wing stall, which can satisfy the constraints on take-off field length and approach speed.However,the maximum lift coefficients are quite lower than that of a conventional configuration.22,23SUGAR-Ray and ERA-0009A of Boeing and NASA’s HWBs adopt slotted Krueger flap at the leading edge with a deployable trailing edge.In the ERA project, an adaptive compliant trailing-edge flap was also investigated. However, the metrics on field performance was not mentioned. SAX-40 and N2A-EXTE adopt deployable drooped leading edge with trailing edge flap eliminated and are trimmed by thrust vectoring. IWB-750 and VELA-3 also adopt the same high-lift devices including slotted slat and simple-hinged flap.The former has take-off filed length 3350 m.The later has the constraint on the take-off filed length less than 3350 m, but it has an approach speed of 165 knots,which is quite high.76,77ACFA-2020 adopts leading edge slat and a single slotted Fowler flap.It was found that the maximum lift is not enough and it is difficult to trim the resulting pitching momentum.Severe lift losses were also observed when the trim elevons were deflected. When a planform resembling X-48C with centerbody section extended was used, the approach performance of ACFA-2020 was improved.101

It can be concluded that the traditional leading edge slat and slotted flap cannot be easily used for BWB configuration due to the resulting high pitching moment, unless unconventional trim device is adopted, such as thrust vectoring. When to trim the resulting pitching moment, a severe lift loss leads to higher take-off and landing angle of attack due to the short lever arm.Higher angles of attack were not preferred,because passenger comfort rapidly deteriorates at or beyond approximately 8° and further problems during approach can arise from crosswinds causing the wing tips to touch the ground before the landing gear.102

Additionally,BWB configuration is also susceptive to gust,and specifically can be exaggerated at high angle of attack by low speed. To alleviate thise problem, the planform modifications are properly required. However, a trade-off with cruise performance should be balanced.Therefore,low-speed performance,stability and control design needs to be integrated with cruise point design during the first stages of the design process.To improve the low-speed performance,several design adaptations can be considered

(1) Extending the centerbody trailing edge to improve trim capability by enlarging the lever arm, which can also improve noise shielding when the engines are integrated.However, the increased wetted area and the resulting increased friction drag should be considered.36

(2) A leading edge Krueger flap is more preferred than a conventional slat,since it can enable a laminar flow wing by providing protection from insect and debris accretion.A well-integrated slotted Krueger flap has the same capability for low-speed stall protection. However, its noise characteristics including Krueger cavity need further investigations.103

(3) Simple-hinged flap is much more preferred than a conventional slotted flap due to its favorable pitching moment. If the slotted flap is required, it is better to lay it near the Center of Gravity (CG) in the absence of a conventional horizontal stabilizer.

(4) Unconventional active control methods such as thrust vectoring,70flap blowing,99belly-flaps104or boundary layer control blowing system at the leading and trailing edge can be used to improve low-speed high-lift,control and trim. However, the weight, complexity, reliability,cost and safety may be challenging in the consideration of practical implementation.

(5) To realize the full potential of the clean planform of BWB further investigation need to compromise the requirements of low-speed and high-speed performances. This is mainly because there is no functionally independent horizontal stabilizer and control surfaces.

4.1.3. Comments on spanwise lift distribution and multidisciplinary design optimization

Besides the wetted area reduction by the integration of the wing and multi-functional body, the high-speed aerodynamic efficiency has to be realized by using well designed spanwise lift distribution and multidisciplinary design optimization methods. Several modern transports have a more triangle loading that increases the induced drag at cruise, but is based on the reduction of the wing root bending moment and the consequent lighter structural weight, as well as approach and high-speed buffet aerodynamic characteristics. In the MOB project, the triangle, elliptic and an average of triangle and elliptic spanwise lift distributions were studied on a given planform to maximize the cruise efficiency.105The average type was preferred for lower wave drag compared with elliptic type and lower induced drag compared with triangle type.However,this conclusion could be plausible and misleading. Because the modern aircrafts have a trend to increase aspect ratio but trading for increasing operating empty mass.106BWB configuration has low wing loading and potential local-load balancing.If triangle or mixed type distribution is used, the required larger lift over centerbody have to be provided with higher positive twist, which results in higher deck angle. Therefore, most of the BWB aircrafts have the elliptic spanwise lift distribution.

Aerodynamic advantages are realized by reduced wetted area,structurally efficient use of large wing span,relaxed static stability, and optimum spanwise loading. Therefore, to better compromise and balance different requirements on cruise efficiency, SC and low-speed performance, MDO method should be used in the concept design stage.Because of the lack of fullscale test data, validation is a major challenge, and relatively few system-level studies have been performed. Some fundamental questions, like for specified cabin floor area, the optimal width and the lift of the center body, are still open.Most of the designs are achieved by single-point optimization at cruise condition with specified planform, but without constraints on requirements of the low-speed and SC. However,based on past designs77,84,85and low-speed high-lift design practice,102the planform has the first order effect on the low-speed aerodynamic performance. Therefore, when performing MDO, the planform should be parameterized and can be adjusted to satisfy the constraints. Further details on review of MDO methods and a design practice to realize the low- and high-speed performances were presented in detail in Ref.107in this volume.

4.2. Stability and control and flying qualities

In early study of BWB, the challenges related to the SC on high control power,22low longitudinal and lateral stabilities,as well as small natural yaw damping78were raised,which have aircraft system level impact on aerodynamic efficiency, power requirement and weight of actuation system. BWB aircrafts are inherently tailless, which possess some SC characteristics of FW type tailless aircrafts.97

The SC characteristics of bare BWB are consequences of its unique aerodynamic configuration, mass distribution, propulsion and the layout of the stabilizer and control surfaces,which are the bases for the development of the full flight control laws to satisfy the flying quality requirements,and the preliminary actuation system and failure requirements. Based on the gradually deeper investigation and more experiences, the perspectives on challenges, control surface layout and design methods, and the related flying qualities are drawn.

The BWB is an unconventional configuration lacking in historical database and design knowledge that can enable the conceptual design phase as TAW configurations. In the absence of historical data,a physics-based method must bridge the gaps to generate new knowledge, data, and insights to enable the design.

One of the new constraints on BWB design is to achieve trimmed flight and acceptable static stability throughout the flight envelope. The trim drag of a BWB can be reduced by allocating its CG further aft compared to an inherently stable TAW. If the negative static margin is too large, flight-critical stability augmentation would be required as in the cases of the first- and second-generation BWB. However, the BWB-450 layout has been balanced by means of optimization of the lift distribution along the span and application of washout,resulting in a configuration trimmed at a stable CG with all control surfaces in their neutral position.22This feature results in a very small trim drag in cruise and low-speed conditions.The degree of static stability selection depends on the design philosophy and the use of stability augmentation with active controls.

4.2.1. Stability and control challenges

Some of the key challenges in controllability of BWB aircraft are summarized as in the following.

(1) Control surface layout

Control surface architecture is main challenge of BWB design.The options can be partially tailless with only horizontal or vertical stabilizer,or can be truly tailless without any stabilizers. If both stabilizers are adopted, the additional wetted surface area would lead to degenerated aerodynamic efficiency.The BWB configuration typically features redundant elevons located at the trailing edge of the center body and wing.These control surfaces are multi-functional,including trim,longitudinal and lateral control,pitch and directional stability augmentation, and wing loading alleviation.

(2) Difficult to choose and realize the static stability margins

Because of much large reference surface area and chord length, as shown in Fig. 2, the static stability margin of conventional civil aircraft cannot be used as a reference. Meanwhile the longitudinal and directional stability is difficult to be realized, as they are highly interrelated with aerodynamic efficiency, due to the inherent wing-body integration. Usually to have high aerodynamic efficiency, the longitudinal stability is neutral or unstable. The directional stability is also nearly neutral stable.

(3) Limited control authority due to short moment arm

Due to the integration of the wing, body and consequent planform,the moment arms for movables and control surfaces are short compared to the conventional TAW,which limits the capability and authority of control surfaces,both in longitudinal and lateral directions.22

(4) Excessive power consumption and actuator mass penalty

To alleviate the control authority limitations due to short moment arm, larger control surface can be used to obtain larger moments. However, the corresponding hinge moment will increase with the cube of the scale.43Combined with high deflection rates to satisfy the requirements of augmentation,it can result in large secondary power consumption and actuator weight penalty.Therefore,it is highly desirable at the preliminary design level to minimize control surface area, while ensuring adequate closed-loop handling qualities, with limited deflections and deflection rates.This challenge has been recognized as an enabling technology for the BWB configuration by Boeing.23

(5) Complex failure case analysis of actuators and control surfaces

Failure cases of actuators or control surfaces induce a loss of authority on pitch,roll and yaw axis.The failure case analysis then tends to be more complex than that of classical aircraft configurations, where the functions are segregated and almost independent for each axis.

(6) Requirement of new control-surface sizing method

Conventionally, control surfaces sizing considers simplified open-loop handling qualities criteria,such as the pitch rate target for the elevator and the roll rate target for the ailerons.However, for BWB it is not obviously to size the individual control surface, as the functions of control surfaces are not independent. Therefore, the conventional control-surface sizing methods based on volume coefficient method may be invalid and new sizing methods are required.108

4.2.2. Stability and control architecture

(1) Static stability margin selection

As there is no tail and the control surfaces are part of the wing,the aspects of stability,trim and control need to be considered simultaneously to realize a high aerodynamic efficiency for BWB design.109,110The static stability and control are strongly interrelated and conflict with one another.The degree of stability determines the magnitude of the control action.The right amount of both stability and control has to be found. Consequently, the solution is inevitably a compromise.97

Too much positive static margin entails poor maneuverability and trim capability.Particularly,the pitch control may not be powerful enough to raise the nose wheel off the ground at take-off. Furthermore, the elevator deflection required to trim the aircraft in level flight may severely reduce the aerodynamic efficiency and with a consequent loss in performance.A previous study shows that the more unstable an aircraft is,the faster its control surfaces need to move in order to maintain the equilibrium under disturbance.111Therefore, a too unstable aircraft needs an augmentation system that results in a very high requirement of control power, even possibly problematic flight safety.112The static margin for TAW is of a magnitude 0.1-0.2, which appears too large for BWB. Consequently, a range of ultimate static margin from 0.02 to 0.08 appears to be reasonable for tailless airplanes, as suggested by Thorpe113and Donlan.114

Since the development of control theory, an unstable margin as low as -0.15 for early BWB have been considered.22However,Bolsunovsky et al.77argued that due to the problems relating to the reliability of a fly-by-wire control system and flight safety, the stability margin should be limited to larger than -0.03 in the cruise flight, and a value close to zero can be used in take-off and landing regimes. The highest L/D was realized at stability margin around-0.14 to-0.12 in that work. However, the stability margin was finally limited with the penalty on L/D no more than 0.5.77

Based on the design tool development,the BWB-4501L was finally designed having stability margin of 0.05,which is corresponding to a minimum static margin for conventional passenger transports at aft CG position. As the MAC of BWB is around 2.5 times than that of TAW, as shown in Fig. 2, the magnitude of several percent of stability margin is also quite large, considering the absolute chord length. When the SC characteristics of the aircraft is given, the CG range provides a measure for tracing the capability of the flight control system to provide control. For conventional passenger transports a general CG range is of 20%-25% MAC. The allowable CG range for IWB-750 cannot exceed 6%-6.5% of MAC in takeoff and landing regimes.Even the MAC of BWB is larger than that of TAW, the corresponding CG range is still lower than that of TAW.77As the stability margin for BWB is small,the effect of aeroelasticity on the position of aerodynamic center must be seriously considered while defining an allowable CG range.

The flying and handling qualities are also closely related to the aircraft stability and control characteristics. When the inherent stability is high, very high control power is required to change the aircraft equilibrium. If the inherent stability is low or neutral, small control inputs or perturbations can change the aircraft equilibrium easily. If the aircraft is unstable, any control input or external perturbation will cause the aircraft diverge when the aircraft is not augmented. However, a statically unstable aircraft is always unacceptable, especially for civil transports, unless the period of the associated dynamic mode is very long, or an augmentation system is installed to artificially restore stability.Therefore, it is important to understand these inherent aircraft characteristics in order to properly understand its flying and handling qualities.97

The degrees of lateral and directional static momentum derivatives are also crucial for SC characteristics of BWB.Due to the large span and high swept angle of BWB, it is not difficult to realize a specified static lateral stability Clβ＜0. If the value becomes overly negative, the aircraft will suffer from Dutch roll mode.A positive value would cause the plane to spiral while entering a sideslip. However, the directional stability cannot be deduced in a straightforward manner for existing requirements,mainly due to the unique configuration, the lack of directional stabilizer or the limitation on its size. For TAW transports a large directional stability in the range 0.10-0.25 is recommended. However, it is too large to be realized on BWB. A minimum value of 0.03 is somewhat recommended, but this is generally inadequate for ensuring a well-damped Dutch roll mode.Donlan114found a quite limited region of best flight characteristic for FW when-0.057 ＜Clβ＜0 and Cnβ＞0.057 in Langley free-flight tunnel.

Theoretically,the static stability is just the necessary condition for dynamic stability.Static stabilities in three-axial directions do not necessarily result in dynamic stability and good flying qualities. Because the damping derivatives and the mass distribution are also critical to the dynamic stability. Therefore, the thorough static and dynamic stability analysis can only be done for specific well-defined configuration.

It can be concluded that the design of a stable and cruise efficient BWB is still an open issue, but significant improvements were made recently thanks to the advancement of aerodynamic optimization tools considering the effects of stability and control.Whether the BWB is designed stability or not,an augmentation system is required due to the static stability limitations imposed by the realization of high aerodynamic efficiency and control authority.

(2) Stabilizer and control surface layout

First, a design decision should be made on whether traditional horizontal and vertical stabilizers are required or not.Then the control surfaces layout can be decided on the requirements of high-lift, pitch, yaw, roll and drag controls. The layouts of stabilizer and control surface for BWB are shown in Fig. 1, and a survey on the details are given in Table 10.

No horizontal stabilizer is present due to its additional wetted area and the conflict with the integration of the body and wing.In the early design,such as BWB-800-II and BWB-450,22there is no vertical tail on the center body, but on the wingtip as winglets. However, it is found that the directional stability and control authority of winglets is not enough, therefore,additional control provided by split drag rudder is required.Most of recently studied concepts use the centerbody mounted inclined twin vertical tails for directional stability and control.However, the different control-surface layouts do not imply that the purely tailless or twin-tail layout is more suitable for BWB concepts. All BWBs have leading edge high-lift devices,but only several concepts adopt trailing flaps, due mainly to the limitation of the pitch control authority.A system of trailing edge elevons used for flight control is an obvious common feature, which is usually justified by their short moment arms and low individual control authority.

Normally, pitch control is provided by simply hinged elevons at the trailing edge of the centerbody and additional similar ones outboard on each side.The center elevons can also be used for load alleviation.There is also the potential to use symmetrical deflection of the inclined vertical stabilizers to augment pitch control. Yaw control is provided by the rudders on the two vertical stabilizers. Additional yaw control is realized by the outboard split drag rudders.Three unconventional yaw devices, split drag rudders, thrust vectoring, and winglets with rudders, were evaluated for compensating yaw moment induced by one engine inoperative operations. The most promising solution is a combination of winglets and nonpermanent thrust vectoring based on the criterion of direct operating cost.70Roll control is provided by asymmetrical deflection of the trailing edge elevons on each outboard wing.However, the aeroelastic effects of the control surface deflection should be considered while allocating the elevons.

Since the pressure center of the inclined or vertical stabilizer lies above the CG, there is coupling between yawing and rolling moments, as for TAW. The coupling between pitching and/or yawing can also appear when deflecting the outboard elevens for rolling control.115

It can be concluded that the redundancy,the use of multiple control surfaces that provide similar control moments,appears to be a universal design decision made on BWB designs.Although the redundancy increases complexity, it is still helpful to finely control lift distributions and to flexibly avoid aeroelastic mode coupling or structural load issues. The control surfaces have multiple combined allocations, the consequent multiple control effects and the couplings among them.Meanwhile,as the control surfaces are very close to each other,the nonlinear interactions among them were observed.47Therefore, the control surface layout and allocation are very crucial.

4.2.3. Control surface design and control allocation

Control surface design is to satisfy the requirement of control authority during the initial design process.An implicit assumption is that the subsequent flight control system can be realized to cope with the aircraft without too much additional control authority that would limit the ability of the aircraft to perform critical controls. However, the control system architecture is closely related to the requirement of control authority.

If an open-loop control is used,the design criterions should be used for control surface design. If a close-loop control is adopted, the requirement of augmentation on control authority should be satisfied. Additionally, in the case of close-loop control, once the control surfaces are operating at actuatorlimited rates, against the surface stops, or actuator failure,then the design reverts back toward the unaugmented case.The SC characteristics of the bare configuration will be restored.

Table 10 Selection of stabilizer and control surfaces for some BWBs.

Therefore, in order to develop the necessary control laws,the characteristics of BWB SC need to be well understood and actuation system requirements need to be well defined.To ensure efficient control, the control allocation method needs to be specified. When the control allocator is designed in the flight control laws, it must provide at least the control authority in each axis that has been assumed when the SC control authority analysis was performed.

To ensure safety the failure scenario analysis must be thoroughly performed. Failure scenario analyses for elevon layout and sizing were addressed by Garmendia et al.116and in the ACFA2020 studies.117The large control surfaces of original ACFA-2020 were splitted into smaller ones to eliminate the failure critical elevons.

(1) Critical criteria

To satisfy the control-authority requirements of critical trims, maneuvers and augmentations, critical criteria are applied to evaluate the control authority. The moments required by trim, maneuver, and Stability Augmentation System (SAS) specify the requirements of the control system,which must be less than or at least equal to the control authority available at each critical off-design point. These requirements are the constraints that drive the planform of control surface design.

Control-authority requirements of an infinite number flight conditions need to be evaluated, however, not all of them are critical. For unconventional aircraft like BWB, it is uncertain which flight conditions are most likely to violate the control authority constraints. The more flight conditions evaluated are the better,whereas the data and time are quite limited during the conceptual design stage. A limited number of evaluations poses a risk of missing certain control authority constraints,which could lead to severe consequences including degraded performance, accidents, expensive redesign and delays, or even program cancellation.

Control authority analysis can be broken down into the longitudinal and lateral-directional conditions. Based on the experiences in the flight experiments of X-48B/C,Boeing summarized a series of crucial criteria for longitudinal, lateraldirectional control authorities for BWB, as shown in Table 11.23The simulations of full scale BWB aircrafts and actual flight testing of the X-48B have shown that this set of critical maneuvers defines a CG envelope in which a BWB can safely operate. When all maneuvers are evaluated, the most restrictive set is used to define the allowable flight CG range. The airplane must be loaded within this CG envelope or it will not be able to perform all required maneuvers. For AVECA-BWB it was found that the most critical issue is the rotation at take-off, while the clean configuration has to provide a minimum nose-up pitching moment to enable the rotation.110

The engine out and crosswind landing trim requirements are only marginal by themselves for determining whether a BWB can operate safely. These requirements do not provide enough control authority to simultaneously stabilize the yaw axis and provide enough control for these maneuvers. Study has shown that the yaw axis requires the most augmentation to provide desired stability for the BWB.23It is well known that the B-2 was designed for engine out capability down to the stall speed.This should be considered as a target for design of the yaw control surfaces for BWB.

During these criteria checking a system way for selection the type and initial control-surface layout is required. If the control authority is not enough, the planform modification may be the best option for relieving those problems.

Table 11 A list of criteria for control authority.23

(2) Control surface sizing methods

Control surface sizing is a key challenge for BWB design,mainly due to unconventional flight dynamics connected with multiple redundant control surfaces.The control surface sizing not only need to satisfy the control authority requirements but also need to provide level 1 safe handling qualities for civil aircraft.112

The adequacy of these control surfaces is determined in part by their size,but also by the planform shape.For conventional TAW, the sizing of stabilizer and control surfaces are almost independent after the wing design due to the convenience to shifting wing or tail position individually. In a quite recent study a conventional volume coefficient method was used to size twin vertical and inclined stabilizers for BWB.Both have dimensions resembling A380. The CFD results showed that stability derivatives are similar for both twinstabilizer configurations. The inclined configuration provided a smoother response, but its drag is higher. It was concluded that a twin-stabilizer design is suitable for BWB aircraft.115

Generally,two complementary approaches for BWB stability and control design were studied. The first method is in the framework of MDO integrating a stability and control module, or considering stability constraints during the planform optimization.109,118The second method treats BWB as a control-configured vehicle. This approach takes advantages of optimization tools to simultaneously optimize a controller and some physically meaningful parameters. In the quite recent work of Denieul et al.108the integrated design of control-surface sizes and flight-control laws for an unstable BWB were constructed to optimize multicontrol surfaces under handling quality constraints. Significant gains in terms of the outer elevon’s span that satisfies the closed-loop handling quality constraint were demonstrated.

In the work of Garmendia et al.116the tradeoffs between drag, control authority, actuator weight, and actuation power requirements as a function of the number and spacing of elevons were studied. The actuators were sized based on hinge moments computed during nominal and failed control flight conditions. N2A-EXTE was used to demonstrate these tradeoffs.The study concluded that adjacent elevons could be combined to achieve reductions in weight, power usage, and fuel burn. This resulted in a reduced number of elevons from the baseline and unequal span fractions. In the design of ERA-0009A, explicit vertical tail sizing was not performed. Instead,the vertical tails were sized to provide the same volume coefficient as the X-48C of a value 0.01279. For X-48C the explicit vertical tail sizing was performed to provide more crosswind and engine-out control margins than that of the X-48B corresponding to the pilot comments.23BWB-OREIO also has the similar vertical tail volume coefficient.39

After the stabilizer and control surfaces sizing, a series of CFD and/or wind-tunnel tests need to be conducted to develop an aerodynamic database. The aerodynamic database is needed to develop the aerodynamic model and associated simulation,from which the bare airframe SC characteristics can be understood and assessed.

(3) Flight control system design and control allocation strategies

Based on the bare airframe stability and control characteristics,a full authority augmented flight control system needs to be designed to meet the flying qualities requirements, and preliminary actuation system and failure requirements. Most of the studied BWBs are of relaxed stability or are barely unstable. Hitting deflection stops or rate limits on an unstable aircraft is potentially catastrophic. In this situation the aircraft will revert back to its open-loop,i.e.unstable,behavior.22,71,119Therefore, SAS is compulsory for handling qualities and safety. The essential element of the SAS is the control law,which is often implemented in a FBW system.For aircraft with significant stability augmentation, the flight control system introduces additional dynamics resulting in a higher order characteristic equation.Interpretation of high order characteristic equations can be challenging.

In the control law design,a proper control allocation is very crucial, for it will determine pitch, roll and yaw capability of the aircraft. It is also difficult because of the intrinsic redundancy feature of BWB control. When the control allocation optimization is performed, the control authority, hinge moments, actuation power demands, and actuator forces and moments should be taken into consideration.

Cameron and Princen120comprehensively studied control allocation requirements for BWB. Goldthorpe et al.121describes control allocation on the X-48B,featuring some kind of ganging method with tabulated tables to avoid any online computations. This method is purely deterministic. But it has drawbacks on a lack of flexibility and implications of high deflections and deflection rates. Waters122implemented different control allocation strategies on a BWB.He concluded that direct control allocation provides the smallest errors for it yields smaller deflections, therefore control surfaces act more in their linear efficiency domain. Most recently, an artificial neural network and genetic algorithm were used for the control allocation optimization problem of a BWB to minimize the sum of absolute values of hinge moments.123

4.2.4. Flying and handling qualities

The Flying and Handling Qualities (FHQ) of BWB inherently depend on its stability and control characteristics and the flight control system. When the flight control system has been designed,a series of piloted simulation tests can be used to verify whether handling qualities requirements can be satisfied under certain specified criteria. This can significantly reduce the risk of unexpected flight controls problems before design decision-making.

The tailoring of FHQ requires clear and adequate criteria.FHQ terms present in the military standard, like MIL-STD-1797. The civil requirements, like FAR-25, are qualitative in nature, and do not even refer to the FHQ field explicitly at all. Therefore, the trend in the civil transport aircraft industry has been for manufacturers to design aircraft with a wide range of different flying qualities due mainly to the fairly loose constraints. However, a failure to provide good flying qualities may lead to unsafe operations, especially in bad weather and emergency procedures.

Handling qualities constraints are expressed both in terms of maximum authority to trim the aircraft and maneuvering in the whole flight envelope, as well as constraints on openand closed-loop poles characteristics. The longitudinal Bandwidth/Phase delay/Gibson drop back criteria, as suggested by the military standards, together with the Generic Control Anticipation Parameter (GCAP) were proved possible to be used to assess FHQs of BWB aircraft. For the lateraldirectional motion, the MIL-F-8785C criteria were used.Although it is possible to assess the FHQ of BWB configurations using these criteria, more research is recommended specifically on the lateral-directional FHQs criteria and requirements of highly augmented large transport aircraft.97

A generic handling quality tool to assess control authority in conceptual design for unconventional aircraft, like flying wings and non-symmetric airplanes, was developed by Chudoba.124This approach was further developed by Coleman and Chudoba.125De Castro conducted piloted-handling trials of a FBW BWB civil aircraft in a fixed-base simulator.97The prescribed solution was a slightly unstable aircraft.Both longitudinal and lateral handling qualities were studied. The longitudinal handling qualities were predicted by using the bandwidth/phase-delay criterion, drop-back criterion, and the GCAP criterion. The lateral handling qualities were determined by adopting several criteria from the military specification MIL-F-8785C. The piloted handling trials reveal that the selected BWB configuration has essentially Level 1 or Level 2 FHQs, depending on the task. The main contributions for the Level 2 deterioration were identified as residual lateraldirectional activity, unconventional flare, ground to flight model discontinuity, flight path PIO, and insufficient flight path control when one engine has failed. Peterson and Grant126used much of the data from de Castro’s work97to determine the handling qualities of the BWB, using piloted-handling trials and different criterion on a moving-base simulator.Three pilots rated the same longitudinal and lateral handling qualities as that of de Castro. It was concluded that the simulator motion did not have a significant effect on the ratings. These studies only evaluated some low-speed maneuvers at takeoff and landing phases. Further studies on highspeed handling qualities such as cruise and initial descent are expected.

4.3. Cabin structure concepts and weight prediction

For BWB,apart from the high aerodynamic efficiency realized by the outer mold line of BWB,an efficient structural design of lighter weight than TAW is also a critical technology for advantages of BWB. The BWB concept was promoted while releasing constraint on using cylindrical pressure vessel,22,42and exhibits distinct aerodynamic shape. The outer mold line defined by the aerodynamic configuration is a compulsory constraint for BWB structure design, because the high aerodynamic efficiency is the primary advantage and is of first order effects. Although the spanwise distribution of lift is favorable to balance local inertia loading and results in weight saving of wing structures, the weight penalty of the noncircular pressurized cabin is still challenging. The weight penalty of the body is 21% and 28.7% for early concept BWB-45022and a recent concept ERA-0009A,23respectively.

The pressurization load on the compound curvature noncircular body is resisted by very high out-plane bending stresses, but not by most efficient hoop tension as circular body of TAW.53More severely the longitudinal body bending and spanwise wing bending result in comparable bi-axial loads on the body shell.23The cabin pressure load is experienced on every flight, and thus fatigue becomes the design criterion.Since the large secondary-bending effects of the cabin pressure loads, the using of metallic materials is precluded, while composites are essentially exempt from fatigue and hence would suffer no penalty.28,127However, the high bi-axial loads make the adoption of conventional prepreg composite materials that are susceptible to resin-dominated interlaminar failures unfeasible.Additionally,the compound curvature shape exacerbates manufacturing costs, particularly if conventional multi-piece aircraft construction techniques are adopted. The bi-axial loads make the construction of continuous loading paths much more difficult. Therefore, the structure design of BWB mainly concentrated on the structural concept of non-circular pressure cabin.

Based on the complex loads on the body resisted by different structure forms, different concepts were introduced with respect to the BWB centerbody. Weight and cost are the primary figures of merit. Four concepts including the integrated skin and shell,22segregated skin and shell,22,52,53,127-129hard/soft shell130and oval concepts131,132were studied. From the aspects of concept simplicity, safety, passenger experience,structural efficiency,the capability to keep OML,manufacturing, space waste, structure forms and load balancing, all the presented concepts have their pros and cons.From the studies,it can be found that the segregated inner and outer shell is difficult to manufacture and has high weight penalty133; the hard/soft shell concept has large dead weight penalty130; the oval concept is inferior to satisfy the requirement on OML.131,132Therefore, the integrated skin and shell concept is a compromised choice as previously made by Liebeck22and Mukhopadhyay.53However, the integrated concept still has large weight penalty.53,133Until now the optimal structural layout of BWB is not yet fully understood. A topology optimization method was used to determine the structure layout of BWB passenger aircraft, which is valuable to acquire more knowledge for optimal structural layout.134

Ultimately,weight is the primary measure of structural performance. The Boeing and NASA focused on developing the highly integrated PRSEUS concept under NASA SFW135,136and ERA project.137,138The finite element analysis and trade studies of centerbody section using PRSEUS demonstrated that BWB aircraft can be structurally as efficient as the conventional cylindrical skin-stringer-frame construction.139,140Although PRSEUS structure concept offers potential benefits,the issues of manufacturing and repair, high maximum stress and strain levels, and adverse aerodynamics effect due to the deformation should be addressed.141

Recently a concept of structure layout based on the procomposite hybrid double lattice-rods panels was used for BWB centerbody.A weight saving of 22%-25%was estimated from the preliminary design.142The feasibility of the manufacturing methods using currently available processes of aircraft production was further analyzed. It was found that the manufacturing scheme decreases the labor input necessary for manufacturing and assembling the flat double-lattice panel of the pressurized cabin.79

Weight engineering is a principal discipline involved in aircraft design. An accurate weight prediction in the conceptual design stage is very crucial to determine the aircraft performance. Weight depends on key factors including aircraft size and geometry, internal structural arrangement, the limit load factor, and the choice of material. It can be obtained statistically by empirical method, analytically by semi-empirical method, computationally by Finite Element Method (FEM)or jointly by combination of these methods. For new aircraft configurations the empirical and semi-empirical method are quite limited due to lack of the historical data and primary structure concept. The accuracy of FEM depends on the type of structure and the details of the model.Usually common calibration factors of 1.25-2.0 are used to correlate the theoretical FEM weight to the actual weight.

As an unconventional concept, BWB aircraft lacks historical data that is typically used for sizing and weight estimation in the conceptual design phase. Furthermore, the centerbody section is of particular concern due to its unique loading characteristics and adoptions of new structure concepts, which makes sizing and weight estimation even more difficult and of high uncertainty.143,144Inaccuracy and uncertainty of weight evaluations could dissatisfy mission requirements and degrade the overall viability of BWB concepts. Therefore,weight prediction has been identified as a high-risk item.54

The developed weight prediction methods for BWB are summarized in Table 12. The earlier methods are all semiempirical, which are quite limited. The recent methods are exclusively FEM based methods but have high computational cost.A coarse-FEM based method was promoted very recently to reduce the cost. All the methods are for specific structural concept and materials. Therefore, when one specific method is adopted, the corresponding limitations should be considered. The predicted structural weight/mass ratio (Mst/MTO)is from 18% to 26%, which is reasonable compared to that of TAW.

Table 12 Weight prediction methods for BWB.

Fig. 5 Comparisons of OEM/MTO for BWB and TAW.

The comparisons of the ratio of Operational Empty Mass(OEM) to maximum take-off total mass (MTO) between BWB and TAW are shown in Fig.5.For TAW the ratio exhibits a decreasing trend with increasing aircraft size. The ratio of BWB is of large scatter and is generally larger than that of TAW, except several middle size concepts. This scatter and general trend may be due to the uncertainty of the prediction method and the weight penalty of the centerbody,respectively.

4.4. Noise reduction

In the early development stage of BWB,the high aerodynamic efficiency was thoroughly pursued, therefore its noise reduction potential was not fully appreciated and understood,specifically its PAA effects. For BWB-800-II and BWB-450 the engine exhaust was aft of the trailing edge,22which makes the shielding of the aft radiated engine noise impossible. Aircraft noise can be categorized into the airframe noise, the engine noise and the PAA interactions. Considering noise certification required by the regulations, the flight operations are also very critical. BWB has specific noise features due to its unconventional configuration and performance.

The airframe noise mainly includes the noise sources of the high-lift system and the landing gears.Because the elimination of trailing edge slotted flap, BWB has no this noise source.Normally in order to realize laminar flow wing and to improve low-speed performance, a slotted Krueger flap is adopted as the leading edge high-lift device to delay outer board wing separation and to enlarge stall angle.If a drooped leading edge is adopted,further noise reduction can be realized.However,the noise features of the Krueger flap and drooped leading edge are not well understood. The noise could be lower than traditional slat due to its shallow cove for the Krueger flap. But it has additional resident cavity and larger brackets than the slat tracks. These features can bring additional noise sources.

The main landing gears are mounted at the transition region between the centerbody and the outer board wing on BWB,where local flow speed can be as large as or higher than freestream flow velocity at approach condition. For the conventional TAW configuration, the local flow speed is almost 20% lower than the freestream velocity. This local flow velocity is very important because it controls the noise level,following a sixth power law.149Since the engines are mounted on the rear upper centerbody,the landing gears can be shortened and partially podded,which are quite new realizable potentials and need further investigations.150

To know the airframe noise characteristics of the BWB,extensive wind tunnel experiments were performed on leading edge slat,151Krueger flap,152,153comparisons between them,103and drooped leading edge with landing gears.154The computational aeroacoustic (CAA) methods were also used to predict the Krueger flap noise.155,156Most of the experiments were done by Boeing and NASA. The noise prediction methods for Krueger flap,157and the landing gears149were updated based on the experimental and computational data.

In the timeframe of next generation civil aircrafts,the UHB geared turbofan could be adopted for its low TSFC and noise feature.However,it is difficult to get proprietary data on noise features of future UHB engines. Engine noise sources have changed from the dominated compressor and jet noise of turbojet engines to the mixture of fan and jet noise according to the introduction of high bypass turbofan engines and the increment of BPR, as shown in Fig. 6.158UHB engines have the noise features of mixing fan and jet noise. Fan noise is very tonal and has a well-defined directivity around the engine.Jet noise is a distributed source from the plume that extends five to seven fan-nozzle diameters downstream the engine. As the fan diameter is very large, to relieve the mounting loss of UHB engines, a short nacelle is preferred. However, it is hard to lay acoustic liner in a shortened nacelle.

PAA effects of a BWB with UHB engines mounted on aft upper rear centerbody are of distinct features comparing with that of the conventional TAW configurations with under wing mounted engines. On the one hand, the engine position of BWB is favorable for adoption of UHB engine. On the other hand, the centerbody can provide very effective shielding of upstream propagated noise. However, due to the characteristics of the jet noise, it is hard to be shielded. The PAA effects mainly depends on the distance of the engine exhauster to the trailing edge of configuration and the characteristics of noise source, like spectra and directivity. Since it is very expensive to model the PAA effects using high fidelity CAA methods and there is no historical database.Therefore,many wind tunnel experiments were performed to obtain the PAA characteristics of BWB using simplified point noise source in nacelle,159jet noise shielding and jet noise source position mitigation,160two dual-stream, heated Compact Jet Engine Simulator(CJES),161Broadband Engine Noise Simulators (BENS),162an Ultrasonic Configurable Fan Artificial Noise Source(UCFANS)163and open rotor noise.164It was found that the upstream propagating fan noise can be effectively shielded,and that the pylons and their orientation, and the chevrons have favored effects on jet noise reduction and shielding.160

Flight operational conditions play an important role on aircraft system noise assessment, because the flight parameters,such as the flight Mach number and the angle of attack,determine the noise source levels. Meanwhile, the flight path determines the distance of the noise propagation,and consequently,the amplitude of the noise received at the measurement locations. The flight operations of BWB are quite different with that of the conventional aircraft.The angle of attack at takeoff and landing states,normally 10-13°,are much higher than that of conventional TAW aircraft, normally 4-8°. The higher angle of attack has adverse effects on noise reduction. The approach speed is normally lower than that of TAW configuration due to the BWB’s low-wing loading.

Fig. 6 Evolution of engine noise sources with BPR increasing.158

Based on the accumulative configuration improvements and database on airframe noise and PAA effects,NASA noise assessment of BWB began formulation in 2003 and was first published in 2009,165then updated in 2010,582012,166and 2016167on NASA HWB. Most recently the uncertainty of BWB aircraft system noise prediction was studied using the direct Monte Carlo method.168NASA also performed system noise assessment of N2A-EXTE based on abundant experimental data of airframe noise and PAA effects. Boeing began its BWB system noise assessment in 2011 and first published the results of BWB with podded engines in 2014,23,169and on BWB with open rotor engines in 2015.170These assessments were implemented in the SFW and ERA projects.

The cumulative noise levels of BWB are compared with that of conventional aircrafts in cumulative Effective Perceived Noise Level (EPNL) dB,as shown in Fig.7.It is obvious that the noise level of BWB is quite lower than that of conventional aircrafts,except the BWB with open rotors.However,they still cannot reach the NASA goals, which indicates that the noise shielding is not enough, but further reduction on airframe noise is required.A typical tone corrected perceived noise level was presented by Guo et al.169It was shown that the noise sources of the main landing gear and Krueger flap are dominant at approach. Whereas at cutback the dominant component is slat noise, and at sideline the sources of aft propagation fan noise,landing gear and Krueger are the comparably dominant components. Clearly, the engine components are not dominant at any of the three operation conditions, even though they still make noticeable contributions at cutback and sideline conditions where the aft fan noise is comparable to the slat noise.23,169

Fig.7 Comparisons of noise margins of BWB with conventional TAW aircrafts (ICAO certification data).

The potential contributions of different technologies on the noise reduction23was studied. A future conventional TAW aircraft with geared turbofan engines at timeframe 2025 has an 18 dB reduction relative to the 1998 aircraft for nearly cumulative 29 EPNL dB below Stage 4. This improvement is attributed primarily to the geared turbofan engine. The Boeing BWB aircraft ERA-0009A achieves additional 5 dB reduction by acoustic shielding to cumulative 34 EPNL dB below Stage 4. Further technologies are required on landing gear and leading-edge device to reduce noise level to the N+2 goal.

A post-ERA Advanced Air Transport Technology(AATT)project was followed to study the technologies to further reduce noise. In this project the ground noise footprint area of NASA’s HWB301-GTF was predicted, which is 82.5% less than that of a B777-like TAW aircraft, with upper and lower level of uncertainty 70% and 85.9%.171A new set of noise reduction technologies was also selected and evaluated on HWB301-GTF for the 2035 timeframe of the NASA farterm goal, as shown in Table 2. When eighteen configurations with different noise reduction technologies were added to form the final aircraft concept (HWB-FT-2017), a margin of 50.9 EPNL dB below Stage 4 was achieved, as shown in Fig. 7.The most noticeable contributions were internal nacelle liner,Krueger flap bracket alignment, Krueger flap cove filler and partially podded main gear.172A noise reduction technology roadmap for a Mid-Fuselage Nacelle (MFN) aircraft concept was also established to achieve NASA’s noise goal of the far term timeframe.173

It can be concluded that noise reduction is an obligation for future civil aviation reflected on the more stringent noise regulation. BWB has inherent features with aft upper centerbody mounted engines,low wing loading and simple-hinged trailing edge flap, which can help noise reduction. The following benefits can be expected

(1) Engine noise can be shielded by the airframe.

(2) The landing gear can potentially be shortened.

(3) Low-wing loading results in low approach speed.

(4) The trailing edge flap noise source can be eliminated.

(5) If Krueger flap is used, it has shallow cove and weak separation, which can reduce noise.

However, BWB has also some flight operation related disadvantages for noise reduction

(1) Much higher angles of attack at take-off and landing,usually 10-13°, than that of TAW aircraft, normally 4-8°. Higher angle of attack can lead to higher slat/Krueger noise than conventional slat at low angle of attack.

(2) The main landing gears are mounted at the transition region from the outer board wing to the centerbody,where local flow speed at approach condition is higher than that of TAW aircraft. This difference makes the main landing gear noise source higher.

(3) If the Krueger flap is used, its brackets are larger than the tracks of slats, and the road can lead to local flow separation when they are not along local flow direction.These additional features of the Krueger can introduce higher noise level.

(4) To relieve the mounting loss of UHB engines, short nacelle is preferred. It is hard to lay acoustic liner in the shortened nacelle.

(5) Due to the adoption of UHB engines and the favored engine noise shielding by centerbody,the airframe noise sources emerge as dominant components, which results in distinct noise features for BWB aircrafts.

4.5. Engine technologies

Fuel efficiency is always a driven design matrix for commercial aircrafts because it is an important determinant of aircraft range, size, economics, and emissions. Historically, the fuel burn per seat kilometer of gas-turbine powered commercial aircrafts has been reduced by 70%since the commercial service started in the 1950s.7It exhibits an average rate of about 2%per year since 1970. The improvements of the airframe technologies contribute about half of the gain, whereas the advancements of the engines contribute the rest. Therefore,engine is one of the dominant design factors and its technologies were recently analyzed in detail concerning green aviation.174

The contribution of engines to the aircraft efficiency through engine overall efficiency ηo, engines’ weight included in OEM and installation drag penalty. However, the turbofan engine is a common technology for different aircraft concepts.It needs to improve engine efficiency and to reduce noise and NOxemission simultaneously for sustainable aviation. These aspects are analyzed in a historical perspective of advancement and recent BWB studies.

The overall engine efficiency ηo=ηthηprηtris primarily a function of the thermodynamic efficiency ηthand the propulsive efficiency ηpr,ηtris generally fixed and close to 1.The large turbofan engines for the most efficient commercial aircraft in service, as shown in Fig. 8, have thermodynamic efficiencies up to 55% and propulsive efficiencies lying between 70%and 80%. The resulting overall efficiency is around 40%.

Fig. 8 Commercial aircraft gas turbine engine efficiency trend,bypass ratio.174

The thermodynamic efficiency can be improved by increasing OPR to a possible higher value around 65-70%, combining with the development of new materials, architectures,and component technologies. The historical OPR has an increasing trend174for engines of GE, Pratt and Whitney(PW) and Rolls-Royce (RR), as shown in Fig. 9(a). In the BWB aircraft design, both Direct-Drive (DD) and GTF engines were studied in a series of investigations.12,175,176The OPR was increased from an early design of 46 to a later design of 60.The future projections have larger values up to 70. However,the higher OPR results in higher combustion inlet temperature T3, which will lead to higher NOxemission. In the ERA project,the highly loaded front stages of compressor were demonstrated to realize OPR in a range from 60 to 70,177and a new combustor was successfully tested to reduce LTO NOxemissions 88% under the ICAO CAEP/6 standard.178,179

The propulsive efficiency can be improved to a possible value of 90-95% by reducing fan pressure ratio (FPR=1.35 or below)with consequent dropping of the fan exhaust velocity and increasing BPR,combining with the reduction of the pressure losses along the internal flow path. The historical BPR also has an increasing trend,as shown in Fig.9(b).The highest BPR of in-production geared turbofan is larger than 12,and a near future projection is around 15.In the ERA project,a BPR of 20.6 for a geared turbofan was studied.12,175,176However,for a constant level of thrust, this technology requires that the effective fan area increases to avoid commensurate increments in weight, drag, integration losses and fan noise. At the current state of the art,high flight-speed unducted propulsors, such as open rotors, face significant noise, mechanical complexity, and installation safety concerns that need to be overcome before they can be considered attractive alternatives to ducted fans.

The cycle parameters and performance of engines adopted for BWB designs are given in Table 13. Two architectures,DD and GTF,are considered.Most of the conceptual engines have OPR 60,FPR as low as 1.35,and BPR larger than 12.A lower SFC can be expected.

It can be seen that during the past 70 years,a great progress has been made on engine design, contributing around half of the aircraft fuel efficiency. This trend most likely continue by increasing OPR and BPR. However, the increasing of OPR and the requirement of NOxreduction should be harmonized.It will be more difficult for TAW aircraft to adopt larger BPR engines using under-wing mounting positions. For BWB aircrafts it provides an opportunity to adopt larger BPR engines because of upper-surface mounting position. However, this opportunity also brings challenges which are analyzed in the following section.

4.6. Propulsion and airframe integration

The layout of BWB aircraft opens new opportunity of propulsion and airframe integration, which can result in system benefits of fuel efficiency and noise reduction,but also consequent special challenges.As new integrations are quite different with conventional under-wing mounted engine of TAW, no much database and experiences have been accumulated. Therefore,most of the studied were carried out by using CFD and wind tunnel tests.

Fig. 9 Historical trend of overall pressure ratio and bypass ratio of gas turbine engines.

Table 13 Engine cycle parameters and performances used in BWB studies.

The BWB concept has been studied with a variety of propulsion options,including embedded engines,podded engines in nacelles, direct-drive turbofans, geared turbofans and open rotors.The engines can also be mounted at different positions, including conventional under-wing, aft-upper surface of centerbody, upper wing trailing edge, embedded in the body with BLI and distributed along the span, as listed in Table 5 to Table 8 for existing BWB concepts. The characteristics of these concepts are given in Table 14. Upper surface mounted concepts are not well studied,lacking design methods,difficult for maintenance and operations, and having some safety concerns. But all can provide better noise shielding effects. The most adopted concept is aft-upper-centerbody position that is analyzed in detail in the following. Several studied are also summarized.

Table 14 PAI concepts and their characteristics.

The location of engines on the aft-upper surface of the centerbody has a unique feature that can provide propulsion noise shielding, enabling very low flyover noise. However, it also leads to more complex maintenance and operational issues comparing with that of the conventional under-wing mounted engines. In consideration of safety issues, the challenges associated with uncontained engine blade burst appear for turbofans and much more severely for open rotor. This risk could be minimized with newly developed technology and the use of redundant structure, but having weight penalty. There is also a potential issue with nose wheel liftoff from pitch-up when current types of thrust reversers are used.

In the early investigations, the embedded type integration on the aft-upper surface were heavily studied,45-47which can have obvious benefits of reducing friction and ram drag because of wetted area reduction and low momentum wake filling, respectively. However, the benefits are also of high risk due to the penalties on the fan efficiency and stall margin caused by high total pressure loss and inlet flow distortion.Hence, the investigations on the fan forced response and distortion tolerant fan are highly required. As early as in 2005 CFD based design methods for PAI was evaluated within the UEET project on BWB-450-1L configuration with three embedded boundary-layer ingestion engines at transonic condition of Mach number 0.85.Their effectiveness was generally confirmed by high Reynolds number wind tunnel tests. However, further sensitive numerical and experimental design methods were recommended to capture small design changes.45

Based on the requirement of ERA,the open rotor was chosen for its potential to provide the greatest fuel burn reduction relative to all other potential propulsion options for BWBOREIO.39The BWB provides the potential of noise shielding when the engines are properly mounted on the upper surface,which can alleviate one of the main drawbacks of the open rotor. Therefore, the PAI problem of open rotor integrated with BWB was studied in detail by using CFD method. Open rotor engines have unique problems relative to turbofans due to the direct exposure of rotors to flow. When the engines are mounted on the aft-upper surface of centerbody, where the local flow may be of higher Mach number than the freestream flow,and can be highly distorted at high angle of attack or sideslip angle.Both of these features can increase noise and decrease aircraft performance. At cruise condition, the pylons have significant amounts of interference drag and a significant amount of separation happens due to the appearance of strong shock. At higher power conditions such as takeoff or climb out, the stream tube of air that goes through the rotors contracts so rapidly that the adverse pressure gradient can be high.This feature can cause the boundary layer separation and consequence of performance penalty. It was concluded that the BWB with open rotor must be designed to mitigate these problems in order to reach fuel burn and noise goals.

To reduce the risk for near term application, podded turbofan engines have been mostly considered. Because the inlet flow of engines is in the downstream flow of centerbody, the integration of engines on the pylons near the trailing edge of upper surface is quite challenging. The interferences between the propulsion and airframe can be quite complex. On the one hand, the mounted engine can lead to strong local flow distortions, which can result in high airframe drag due to shock formation and/or local flow separation. On the other hand, the inlet flow of engines can be highly distorted at high angle of attack or high sideslip angle, which can result in fan flow instability and engine efficiency reduction. The longitudinal position of the engine is also crucial to the noise shielding effects of fan and jet noise. Additionally, due to the adoption of UHB turbofans, the increase in the nacelle drag and weight are detrimental to the aircraft performance. More challenges on the short nacelle design, noise reduction and integration are recognized and strongly depend on engine position.

Several different mounting locations were tested by TsAGI on IWB configuration,as shown in Fig.1078Wind tunnel tests revealed that the configuration with two under-wing pylonmounted engines is of the most favorable aerodynamic interfaces. Four under-wing engines are also acceptable. However,both configurations with over-wing initial engine positions,suffer from the early onset of wave drag divergence, although they can provide engine noise shielding. The configuration with aft-upper surface mounted engine can be improved by some modifications, but it still inferior to the configuration with under-wing mounted engine.78

In the N2A-EXTE development, the PAI was also investigated in detail by using CFD.36The results of original N2A show that the local Mach number at the inlet of the nacelle can be larger than 0.9 at cruise Mach number 0.8, as shown in Fig.11(a).Therefore,the nacelle design method for conventional wing mounted engines should be revised.

Fig. 10 Experimental aerodynamic model of IWB configurations with different engine positions.78

Fig. 11 Challenges of PAI of N2A.36

To achieve shielding of downwards propagating engine noise,the engine pods are in local flow of high Mach number,which leads to appearance of strong shocks over the nacelles and corresponding flow separations, as shown in Fig. 11(b).This introduces further challenges to minimize adverse PAI effects. Meanwhile, a significant loss in lift is observed due to that the inlet spillage disrupts the lift generating flow circulation over the centerbody.The remedies by shifting nacelle axial location and height over the body and by increasing inlet mass flow to reduce flow spillage were found ineffective.To alleviate these adverse PAI effects, the aft-body was extended by 10%to allow engine nacelle locating in lower Mach number flow with additional benefits of noise shielding and longitudinal control authority enhancing.Redesigns of the centerbody lines with dishing and outer cowl lines were implemented. The final design can recover the L/D of N2A with less compressibility drag compensating the friction drag due to aft-body extension.However,the increased surface area did increase weight as well as absolute drag so there was an increase in fuel burn. However, N2A-EXTE configuration shows unacceptable flow characteristics for engine operability.180

In the framework of EAR projects, the PAI problems including drag penalty at cruise condition, inlet flow distortion, power-on effects on stability and control, and engine operability were studied in detail by combining numerical and experimental methods.181Extensive CFD analyses and optimization studies were performed to minimize installation drag of the engine nacelle at transonic conditions.182The contours of centerbody line and nacelles were optimized to eliminate the shock interactions between the nacelles and the body,as shown in Fig. 12. The refined configuration ERA-0009H1 from the baseline ERA-0009A has only 1.4% drag penalty of engine installation estimated from CFD analysis.

At low speed a 5.75% scale model183was tested with flow through nacelles, powered ejectors and turbine powered simulators to study the high-lift system,119flow distortion at the inlet face184and power effects,185respectively.The inlet distortion having total pressure ratio as low as 0.95 was observed in the wind tunnel test of 5.75%scale model of ERA-0009G configuration mounted with flow-through nacelle at a high angle attack combing a slight sideslip.119However, the test results using powered ejectors showed the distortion and pressure recovery levels were acceptable for engine operability.184

Fig.12 Nacelle and fuselage reshaping to reduce transonic drag at cruise condition.182

Inlet flow characteristics of the model scale and full scale were also performed using CFD method to compare measured and predicted distortion levels, respectively.186Compared to flow-through nacelle testing on the same BWB model,the control surface effectiveness, pitching moment and drag were found to increase with the turbine powered simulator units operating. However, the improved control surface performance must be balanced with engine performance and acoustics in a comprehensive airframe design.185The engine response to the inlet distortion was predicted based on a coupled inlet-fan CFD simulation. Separate analyses were performed to assess performance, fan operability, core engine operability, and fan blade vibratory stress. It was concluded that, for all the inlet distortion cases that were within the expected operational envelope of the PSC aircraft, the engine operability and fan blade stress metrics were determined to be within acceptable limits, and there were relatively small impacts to engine performance.187

It can be concluded that the aft-upper centerbody mounting position of the podded engines is an option with most possibility and low risk. However, the interactions between airframe and the engines should be studied in detail and be enhanced at off-design conditions on aircraft stability and control and engine operability. A detailed aerodynamic design of PAI was performed on NPU-330 and was presented in Ref.188in this volume.

5. Overall efficiency evaluation

Aircraft fuel efficiency is very critical for unconventional BWB aircraft, which is an aircraft level measure in terms of engine,aerodynamic and structural performance. It is also closely related to the emissions for environmental concerns.Only providing higher aircraft fuel efficiency BWB aircraft can earn its way for future application.

Fig. 13 Comparisons of ETRW for TAW and BWB aircrafts.

Different parameters can be used to indicate aircraft fuel efficiency. Energy intensity in terms of energy consumed per seat kilometer for various aircraft types was used by Lee et al.93as an indication.It was also extended as a metric to assess various scenarios of aviation development.A fuel efficiency metric named Payload Fuel Efficiency(PFE)was defined by Green189using Breguet equation in terms of range,payload weight,and mission fuel weight.It was used as a figure of merit to examine optimal aircraft design range for minimal fuel use. A Payload Fuel Energy Efficiency (PFEE) was introduced by Hileman et al.190as a metric to quantify the energy efficiency of aviation on the fleet level for aviation environmental performance.

Through literature review, it can be observed that the predicted fuel benefit of BWB over TAW varies significantly between studies,due to differences in design requirements,system architectures,as well as study ground rules and technology assumptions. A non-dimensional fuel efficiency parameter,ETRW, introduced by Poll24based on Breguet equation is adopted here to evaluate the fuel efficiency of BWB aircraft,in terms of engine overall,aerodynamic and structural efficiencies. The ETRW can be written as

The comparisons of fuel efficiency between TAW and BWB aircrafts across whole size range are shown in Fig. 13. To exclude the effects of early TAW the aircrafts of Boeing and Airbus are included after first flight year 1981 and 1987,respectively. The mean ETRW for TAW of Boeing and BWB concepts with ηo=0.4 are 1.014, 1.011 and 0.694,respectively, as shown in Table 15. The fuel efficiency of BWB is about 31.5% better than that of TAW almost across the aircraft size. The worst BWB concept is AGILE-BWB due to its inferior L/D of 18.73When more advanced engine technologies resulting ηo=0.45 is used, the savings can be as high as 40.0%.To exhibit the effects of the payloads,the mean values of ETRW across the aircrafts with different PAX and the assessments at the end of ERA project12are compared in Table 15. The benefit of BWB increases from 25.6% to 45.3% as PAX increases from the range 100-199 to the range 200-299 at ηo=0.4, which is 34.7-52.2% at ηo=0.45 correspondingly. When the PAX is equal or larger than 400, the benefit of BWB decreases in the present prediction, which is due to that most of the large BWBs are studied much earlier than the smaller versions. However, the recent NASA assessments still confirm the aforementioned conclusion.12

Table 15 ETRW comparisons of across different aircraft sizes between BWB and TAW.

6. Conclusions and outlook

Blended-wing-body aircraft has emerged from past decades’investigations as potential architecture replacing conventional tube-and-wing aircraft to satisfy the requirements of growing markets, more stringent regulations and sustainable aviation.The developments of this concept in different nations were analyzed in a historical perspective.The enabling technologies including aerodynamic design, structure concepts for noncircular pressurized cabin, stability and control, and propulsion and airframe integration were scrutinized. The enhancing technologies including noise reduction, engine advancements were also discussed. The aircraft-level fuel efficiency was evaluated and compared with that of conventional aircrafts. The following conclusions can be drawn

(1) Four different concepts including tube and wing,hybrid wing body,blended wing body and flying wing were categorized based on the principle of the forms following the function.

(2) The high aerodynamic efficiency of blended-wing-body aircraft comes from large mean aerodynamic chord and high wetted aspect ratio, which can be well scaled using these two parameters including that of tube and wing aircraft.

(3) The trade-offs among stability and control, cruise aerodynamic design and low-speed aerodynamic design have been realized but have not been well investigated.A perceptible trend to extend the centerbody length can be a potential solution.

(4) The integrated shell and skin concept using pultruded rod stitched efficient unitized structure is the most promising solution for non-circular pressurized cabin.However, the weight penalty and weight prediction of new structures are still of large uncertainty. The structure efficiency is inferior to that of tube-and-wing aircraft based on the published blended-wing-body design.

(5) There are no static stability margins that can ensure dynamic characteristics. Since stability and control and flying qualities are closely related, clear criteria are required for flying qualities evaluation, stability and control design, and flight control low design.

(6) The airframe noise sources become dominant due to the adoption of ultra-high bypass ratio engines and the noise shielding. The higher operating angle of attack and local flow speed lead to specific noise feature of high-lift device and landing gears.

(7) The propulsion and airframe integration is quite difficult.During the integration, aerodynamic efficiency, noise reduction, and engine operability should be balanced.

(8) At the aircraft-level the mean fuel efficiency of blendedwing body is more than 31.5% better than the existent tube-and-wing design. More improvement can be expected by adopting advanced technologies.

Further investigations are expected

(1) New structural layout of the whole blended-wing-body aircraft needs to be explored using like topology optimization method.

(2) The effects of structural displacement(mainly the deformation of the centerbody) on the cruise aerodynamic efficiency need to be evaluated.

(3) The safety issues related to certification including uncontained engine blade burst,thrust reverser and evacuation need to be investigated in detail.

(4) The effects of flight speed on the noise shielding and airframe noise reduction need to be investigated.

(5) A blended-wing-body concept needs to go further outside of the conceptual design stage to exhibit more potentials and problems.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (Nos. 3102019JC009 and G2016KY0002).