seminar bwb

AERODYNAMIC DESIGN EVALUATION OF BLENDED WING

BODY DESIGN IN AIRCRAFTS AND ITS SCOPE IN

COMMERCIAL USE

A Seminar Report submitted in partial fulfillment of the requirements for the award of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

Under University of Calicut

By

ANOOP M R

APAMEME008

DEPARTMENT OF MECHANICAL ENGINEERING

Aryanet Institute of Technology

Velikkad, Mundur, Palakkad- 678592

MARCH 2016

DEPARTMENT OF MECHANICAL ENGINEERING ARYANET INSTITUTE OF TECHNOLOGY

VELIKKAD, PALAKKAD, PIN 678 592

CERTIFICATE

Certified that the seminar titled AERODYNAMIC DESIGN

EVALUATION OF BLENDED WING BODY DESIGN IN

AIRCRAFTS AND ITS SCOPE IN COMMERCIAL USE is a bonafide

record of the work done by ANOOP M R (APAMEME008) under my

supervision and guidance, and is submitted in March 2016 in partial

fulfillment of the requirements for award of the Degree of Bachelor of

Technology in Mechanical Engineering under University of Calicut.

Seminar Guide Head of the Department

SREEJITH M Prof. V Gopinathan

Assistant professor Department of Mechanical

Department of Mechanical Engineering.

Engineering.

Place : PALAKKAD

Date :

Seminar report BWB Analysis

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Department of Mechanical Engineering, AIT (2012-2016)

ii

ACKNOWLEDGEMENT

While bringing out this seminar to its final form, I came across a number of

people whose contributions in various ways helped my field of research and they

deserve special thanks. It is a pleasure to convey my gratitude to all of them.

I would like to express my deepest gratitude to Dr. M.R. VIKRAMAN,

Principal, Aryanet Institute of Technology, Palakkad for fostering an excellent

academic climate in the college and for his support and encouragement throughout the

course period.

I wish to thank Prof. V GOPINATHAN, Head of the Department,

Mechanical Engineering, Aryanet Institute of Technology, Palakkad, for providing all

the facilities and support for execution of the work.

I am thankful to Mr. SREEJITH M (Seminar Coordinator and Seminar

Guide), Assistant Professor, Department of Mechanical Engineering, Aryanet Institute

of Technology, Palakkad for his advice, encouragement, suggestions, invaluable

supervision and support throughout this seminar work.

I would like to take this opportunity to thank my friends who spent their

valuable time and shared their knowledge for helping me to complete the seminar

with the best possible result.

ANOOP M R


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iii

ABSTRACT

The desire to produce environment friendly aircraft that is aerodynamically

efficient and capable of conveying large number of passengers over long range at

reduced operating cost led aircraft designers to develop the Blended Wing Body

(BWB) aircraft concept. The BWB aircraft represents a paradigm shift in the design

of aircraft. The design provides aerodynamic and environmental benefits and is

suitable for the integration of advanced systems and concepts like laminar flow

technology, jet flap and distributed propulsion. However despite these benefits BWB

aircraft design is yet to be developed for commercial air transport due to several

challenges. The emerging trends in BWB aircraft design highlighting design

challenges that have hindered the development of a BWB passenger transport aircraft

is reviewed. In order to harness the advantages and reduce the deficiencies of a tightly

coupled configuration like the BWB, a multidisciplinary design synthesis

optimization should be conducted with good handling and ride quality as objective

functions within acceptable direct operating cost and noise bounds.

Keywords: Blended Wing Body, Aerodynamics, Laminar flow, Jet flap, Distributed

propulsion


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iv

CONTENTS

SL No. TITLE Page No.

LIST OF TABLES vii

LIST OF FIGURES vii

LIST OF ABBREVATIONS x

1 INTRODUCTION 1

2 LITERATURE REVIEW 2

2.1 Historical Background 2

2.2 Research projects in BWB by European Union 7

3 AERODYNAMICS 9

3.1 What is Aerodynamics 9

3.2 Aerodynamic Drag 9

3.2.1 Drag Coefficient 9


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v

3.3 Aerodynamic Lift 10

3.3.1 Lift Coefficient 10

3.4 Lift to Drag Ratio 11

3.5 Pitching Moment 11

3.5.1 Pitching Moment Coefficient 12

4 AERODYNAMIC DESIGN EVALUATION OF BWB 13

4.1 Design methodology 13

4.2 The First Airframe 13

4.2.1 The Design Approach 13

4.2.2 Baseline Geometry 15

4.3 Aerodynamic Performance 15

4.3.1 Lift Coefficient 16

4.3.2 Drag Coefficient 16

4.3.3 Pitching Moment Coefficient 17


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vi

4.3.4 Lift to Drag Ratio 18

4.3.5 Pressure Distribution 19

4.4 Usable Area 20

4.5 Second Airframe 22

4.5.1 Conceptual Design Approach 22

4.5.2 Baseline Geometry 23

4.5.3 Interior Arrangement 24

4.5.4 Airframe Sections and Control Surfaces 25

5 COMPARISION OF BWB WITH THE

CONVENTIONAL TYPE

26

5.1 Wetted Surface Area Comparison 26

5.2 Moment Arms and Pitch Control Effectiveness Comparison 27

5.3 Aero Structure Comparison 27

6 CONCLUSIONS 29

7 REFERENCES 30


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vii

LIST OF TABLES


4.1 Cruise flight conditions of the first AEROP

frame

14

4.2 Weight estimation of the first AEROPP

airframe

14

4.3 Design constrains of the second AEROPP

airframe

22

LIST OF FIGURES


2.1.1 The D-8 tailless aircraft at the 1914 Farnborough air

show

3

2.1.2 Westland-Hill Pterodactyl V aircraft with fully

moving wingtips

3

2.1.3 The Northrop semi-flying wing aircraft 3

2.1.4 The Northrop N-1M Aircraft 3


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viii

2.1.5 The Northrop N-9 M aircraft 4

2.1.6 Northrop XB-35 piston-engine long-range bomber 4

2.1.7 ‘B2-Spirit’ Stealth Bomber 5

2.1.8 Turbojet powered Ho-229 flying wing aircraft 6

2.1.9 BW-17 radio controlled model aircraft 6

2.1.10 BWB-450 commercial passenger transport aircraft 6

2.2 European Union sponsored BWB-related research

programs

8

3.3 Lift and Drag 10

3.4 Lift to drag ratio 11

4.2.1 Mission profile of the first airframe. 14

4.2.2 The first baseline airframe. 15

4.3.1 Lift coefficient variation with Angle of Attack. 16

4.3.2 Drag coefficient variation with Angle of Attack. 17

4.3.3 Pitching moment coefficient with Angle of Attack 18


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ix

4.3.4(a) Lift-to-drag ratio variation with lift coefficient 19

4.3.4(b) Lift-to-drag ratio variation with Angle of attack. 19

4.3.5(a) Total pressure line 20

4.3.5(b) Total pressure distribution 20

4.4 Schematic view of usable space for the first AEROPP

airframe

21

4.5.2 Technical drawing of the second AEROPP airframe 23

4.5.3(a) Schematic views of the interior arrangement 24

4.5.3(b) Sectional view of the interior arrangement 24

4.5.4 Second air frame layout and types of winglets 25

5.1 Comparison of Wetted Surface Area 26

5.2 Comparison of moment arms and pitch control

effectiveness

27

5.3 Comparison of the aerodynamic and inertia load

distribution

28


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x

LIST OF ABBREVATIONS

BWB Blended Wing Body

L/D Lift to Drag Ratio

NASA National Aeronautics and Space Administration

MDC McDonnell Douglas Corporation

BLI Boundary Layer Ingestion

VELA Very Efficient Large Aircraft

MOB Multidisciplinary Optimizations of a Blended Wing Body

NACRE New Aircraft Concept Research


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1

CHAPTER 1

INTRODUCTION

An aircraft is a machine that is able to fly by gaining support from the air. It

counters the force of gravity by using either static lift or by using the dynamic lift of

an airfoil or in a few cases the downward thrust from jet engines. Before the renewed

interest in unconventional aircraft configurations, the design of aircraft centred on the

improvement of the efficiency of the conventional tube and wing design. However, having

reached the limit of conventional design coupled with a growing demand for an

environmentally friendly, aerodynamically efficient aircraft that can carry large number of

passengers over long ranges at reduced direct operating cost, the BWB aircraft was

conceptualised. The BWB has low noise signature because it does not require flaps or tail

plane for pitch control hence removing the need for trailing edge and possibly leading edge

devices in take-off and landing. It also emits less pollutants due to reduced fuel burn and

propulsive efficiency. The BWB offers increased range and payload capacity due to 27%

reduction in fuel burn per seat leading to reduced direct operating costs. These advantages

are enabled by blending a lift generating centre-body housing the payload with

conventional outer wings, to obtain a compact aerodynamically efficient flying wing

providing structural, aerodynamic and payload synergy. The BWB however differs from a

pure flying wing in that a pure flying wing has straight leading and trailing edges with no

definite fuselage. Payloads in a flying wing aircraft are stored in the main wing structure.

On the other hand, a BWB consists of a flattened fuselage for accommodating payload.

Other advantages of the BWB design includes 15–20% increased lift to drag ratio (L/D)

due to reduced drag resulting from a 33% lower wetted surface area compared to a

conventional tube and wing aircraft.

https://en.wikipedia.org/wiki/Machine

https://en.wikipedia.org/wiki/Flight

https://en.wikipedia.org/wiki/Air

https://en.wikipedia.org/wiki/Buoyancy

https://en.wikipedia.org/wiki/Lift_(force)

https://en.wikipedia.org/wiki/Airfoil

https://en.wikipedia.org/wiki/Powered_lift

https://en.wikipedia.org/wiki/Jet_engine


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2

CHAPTER 2

LITERATURE REVIEW

2.1 Historical Background

The first recorded tailless flying wing aircraft was the D-8 aircraft designed by John

Dunne in 1911. The D-8, shown in Fig.2.1.1, is a tailless biplane with swept wing and

washout to prevent premature tip stall and improve pitch stability. In the years between

1924 and 1931, (Later a Professor) Hill designed a series of tailless aircraft (Fig. 2.1.2)

known as the Hill's Pterodactyl. These aircraft culminated in the MKIV, the first tailless

aircraft capable of looping and rolling manoeuvres. The main features of the Pterodactyl

series are its flight-controlled variable sweep, operated to trim the aircraft at different

loading conditions.

Convinced of the aerodynamic benefits of fewer non-lifting surfaces, Jack Northrop

established the Northrop's Corporation in 1927 to explore the potentials of the flying wing

configurations. The Corporation developed a semi-flying wing aircraft in 1928 and the N-

1M pure flying wing in 1940. The semi-flying wing aircraft shown in Fig. 2.1.3 comes

fitted with external control surfaces and curried outrigger twin booms. The N-1M ‘pure’

flying wing aircraft, shown in Fig. 2.1.4, incorporates ground-controlled variable sweep,

dihedral and control surfaces. The aircraft canal so change its centre of gravity location and

tip configuration while on the ground. Overall, the N-1M, with its elevons and wing-tip

drag rudders, performed creditably well and proved the possibility of the flying wing

concept. However, the engines hidden in the air foil suffered from overheating while the

drooped wing tips used for stability were found to be unnecessary. None the less, the

performance was sufficient to convince the United States Air Force to award Northrop

Corporation the contract to assess the feasibility of a flying wing bomber.


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3

Fig. 2.1.1. The D-8 tailless aircraft at the 1914 Farnborough air show

Fig. 2.1.2. Westland-Hill Pterodactyl V aircraft with fully moving wingtips

Fig. 2.1.3. The Northrop semi-flying wing aircraft (Source: Smithsonian NASA Museum).

Fig. 2.1.4. The Northrop N-1M Aircraft (Source: Smithsonian NASA Museum).

The N-9M (Fig. 2.1.5) was developed as a scaled mock-up of the proposed bomber. The

N-9M is an 18 m span twin-engine aircraft with a take-off weight of 6326 kg. This is

approximately one-third the size of the subsequently developed long range heavy bombers,

the XB-35 and YB-35.


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4

The XB-35 aircraft (Fig. 2.1.6), which came into service in 1946, is powered by four piston

engines, each driving two contra rotating four-blade pusher propeller through along shaft

and gearbox. The YB-35, on the other hand, is powered by jet engines. Both the XB-35

and the YB-35 were unstable with poor handling qualities. Additionally, they suffered from

engine/gear box problems. Despite these defects, they provided practical knowledge on the

design of flying wing bombers which later proved useful in the development of the YB-49

in 1947 and the famous Northrop- Grumman B2 Spirit in 1981. The success of the

Northrop-Grumman B2-Spirit shown in Fig. 2.1.7 spurred renewed interest in the BWB

configuration.

Fig. 2.1.5. The Northrop N-9 M aircraft (Source: Smithsonian NASA Museum).

Fig. 2.1.6. Northrop XB-35 piston-engine long-range bomber (Source: Virtual Aircraft Museum).


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5

Fig. 2.1.7. ‘B2-Spirit’ Stealth Bomber (Source: Xairforces Military Aviation Society).

Other notable proponents of the flying wing concept were the Horten brothers. The Horten

brothers, Walter and Reiman Horten, worked on the flying wing concept from 1931 until

1944. In the process they developed the Ho-series flying wing aircraft. These aircraft

incorporates in board flaps, elevons and tip-mounted drag rudders. The Horten brothers are

credited with the development of the world's first turbojet-powered flying wing aircraft,

the Ho-IX shown in Fig. 2.1.8.

The BWB aircraft, as it is known today, was conceptualised in 1988 by Robert Liebeck of

the then McDonnell Douglas Corporation (MDC) now Boeing Company. The aircraft

provides improved aerodynamic efficiency by merging the fuselage and wing sections into

a single lifting surface. Subsequently, in 1997, the BWB configuration was adopted in the

design of a 17ft span, radio-controlled model aircraft, the BWB-17 (Fig. 2.1.9), by a

combined team of researchers from MDC, NASA and Stanford University.

Following the success of the BW-17, NASA further explored the possibility of applying

the BWB tailless aircraft concept to commercial passenger transport. This led to the birth

of the BWB-450 in 2003. The BWB-450 (Fig. 2.1.10), is a 450 passenger capacity

commercial transport airplane incorporating an ultra-efficient engine technology with

Boundary Layer Ingestion (BLI) engine inlets and Active Flow Control. The BWB-450

design heralded several NASA programs on the feasibility of the BWB concept for

commercial passenger transport. The 450-passenger capacity was selected to permit the


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6

validation of developed design tools and methods with the A380 conventional tube and

wing aircraft.

Fig. 2.1.8. Turbojet powered Ho-229 flying wing aircraft (Source: Military Factory).

Fig. 2.1.9. BW-17 radio controlled model aircraft (Source: Stanford University).

Fig. 2.1.10. BWB-450 commercial passenger transport aircraft (Source: NASA).


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7

2.2 Research projects in BWB by European Union

The anticipated benefits of the BWB concept spurred several organisations into researches

on various aspects of the aircraft. Notable among the researches is the European Union

(EU) 5-tiered project to develop innovative, efficient long range, large capacity passenger

transport and cargo aircraft. The project comprised 3 wholly EU framework Programs and

a program jointly funded by the EU and the United States. The 3 EU Framework Programs

are the 5th, 6th and 7th EU Framework Programs. The 5th EU Framework Program consists

of the Multidisciplinary Optimisation of a Blended Wing Body (MOB) and the Very

Efficient Large Aircraft (VELA) projects. The MOB project is a 3 year research project

carried out in distributed environments across 4 European countries. The project involved

15 partners. This includes 3 aerospace companies, 4 research institutes and 8 universities.

The aim of the MOB project was to develop tools and methods that will enable distributed

design teams to create innovative and complex aeronautical products using either

commercial off the shelf methods or proprietary codes. The VELA project was setup to

develop the necessary skills set, capabilities and methodologies appropriate to the design

and optimisation of Very Efficient Large Aircraft concepts. The VELA project, which ran

from 2002 to 2005, investigated 2 extremes of a BWB configuration, in terms of the

placement and blending of the outboard wing.

The 6th Framework Programme also known as the New Aircraft Concept Research

(NACRE) began in 2005 and was completed in 2009. The NACRE Integrated Project was

undertaken to integrate and validate technologies that enable new aircraft concepts to be

assessed. The NACRE project, which was led by Airbus, involved 36 partners from 13

European countries. The NACRE project advanced the design of BWB aircraft through its

Passenger- driven Flying Wing (PFW) configuration.

Cambridge and MIT investigated the feasibility of an ultra-low noise, fuel efficient BWB,

dubbed the Silent Aircraft Initiative (SAI). The SAI was an ambitious 3-year project by a

team of 35 researchers, beginning in 2002. The research was aimed to design an airplane

that is radically quieter than current passenger transport aircraft.


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8

The 7th Framework Program focuses on the development of innovative active control

concepts for advanced 2020 aircraft configuration. The project called Active Flight Control

for Flexible Aircraft 2020 (ACFA2020) involves 13 partners from 11 European countries.

The mandate for ACFA20, which is derived from the strategic goal of the Advisory Council

for Aeronautics Research in Europe (ACARE), is the design of an innovative ultra-efficient

450 passenger aircraft together with a robust, adaptive multichannel control architecture

suitable for the aircraft.

Fig. 2.2. European Union sponsored BWB-related research programs


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9

CHAPTER 3

AERODYNAMICS

3.1 What is Aerodynamics?

Aerodynamics is the study of how gases interact with moving bodies. Because the

gas that we encounter most is air, aerodynamics is primarily concerned with the forces of

drag and lift, which are caused by air passing over and around solid bodies. Engineers

apply the principles of aerodynamics to the designs of many different things, including

buildings, bridges and even soccer balls; however, of primary concern is the aerodynamics

of aircraft and automobiles. Aerodynamics comes into play in the study of flight and the

science of building and operating an aircraft, which is called aeronautics. Aeronautical

engineers use the fundamentals of aerodynamics to design aircraft that fly through the

Earth's atmosphere.

3.2 Aerodynamic Drag

The most significant aerodynamic force that applies to nearly everything that moves

through the air is drag. Drag is the force that opposes an aircraft's motion through the air.

Drag is generated in the direction the air is moving when it encounters a solid object. In

most cases, such as in automobiles and aircraft, drag is undesirable because it takes power

to overcome it.

3.2.1 Drag Coefficient

The drag coefficient is a dimensionless quantity that is used to quantify the drag or

resistance of an object in a fluid environment, such as air or water. It is used in the drag

equation, where a lower drag coefficient indicates the object will have less aerodynamic or

hydrodynamic drag. The drag coefficient is always associated with a particular surface

area.


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10

The drag coefficient Cd is defined as:

𝐶𝑑 =2𝐹𝑑𝜌𝑢2𝐴

Where:

Fd is the drag force, which is by definition the force component in the direction of

the flow velocity

ρ is the mass density of the fluid

u is the flow speed of the object relative to the fluid,

A is the reference area

3.3 Aerodynamic Lift

A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this

force that is perpendicular to the oncoming flow direction. Lift is most commonly

associated with the wing of a fixed-wing aircraft, although lift is also generated by

propellers, kites, helicopter rotors, rudders, sails and keels on sailboats, hydrofoils, wings

on auto racing cars, wind turbines, and other streamlined objects. Lift is also exploited in

the animal world, and even in the plant world by the seeds of certain trees.

Fig.3.3.Lift and drag

3.3.1 Lift Coefficient

The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting

body to the fluid density around the body, the fluid velocity and an associated reference

area.


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11

𝐶𝑙 =2𝐿

𝜌𝑣2𝑆

Where

L is the lift force, ρ is fluid density, v is true airspeed and S is the relevant plan area

3.4 Lift to Drag Ratio

In aerodynamics, the lift-to-drag ratio, or L/D ratio, is the amount of lift generated by a

wing or vehicle, divided by the aerodynamic drag it creates by moving through the air. A

higher or more favorable L/D ratio is typically one of the major goals in aircraft design;

since a particular aircraft's required lift is set by its weight, delivering that lift with lower

drag leads directly to better fuel economy in aircraft, climb performance, and glide ratio.

Fig.3.4.Lift to drag ratio

3.5 Pitching Moment

In aerodynamics, the pitching moment on an airfoil is the moment (or torque) produced by

the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied,

not at the center of pressure, but at the aerodynamic center of the airfoil. The pitching

moment on the wing of an airplane is part of the total moment that must be balanced using

the lift on the horizontal stabilizer.


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12

3.5.1 Pitching Moment Coefficient

The pitching moment coefficient is important in the study of the longitudinal static stability

of aircraft and missiles. The pitching moment coefficient Cm is defined as

𝐶𝑚 =𝑀

𝑞𝑆𝑐

Where M is the pitching moment, q is the dynamic pressure, S is the wing area, and c is

the length of the chord of the airfoil.


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13

CHAPTER 4

AERODYNAMIC DESIGN EVALUATION OF BWB

4.1 Design Methodology

The first BWB airframe is obtained simply through extruding an S-shaped airfoil

along the span. Provisionally, for opening possible future research opportunity, one-

percent scaled airframe is considered at initial conceptual design layout. Meanwhile,

because aft portion of the central body is appropriate for engine installation and total

pitching moment of the airframe needs to be trimmed in cruise condition, an airfoil with

S-shaped chamber line is chosen.

The second BWB airframe is designed based on computational aerodynamic analyses of

the first airframe. Meanwhile, aircraft conceptual design approach is used for designing the

full-scaled configuration. Accordingly, mission profile includes main and reserved sections

for this airframe. In addition, the interior arrangement and control surfaces are being sized.

The modified airframe is obtained with arranging S-shaped airfoils from central body

toward the span, and then smoothly converting their camber line into form of supercritical

curvature near the outboard wing.

4.2 The First Airframe

The design procedure, including conceptual design approach and baseline geometry, for

the first airframe are introduced. The first airframe is assessed using computational fluid

dynamics. The computational aerodynamic assessment, grid over the baseline geometry,

and implemented computational schemes are also discussed. Meanwhile, longitudinal

stability and pitching moments of the first airframe is investigated afterward. At the end,

usable space is discussed as a major design constraint.

4.2.1 The Design Approach

Typical mission profile includes takeoff, climb, cruise, loiter, ap-proach and landing

segments. The mission profile is schematically shown in Fig.4.2.1. Furthermore, range,


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14

cruise speed, and altitude are stated in Table 4.1.and and static weight estimation is

demonstrated in Table 4.2

Fig.4.2.1.Mission profile of the first airframe.

Table.4.1. Cruise flight conditions of the first AEROP frame

Table.4.2. Weight estimation of the first AEROPP airframe

The Selig S5010 airfoil has been chosen for extrusion along the span. This airfoil is

designed for low Reynolds number. Its zero pitching moment coefficient is close to zero,

which is appropriate for longitudinal stability of the airframe.


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15

4.2.2 Baseline Geometry

The first baseline geometry, as already mentioned, is obtained through extruding the Selig

S5010 airfoil along the span with re-spect to a planform. The first baseline airframe is

illustrated in Fig.4.2.2. The airfoil-liked body is connected smoothly with the wings, and

they are connected to a winglet in wingtips. The winglet improves lift distribution by

weakening the wingtip vortices. They are important in reducing induced drag. For this

reason, they are included in the first baseline airframe. For sizing the winglets, vertical tail

volume coefficient is used.

Fig.4.2.2.The first baseline airframe.

4.3 Aerodynamic Performance

The computational aerodynamic analysis includes variation of lift coefficient, drag

coefficient, pitching moment coefficient, lift-to-drag ratio, and center of pressure location

with angle of attack.


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4.3.1 Lift Coefficient

The lift coefficient variation with angle of attack is demonstrated in Fig.4.3.1. As the angle

of attack increases, the lift coefficient shows smooth variation. Up to 24º, no stall behavior

is observed for this airframe in this interval. Among the geometric parameters, low Aspect

Ratio and high sweepback angle are responsible for this stall behavior. This issue shows

their inappropriateness in the low Reynolds number flow regime for this airframe.

Fig.4.3.1.Lift coefficient variation with Angle of Attack.

In general, the BWB configuration introduces high lift-to-drag ratio in a cruise flight

condition. The high lift-to-drag ratio increases aerodynamic performance of an airframe.

In the first airframe, desired lift coefficient is not satisfied in the cruise flight condition

with zero angle of attack. Typically, angle of zero lift is approximately equal to percent

camber of an airfoil. In this airframe, the angle of zero lift is 4.5º.

4.3.2 Drag Coefficient

The drag coefficient variation with angle of attack is presented in Fig.4.3.2. Although,

angle of attack in cruise flight condition is zero, minimum drag coefficient is obtained in a

negative angle with positive lift coefficients. One of the major reasons for obtaining

minimum drag in a negative angle is high angle of incidence in this airframe. For this

purpose, cruise flight condition in zero angle of attack is not economically efficient. The


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17

thrust is not minimized in this condition and fuel consumption will raise. Additionally,

maxi-mum lift-to-drag ratio is not gained, which affects the aerodynamic performance of

the airframe. In the second airframe, it is expected choosing appropriate incidence angle

obtains the minimum drag coefficient in between 0ºand 2º angle of attacks.

Fig.4.3.2.Drag coefficient variation with Angle of Attack.

4.3.3 Pitching Moment Coefficient

The pitching moment coefficient variation with angle of attack is demonstrated in

Fig.4.3.3. As angle of attack increases, the pitch-ing moment decreases. According to the

right-hand rule, positive direction of pitching moment around lateral axis is associated with

the counterclockwise rotational direction, indicating downward rotation of nose. The

pitching moment coefficient is negative after −4º, and also the curve slope is negative

before 5º and after of 11º. Therefore, as angle of attack increases, pitching moment

coefficient decreases, implying the airframe’s nose is turning upward. In between 5ºand

11º, pitching moment coefficient is almost con-stant because its curve slope is almost zero.

For this reason, the airframe needs to be considered for pitch up avoidance in climb

condition. In addition, the static margin is negative ratio of pitching moment curve slope

to lift curve slope. From the aforemen-tioned points, the second airframe is expected to


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18

obtain at least 1% positive static margin with appropriate distribution of airfoil stack across

the span.

Fig.4.3.3.Pitching moment coefficient variation with Angle of Attack.

4.3.4 Lift to Drag Ratio

The lift-to-drag ratio variation with lift coefficient is illustrated in Fig.4.3.4(a). The

maximum lift-to-drag ratio is obtained in 0.175 lift coefficient. Accordingly, higher lift

coefficient and lower drag co-efficient increases the ratio, which is desirable for decreasing

the fuel consumption and increasing the range.

The lift-to-drag ratio variation with angle of attack is illustrated in Fig.4.3.4(b). Its

maximum value is 8.5obtained in zero angle of attack. Setting the maximum lift-to-drag

ratio in cruise flight condition improves the aerodynamic performance; however, the

maximum value is not desirable. The maximum lift-to-drag ratio is relatively lower than

the desirable value for the airframe, resulted by small lift-curve slope with large drag

coefficient.


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19

Fig.4.3.4.(a) Lift-to-drag ratio variation with lift coefficient

Fig.4.3.4.(b) Lift-to-drag ratio variation with Angle of attack.

4.3.5 Pressure Distribution

The pressure line distribution is demonstrated in Fig.4.3.5(a). Based on the pressure line

pattern in the interconnection between swept forward body and swept backward wing,

high-pressure flow escaped toward upper surface in this region. Therefore, smoother

interconnection diminishes induced drag and viscous separation drag, which is desirable in

the second planform.

The pressure distribution over upper and lower surface of the airframe is provided in

Fig.4.3.5(b). Over the lower surface, pressure coefficient fairly remains uniform, except


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20

there is a slight variation narrowly in the leading edge. Over upper surface, there is a lit-tle

gap between low and high pressure coefficients in trailing-edge interconnection between

the swept forward body and the swept backward wing. The gap between low and high

pressure coefficients develops toward wingtips, which makes airflow vulnerable to

separate. The gap produces vortices which extract energy from the flow and develop

induced drag.

Fig.4.3.5.(a) Total pressure line at AoA=2ºand Re=200K over the upper surface.

Fig.4.3.5.(b) Total pressure distribution at AoA=2º and Re=200Kover upper and lower surfaces.

4.4 Usable Area

The blended-wing-body configuration provides adequate space for accommodating

passengers and cargo. The space is not us-able unless an airfoil with appropriate thickness-

to-chord ratio to be used in the central body. The first airframe does not provide adequate

space for accommodating passengers and cargo, which is shown in Fig.4.4, and the Selig


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S5010 airfoil with 10% thickness-to-chord ratio is not proper. It is expected the second

airframe to be designed with appropriate airfoil, which provides sufficient thickness-to-

chord ratio for accommodating 800 passengers and cargo. Meanwhile, because the central

airfoil-liked body has small compressibility drag, it can be thicken enough to accommodate

the passengers in a double-deck interior and participate in the lift generation. The thick

central body hold major load over the airframe allowing outboard wing to be thinned

adequately and preventing wave drag development over the wings.

Fig.4.4.Schematic view of usable space for the first AEROPP airframe.


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4.5 The Second Airframe

The second airframe is designed and modified based on evaluation of the first airframe. In

this section, design procedure of the second airframe is described which includes the

conceptual design approach, baseline geometry, interior arrangement, and control surfaces.

4.5.1 Conceptual Design Approach

In the second airframe, required space for 800 passengers is ac-quired by choosing a very

thick airfoil in the central body. Further, thickness-to-chord ratio of the airfoil stack is

changed from 18% to 10% across the span. Other modifications are applied to this air-

frame based on the assessment of the first airframe. Among them, Aspect Ratio of the

wings are increased to improve the maximum lift-to-drag ratio, in particular wetted Aspect

Ratio. Moreover, five different airfoils with different thickness-to-chord ratio are situ-ated

across the span. With respect to the increase of Aspect Ratio, the span is decreased in

comparison with the first airframe. How-ever, the wing area is increased just 33%

comparatively. Design constraints for this airframe is demonstrated in table 4.3.

Table.4.3.Design constrains of the second AEROPP airframe


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4.5.2 Baseline Geometry

The full-scaled baseline geometry of the second airframe is demonstrated by its technical

drawing in Fig.4.5.2. It shows dimensions of the second configuration in top, front, and

side views. The control surfaces are also sized in this airframe, and they are indicated in

this figure.

Fig.4.5.2.Technical drawing of the second AEROPP airframe.


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4.5.3 Interior Arrangement

The lower and upper passenger cabins are demonstrated in Fig.4.5.3(a) in the second

airframe. The upper deck includes first, busi-ness, and economic seats class located in 7

cabin bays. In every bay, galley and lavatory are located at the rear. The lower deck is

accessible through two stairs in front of the upper deck. Additional stairs toward the lower

deck are also situated in rear of the each bay.

The first seats class in the upper deck includes two cabin bays. One bay is located in front

of the deck and another one is located in one of the central bays. The lower deck includes

first and economics seats class included in 5 bays. The first seats class in the lower deck is

located in front with wide view toward the sky, and other bays in this deck are dedicated

to the economic seats class. The entire interior of the second airframe, including passenger

cabins and cargo bays, are illustrated by the middle sectional view in Fig.4.5.3(b). The

passenger bays in the center are surrounded by the cargo bays. As it is shown, the double

deck interior arrangements appropriately fits in the provided space in this airframe.

Fig4.5.3(a) Schematic views of the interior arrangement


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Fig.4.5.3.(b) Sectional view of the interior arrangement.

4.5.4 Airframe sections and control surfaces

The airframe layout of the second airframe is demonstrated in Fig.4.5.4a. Accordingly,

passengers and cargo are placed in the central body, and fuel tanks are situated in the

outboard wings. Moreover, tip-to-tip control surfaces are sized for this airframe for

providing 10% extra controllability. Among the tip-to-tip control surfaces, a single slotted

fowler flap is placed between the two engines. In the trailing interconnection between

swept back wing and swept forward body, a double slotted flap is situated. In this region,

horizontal line of the trailing edge improves its performance. Between the fowler and

double slotted flaps, elevators are sized. In addition, ailerons are located toward the tips at

the endof the double slotted flap. In the outboard wing, a leading edge slot is sized. Similar

to the first airframe, a Split Drag-Rudder and a Double-Hinged Rudder are situated on the

winglets demonstrated in Fig.4.5.4b.

Fig.4.5.4.Second air frame layout and types of winglets


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CHAPTER 5

COMPARISON OF BWB DESIGN WITH THE CONVENTIONAL

TYPE

5.1 Wetted Surface Area Comparison

For comparing the effect of reduced wetted surface area on the lift coefficient of an 800

passenger BWB and a conventional tube and wing aircraft of same capacity. This

phenomenon was investigated by transforming a 650 square meter ball into a cylinder and

a lifting body and then sizing the streamlined options to accommodate 800 passengers.

Subsequently, following the integration of the wing, empennage and engines, showed that

a conventional aircraft with 4 under the wing engines has a wetted surface area of 4100 m2

while the BWB with trailing edge BLI engines has only 2800 m2 for the same passenger

capacity. The 33% reduction in wetted surface area, lowers the BWB profile drag thus

increasing the L/D by 10 – 15% when compared to a conventional configuration.

Fig.5.1. Comparison of Wetted Surface Area


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5.2 Moment Arms and Pitch Control Effectiveness Comparison

The BWB has low pitch and yaw control authority due to its short moment arm. Hence,

multiple, rapidly moving control surfaces are required to provide sufficient control force

for longitudinal and lateral control. Large down force generated by negative elevon

deflection however creates substantial loss in lift causing the BWB to first plunge before

pitching to the desired angle of attack. This degrades flight path control especially during

take-off rotation and landing flare. Furthermore, excessive power is required to actuate

large multifunctional control surfaces with high hinge moments. This feature of the BWB

increases the challenge of improving lateral and longitudinal stability.

Fig.5.2. Comparison of moment arms and pitch control effectiveness with gears on ground and in-flight

between a conventional aircraft and a BWB

5.3 Aero Structure Comparison

The BWB provides efficient payload distribution and permits over the wing engine

placement. Additionally, the BWB center body generates lift due to its low aspect ratio

there by reducing the wing load. These features minimize wing bending moment and shear

force, thus creating favorable inertia relief and hence reduced structural weight.

Additionally, by blending the fuselage and outer wings, lower wetted surface area is

obtained. This translates to a higher wetted aspect ratio and hence a structurally more

efficient wing.


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Fig. 5.3. Comparison of the aerodynamic and inertia load distribution between a conventional aircraft

configuration and the BWB


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CHAPTER 6

CONCLUSIONS

Combining wing and body in the blended-wing-body configuration is an innovative idea

which benefits from its inherent aerodynamic potential. However, it needs creative and

careful re-vision at its stage of evolution as a proper candidate for the future generation of

commercial transport. Aerodynamic performance of a blended-wing-body airframe is

studied with aim of improvement in early stage of the conceptual design. The key design

parameters are identified for two sequential airframes using computational fluid dynamics.

The first airframe is designed as a simple research model and its aerodynamic performance

for a mission profile is investigated. The aerodynamic performance investigation includes

variation of lift, drag, and pitching moment coefficients. Also lift-to-drag ratio, pressure

distribution and usable space are also studied in this airframe. Through this investigation,

advantages and disadvantages of the geometric parameters are identified in the first

airframe. Then, based on assessing the first airframe, the second airframe is designed and

modified. In this airframe, interior arrangement and control sur-faces are also investigated.

In summary, baseline geometries and planform of the first and the second airframes are

compared, and the second airframe is proposed as a high-capacity long-range blended-

wing-body commercial transport. As the future works, aerodynamic shape optimization of

the span wise airfoils and multi objective planform optimization improve performance of

the air-frame further, which are currently under investigation.


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REFERENCES

[1] Payam Dehpanah, Amir Nejat, ‘THE AERODYNAMIC DESIGN EVALUATION

OF A BLENDED-WING-BODY CONFIGURATION’, Aerospace Science and

Technology 43 (2015) 96–110

[2] P. Okonkwo, H. Smith, ‘REVIEW OF EVOLVING TRENDS IN BLENDED WING

BODY AIRCRAFT DESIGN’, Progress in Aerospace Sciences (2016),

http://dx.doi.org/10.1016/j.paerosci.2015.12.002

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