As outlined in Chapter 2, in order to implement the “Systems Engineering” discipline (based on Ref. 1), the aircraft (i.e. system) design process includes four major phases: 1. Conceptual Design, 2. Preliminary Design, 3. Detail design, and 4. Test and evaluation. The purpose of this chapter is to present the techniques and selection processes in the aircraft conceptual design phase. Conceptual design is the first and most important phase of the aircraft system design and development process. It is an early and high level life cycle activity with potential to establish, commit, and otherwise predetermine the function, form, cost, and development schedule of the desired aircraft system. The identification of a problem and associated definition of need provides a valid and appropriate starting point for design at the conceptual level. Selection of a path forward for the design and development of a preferred system configuration, which will ultimately be responsive to the identified customer requirement, is a major responsibility of conceptual design. Establishing this early foundation, as well as requiring the initial planning and evaluation of a spectrum of technologies, is a critical first step in the implementation of the systems engineering process. Systems engineering, from an organizational perspective, should take the lead in the definition of system requirements from the beginning and address them from a total integrated lifecycle perspective.0
Introduction
The aircraft design process, as detailed in Chapter 2, consists of four key phases: Conceptual Design, Preliminary Design, Detail Design, and Test and Evaluation This chapter focuses on the techniques and selection processes crucial to the Conceptual Design phase, which is the initial and most critical stage in the aircraft system design and development lifecycle During this phase, significant decisions regarding the function, form, cost, and development timeline of the aircraft are established, making it essential for identifying the problem and defining the need that guides the design process.
The selection of a path for designing and developing a preferred system configuration is a key responsibility of conceptual design, ensuring responsiveness to customer requirements This foundational step involves initial planning and evaluating various technologies, which is crucial for implementing the systems engineering process From an organizational standpoint, systems engineering should lead in defining system requirements from the outset, addressing them with a comprehensive integrated life-cycle approach.
The aircraft design process generally commences with the identification of a “what” or
Desire for a solution stems from a real or perceived deficiency, leading to the definition of system requirements, prioritization for implementation, and resource estimation for acquiring the new system A thorough problem statement should be articulated in both qualitative and quantitative terms, providing sufficient detail to justify moving forward For further insights on need identification and formulation, refer to Chapter 2.
The aircraft conceptual design phase focuses on creating an initial design configuration based on general requirements This stage involves a systematic process to develop a satisfactory aircraft design, with the primary tool being the selection of key design elements.
Aircraft Design Requirements (Mission, Performance, Stability, Control, Cost, Operational, Time, Manufacturing)
Aircraft approximate 3-view (without dimensions) Identify major components that the aircraft requires to satisfy the design requirements
Conceptual Design 3 involves various evaluations and analyses, but it requires minimal calculations The success of this phase heavily relies on the past design experience, making it essential for the team to consist of the most experienced engineers within the corporation Detailed discussions on the advantages and disadvantages of each configuration are provided in chapters 5 through 11.
The conceptual design phase, as depicted in Figure 3.1, encompasses key activities that culminate in an approximate three-view representation of the aircraft's configuration Section 3.2 focuses on the primary functions and roles of each aircraft component, including the wing, fuselage, tail, landing gear, and engine Configuration alternatives for these components are explored in Section 3.3, while Section 3.4 reviews aircraft classifications from various perspectives The principles of trade-off analysis to identify the most satisfactory configuration are introduced in Section 3.5 Finally, Section 3.6 delves into conceptual design optimization, highlighting the application of multidisciplinary design optimization techniques.
Primary Functions of Aircraft Components
An aircraft consists of several key components, including the wings, horizontal tail, vertical tail, fuselage, propulsion system, landing gear, and control surfaces To effectively design each component's configuration, the designer must understand the specific functions of these parts Additionally, it's crucial to recognize the inter-relationships among components, as they can influence and impact each other's performance.
1 Wing: The main function of the wing is to generate the aerodynamic force of lift to keep the aircraft airborne The wing tends to generate two other unwanted aerodynamic productions: an aerodynamic drag force plus an aerodynamic pitching moment Furthermore, the wing is an essential component is providing the aircraft lateral stability which is fundamentally significant to the flight safety In almost all aircraft, the aileron is arranged to be at the trailing edge of the outboard section Hence, the wing is largely influential in providing the aircraft lateral control
2 Fuselage: The primary function of the fuselage is to accommodate the payload which includes passengers, cargo, luggage, and other useful loads The fuselage is often a home for pilot and crewmembers, and most of the times, fuel tanks and engine(s) Since the fuselage is providing a moment arm to horizontal and vertical tail, it plays an influential role in longitudinal and directional stability and control If the fuselage is decided to be short, a boom must be provided to allow for the tails to have the sufficient arm
3 Horizontal tail: The horizontal tail primary function is to generate an aerodynamic force to longitudinally trim the aircraft Furthermore, the vertical tail is an essential component is providing the aircraft longitudinal stability which is a fundamental requirement for flight
Conceptual Design 4 safety In majority of the aircraft, the elevator is a movable part of the horizontal tail, so longitudinal control and maneuverability is applied through horizontal tail
4 Vertical tail: The vertical tail primary function is to generate an aerodynamic force to directionally trim the aircraft Furthermore, the vertical tail is an essential component is providing the aircraft directional stability which is a fundamental requirement for flight safety In majority of the aircraft, the rudder is a movable part of the vertical tail, so directional control and maneuverability is applied through vertical tail
5 Engine: The engine is the main component in the aircraft propulsion system to generate the power and/or thrust The aircraft requires a thrust force to move forward (as in any other vehicle), so the engine primary function is to generate the thrust The fuel is assumed to be a necessary section of the propulsion system and it sometimes constitutes a large part of aircraft weight An aircraft without engine is not able to take-off independently, but it is capable of gliding and landing, as are performed by sailplanes and gliders Sailplanes and gliders are taking off with the help of other aircraft or outside devices (such as winch), and are gliding with the help of wind and thermal currents
6 Landing gear: The primary function of the landing gear is to facilitate the take-off and landing During the take-off and landing operations, the fuselage, wing, tail, and aircraft components are kept away from the ground through the landing gear The wheels of the landing gear in land-based and ship-based aircraft are also playing a crucial role in the safe acceleration and deceleration Rolling wheels as part of landing gear allows the aircraft to accelerate without spending a considerable amount of thrust to overcome the friction
No Component Primary function Major areas of influence
Aircraft performance, longitudinal stability, lateral stability, cost
2 Wing Generation of lift Aircraft performance, lateral stability
3 Horizontal tail Longitudinal stability Longitudinal trim and control
4 Vertical tail Directional stability Directional trim and control, stealth,
5 Engine Generation of thrust Aircraft performance, stealth, cost, control
6 Landing gear Facilitate take-off and landing
Table 3.1 Aircraft major components and their functions
The six primary components identified are considered essential to an air vehicle, yet there are additional parts within an aircraft that are not classified as major components The functions of these supplementary components will be detailed in the subsequent sections.
Table 3.1 provides a comprehensive overview of the major components of an aircraft, detailing their primary functions, secondary roles, and the key design requirements influenced by each component While the table focuses on the main functions, it does not cover the secondary functions, which are elaborated in Chapters 5 through 12.
Traditional aircraft design focuses on enhancing performance and lowering operating costs by reducing maximum takeoff weight However, for aircraft manufacturers, this strategy does not ensure the financial success of an aircraft program A more effective design approach should consider not only performance and manufacturing costs but also factors like flying qualities and systems engineering criteria.
Minimizing gross take-off weight (GTOW) has historically been a key objective in aircraft design to enhance performance and reduce operating costs, particularly through lower fuel consumption However, this approach does not ensure that an aircraft design meets the optimal needs of consumers In a competitive aircraft market, manufacturers should focus on improving systems engineering and technical merit before committing to significant investments in aircraft development.
Aircraft Configuration Alternatives
Once the necessary aircraft components are identified and a list of major components is prepared, the selection of their configurations begins Each major component can have multiple alternatives that meet design requirements, but each alternative has its own set of advantages and disadvantages, affecting the level of satisfaction of those requirements Given that each design requirement carries a unique weight, different configuration alternatives will yield varying levels of satisfaction This section will explore the configuration alternatives for each major component, with detailed descriptions of their advantages and disadvantages covered in Chapters 5 through 12.
In general, wing configuration alternatives from eight different aspects are as follows:
7.2 Strut-braced (a faired, b un-faired)
Chapter 5 outlines the pros and cons of various wing configuration alternatives, along with methods for selecting the optimal design to fulfill specific requirements The key effects of these wing configurations significantly influence overall performance and design outcomes.
1 high wing 2 mid-wing 3 low wing
1.Rectangular 2 Tapered 3 Swept back 4 Delta
Conceptual Design 7 on cost, the duration of production, ease of manufacturing, lateral stability, performance, maneuverability, and aircraft life Figure 3.2 illustrates several wing configuration alternatives
In general, tail configuration alternatives from three different aspects are as follows:
Chapter 6 outlines the pros and cons of various tail configuration alternatives, highlighting their influence on key factors such as cost, production time, manufacturing ease, and both longitudinal and directional stability and maneuverability Additionally, these configurations significantly affect the overall lifespan of the aircraft For visual reference, Figure 3.3 presents several tail configuration options.
In general, propulsion system configuration alternatives from four different aspects are as follows:
2 Engine and the aircraft cg
1 Aft tail 2 Canard 3 Three surfaces
4.1 In front of nose (inside)
4.7 Side of fuselage at aft section
Chapter 9 discusses the pros and cons of various propulsion system configurations, along with a method for selecting the optimal engine configuration to fulfill design requirements Key factors influenced by these engine alternatives include flight operation costs, aircraft production expenses, performance metrics, production timelines, manufacturing simplicity, maneuverability, and overall aircraft lifespan Several engine configuration options are depicted in Figure 3.4.
In general, landing gear configuration alternatives from three different aspects are as follows:
2.2 Tail gear (tail dragger or skid)
1 Tractor (single engine) 2 Pusher (twin engine) Prop-driven jet
1 Tri-engine 2 Four engine (under wing) Figure 3.4 Engine configuration alternatives
3 Multi-gear 4 Bicycle Figure 3.5 Landing gear configuration alternatives
The design of landing gear is significantly influenced by the type of runway, which can be categorized into five main types Various landing gear configuration alternatives are illustrated in Figure 3.5.
3.5 Shoulder-based (for small remote controlled aircraft)
Chapter 4 introduces various types of runways, highlighting their influence on engine, wing, and fuselage design Chapter 8 discusses the advantages and disadvantages of different landing gear configurations, providing techniques for selecting the best option based on design requirements The choice of landing gear configuration significantly impacts flight operation costs, aircraft production expenses, overall performance, production duration, manufacturing ease, and the aircraft's lifespan.
In general, fuselage configuration alternatives from three different aspects are as follows:
Chapter 7 discusses the advantages and disadvantages of various fuselage configuration alternatives, emphasizing their significant impact on factors such as flight operation costs, aircraft production expenses, performance, production duration, manufacturing ease, passenger comfort, and overall aircraft lifespan Additionally, Figure 3.6 provides a visual representation of these fuselage configuration alternatives to aid in selecting the most suitable option to meet specific design requirements.
1 Side-by-side 2 Tandem Figure 3.6 Fuselage configuration alternatives
In general, manufacturing configuration alternatives from four different aspects are as follows:
This book does not cover the specifics of engineering materials and manufacturing processes For detailed information on these topics, readers should refer to relevant sources, including References 17 and 18 The main effects of alternative materials and techniques are related to production costs, time efficiency, manufacturing simplicity, and the lifespan of aircraft.
In general, subsystems configuration alternatives from five different aspects are as follows:
2.1 High lift device (e.g flap, slat, slot)
4.2 Inside wing (both sides) ( a between two spars, b in front of main spar)
Chapters 5 to 12 detail the pros and cons of various configuration alternatives, along with a method for choosing the optimal subsystem configuration to satisfy design requirements A summary of these configuration alternatives for key aircraft components is provided in Table 3.2.
1 Fuselage - Geometry: lofting, cross section
- What to accommodate (e.g fuel, engine, and landing gear)?
2 Wing - Type: Swept, tapered, dihedral;
- Location: Low-wing, mid-wing, high wing, parasol
3 Horizontal tail - Type: conventional, T-tail, H-tail, V-tail, inverted V
- Location: aft tail, canard, three surfaces
4 Vertical tail Single, twin, three VT, V-tail
5 Engine - Type: turbofan, turbojet, turboprop, piston-prop, rocket
- Location: (e.g under fuselage, under wing, beside fuselage)
6 Landing gear - Type: fixed, retractable, partially retractable
Separate vs all moving tail, reversible vs irreversible, conventional vs non-conventional (e.g elevon, ruddervator)
Table 3.2 Aircraft major components with design alternatives
3 Propulsion 1 Turbojet, 2 Turbofan, 3 Turboprop, 4 Piston prop, 5 Rocket
4 Number of Engine 1 Single engine, 2 Twin engine, 3 Tri-engine, 4 Four-engine, 5
5 Engine and aircraft cg 1 Pusher, 2 Tractor
6 Engine installation 1 Fixed, 2 Tilt-rotor
7 Engine location 1 Under wing, 2 Inside wing, 3 Above wing, 4 Above fuselage, 5
8 Number of wings 1 One-wing, 2 Biplane, 3 Tri-plane
9 Wing type 1 Fixed-wing, 2 Rotary-wing (a helicopter, b gyrocopter)
10 Wing geometry 1 Rectangular, 2.Tapered, 3 Swept, 4 Delta
11 Wing sweep 1 Fixed sweep angle, 2 Variable sweep
12 Wing setting angle 1 Fixed setting angle, 2 Variable setting angle
13 Wing placement 1 High wing, 2 Low wing, 3 Mid-wing, 4 Parasol wing
14 Wing installation 1 Cantilever, 2 Strut-braced
15 Tail or canard 1 Tail, 2 Canard, 3 Three-surfaces
16 Tail type 1 Conventional, 2 T shape, 3 H shape, 4 V shape, 5 + shape,…
17 Vertical tail 1 No vertical tail (VT), 2 One VT at fuselage end, 3 Two VT at the fuselage end, 4 Two VT at the wing tips
18 Landing gear 1 Fixed and faired, 2 Fixed and un-faired 3 Retractable, 3
19 Landing gear type 1 Nose gear, 2 Tail gear, 3 Quadricycle, 4 Multi-bogey, …
20 Fuselage 1 Single short fuselage, 2 Single long fuselage, 3 Double long fuselage, … 21a Seating (in two-seat) 1 Side-by-side, 2 Tandem
21b Seating (in higher number of passengers)
22 Luggage pallet Based on types of luggage and payload, it has multiple options
23 Cabin or Cockpit 1 Cabin, 2 Cockpit
1 Tail and elevator, 2 All moving horizontal tail
1 Vertical tail and rudder, 2 All moving vertical tail
26 Wing control surfaces 1 Aileron and flap, 2 Flapron
27 Wing-tail control surfaces 1 Conventional (elevator, aileron, rudder), 2 Ruddervator, 3
Elevon, 4 Split rudder, 5 Thrust-vectored
28 Power system 1 Mechanical, 2 Hydraulic, 3 Pneumatic, 4 FBW 1 , 5 FBO 2
29 Material for structure 1 Full metal, 2 Full composite, 3 Primary structure: metal, secondary structure: composite
1 Trailing edge Flap, 2 Leading edge slot, 3 Leading edge slat
Table 3.3 Configuration parameters and their options (set by designer)
1 Fly-By-Wire (Electric signal)
2 Fly-By-Optic (light signal)
Table 3.3 outlines various configuration parameters and their design alternatives, which are finalized by the designer The optimization process identifies the best configuration through a methodology introduced in this paper, allowing designers to select parameters that meet design requirements optimally With 30 groups of configurations available for aircraft designers, the range of design options is notably extensive The Multidisciplinary Design Optimization process, discussed in Section 3.6, is a proven method for optimizing configurations for multidisciplinary purposes.
Aircraft Classification and Design Constraints
Clarifying the aircraft type with a detailed description of specifications is a crucial step for designers, as it streamlines the design process and prevents confusion in later stages The classification of the aircraft type is largely determined by its intended mission and necessary specifications This section explores various aspects of aircraft classifications and types.
When a customer orders an aircraft, it comes with a set of mandatory requirements and constraints that must be adhered to during the design process These requirements can only be modified if the designer can convincingly demonstrate their infeasibility to the customer Additionally, airworthiness standards, such as FAR, JAR, and MIL-STD, impose further requirements that must be integrated into the design Aircraft configurations can be categorized in various ways, depending on different criteria, which may also influence the classification of these requirements.
In configuration design, applying constraints and selecting the appropriate classification and type is crucial Design constraints and requirements, as shown in Table 3.4, are established by the customer and encompass various factors, including aircraft mission, payload type, control type, and performance requirements Designers typically have limited influence over these requirements unless they can demonstrate their impracticality; otherwise, adherence to these constraints is mandatory throughout the design process Visual representations in Figures 3.7, 3.8, and 3.9 illustrate various aircraft types, including civil transport, General Aviation, military fighters, lighter-than-air, heavier-than-air, manned, unmanned, and remote-controlled aircraft.
No Group Design requirements and constraints
1 Standard, 2 Homebuilt (or garage-built)
2 General type 1 Military (MIL-STD), 2 Civil - Transport (FAR 4 25), 3 Civil -
General Aviation or GA (FAR 23), 4 Very Light Aircraft (VLA), …
3 Maneuverability 1 Normal or non-aerobatic, 2 Utility or semi-aerobatic, 3 Aerobatic or acrobatic, 4 Highly maneuverable (e.g Fighters and anti-missile missiles)
4 GA mission 1 General purpose, 2 Hang glider, 3 Sailplane or glider, 4
Agricultural, 5 Utility, 6 Commuter, 7 Business, 8 Racer, 9 Sport,
5 Military mission 1 Fighter, 2 Bomber, 3 Attack, 4 Interceptor, 5 Reconnaissance, 6
Military transport, 7 Patrol, 8 Maritime surveillance, 9 Military trainer, 10 Stealth, 11 Tanker, 12 Close support, 13 Trainer, 14 Anti-submarine, 15 Early warning, 16 Airborne command, 17 Communication relay, 18 Target, 19 Missile, 20 Rocket
6 Density 1 Lighter-than-air craft (a balloon, b airship), 2 Heavier-than-air craft
7 Pilot control 1 Manned aircraft, 2 Unmanned aircraft, 3 Remote Control (RC)
8 Weight 1 Ultra light aircraft (less than 300 kg), 2 Very light (less than 750 kg), 3 Light (less than 12,500 lb), 4 Medium weight (less than 100,000 lb), 5 Heavy or Jumbo (above 100,000 lb)
9 Producibility 1 Kit form, 2 Semi-kit form, 3 Modular (conventional)
10 Take-off run 1 Short Take Off and landing (STOL) (runway less than 150 m), 2
Vertical Take Off and landing (VTOL), 3 Regular
11 Landing field 1 Land-based, 2 Sea-based, 3 Ship-based, 4 Amphibian, 5
13 Term of use 1 Long term (Regular), 2 Experimental (X aircraft) or Research
14 Payload 1 Number of passengers, 2 Payload weight, 3 Store, …
1 Air condition, 2 Weather radar, 3 Parachute, …
1 Number of crew, 2 Ejection seat, 3 Reserve fuel, …
17 Performance 1 Max speed, 2 Range, 3 Ceiling, 4 Rate of Climb, 5 TO run, 6
18 Maneuverability 1 Turn radius, 2 Turn rate, 3 Load factor
Table 3.4 Design constraints and requirements (set by customer)
Figure 3.7 General aviation, civil-transport, and military aircraft
1 Air ship Zeppelin NT (Lighter-than-air craft) 2 Learjet 60 (heavier-than-air craft)
Figure 3.8 Lighter-than-air craft versus heavier-than-air craft
1 PA-28-236 Dakota 2 Global Hawk 3.R adio controlled model aircraft Figure 3.9 Manned aircraft, unmanned aircraft, and remote controlled aircraft
Government regulations significantly influence aircraft design, presenting designers with two choices: to either create an aircraft that adheres to these regulations and standards or to disregard them entirely While designers have the freedom to choose their approach, they must understand the implications of their decision, as it will affect the entire design process.
Conceptual design is influenced by various factors, including government regulations and standards, which can raise costs and complicate the design process Despite these challenges, adhering to these regulations enhances the overall quality of the aircraft, ultimately enabling it to be marketed successfully in the US.
Homebuilt or garage-built aircraft are those not certified by government aviation authorities, typically designed by non-expert individuals for personal use Due to the lack of official airworthiness confirmation, these aircraft carry a higher risk of crashes compared to certified models Their flight permissions are restricted to specific airspaces to minimize the potential for civilian casualties Additionally, homebuilt aircraft are prohibited from being sold in the US market.
Several countries have created official regulatory bodies to oversee aviation issues and establish standards In the United States, the Federal Aviation Administration (FAA) is responsible for regulating aircraft design and manufacturing Meanwhile, certain European nations, including England, France, and Germany, have developed the Joint Aviation Requirements (JARs) to streamline Type Certification processes for joint ventures and facilitate the international trade of aviation products The JARs are acknowledged by the Civil Aviation Authorities of participating countries as a valid basis for demonstrating compliance with national airworthiness regulations.
The Civil Aviation Authorities of several European nations have established Joint Aviation Requirements (JARs) to streamline Type Certification processes for joint ventures, enhance the export and import of aviation products, and simplify maintenance acceptance across borders These comprehensive requirements aim to regulate commercial air transport operations effectively and are recognized as acceptable by the participating countries' Civil Aviation Authorities.
In the US, FAA (Ref 19) of Department of Transportation regulates the aviation standards and publishes Federal Aviation Regulations (FAR) Some important parts of the FAR are:
Part 23: Airworthiness Standards for GA aircraft
Part 25: Airworthiness Standards for Civil Transport aircraft
Part 29: Airworthiness Standards for Helicopters
Part 33: Airworthiness Standards for Aircraft engines
The following countries are included: Austria, Belgium, Cyprus, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Luxembourg, Malta, Monaco, Netherlands, Norway, Poland, Portugal, Romania, Slovak Republic, Slovenia, Spain, Sweden, Switzerland, Turkey, and the United Kingdom.
6 Reference: http://www.jaa.nl/publications/section1.html
Part 103: Airworthiness Standards for Ultralight aircraft
Military aircraft must adhere to military standards, known as MIL-STD or MIL-SPEC, established by the U.S Department of Defense to ensure standardization These standards encompass various documents, including defense specifications, handbooks, and standards, each serving distinct purposes Military specifications outline the physical and operational characteristics of a product, while military standards specify the processes and materials for production Additionally, military handbooks provide compiled information and guidance, contributing to the overall compliance and effectiveness of military operations.
MIL-STD is a crucial document that sets uniform engineering and technical requirements for military-specific or significantly modified commercial processes and practices It encompasses five categories of defense standards: interface standards, design criteria standards, manufacturing process standards, standard practices, and test method standards, with over 33,000 standards currently in existence These defense standards are recognized for their reliability and are frequently adopted by various government agencies, non-government technical organizations, and the broader industry.
MIL-PRF is a performance specification outlining the necessary results and compliance verification criteria without detailing the methods to achieve them It defines the item's functional requirements, operating environment, and interchangeability characteristics In contrast, MIL-KHBK serves as a guidance document, providing standard procedural, technical, engineering, and design information related to materiel and practices within the Defense Standardization Program Additionally, MIL-STD-962 specifies the content and format requirements for defense handbooks.
When choosing design options for aircraft, certain selections can render other alternatives unviable For example, opting for a single engine configuration prohibits the placement of the engine in the side fuselage, as this would create an asymmetric aircraft Similarly, if a designer decides against incorporating a vertical tail for stealth purposes, the use of a ruddervator as a control surface becomes impossible.
Table 3.5 illustrates the connection between major aircraft components and their design requirements, highlighting the component most significantly influenced by each requirement While each design requirement typically impacts multiple components, our focus is on the primary one affected For instance, the requirements for payload, range, and endurance primarily influence the maximum take-off weight, which in turn affects engine selection, fuselage design, and overall flight performance.
The cost of conceptual design is influenced by both payload weight and volume, which have distinct impacts on optimization To effectively manage design constraints, it is crucial for designers to accurately determine the payload's weight and volume Additionally, if the payload can be segmented into smaller units, it simplifies the design process Other performance parameters, such as maximum speed, stall speed, rate of climb, take-off run, and ceiling, also play a significant role in determining wing area and engine power or thrust.
No Design requirements Aircraft component that affected most, or major design parameter
1 Payload (weight) requirements Maximum take-off weight
3 Performance requirements (maximum speed, Rate of climb, take-off run, stall speed, ceiling, and turn performance)
Engine; Landing gear; and Wing
4 Stability requirements Horizontal tail and vertical tail
5 Controllability requirements Control surfaces (elevator, aileron, rudder)
6 Flying quality requirements Center of gravity
8 Cost requirements Materials; Engine; weight, …
Table 3.5 Relationship between aircraft major components and design requirements
Configuration Selection Process and Trade-Off Analysis
Selecting the optimal aircraft configuration requires a comprehensive trade-off analysis, as various trade-offs emerge throughout the design process Key decisions involve evaluating and choosing suitable components, subsystems, automation levels, and commercial off-the-shelf parts, along with maintenance and support policies As the design progresses, considerations may include alternative engineering materials, manufacturing processes, maintenance plans, logistics support structures, and strategies for material phase-out, recycling, or disposal.
To effectively tackle a design problem, it is essential to first define the issue and establish the design criteria for evaluating alternative configurations The evaluation process involves gathering necessary input data and assessing each candidate option Conducting a sensitivity analysis helps identify potential risks, leading to the recommendation of a preferred approach This adaptable process, illustrated in figure 3.10, can be customized at any stage of the project life cycle, with the depth of analysis varying based on the component's complexity.
Figure 3.10 Trade-off analysis process
Select and weight evaluation parameters (Mission, Performance, Stability, Control, Cost, Operational, Time, Manufacturing)
Identify data needs (existing data, new data, estimating relationships)
Select and/or develop a model
Generate data and run model
Identify areas of risk and uncertainty
Trade-off analysis in aircraft design encompasses synthesis, which involves combining and structuring components to form an aircraft system configuration Initially, synthesis aids in developing preliminary concepts and establishing relationships among various aircraft components As the design process progresses and functional definitions become clearer, synthesis is employed to refine the "hows" at a more detailed level This process ultimately leads to the creation of a configuration that may represent the aircraft's final form, although a definitive configuration should not be assumed at this early stage of design.
To evaluate a synthesized configuration, it is essential to assess its characteristics against the initially specified aircraft requirements Necessary changes will be made to arrive at a preferred design configuration This iterative cycle of synthesis, analysis, evaluation, and design refinement culminates in the establishment of both functional and product baselines.
In aircraft configuration design, a crucial initial step is identifying system design considerations, which begins with defining customer requirements at the system level Establishing the functions that the system must perform helps outline these requirements Design criteria, which are essential "design-to" requirements, can be articulated in qualitative and quantitative terms These criteria, often specified or negotiated by customers, serve as target values for technical performance measures, setting the boundaries within which designers must operate.
In the iterative process of synthesis, analysis, and evaluation, it is essential to define both operational functions necessary for executing specific mission scenarios and maintenance functions that ensure aircraft readiness These top-level descriptions are crucial for effective operations and support.
Once a baseline configuration is set following a formal design review, changes are often made for various reasons, such as correcting design flaws, enhancing product features, integrating new technologies, adapting to updated operational needs, or addressing outdated components These modifications can arise from internal project developments or from new external requirements.
Initially, changes may seem minor, such as adjustments to equipment design, software, or processes; however, these seemingly insignificant modifications can significantly impact the entire system's hierarchical structure For example, altering the design configuration of a key component—like its size, weight, repackaging, or performance capabilities—can have far-reaching effects on related components, testing and support equipment, spare parts inventory, technical documentation, and transportation requirements.
Changes to any single aircraft component, such as the horizontal tail, can significantly affect other parts like the wing and fuselage When multiple changes are made simultaneously, it risks compromising the entire system's configuration and requirements traceability Historical data shows that many changes often occur late in the detailed design phase, during production, or early in the system's utilization and support phase Although changes are unavoidable, it is crucial to formalize and control the process to maintain traceability between different configuration baselines.
Multidisciplinary Design Optimization (MDO) is a powerful technique in trade-off studies that integrates multiple engineering disciplines to solve complex design problems This approach, which is continually being refined by researchers across academia, industry, and government, allows for the simultaneous consideration of all relevant disciplines, leading to superior solutions compared to sequential optimization By exploiting the interactions between disciplines, MDO addresses the intricate complexities of modern engineering challenges, although this simultaneous inclusion can significantly increase problem complexity.
No Figure of Merit Military designer
Table 3.6 Design objectives and an example of the priorities for various aircraft designer
Aircraft designers prioritize different objectives based on their specific mission requirements, resulting in four primary groups: military aircraft designers, civil transport aircraft designers, general aviation (GA) aircraft designers, and homebuilt aircraft designers Each group has distinct interests and design criteria that guide their processes Additionally, ten key figures of merit, including production cost, play a crucial role in shaping the design of every aircraft configuration.
Conceptual Design 22 performance, flying qualities, design period, beauty (for civil aircraft) or scariness (for military aircraft), maintainability, producibility, aircraft weight, disposability, and stealth requirement
Table 3.6 demonstrates priorities of each aircraft designer against ten figures of merit This priority allocation is the author’s idea and may be different at some cases References 4 and
Discover five invaluable references that delve into authentic aircraft design stories and the insights gained over six decades These resources highlight various challenges and opportunities encountered throughout multiple designs, making them essential for prioritizing the configuration design process.
In the evaluation of design priorities, grade “1” signifies the highest importance, while grade “10” indicates the lowest, with grade “0” denoting that a criterion is not applicable to the designer As shown in Table 3.6, aircraft performance is the top priority for military aircraft designers, whereas cost is paramount for homebuilt aircraft designers Notably, stealth capability ranks highly for military designers but is deemed unimportant by the other three designer groups These priorities, referred to as weights, highlight the varying significance of each figure of merit from the designer's perspective.
No Figure of Merit Priority Designer # 1
Table 3.7 Three scenarios of weights (in percent) for a military aircraft designer
In design evaluation, establishing a baseline is crucial for assessing various design alternatives This baseline is developed through an iterative requirements analysis process, which includes identifying needs, analyzing feasibility, defining operational requirements, selecting a maintenance concept, and planning for phase-out and disposal The aircraft's mission must be clearly outlined to meet specific customer needs, considering factors such as cycle time, frequency, speed, cost, and effectiveness Additionally, functional requirements should be addressed by integrating appropriate design characteristics into the aircraft.
Conceptual Design 23 configuration components As an example, Table 3.7 illustrates three scenarios of priorities (in percent) for military aircraft designers
1 Cost Minimum direct operating cost
Minimum total manufacturing cost Minimum system cost over X years (life-cycle cost) Maximum profit
Maximum return on investment Maximum payload per $
Maximizing range Maximizing endurance Maximizing absolute ceiling Minimizing take-off run Maximizing rate of climb Maximizing maneuverability
3 Weight Minimum take-off weight
Minimum empty weight Maximum fuel weight
Most controllable Most stable Highest flying qualities Most luxurious for passengers
Smallest fuselage length Smallest aircraft height Most specious fuselage
6 Beauty or scariness Most attractive (civil) or most scariest (fighter)
Most maintainable Most Producible Most disposable (environmental compatibility) Most flight testable
Most stealth Most flexible (growth potential) Most reliable
Minimum duration of design Minimum duration of manufacture Maximum aircraft operating life
Table 3.8 Optimization criteria at group level
Design criteria should be defined for each level within the system's hierarchical structure Table 3.8 illustrates potential optimization objectives for these levels, which are essential for determining the optimal design of a chosen aircraft configuration.
Conceptual Design Optimization
Optimization in mathematics involves finding the maximum or minimum value of a real function by selecting values for real or integer variables within a defined set An optimization problem focuses on determining the optimal value of an objective function while adhering to specified constraints To tackle such a problem, one must first articulate the objective function and constraints, then manipulate and graphically represent the inequalities involved Finally, the optimization problem is solved using appropriate mathematical techniques, often involving linear programming in two real variables.
Optimization involves key elements such as design variables, objective functions, constraints, and design space Even in the absence of uncertainty, optimizing complex problems with numerous design variables and varying types can be challenging, especially when the performance function's structure is not well understood Estimates may hinder the ability to determine the superiority of one design over another, complicating the optimization process Comparing two aircraft configurations is computationally simpler than evaluating multiple designs simultaneously Dynamic optimization aims to minimize or maximize a cost function while adhering to various constraints, including dynamic equations, control inequalities, state equalities, and specified initial and final conditions.
In general a constrained single objective optimization problem (Ref 12) is to
The objective function, denoted as f: R^n → R, is a real-valued function that we aim to optimize, either by maximizing or minimizing its value The vector x, represented as x = [x1, x2, …, xn]ᵀ ∈ R^n, consists of independent variables known as decision variables Additionally, the set Ω, a subset of R^n, is referred to as the constraint set or feasible set, which defines the limitations within which the optimization occurs.
Conceptual design can be framed as a decision-making challenge aimed at identifying the optimal vector x from all possible decision variable vectors within the constraint set Ω The optimal vector, known as the optimizer or extremizer, yields the best value—either the minimum or maximum—of the objective function Typically, the constraint set Ω is defined in a specific manner to facilitate this optimization process.
x:h x 0, g x 0 , where h and g are given functions
Definition: suppose that f :R n R is a real-valued function defined on some set :R n A point x is a local optimizer of f over if there exists 0such that f x f x for all
x x \ and xx A point x is a global minimize of f over if f x f x for all x \ x Strictly speaking, an optimization problem is solved only when a global minimizer (in general, extremizer) is found
Theorem 1 First order necessary condition: let be a subset of R n and f C 1 a real-valued function on If x is a local minimizer of f over , then for any feasible direction d atx , we have
Single-objective optimization involves optimizing one objective function, but many engineering challenges, such as aircraft configuration design, require addressing multiple conflicting objectives These objectives can compete with one another, meaning that improving one may negatively impact another Consequently, multi-objective optimization problems, also known as multicriteria or vector optimization problems, often lack a single optimal solution In such cases, the goal is to identify a decision variable that meets specified constraints while optimizing a vector function composed of the various objective functions.
The formulation of a multi-objective optimization problem is as follows:
7 Proof is given in Ref [18]
Multi-objective optimization problems can generally be categorized into three types: minimizing all objective functions, maximizing all objective functions, or minimizing some while maximizing others Importantly, each of these types can be transformed into an equivalent minimization problem, allowing for a unified analytical approach to solving them.
In certain scenarios, multi-objective optimization problems can be simplified into single-objective optimization problems, allowing for the application of standard optimization techniques One effective approach involves creating a single objective function by using a linear combination of the components of the objective function vector, with positive coefficients.
( 1 ) Equivalently, we form a convex combination of the components of the objective function In other word, we use
The weighted-sum method utilizes the objective function F = T, where c represents a vector of positive components, known as weights These weights indicate the relative importance of each component in the objective vector, allowing for a tailored performance measure based on specific priorities In scenarios where objective criteria are not clearly defined, the weighting process can become subjective, necessitating a careful evaluation of the optimization results to ensure their overall acceptability.
In configuration design, various physical and economic limitations can hinder system optimization, necessitating that decision-makers address these constraints The primary constraints include time, cost, geometry, weight, physical properties, performance, and safety, compelling the search for the best possible solution within these parameters.
Firefighting aircraft are designed to carry a fixed volume of water or a specific liquid, with both payload weight and total volume being constant; however, the total volume can be partitioned into multiple sections In contrast, transport aircraft must accommodate a specific piece of equipment that has a fixed weight and geometry, with the total volume being unchangeable and the payload not divisible into smaller components.
Optimization serves to align mutually exclusive alternatives into comparable states; however, when faced with multiple criteria, neither x optimization nor y optimization alone suffices It is essential to enhance these processes with information regarding how well each alternative satisfies specific criteria A useful method for consolidating and presenting this information is the decision evaluation display approach.
Optimization problems can be categorized based on several criteria, including the presence of constraints, the characteristics of design variables, and the physical structure of the problem Additionally, they can be classified according to the type of equations involved, the allowable values of design variables, the deterministic or stochastic nature of these variables, the separability of functions, and the type of objective functions being used.
Configuration design involves arranging a set of arbitrary objects within a defined space while adhering to spatial constraints and achieving performance goals Optimization practices typically focus on a solution domain defined by design variables, which indicate limited differences in an aircraft configuration, while design parameters reflect complex variations and inter-type differences In aircraft configuration design, parameters are usually fixed during optimization, which aims to find the optimal combination of design variable values to minimize or maximize objectives such as weight or speed The mathematical foundation for higher-level optimization and concept selection is rooted in differential calculus.
This research aims to develop a method for determining a configuration that achieves an optimal solution while adhering to design requirements and constraints, all while minimizing time and costs The focus is not solely on creating the best aerodynamic shape but rather on identifying the configuration that yields the highest design index Additionally, manufacturing technologies such as casting, welding, milling, sheet metal working, riveting, and composite material lay-up can significantly impact the design process As illustrated in Figure 3, the phases of configuration design optimization include a feedback loop, highlighting the iterative nature of the design process.
Figure 3.11 The Phases in the configuration Design optimization
The methodology focuses on estimating system characteristics to enable quantitative comparisons between two designs The configuration optimization model includes parameters and decision variables, where design parameters define the problem and decision variables represent the quantities determined to achieve the optimal configuration These decision variables, also known as design variables, are detailed in Table 3.9 The complexity of the solution increases with the number of variables, which varies based on aircraft classification as shown in Table 3.9.