ME4807 Aircraft Systems Engineering & Conceptual Design Lecture Notes 2024-25 PDF

LECTURE NOTES ACADEMIC YEAR 2024/25 MODULE ME 4807: AIRCRAFT SYSTEMS ENGINEERING AND CONCEPTUAL DESIGN PREPARED BY PROF TREVOR M. YOUNG, SCHOOL OF ENGINEERING, UNIVERSITY OF LIMERICK ...

LECTURE NOTES ACADEMIC YEAR 2024/25 MODULE ME 4807: AIRCRAFT SYSTEMS ENGINEERING AND CONCEPTUAL DESIGN PREPARED BY PROF TREVOR M. YOUNG, SCHOOL OF ENGINEERING, UNIVERSITY OF LIMERICK  Aircraft Design ME4807/Front matter Page 2 Rev. 4 __________________________________________________________________________________________ ACKNOWLEDGEMENT This collection of lecture notes has been printed for students at the University of Limerick and not for profit. Much of the contents presented here has been extracted from the vast volume of published works on the subject (including books, journal and conference papers, reports, regulations and standards). Several figures have been reproduced from these sources, and in such cases the origin of the material is acknowledged. Students should be aware that these notes have been prepared for their personal use and may not be reproduced for any other purpose. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Front matter Page 3 Rev. 4 __________________________________________________________________________________________ CONTENTS 1 Introduction to aircraft systems engineering and design  Original design, systems engineering  Design requirements, mission specifications, RFP (Request for Proposal)  Phases of an aircraft project  Airworthiness regulations, standards 2 General arrangement  Concept sketches  Wing position  Location of engines  Tailplane arrangement 3 Preliminary sizing of aircraft  Takeoff and empty weight estimation  Parametric analysis  Thrust-to-weight ratio (or power loading) versus wing loading  Matching of requirements; design point selection 4 Wing layout: Part 1  Aerofoil selection  Stall  Planform selection, aspect ratio, sweep 5 Wing layout: Part 2  Wing incidence, dihedral  Wing-tips  High lift devices  Control surface layout  Aerodynamic crutches 6 Fuselage layout  Crew station  Passenger compartment  Cargo  Fuselage shape 7 Tail Layout  Sizing of horizontal stabiliser (tail plane)  Sizing of vertical stabiliser (fin)  Stability, trim and control 8 Integration of propulsion system  Powerplant types  Jet engine integration  Inlets and nozzles  Piston-propeller integration Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Front matter Page 4 Rev. 4 __________________________________________________________________________________________ 9 Landing gear (undercarriage)  Landing gear configurations  Layout requirements  Gear retraction  Tyres and shock absorbers 10 Aircraft Weights  Group weight statement  Weights estimation  CG position, load and balance diagram 11 Structures, vulnerability, producibility, maintainability and reliability  Structural layout  Vulnerability and crashworthiness  Producibility  Maintainability  Reliability 12 Environmental considerations  Noise sources and measurement  Regulations and compliance  Emissions and environmental damage  Pathways to mitigate environmental impact 13 Fuel system integration  Conceptual design and fuel systems  Fuel system layout  Reducing trim drag 14 Aircraft shaping / lofting  Fuselage lofting / shape definition  Wing, tail layout and loft  Area rule 15 Cost factors  Life cycle cost  Cost estimation  Design factors influencing cost Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 1 Rev. 4 __________________________________________________________________________________________ CHAPTER 1 INTRODUCTION TO AIRCRAFT SYSTEMS ENGINEERING AND CONCEPTUAL DESIGN 1. SYSTEMS ENGINEERING 1.1 The Air Transportation System The past several decades have seen the development of large, complex and highly interactive civil and military air transportation systems that are on the leading edge of technological development. These systems are more than just the air vehicles; they include the entire infrastructure needed to move people, goods or military hardware, from one place to another. The complete system includes facilities (airports), services (air traffic control, maintenance), personnel (air and ground staff) and the sustainable provision of consumable items (such as fuel). For the system to be effective and sustainable it must interface with other transportation networks (i.e. rail, road, sea) and must consider the environmental impact of the by-products of the operation (e.g. noise, waste, engine emissions). The design of an aircraft is thus part of a bigger picture, which is the provision of a modern air transportation system (Fig. 1.1). These systems have a natural process of evolution, or life cycle, in which actions and decisions taken in the early stages of the development of an element of the system, can mean the difference between success and failure of the project downstream. Designers must understand the entire system in which the envisaged aircraft is to operate. An inaccurate assumption regarding an external factor, such as the future price of fuel, for example, can lead to a non-viable product. The development of any element of this transportation system begins with the identification of a need. The goal of the systems engineering process is to deploy at the appropriate time and to sustain an effective system that satisfies that need at an affordable cost. 1.2 The Systems Engineering Process The systems engineering process is a logical sequence of activities and decisions that will transform the perceived need into a description of system performance parameters and a preferred system configuration that will satisfy the particular requirement. The objective is to ensure that the three critical elements of the programme – viz. product performance, cost and delivery timescales – will meet their targets by structured audits, design reviews and feedback cycles, as stipulated in the System Engineering Management Plan (SEMP). This formal document includes plans and schedules for the product definition, the integration of the design specialities and the key management structures of the prime contractor and subcontractors. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 2 Rev. 4 __________________________________________________________________________________________ Figure 1.1 Systems engineering approach to aircraft design (McMasters ) Systems engineering is primarily concerned with specifying the systems configuration or architecture, and the interfaces and compatibility of the various elements making up the system, rather than with the detail design of each item. Ensuring that these items meet performance goals, as demonstrated by tests or simulation, is an important aspect of systems engineering. Design inevitably involves compromise. The systems engineering framework can be used to maintain a balance between the projected life-cycle cost (the resources required to acquire, Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 3 Rev. 4 __________________________________________________________________________________________ operate, support and finally dispose of the system), the predicted system effectiveness/ performance (the degree to which a system can be expected to achieve the specific mission requirements) and the delivery schedule. The increased sophistication of modern aircraft has implied that design teams are larger and include specialists, whose knowledge of other design disciplines can be limited. The potential for misunderstanding between designers has increased. The management of the design programme must ensure that the interaction between design groups and the data transfer between groups is accurate and appropriate. The process of design must be structured to allow design decisions to be made in the context of the overall requirement, with the responsibilities for decisions decentralised, but nevertheless maintaining accountability. The systems engineering approach can be used in the development of elements at different levels in the air transportation system; it is a valid tool for the design of a complete aircraft, or a single component such as a flight control computer. Three steps are important in the process, as shown in Figure 1.2: (a) Requirement analysis; (b) Functional analysis and requirements allocation; and (c) Synthesis. INPUTS REQUIREMENT - MISSION REQUIR EMENTS - OPERATIONAL ENVIRONMENT AN ALYSIS - SYSTEM CONSTRAINTS - MEASURES OF EFFECTIVENESS FUNC TIONAL ANALYSIS & REQUIREMENTS ALLOCATION OUTPUTS - SYSTEM ARCH ITECTURE / CONFIGUR ATION SYNTHESIS - SCHEMATICS & FLOW CHARTS - PERFORMANC E MEASURES - TRADE-OF F STUDY REPORTS Figure 1.2 Systems engineering process a) Requirements analysis The first step in the process is to analyse and prioritise the initial set of requirements. The process is aided by defining measurable performance goals together with definitions of the mission, life and environmental profiles. In terms of the overall system effectiveness, the performance may be measured by the quantity (of aircraft), quality, timeliness and availability. The mission profile is a description (usually depicted graphically) of the performance requirements as a function of time. It shows the occurrences of scheduled events during a complete operational mission, as well as their sequence and duration. The life profile will cover the period from delivery to the customer to final disposal. The environmental profile describes Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 4 Rev. 4 __________________________________________________________________________________________ the operating environment (e.g. temperature, humidity) to which the item will be subjected during its life cycle. b) Functional analysis and requirements allocation Functions are discrete operations the system must perform to fulfil its intended mission. A process of functional decomposition is typically used to break down the high level operational functions into discrete technical functions. These functions can be implemented by hardware, computer software, personnel, procedural data, or a combination thereof. Each function must be described and shown to be traceable to the original mission requirement. If the system is treated in its totality, the likelihood of functional omissions occurring in the final product is reduced. c) Synthesis This step involves the synthesis of the functions and their allocated characteristics into several system items (e.g. equipment, software, personnel, facilities or procedures). Each discrete function must be allocated to a system item and the resources needed to perform the function, described. The system can be portrayed by schematic diagrams, physical and mathematical models, and computer simulations. The layouts will illustrate the external and internal interfaces of the system and its components. For example, in the design of the avionics system architecture of a modern aircraft, functions will be grouped and allocated to individual hardware components. For each component, the inputs and outputs can be defined. The installation requirements of each “black box”, (e.g. power, weight, cooling) can then be estimated. The initial allocation of budgets (for weight and space) can also be completed. 2. AIRCRAFT DESIGN PROCESS 2.1 Original Design Asked about aircraft design, Burt Rutan (designer of many extraordinary aircraft) replied: “To come up with something new and address a new requirement... you need... to go back pretty much to a sketch board and try different things. Having the courage to try something unusual and then combined with the engineering knowledge [to determine] will it work; that is what is needed. We spent an awful lot of money on how to analyse, but we do not spend much money on creating an environment for creativity. Much of what people do, called design, is really better called analysis. So [aircraft] design is something different. You need to be able to visualise load paths and visualise the [air] flow over an airplane and [to know] just what it needs to do.” Aircraft design is both art and science, it is creative and analytical (judicial). The design process is constituted by these two elements, as illustrated in the schematic in Figure 2.1. Diverge, lateral thinking, sometimes even producing illogical, seemingly bizarre alternatives, is required in the creative process. The uncritical brain storming, the borrowing of unrelated concepts from other engineering disciplines and the observation of nature, are all essential Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 5 Rev. 4 __________________________________________________________________________________________ parts of original design. This irrational thinking process, without rules and constraints, produces design alternatives, some of which, of course, may turn out to be nonsense. Thereafter, when the essence of the concept is sketched out, the judicial (analytical) part of an engineer’s brain takes over from the creative part, and the analysis begins. This part of the process is characterised by rigid scientific and mathematical rules and rational analysis. Convergence of alternatives to single concepts is achieved by design reviews and structured stage (decision) gates. Decisions are made regarding the alternatives and this will often lead to another cycle of creativity. It is these cycles of creation and analysis that often lead to innovative solutions to engineering problems. Figure 2.1 The engineering design process (adapted from McMasters ) 2.2 The Design Process Aircraft design is an iterative process that seldom has a clear starting point and a defined sequential series of events that leads to the final pack of detail drawings. Instead designs evolve in a cyclical process that moves from a list of perceived requirements, through the creative mode, to the analysis (the objective of which is to predict the performance of the envisaged design) and then to loop back to a possible review of the requirements. Many attempts have been made to model the activities that are required to take an original aircraft concept through the various design stages to the final production drawings. There are a great many work elements that are common to all such projects. For example, all projects must include the activities of performance prediction, ergonomic evaluation, weight and CG analysis, and structural design and analysis. Although this list of activities is a common denominator to all projects, aircraft programmes are always, in some way, unique. The relative importance of each element, the manner in which information flows from one activity to another, the sequence in which the tasks are completed, and the number of iteration cycles is always different. The process can be modelled by means of a flow-chart of activities identified in neat blocks, as illustrated in Figure 2.2. The chart gives the impression of a standard pre-defined process that leads to the final product. This interpretation is incorrect. The flow chart merely indicates the common activities that must be completed and some of the links between the activities. In reality no two designs evolve in exactly the same way. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 6 Rev. 4 __________________________________________________________________________________________ Figure 2.2 Design process for an aircraft Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 7 Rev. 4 __________________________________________________________________________________________ 2.3 Key Design Drivers The key parameters that will drive an aircraft design programme need to be clearly identified and ambiguously defined. Next, new technologies that have the potential to impact the key design drivers need to be established. Finally, an assessment of the maturity of these new technologies has to be undertaken to establish the viability of incorporating new, novel technologies within the design programme. Key design drivers for commercial jet transport aircraft are described below. Cruise efficiency In the case of a new jet transport aircraft, cruise efficiency is of fundamental importance. Any new aircraft that does not provide a step improvement in this parameter (compared to the previous generation of aircraft) is unlikely to be successful in the marketplace. A measure of aerodynamic cruise efficiency is the specific air range (ra) , which can be written as follows 1:  L M  ra = a0  D --- [2.1]  c    {W }  θ where a0 is the speed of sound at the ISA sea-level datum; M is the Mach number and L/D is the lift-to-drag ratio; c is the thrust specific fuel consumption (TSFC) (mass-flow based); θ is the relative temperature; and W is the aircraft gross (i.e. instantaneous) weight. By grouping the variables in Equation 2.1 in this way, it is apparent that the specific air range (SAR) is a function of three terms: (1) An aerodynamic term, given by the product of Mach number and aerodynamic efficiency, that is, M ( L D ) ; (2) A powerplant term, given by the corrected TSFC, that is c / θ ; and (3) The aircraft’s instantaneous weight (W), which for a given payload and fuel quantity depends on the aircraft’s empty weight, and hence on the structural design of the aircraft. Each of these three terms can be considered independently. For example, new technologies that could increase the aerodynamic term include hybrid laminar flow control. This technology (still under development) delays the laminar to turbulent transition of the boundary layer – and as laminar boundary layers have significantly lower skin friction drag than turbulent boundary layers, the ratio L/D improves. New engine concepts that would improve (i.e. reduce) the TFSC include ultra-high bypass ratio turbofan engines with powered gearboxes. Reducing structural airframe weight can be accomplished in many different ways – one of the most straightforward is by employing advanced lightweight materials. It is also possible to reduce the structural 1 See Equation 13.9a in Young , Section 13.2. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 8 Rev. 4 __________________________________________________________________________________________ weight of the wing by the use of active gust alleviation technology, which reduces the loads to which the structure has to be designed, for example. Environmental factors The key environmental factors are noise and engine emissions. Carbon dioxide production is directly linked with fuel burn – by reducing the amount of fuel consumed, CO2 is reduced. But it is not that straightforward with certain other exhaust emissions. The production of nitrous oxides, for example, is linked to the physical conditions under which the fuel is burned in the combustion chamber. Changing these conditions (i.e. temperature and pressure) to reduce NOx can, in certain conditions, lead to an increase in CO2 production. Reducing engine noise, has been a key design driver for aircraft manufacturers for several decades now. Significant improvements were made by increasing engine bypass ratio. It is now becoming harder to continue with this downward trend. Airframe noise has become is more important factor – but design solutions to mitigate airframe noise can sometimes work against improving fuel efficiency. The payload fuel energy intensity (PFEI) is an efficiency metric has been used as the objective function in certain airplane design optimization studies that are directed at minimizing carbon dioxide production (Drela ). It is, essentially, the ratio of the total energy required for the mission divided by a productivity metric (Young ). The selected metric is the total payload mass (or weight) multiplied by the great circle distance 2 between the departure and arrival airports. The PFEI is defined as follows: Ef E I , pl = --- [2.2] m pl d gc where E I , pl is the PFEI (typical unit: kJ kg-1 km-1); mpl is the total payload mass (typical unit: kg); and d gc is the great circle distance between the departure and arrival airports (typical unit: km). The total fuel energy (Ef) required for a mission is the product of the total (i.e. mission) fuel mass (typical unit: kg) and the net heating value of the fuel 3 (typical unit: MJ/kg). Passenger experience A major factor in the design of a new airliner is the passenger compartment. From a passenger perspective, jet transport aircraft in the economy class is a cramped environment. Providing more space per passenger (for the same number of passengers) clearly requires a larger, heavier, more expensive aircraft that will cost more to operate. Manufacturers thus seek to improve the passenger experience without increasing the overall fuselage dimensions (e.g. through improved lighting, infotainment and other passenger services). 2 See Young , Section 6.2.4. 3 See Young , Section 22.7.2. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 9 Rev. 4 __________________________________________________________________________________________ Manufacturing cost and time Even though Airbus and Boeing effectively enjoy a duopoly in the manufacture of jet transport aircraft, competition to sell aircraft into this market is fierce. The cost of manufacturing the aircraft influences the price that the customers will pay for the product and this impacts sales. Consequently, manufacturing cost has to be seen as a primary design driver for new aircraft. A closely coupled factor is the time to manufacture the aircraft. With a combined production rate of the single-aisle A320 and B737 aircraft types exceeding 100 unit per month, there is enormous pressure on manufacturers to reduce assembly time. Operating costs Fuel is a big part of the operating cost for an airline 4. Optimising the design to reduce fuel burn will thus reduce operating costs. Flight crew costs are another significant factor for an airline. Consequently, ongoing research is exploring the technical viability (and public acceptance) of single flight crew operations for airliners. Reducing the flight crew from two to one will introduce cost savings for the airline, but it requires significant technological solutions to be developed. Trade-offs and synthesis Aircraft design involves trade-offs, in which design concepts that have prioritised different design drivers are compared and evaluated against each other. A synthesis of these sometimes- competing factors has to be made during the conceptual design phase. 2.4 Requirement for a New Aircraft For the manufacturer many projects start with a Design Requirement – which is either supplied to the company or self-generated. Very occasionally, the design will begin as an innovative idea rather than as a response to a requirement, as was the case with the Northrop flying wings – a concept pioneered by John Northrop. At some stage in the design process a definitive set of specifications for the new aircraft will be spelled out. (This list of requirements will probably not be complete during the early stage of the design and very often, calculated guesses will be needed to “fill in the blanks”.) Historically the specifications were rather brief, listing only the critical weight, power and performance requirements, as shown in Figure 2.3 (the specification for the Douglas DC-1). Figure 2.4 is an example of a design requirement or a supersonic transport aircraft concept explored by Boeing, several decades ago. 4 See Young , Section 18.2. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 10 Rev. 4 __________________________________________________________________________________________ Figure 2.3 General performance specification for the Douglas DC-1 aircraft (McMasters ) Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 11 Rev. 4 __________________________________________________________________________________________ Figure 2.4 Design Requirements for a transpacific supersonic transport aircraft (McMasters ) For commercial aircraft the specification is usually in the form of a Design Requirement (or Aircraft Requirement), while for military aircraft, a Request for Proposal (RFP) would be issued by the procurement agency. For a military programme, an Operational Requirement may first be drafted by the end user (usually at squadron level) after internal trade studies and then conveyed through the Defence Procurement Agency to the contractor in the form of an RFP. The term Mission Specification is also used by some authors to describe the Design Requirement, although in this series of notes this term will refer only to the requirements pertaining to the aircraft’s mission, in the strict sense of the word. Figure 2.5 is an illustration of the different design objectives and constraints for transport aircraft for civil and military customers. With the increase in complexity of aircraft designs, modern specifications can be large and complex documents, spelling out a wish list of requirements, some of which may be conflicting. These conflicting requirements will require trade-off (or simply trade) studies to determine the optimum mix of performance goals. The establishment of a comprehensive set of requirements for a modern commercial airliner may take a long time, sometimes extending to a few years as in-depth market studies and customer negotiations take place. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 12 Rev. 4 __________________________________________________________________________________________ Figure 2.5 Transport aircraft design objectives and constraints (McMasters ) 3. PHASES OF AN AIRCRAFT DESIGN PROJECT Aircraft design can be broken into a number of phases. The terminology used to describe these phases varies from organisation to organisation and from country to country. The one used in this note is typical and is shown in Figure 3.1. Also, the boundaries indicating where one phase ends and another starts are highly subjective, and several alternative definitions are available. In practise, the phases tend to overlap thus destroying the neat and logical appearance presented in the figure. The United States Department of Defense spells out a definitive process for US military contractors. This approach has been adopted by non-military organisations for the management of large projects, and much of the terminology has been adopted by other sectors of the industry. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 13 Rev. 4 __________________________________________________________________________________________ Design Requirement or RFP CONCEPTUAL DESIGN Initial layout of basic configurations Initial sizing Thrust, Weights, Wing planform, CLmax , etc. Sensitivity analysis Initial cost & timescales Functional Baseline PRELIMINARY DESIGN (DEFINITION/VALIDATION PHASE) Shape definition / Lofting Initial inboard profile Analytical models - Performance, Stress, Loads, Fluid Dynamics, Flight Control Mockups and CAD models Preliminary equipment specification Testing - Wind tunnel, Specimens, Breadboard Preliminary design of critical items Manufacturing process definition Programme cost & timescales Configuration freeze Allocated Baseline DETAIL DESIGN (FULL SCALE DEVELOPMENT PHASE) Detail design (to the last rivet) Design of Jigs, Tools & Fixtures Definitive weights & performance prediction On-board equipment definition Mockups, System test rigs, Iron bird Testing - Control laws, Structural, Systems Manufacturing process planning Manufacturing cost & timescale ILS (Integrated Logistic Support) definition Manufacturing Baseline INDUSTRIALISATION AND PRODUCTION Figure 3.1 Typical phases of an aircraft design programme Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 14 Rev. 4 __________________________________________________________________________________________ 3.1 Conceptual Design Phase Conceptual design deals with establishing the basic configuration of the aircraft, answering questions of: size, weight, what is looks like, basic performance, engine thrust, maximum lift coefficients, etc. One critical aspect is cost. If an affordable aircraft cannot be built, then the requirements must be amended. A flow chart of activities is shown in Figure 3.2. Figure 3.2 Aircraft conceptual design process (adapted from McMasters ) Parametric studies are undertaken that correlate various parameters, such as the sensitivity of performance to changes in wing loading, powerplant, aspect ratio, sweep, high-lift devices et cetera. The methodologies used to determine the parametric variations require cross checking (or calibration) to determine if the numerous assumptions regarding aerodynamics, propulsion efficiency, weight, stability and control and so forth, are valid and justified. This can be done by means of Referee Designs. A few selected points on the parametric curves can be used as input to a detailed study using more sophisticated analysis techniques. One important sensitivity analysis that may be performed is the weight growth factor. This is defined as the increase in gross weight that results from a unit increase in the weight of some component, onboard equipment (or payload). High weight growth factors (above 10) are cause for concern, especially where unproven technologies are involved, as the aircraft design weight can end up spiralling upwards with each design cycle, rendering the design concept worthless. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 15 Rev. 4 __________________________________________________________________________________________ Optimisation is often performed based on minimum weight. This is acceptable under most circumstances as both aircraft construction costs and operating costs display a close relationship with aircraft weight. Graphical representation of several parameters can be used to define a region where all requirements are met (discussed later in Chapter 2 of this lecture series). For more precise optimisation, it is necessary to model the relationships using computer programs and to vary a much larger range of parameters. 3.2 Preliminary Design Phase Following the establishment of an aircraft conceptual layout the preliminary design starts. This initial layout or drafting of a new aircraft is sometimes referred to as the Project Design phase because it is performed by the Projects or Future Projects Group in a large company. Minor revisions to the configuration occur. At some point towards the end of this phase the configuration is frozen. Whereas during the conceptual phase the work can be performed (to a large extent) by a small team of multi-disciplinary people, the preliminary design involves specialists in all the aircraft disciplines – structures, propulsion, aerodynamics, systems, performance, weights, loads, flight control, et cetera The exact geometrical definition of the outside skin of the aircraft is defined during this phase, using the process of lofting. Today this is performed largely on CAD with the resultant surface definition being modelled mathematically. Testing of wind tunnel models and flow visualisation using CFD (Computational Fluid Dynamics) techniques is done. An inboard profile depicting the layout of primary structural members and installed equipment is developed. This drawing (illustrated in Figure 3.4) is an allocation of space inside the aircraft and it depicts the size and position of all major items. One of its uses is the determination of the aircraft’s CG position (and CG movement, as fuel is used). This result is a critical input required for the longitudinal placement of the wing, which in turn affects the internal layout. The structural analysis of critical items (such as the main wing spar) is undertaken and the evaluation of new manufacturing techniques by specimen testing is performed. Control laws and aircraft handling characteristics are analysed. Specification of all onboard equipment is completed. Simple proof-of-concept mockups and test articles are produced. In essence, the objective of this Preliminary Design Phase is to develop the design to a point where detail design can start. During the detail design many areas of the aircraft are designed simultaneously, so it is important to evaluate all critical aspects, so that changes that occur during the detailing have little impact on other areas of the design. The time required for this phase of the design may extend from a few thousand person-hours to over 500 000 person-hours for complex and radically new aircraft types. For defence contractors the phase ends with the contractor submitting a proposal for full-scale development Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 16 Rev. 4 __________________________________________________________________________________________ which will spell out what will be built, how it will perform, how much it will cost and how long it will take. (It is possible for competing companies to stake the continued viability of their company on the details of such proposals, as the cost of overrun contracts or lack of sales can exceed the net worth of the company.) Figure 3.3 Example of inboard profile (Raymer ) 3.3 Detail Design Phase / Full Scale Development The majority of aircraft design projects do not get further than preliminary design stage. Superior designs will eliminate other project aeroplanes in design competitions. Many projects terminate at this stage due to changes in the political scenario or in the projected market. Some projects get cancelled due to their inability to meet the specifications; others may be cancelled due to a lack of funding or a change of mind by the government or potential customer. During full scale development the design is detailed so that each individual piece can be manufactured in a repeatable manner. A detailed weights breakdown is developed. The Production Engineering department is heavily involved in determining how the components will be manufactured and assembled. Changes in the design to facilitate ease of manufacture are numerous – many of which have an impact on the performance and weight of the aircraft – hence trade-offs need to be made. The design of jigs, tools and fixtures takes place. Detailed production planning is done. Control laws for the flight control system are tested on an iron-bird simulator – a functional model of the hydraulics systems, actuators and flight controls. Flight simulators are built and "flown". Traditionally physical mockups were routinely produced (using the correct materials to craft standard tolerances). Today, high fidelity digital mockups are produced instead; nonetheless, physical mockups remain useful in certain circumstances (e.g. for human interfacing and customer demonstration). The cumulative programme costs rise dramatically during the detail design phase. Upwards of 90% of the total programme cost will occur in this phase. Commensurate with the increase in cost, there is a decline in the flexibility to change the design in any significant way (Fig. 3.4). Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 17 Rev. 4 __________________________________________________________________________________________ The detail design phase ends with the manufacture of prototype aircraft. (In any programme, design work on the aircraft will continue through the production run and some design will be required as a support function to in-service aircraft in the way of modifications and repairs.) Figure 3.4 a) Cost management of a typical commercial aircraft programme, b) Milestone chart for the Boeing 757 and 767 (McMasters ) Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 18 Rev. 4 __________________________________________________________________________________________ APPENDICES APPENDIX A: AVIATION ORGANISATIONS AND REGULATIONS A.1 Introduction This appendix introduces the role of the International Civil Aviation Organisation, the main aviation regulatory bodies (viz. FAA, EASA) and the key documents that describe the requirements for aircraft certification. Further information can be found in Young , Chapter 23 and Schmitt and Gollnick. A.2 International Civil Aviation Organization The International Civil Aviation Organization (ICAO), based in Montréal, Canada, is a specialized agency of the United Nations, which is charged with coordinating and regulating international air travel. It has a membership of nearly 200 contracting states (in 2016), including all states (countries) with a significant aviation industry. ICAO has a mandate to administer the principles of the Convention on International Civil Aviation – an international agreement signed in Chicago in 1944 (and revised several times since). ICAO is responsible for the development and implementation of aviation policy documents, which include Standards And Recommended Practices (SARPs) and Procedures for Air Navigation Services (PANS). The Convention on International Civil Aviation defines a broad-ranging set of standards relating to the international operations of aircraft. The convention is supported by nineteen annexes containing SARPs; these documents are regularly updated. SARPs do not have a legal standing; countries that adopt these regulations translate the SARPs into their own laws. Contracting states are obliged to publish in their Aeronautical Information Publications any significant differences between their adopted procedures and the related ICAO procedures. A.3 Civil Aviation Authorities and Airworthiness Regulations Aviation authorities (known as civil aviation authorities in many countries) are statutory organizations, established by governments of states, to oversee the approval, regulation and safe operation of civil aviation within the jurisdiction of the state. Aviation authorities are entrusted by their respective states to maintain the national registry of aircraft, and to issue and renew type certificates and certificates of airworthiness. Aviation authorities typically regulate the following activities: design and manufacture of aircraft, maintenance of aircraft, licensing of personnel, licensing of airports and ground-based navigational aids, operation of aircraft, and the use of the country’s air space. Aviation authorities are often referred to as regulatory authorities due to their role in developing and/or implementing regulations. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 19 Rev. 4 __________________________________________________________________________________________ Federal Aviation Administration The Federal Aviation Administration (FAA), with headquarters in Washington, DC, is an agency of the US Department of Transportation. The FAA was established in 1958, absorbing the responsibilities of the former Civil Aeronautics Administration. It is responsible for overseeing all aspects of civil aviation in the US. One of the FAA’s key activities concerning the airworthiness of aircraft is the development and implementation of Federal Aviation Regulations (FARs) and supporting documentation, such as Advisory Circulars (ACs). The FARs address a diverse range of aviation activities, which includes the certification and operation of aircraft. European Aviation Safety Agency The European Aviation Safety Agency (EASA), with headquarters in Cologne, Germany, is responsible for civil aviation safety in the European Union (EU). EASA was created in 2003 and it progressively took over from the Joint Aviation Authorities (JAA) the responsibility for regulating airworthiness issues within EU member states 5. The JAA (which was initially called the Joint Airworthiness Authorities) was created in 1970 as an associated body to represent the national civil aviation authorities of several European states. A key objective was the development of common regulatory standards and procedures within member states. Other aviation authorities The Interstate Aviation Committee (denoted by the abbreviation IAC or, alternatively, by the Russian equivalent MAK) with headquarters in Moscow, Russia, which was created in 1991 by an inter-governmental agreement between a number of countries of the former USSR (Union of Soviet Socialist Republics), is responsible for civil aviation safety in signatory countries 6. The regulatory authority in the Republic of Ireland is the Irish Aviation Authority (IAA), based in Dublin. A.4 Military Aircraft Design Requirements United States military aircraft are designed to the US Military Specifications (MIL SPECS). The almost endless list of Military Specifications and Handbooks provide a wealth of information on aircraft design, which is used by both military and civil aircraft designers. For example, MIL-C5011A defines aircraft performance and design constraints, while MIL HDBK 5 has been the definitive source of data on aircraft structural materials (metals) for decades 7. British military aircraft are designed to Defence Standard 00-970 (DEF STAN 00-970) while French Military aircraft are designed to the AIR standards. 5 EASA member states are Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Poland, Portugal, Romania, Slovak Republic, Slovenia, Spain, Sweden, The Netherlands, and United Kingdom. In addition, the non-EU European countries Iceland, Liechtenstein, Norway and Switzerland participate in the activities of EASA under a separate agreement. 6 The countries served by the IAC (as of 2014) are Azerbaijan, Armenia, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Russian Federation, Tadjikistan, Turkmenistan, Ukraine and Uzbekistan. 7 MIL-HDBK 5 has been replaced by the Metallic Material Properties Development and Standardization (MMPDS) Handbook. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 20 Rev. 4 __________________________________________________________________________________________ A.5 Regulations Aviation authorities provide the regulatory framework for the approval of new aeronautical products. They are responsible for the issue of type certificates. They are also responsible for the approval of organizations (and, in specific cases, personnel) involved in the design, manufacture, maintenance, or operation of aircraft. The authorities, over many years, have established a series of regulations to ensure that all aircraft engaged in public transportation meet a minimum standard of safety, which has been deemed appropriate for the intended operation of the aircraft. These regulations – which are variously titled regulations, requirements, specifications, or standards – are informally referred to as the rules. Two complementary sets of measures exist. The first set of measures is concerned with the certification of new aircraft; the second set is concerned with the safe operation of the aircraft. In the US, the regulations pertaining to the initial certification and subsequent operation of jet transport aeroplanes are published under Chapter 1 of the Aeronautics and Space (Title 14) section of the US Code of Federal Regulations (CFR) 8. An individual regulation, which is understood by the abbreviation FAR (Federal Aviation Regulation), is called a part. In Europe, EASA 9 publishes Certification Specifications (i.e. the rules for initial aircraft certification) and technical documents for continued airworthiness and safe operation. Most of the topics addressed by EASA in these specifications and documents roughly match those contained in the FARs (although titles, headings, and specific details may differ) 10. Note that airworthiness regulations carry legal status – compliance is mandatory and enforceable by law. In the case of EASA, the regulations/specifications are enforceable in EU member states through non- legislative acts (regulations) passed by the European Parliament and in non-EU EASA member states through parallel actions undertaken by respective national governments. A.6 Certificate of airworthiness and type certificate A certificate of airworthiness (C of A) is issued by the national aviation authority of the country in which the aircraft is registered. It grants authorization for a particular aircraft (as identified by the manufacturer’s serial number) – to operate in that country’s airspace, and, through international agreements, in the airspaces of other countries. This is described in Article 31 of the ICAO Convention on International Civil Aviation, which states that “every aircraft engaged in international navigation shall be provided with a certificate of airworthiness issued or rendered valid by the State in which it is registered”. Renewal of the certificate of airworthiness is subject to demonstration of compliance of the requirements (e.g. periodic inspections) for continued airworthiness, as stipulated the aviation authority of the country in which the aircraft is registered. 8 Current aviation regulations are available as Electronic Code of Federal Regulations (e-CFR) at the US Government Printing Office (GPO) website www.ecfr.gov/. 9 Current EASA specifications and regulations are available at the EASA website www.easa.europa.eu/. 10 The FAA and EASA publish lists of significant standards differences (SSD) and non-significant standards differences (non-SSD) that exist between their respective regulatory documents – for example, between FAR 25 and CS 25. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 21 Rev. 4 __________________________________________________________________________________________ Only an aircraft for which the manufacturer holds a type certificate (TC) and which was produced according to the approved design is eligible for the issue of a certificate of airworthiness. A type certificate is issued by an aviation authority of a country to an aircraft manufacturer to certify that the design of the aircraft type meets the appropriate airworthiness requirements (i.e. certification standards) of that country. The certification process involves a detailed investigation, which is conducted in accordance with stipulated procedures, into all aspects of the design, manufacture, and performance capabilities of the aircraft to demonstrate compliance with the airworthiness requirements. Flight testing is an essential part of the validation of the aircraft’s performance. FAA and EASA certification is usually undertaken simultaneously (this usually marks the commencement of serial production of the aircraft type). Once a type certificate has been issued, the design cannot be changed without the approval of the aviation authority. Modifications or design changes to the aircraft type are approved through the issuing of a supplemental (or supplementary) type certificate (STC) once compliance to the authority’s requirements has been demonstrated. The STC describes the product modification and identifies any changes to the certification basis. A.7 Requirements for aircraft certification The requirements for the issue of a type certificate are described in formal documents (variously known as regulations, requirements, standards, or specifications) published by the regulatory authorities. Transport category aircraft: The set of regulations/specifications for the certification of transport category aeroplanes by the FAA and EASA include the following key documents 11: FAR 25 (Federal Aviation Regulation Part 25), Airworthiness Standards: Transport Category Airplanes. EASA CS-25 (EASA Certification Specification 25), Certification specifications and Acceptable Means of Compliance for Large Aeroplanes: Book 1 12. FAR 33 (Federal Aviation Regulation Part 33), Airworthiness Standards: Engines. EASA CS-E (EASA Certification Specification E), Certification Specifications and Acceptable Means of Compliance for Engines: Book 1. These documents are regularly updated. Normal category aircraft: The regulations/specifications for the certification of smaller aircraft with a maximum seating capacity of 19 passengers and a maximum certificated takeoff weight (MTOW) of 19 000 lb (8618 kg) is described in the following key documents: FAR 23 (Federal Aviation Regulation Part 23), Airworthiness standards: Normal category airplanes. 11 The FAA regulations (i.e. FAR 25 and FAR 33) and the corresponding EASA specifications (i.e. CS-25 and CS-E) for airplane and engine certification are almost identical, but there are some differences between the two sets of requirements which manufacturers and operators need to be aware of. For example, the FAA primarily uses USC units, whereas EASA uses a mix of SI and USC units. 12 CS-25 and CS-E are each divided into two books: Book 1 contains the Certification Specification (CS) and Book 2 contains the corresponding Acceptable Means of Compliance (AMC). Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 22 Rev. 4 __________________________________________________________________________________________ CS-23 (EASA Certification Specification 23), Certification specifications for normal- category aeroplanes. A major revision of FAR 23 was concluded in 2016, with the issue of Amdt. 23-64. The new “rulebook” substantially revised the certification process for these aircraft types 13 The new rules introduced new subcategories of aircraft applicable to this certification class: Level 1 for aeroplanes with a maximum seating configuration of 0 to 1 pax (passengers), Level 2 for 2 to 6 pax, Level 3 for 7 to 9 pax, and Level 4 for 10 to 19 pax. Aircraft are also separated into two performance levels (categories), as follows: (1) Low speed, for aeroplanes with a VNO and VMO ≤ 250 KCAS and a MMO ≤ 0.6. (2) High speed, for aeroplanes with a VNO or VMO > 250 KCAS or a MMO > 0.6. Supporting documentation: The regulatory authorities publish supplemental information that provides guidelines for manufactures to demonstrate compliance to the regulations/ specifications. These supplemental documents provide information on methods, procedures, and practices that are acceptable to the authorities. The FAA issues Advisory Circulars (ACs) for such purposes and the European authorities issue Acceptable Means of Compliance (AMC) and Guidance Material (GM). Uncertified aircraft: Experimental and historic aircraft, which do not comply with the requirements for aircraft certification can be issued a permit to fly by the appropriate aviation authority, subject to the aircraft satisfying a specific set of requirements/conditions. By international agreements, such “permit” aircraft are often allowed to operate in other states (countries), albeit, sometimes, with limitations. 13 Many industry sepecialitsts considered this exercise to be long-overdue. Historically, Part 23 differentiated aircraft on the basis of engine type and aircraft weight and did not address the operational capabilities of modern high-performance small aircraft (which did not exist when the document was first prepared). Also, there was significant pressure from industry to revise the certification process, which was widely considered to be overly complicated and expensive for simple, low-speed 2-4 seat aircraft. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 23 Rev. 4 __________________________________________________________________________________________ APPENDIX B: AIRCRAFT DESIGN BOOKS Brandt, S.A., Introduction to aeronautics: A design perspective, AIAA Education Series, 2nd ed., American Institute of Aeronautics and Astronautics, Reston, VA, 2004. Fielding, J.P., Introduction to aircraft design, 2nd ed., Cambridge University Press, New York, NY, 2017. Howe, D., Aircraft conceptual design synthesis, Wiley, Chichester, UK, 2005. Heinemann, E.H., Rausa, R., and Van Every, K., Aircraft design, Nautical & Aviation Publishing Co., Baltimore, MD, 1985. Jenkinson, L.R., Simpkin, P., and Rhodes, D., Civil jet aircraft design, Butterworth- Heinemann, Oxford, UK, 1999. Nicolai, L.M. and Carichner, G.E., Fundamentals of aircraft and airship design, Vol. 1 – Aircraft design, AIAA Education Series, American Institute of Aeronautics and Astronautics, Reston, VA, 2010. Raymer, D.P., Aircraft design: A conceptual approach, AIAA Education Series, 6th ed., American Institute of Aeronautics and Astronautics, Reston, VA, 2018. Roskam, J., Airplane design part I: Preliminary sizing of airplanes, DARcorporation, Lawrence, KS, 1985. Roskam, J., Airplane design part II: Preliminary configuration design and integration of the propulsion system, DARcorporation, Lawrence, KS, 1997. Roskam, J., Airplane design part IV: Layout design of landing gear and systems DARcorporation, Lawrence, KS, 1997. Roskam, J., Airplane design part VI: Preliminary calculation of aerodynamic, thrust and power characteristics, DARcorporation, Lawrence, KS, 2000. Roskam, J., Airplane design part III: Layout design of cockpit, fuselage, wing and empennage: Cutaways and inboard profiles, DARcorporation, Lawrence, KS, 2002. Roskam, J., Airplane design part VII: Determination of stability, control and performance characteristics, DARcorporation, Lawrence, KS, 2002. Roskam, J., Airplane design part VIII: Airplane cost estimation: Design, development, manufacturing and operating DARcorporation, Lawrence, KS, 2002. Roskam, J., Airplane design part V: Component weight estimation, DARcorporation, Lawrence, KS, 2003. Roskam, J., Lessons learned in aircraft design: The devil is in the details, DARcorporation, Lawrence, KS, 2012. Sadraey, M., H., Aircraft design: A systems engineering approach, Aerospace Series, John Wiley & Sons, Chichester, UK, 2013. Sforza, P.M., Commercial airplane design principles, Elsevier Aerospace Engineering Series, Butterworth-Heinemann, Oxford, UK, 2014. Stinton, D., The design of the aeroplane, 2nd ed., Blackwell Science, Oxford, UK 2001. Torenbeek, E., Synthesis of subsonic airplane design, Delft University Press, Delft, the Netherlands, 1982. Torenbeek, E., Advanced aircraft design: Conceptual design, analysis and optimization of subsonic civil airplanes, John Wiley & Sons, Chichester, UK, 2013. Whitford, R., Design for air combat, Jane's Information Group, London, UK, 1987. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 1 Page 24 Rev. 4 __________________________________________________________________________________________ REFERENCES McMasters, J.H., "BCAG Engineering Division summer intern training program", Boeing Commercial Airplane Group, Seattle, WA, 1991. Anon, "Aircraft design: Aircraft project design", Lecture note DES 8108, College of Aeronautics, Cranfield Institute of Technology, UK, 1986. Young, T.M., Performance of the jet transport airplane: Analysis methods, flight operations, and regulations, John Wiley & Sons, Chichester, UK, 2017. Drela, M., "Design drivers of energy-efficient transport aircraft", SAE International Journal of Aerospace, Vol. 4, Iss. 2, pp. 602-618, 2011. Raymer, D.P., Aircraft design: A conceptual approach, AIAA Education Series, 3rd ed., American Institute of Aeronautics and Astronautics, Reston, VA, 1999. McMasters, J.H., "BCAG Engineering Division summer intern training program", Boeing Commercial Airplane Group, Seattle, WA. Schmitt, D. and Gollnick, V., Air transport system, Springer-Verlag, Wein, Austria, 2016. ICAO, "Convention on international civil aviation", 9th ed., Doc. 7300, International Civil Aviation Organization, Montréal, Canada, 2006. FAA, Airworthiness standards: Transport category airplanes, Federal Aviation Regulation Part 25, Amdt. 25-143, Federal Aviation Administration, Washington, DC, 24 Jun. 2016. Latest revision available from www.ecfr.gov/ under e-CFR (Electronic Code of Federal Regulations) Title 14. EASA, Certification specifications and acceptable means of compliance for large aeroplanes, CS-25, Amdt. 18, European Aviation Safety Agency, Cologne, Germany, 23 Jun. 2016. Latest revision available from www.easa.europa.eu/ under Certification Specification. FAA, Airworthiness standards: Aircraft engines, Federal Aviation Regulation Part 33, Amdt. 33-34, Federal Aviation Administration, Washington, DC, 4 Nov. 2014. Latest revision available from www.ecfr.gov/ under e-CFR (Electronic Code of Federal Regulations) Title 14. EASA, Certification specifications and acceptable means of compliance for engines, CS-E, Amdt. 4, European Aviation Safety Agency, Cologne, Germany, 12 Mar. 2015. Latest revision available from www.easa.europa.eu/ under Certification Specification. FAA, Airworthiness standards: Normal category airplanes, Federal Aviation Regulation Part 23, Amdt. 23-64, Federal Aviation Administration, Washington, DC, 30 Dec. 2016. EASA, Certification specifications for normal-category aeroplanes, CS-23, Amdt. 5, European Aviation Safety Agency, Cologne, Germany, 29 Mar. 2017. Latest revision available from www.easa.europa.eu/ under Certification Specification. FAA, "Part 23 – Small airplane certification process study: Recommendations for general aviation for the next 20 years", Federal Aviation Administration, Washington, DC, July 2009. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 1 Rev. 3 __________________________________________________________________________________________ CHAPTER 2 GENERAL ARRANGEMENT 1. INTRODUCTION This lecture note discusses the general arrangement of aircraft and covers the following topics: Concept sketches Wing position Location of engines Tailplane arrangement 2. CONCEPT SKETCHES The first drawings of a new aircraft are often nothing more than a conceptual sketch, giving a rough idea of what the aircraft could look like (Fig. 2.1). A good concept sketch will indicate the approximate wing and tail geometries, fuselage shape, the position of critical elements such as the crew station, payload/passenger compartments, engine, landing gear and perhaps the fuel tanks (Fig. 2.2). The concept sketches can be used to estimate the basic aerodynamic and weight characteristics needed for the initial sizing exercise. The layout will be revised many times and by the end of the Concept Phase, a firm configuration layout will have evolved from the best of the concept sketches. Figure 2.1 Design concept sketch of a twin engine supercruise fighter, developed by Raymer Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 2 Rev. 3 __________________________________________________________________________________________ Figure 2.2 Initial design concept of an ultra-short-haul airliner (30 passengers), developed by Torenbeek 3. WING POSITION With respect to the fuselage, the wing can be described as being in a high, mid or low position. From a design perspective, the wing location is, to a large extent, dictated by operational requirements, rather than aerodynamic, structural or systems requirements. The aerodynamic and structural differences, although significant, are only deciding factors if the choice is not made by operational considerations. 3.1 High wing For military transport and utility aircraft the loading and un-loading requirement favour a high wing as this can reduce the height of the cargo floor from the apron. Loading by means of a rear door ramp is common with this layout, as typified by the C-130 Hercules, C-160 Transall Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 3 Rev. 3 __________________________________________________________________________________________ and Lockheed C-5A aircraft types. The floor of the C-5A is only about 2.5 m above the apron, compared to a distance of about 5 m for a B747. For the latter configuration, the special loading equipment that is required is unacceptable for a military operator. A similar argument is true for some smaller utility aircraft such as the PBN Islander, CASA Aviocar and Shorts 330. Ground clearance of engines and propellers are also factors favouring the high wing layout. The decision of wing position is always taken after considering the implications on the undercarriage design (Fig. 3.1). For small and medium sized transport aircraft, it is possible to retract the gear into wing mounted engine nacelles (e.g. Fokker 27, DHC-5 Buffalo), but for a large aircraft such as the C-5A (Fig. 3.2) , this is not possible. Mounting the gear on the fuselage requires a strengthening of the fuselage and hence an increase in weight. An associated problem is that it may be difficult to achieve a sufficiently wide track. Although this weight increase is partly off-set by the lighter wing and shorter gear, the high wing tends to result in a net increase in aircraft empty weight and increased structural complexity over the low wing layout. (Torenbeek ). The high wing can incorporate anhedral, which is desirable on swept-back wings for aircraft stability reasons, without compromising the undercarriage design (e.g. BAe 146). Braced monoplanes are generally of a high wing layout (e.g. Cessna 152, 172, 182, Maule, Piper Cub), as this leads to a lighter overall solution. The exception to this is found in agricultural aircraft (e.g. Piper Pawnee, Norman Fieldmaster) where the spraying system is mounted on the trailing edge of a low wing. STOL aircraft are of a high wing layout as a low wing would cause the aircraft to "float" in ground effect making short precise landings more difficult (e.g. Pilatus Porter, Storch). For Ground Attack and Strike aircraft, the requirement to carry external stores (e.g. fuel tanks, ordnance) favours the high-wing position (e.g. Sepecat Jaguar, AMX). Figure 3.1 Landing gear installation options (Raymer ) Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 4 Rev. 3 __________________________________________________________________________________________ Figure 3.2 Lockheed C-5A undercarriage installation (Torenbeek ) 3.2 Mid-wing This layout results in a reduction in interference drag between the wing and the fuselage. Also, the wing root fairing can be reduced in size because the wing meets the fuselage at roughly 90°. Mid-wing layouts are common for military fighters and trainers where good high-speed performance is important (e.g. Lockheed F-16, Mig 21, Lockheed SR-71 Blackbird). The structural complexities of this layout are not insignificant and weight and cost penalties result. Load transfer from the wing is more difficult as continuous wing carry-through structures are not possible in most cases (Fig. 3.3). Heavy machined ring frames are used to transfer loads around fuselage mounted engines (e.g. Mirage F1, Lovaux Optica). The mid- to low- wing layout is favoured by aerobatic aircraft builders where superior manoeuvre characteristics are claimed (e.g. SU-29). 3.3 Low Wing This layout offers a number of advantages. The primary load carrying members of the wing can continue uninterrupted through the lower part of the fuselage. For many commercial transport aircraft, the wing layout approaches a mid-wing position as the wing is located just below the cabin floor (which is itself positioned relatively high in the fuselage cross section). On small commuter aircraft, such as the BAe Jetstream 31, a step in the aisle is caused by the wing carry-through structure. (This undesirable feature resulted in the lowering of the wing and a major re-design by BAe for the Jetstream 41.) Retraction of the gear can be easier for a low wing configuration and simpler (and lighter) solutions are possible. Under crash conditions the low wing is better as the wing and engines absorb energy, but under a moderate impact the high wing solution is preferable, as the risk of fuel spillage (and fire) is lower. The case of a forced landing in water favours a low wing as the fuselage will not be submerged. Regarding lift and drag characteristics Torenbeek states that “… there are admittedly differences between the high and low wing locations, but these may be minimised by proper Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 5 Rev. 3 __________________________________________________________________________________________ use of fillets and fairings. Even so the high wing is superior in this respect to the low wing, particularly where induced drag at high lift is concerned”. Arguments for and against a low wing location for GA aircraft are frequently voiced. Owners of Cessna aircraft maintain that visibility is better with a high wing aircraft. Those who favour a Piper or a Socata aeroplane reply that in a turn inward visibility is obscured by the lowered wing (which can be dangerous). Both arguments have merit. Traditions within companies such as Piper, Cessna and Beechcraft have prevailed over the years with new aircraft being built along the lines of previous models. Figure 3.3 Wing “carry through” structural concepts (Raymer ) 4. ENGINE LOCATION 4.1 Pod Mounted Versus Internally Installed (Buried) Engines Aircraft engines are either mounted externally in a pod (nacelle) or internally within the fuselage or wing or wing/fuselage junction. Alternative locations for installing the engine are illustrated in Figures 4.1 and 4.2 (reproduced from Raymer ). Figure 4.1 Pod-mounted engine installation concepts (Raymer ) Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 6 Rev. 3 __________________________________________________________________________________________ Figure 4.2 Internally-mounted (buried) engine installation concepts (Raymer ) 4.2 Jet Transport Aircraft In the case of civil jet transport aircraft (and bomber aircraft) two schools of thought emerged in the late 1940s and early 1950s (Torenbeek ): Engines buried in the wing roots, with the air intake in the leading edge and the exhaust at the trailing edge (e.g. DeHavilland Comet, Avro Vulcan). Engines pod mounted below the wing or on the rear of the fuselage (e.g. Boeing B-47, BAC Super VC 10). Locating the engine is not an isolated design consideration but rather part of the overall wing/fuselage design concept. The large delta wing (with low wing loading) of the Vulcan provided ample room to house the engines, which was not possible with the B-47, which was designed with a significantly smaller wing (optimised for high wing loading). The development of high bypass ratio engines and improvements in the design of high lift devices resulted in the demise of the buried engine configuration for this class of aircraft. Several design concepts have been considered for locating engines as shown in Figure 4.2; some of these, however, are not practical. Pod mounted engines on the rear of the fuselage (e.g. Fokker 100, DC-9/MD-80) have the obvious advantages of a clean wing, low door sill height, and little asymmetric thrust after engine failure. This engine position is often combined with a T-tail, the resultant configuration requiring careful design to avoid the problem of deep stall. The wing mounted engine provides bending moment relief on the inner wing and better accessibility for maintenance. Comparing aircraft empty weights Torenbeek claims that a weight saving is possible with rear mounted engines. An overall assessment of performance and weight of the two configurations however indicates that the differences are modest and insufficient to drive future designs in favour of the one layout over the other. Installing the third engine in a three-engined configuration results in burying the engine (e.g. Lockheed L-1011) or pod mounting it at the base of the fin (e.g. MD-11). The layouts are Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 7 Rev. 3 __________________________________________________________________________________________ comparable regarding performance and weight. Engine access is more difficult for the third engine. Today, four engine aircraft struggle to compete economically with two engine aircraft. Modern jet engines are so reliable that the previous justification for installing four or even three engines on civil transport aircraft – that is, based on in-flight shutdown rates – is no longer valid. 4.3 Regional Jet Aircraft Pod mounting engines below the wing (Embraer E170/190, Sukhoi Superjet 100), or on the rear fuselage (Sud Aviation Caravelle, Fokker 100) are widely adopted solutions. Most regional jets are of a low configuration; the high wing Bae 146, however, featured four pod mounted engines installed below the wing. Mounting the engines in pods on pylons above the wing is rare (adopted for the VFW 614). 4.4 Piston Engined Aircraft Piston engines are mostly installed on single-engined aircraft in the nose mounted tractor layout and in dual-engined aircraft on the wings. Locating the propeller in front of the wing has a favourable impact on stall and increases the wing lift. Engine failure, however, causes undesirable disturbance of the airflow over the wing before the propeller is feathered. Pusher configurations (e.g. Lovaux Optica) are less common in the Commercial and General Aviation sectors, although they are popular with home builders (e.g. Rutan Long EZ, Quickie). The pusher-puller layout (e.g. Cessna 337 Skymaster, Rutan Defiant) is seldom used. The pusher layout is liked by the microlight fraternity (e.g. Quicksilver, Aviasud Mistral), as it is quieter and provides an unrestricted forward visibility but is poor with regard to crashworthiness. An odd-ball layout is the high wing Dornier Do-28 Skyservant, which features two tractor engines pod-mounted off the fuselage. 4.5 Turboprop Powered Aircraft Some of the general comments made in Section 4.3 above are also true for turboprop-powered aircraft. Both single and twin-engine designs are popular. The single-engine tractor solution is favoured by certain manufacturers due to its simplicity (Pilatus PC 12, Cessna 208 Caravan). Twin-engine wing mounted tractor installations are by far the most common (e.g. Embraer Bandeirante, Piper Cheyenne); however, the twin turboprop pusher layout as illustrated by the Beechcraft Starship and Piaggio Avanti is claimed to have performance advantages in this class of aircraft. Powerplants installed on a high wing present fewer problems to the designers than those installed on low wing aircraft. To achieve adequate ground clearance the powerplant must be located in a relatively high position with regard to the wing and this may lead to both structural Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 8 Rev. 3 __________________________________________________________________________________________ and aerodynamic problems. Also, it may be necessary to duct the hot gasses over the top of the wing box, adding weight and complexity (Fig. 4.3). Amphibian aircraft (either piston or turboprop) require a high mounted engine installed in a pod above the fuselage (e.g. Claudius Dornier Seastar) or mounted above the wing (e.g. Canadair CL-215). Engine installations of this type suffer from undesirable pitch changes with changes in thrust and are not common except in this aircraft category. Figure 4.3 Three alternative turboprop engine installation designs(Torenbeek ) 4.6 Jet Fighters Jet engine installation on fighters depends on the aircraft role (e.g. Air Superiority, Intercept, Ground Attack, Strike) and specific design requirements (e.g. stealth, vulnerability). The classic fighter has one or two engines installed in the fuselage. The engine position and internal structural layout impact the intake design (which can be a complicated and costly element of the overall design). The integration of the intake and inlet duct into the fuselage has resulted in several different intake positions being used: nose (e.g. Mig-21, BAe Lightning); Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 9 Rev. 3 __________________________________________________________________________________________ chin (Vought A-7, Lockheed F-16); wing-root (BAe Hunter); side of fuselage (Saab Gripen, Mirage III); and lower fuselage (MD F/A-18, Dassault Rafale). A long exhaust (Aermacchi 339) adds weight and results in a loss of thrust, while a short exhaust (e.g. MD F-4 Phantom) requires special precautions to prevent the hot gas from damaging the aircraft outer skin. Special measures and ground equipment are required for removing fuselage installed engines. (The BAe Harrier has the engine removed vertically.) Installing the engines below the wing (e.g. Mig-29, Su-27) eliminates some installation problems but can introduce others like FOD (Foreign Object Damage). Special design requirements like a low radar cross-section and/or infrared signature can influence the position (and design) of the engine, intake and exhaust (e.g. Fairchild A-10, Northrop B-2). 5. TAIL AND CANARD CONFIGURATIONS 5.1 Design Influences The tail must provide trim, stability and control under normal operating conditions and in the event of an emergency (e.g. engine failure). The wing layout and relative position of powerplant(s) thus has a strong influence on the design of the tail. Jet efflux and prop-wash require consideration. The tail must be sized to provide adequate control power at all critical conditions. These critical conditions for the horizontal tail (or canard) include nosewheel lift-off, low speed flight with flaps down and transonic manoeuvring. For the fin, the critical conditions include engine-out at low speed, maximum roll rate and spin recovery (Raymer ). 5.2 Tail Configurations A number of tail configurations are illustrated in the Figure 5.1. The so-called conventional layout of a single fin and horizontal stabiliser mounted on the fuselage is by far the most common (e.g. Boeing 747, Airbus A-310). For many aircraft this layout satisfies all requirements at the lightest weight. The T-tail is also used widely (e.g. MD-80, Boeing 727). Although inherently heavier than a conventional tail, the T-tail configuration can result in a smaller fin (due to the end-plate effect) and smaller horizontal tail (increased efficiency due to reduced wing wake and prop-wash). It does, however, require special attention regarding flutter and deep stall. The cruciform tail (e.g. Rockwell B-1B) imposes a lower weight penalty than the T-tail and provides sufficient rudder below the horizontal tail for spin recovery. However, the end-plate effect is not applicable to the fin in this case. The H-tail (e.g. Fairchild A-10) is heavier than the conventional tail; although, it does reduce the size of the horizontal tail. The fins can be positioned so that they operate in undisturbed air in high alpha (i.e. angle of attack) conditions. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 10 Rev. 3 __________________________________________________________________________________________ The V-tail layout (e.g. Aerospatiale Magister, Beechcraft Bonanza) is claimed to reduce the wetted area of the tail, but investigation by NASA indicated that this is ill-founded (Raymer ). However, the layout reduces interference drag but a complexity in control of the ruddervators results as the control inputs of pitch and yaw must be mixed. The V-tail suffers from adverse roll-yaw coupling. This is not the case with an inverted V-tail, but due to ground clearance requirements on rotation, this layout is difficult to achieve. For Naval aircraft the twin vertical tail is desirable as it reduces the overall aircraft height (e.g. Grumman F-14, MD F-18). Although heavier than a single fin, the layout is sometimes unavoidable for large fighters (e.g. MD F-15, SU-27) because of “blanking” of the fin by the wing and fuselage at high alpha. Boom mounted tails are generally heavier than a conventional layout but are desirable for certain applications, like pusher aircraft (e.g. Cessna 337 Skymaster, Lovaux Optica) or where rear fuselage loading is required (e.g. IAI Arava). Figure 5.1 Aft tail (empennage) concepts (Raymer ) 5.3 Canards For a control canard, the wing carries most of the lift and the canard provides control input (e.g. Typhoon, Wright Flyer). However, a lifting canard operates at some angle of attack and provides lift. For the latter, the aircraft's centre of gravity is positioned well forward of the typical location. The lifting canard arrangement is theoretically more efficient than an aft-tailed aircraft because the lift of the canards reduces the lift that must be produced by the wing, resulting in a smaller wing and reduced total drag due to lift. However, the advantages are offset by some drawbacks. Achieving sufficient additional lift to balance the pitching moment caused by flap deflection at low speed can be a problem (e.g. Beechcraft Starship). As the distance from aircraft's CG (centre of gravity) to the canard is less than that from the CG to the Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 11 Rev. 3 __________________________________________________________________________________________ elevator on a conventional layout, greater control deflections are required, producing more trim drag (Raymer ). 5.4 Spin Recovery Spin recovery depends on having sufficient rudder control at high angles of attack to stop the rotation and reduce the sideslip angle. In a spin the horizontal tail is stalled and produces a turbulent wake rendering any part of the rudder within this wake useless (Fig. 5.2). Correct relative positioning of the rudder and horizontal tail is required (discussed later in Chapter 7 of this lecture series). Figure 5.2 Relative position of horizontal and vertical tailplanes for spin recovery (Raymer ) 5.5 Tailless Aircraft The tailless delta configuration (e.g. Mirage 2000, Concorde) and flying wing (e.g. Northrop B-2) have an obvious weight saving and drag advantage (due to the reduced wetted area) in eliminating part or all of the tail. Tailless designs, however, are sensitive to CG location and have a small permissible CG range, restricting the location of fuel and payload. REFERENCES Raymer, D.P., Aircraft design: A conceptual approach, AIAA Education Series, 3rd ed., American Institute of Aeronautics and Astronautics, Reston, VA, 1999. Torenbeek, E., Synthesis of subsonic airplane design, Delft University Press, Delft, the Netherlands, 1982. Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2 Page 12 Rev. 3 __________________________________________________________________________________________ Module lecture notes by Prof. T.M. Young, University of Limerick, Limerick V94T9PX, Ireland. Not to be reproduced without permission.  Aircraft Design ME4807/Ch. 2. Page A1 APPENDIX A' EXTRACT FROM JANE'S WORLD AIRCRAFT RECOGNITION HANDBOOK, D. WOOD Lockheed F-117A  Aircraft Design ME4807/Ch. 2. Page A2 BAe One-Eleven "1' BAe VC10/Super VC10 -t McDonnell Douglas MD-80 + -- '----- Suan (Series 500) 28 Sm Leng/h 32 61 rn Span 44 55m Leng1h (Super VC 10) 52 32m Span 32 85m Length 41.3m I ------£ti'i2-- 41' j· Boeing 747 1' Boeing 727 -------------- -------------- --- -- ---- ----- Leng1h 70 51m BAe 146 t- Span 59 64m Power 4 x ALF502 turbofans Span 26.34m Power: 3 x JT80 turbofans Span. (-200): 32.92 --·-- I 0 -ii!- o- o 0 @ @ m @ @ J;. Ref. D. Wood, Jane's World Aircraft Recognition Handbook  Aircraft Design ME4807/Ch. 2. Page A3 1' Airbus A310 Power 2 x CF6 or JT9D turbofans Lockheed L-1011 TriStar Span 43 gSpan 47 35m + Length (except -500) 54 17m McDonnell Douglas MD-11 Span 51.66m + Length 61.21m 1. ---@- -- @ ____............ ,_ __ liil + DHC DHC-5 Buffalo, Power 2 x CT64 turboprops -j:- Span 29.26m Transporter Allianz C-160 Transall Lockheed C-130 Hercules Span 40.41m T Length. 29. 78m Ref. D. Wood, Jane's World Aircraft Recognition Handbook  Aircraft Design ME4807/Ch. 2. Page A4 + Piaggio P.180 Avanti Powe, 2 x PT6A turboprops Span 13 84m t- Beechcraft Starship 1 Power 2 x PT6A turboprops Span 16 6m Fokker F27 Friendship Power 2 x Dart turbopro ps Span 29m -- /__=·re-·- ="'· ·=-·--=-