Aircraft Design Projects for engineering students(download)

Aircraft Design Projects for engineering students




Introduction
It is tempting to title this book ‘Flights of Fancy’ as this captures the excitement and
expectations at the start of a new design project. The main objective of this book is
to try to convey this feeling to those who are starting to undertake aircraft conceptual
design work for the first time. This often takes place in an educational or industrial
training establishment. Too often, in academic studies, the curiosity and fascination of
project work is lost under a morass of mathematics, computer programming, analytical
methods, project management, time schedules and deadlines. This is a shame as there
are very few occasions in your professional life that you will have the chance to let your
imagination and creativity flow as freely as in these exercises. As students or young
engineers, it is advisable to make the most of such opportunities.
When university faculty or counsellors interview prospective students and ask why
they want to enter the aeronautics profession, the majority will mention that they want
to design aircraft or spacecraft. They often tell of having drawn pictures of aeroplanes
since early childhood and they imagine themselves, immediately after graduation, producing
drawings for the next generation of aircraft. During their first years in the
university, these young men and women are often less than satisfied with their basic
courses in science, mathematics, and engineering as they long to ‘design’ something.
When they finally reach the all-important aircraft design course, for which they have
yearned for so long, they are often surprised. They find that the process of design
requires far more than sketching a pretty picture of their dream aircraft and entering
the performance specifications into some all-purpose computer program which will
print out a final design report.
Design is a systematic process. It not only draws upon all of the student’s previous
engineering instruction in structures, aerodynamics, propulsion, control and other
subjects, but also, often for the first time, requires that these individual academic
subjects be applied to a problem concurrently. Students find that the best aerodynamic
solution is not equated to the best structural solution to a problem. Compromises
must be made. They must deal with conflicting constraints imposed on their design
by control requirements and propulsion needs. They may also have to deal with real
world political, environmental, ethical, and human factors. In the end, they find they
must also do practical things like making sure that their ideal wing will pass through
the hangar door!


An overview of the book
This book seeks to guide the student through the preliminary stages of the aircraft
design process. This is done by both explaining the process itself (Chapters 1 and 2)
and by providing a variety of examples of actual student design projects (Chapters 3
to 10). The projects have been used as coursework at universities in the UK and the US.
It should be noted that the project studies presented are not meant to provide a ‘fill in
the blank’ template to be used by future students working on similar design problems
but to provide insight into the process itself. Each design problem, regardless of how
similar it may appear to an earlier aircraft design, is unique and requires a thorough
and systematic investigation. The project studies presented in this book merely serve
as examples of how the design process has been followed in the past by other teams
faced with the task of solving a unique problem in aircraft design.
It is impossible to design aircraft without some knowledge of the fundamental theories
that influence and control aircraft operations. It is not possible to include such
information in this text but there are many excellent books available which are written
to explain and present these theories. A bibliography containing some of these books
and other sources of information has been added to the end of the book. To understand
the detailed calculations that are described in the examples it will be necessary to use
the data and theories in such books. Some design textbooks do contain brief examples
on how the analytical methods are applied to specific aircraft. But such studies are
mainly used to support and illustrate the theories and do not take an overall view of
the preliminary design process.
The initial part of the book explains the preliminary design process. Chapter 1 briefly
describes the overall process by which an aircraft is designed. It sets the preliminary
design stages into the context of the total transformation from the initial request for
proposal to the aircraft first flight and beyond. Although this book only deals with
the early stages of the design process, it is necessary for students to understand the
subsequent stages so that decisions are taken wisely. For example, aircraft design is
by its nature an iterative process. This means that estimates and assumptions have
sometimes to be made with inadequate data. Such ‘guesstimates’ must be checked when
more accurate data on the aircraft is available. Without this improvement to the fidelity
of the analytical methods, subsequent design stages may be seriously jeopardized.
Chapter 2 describes, in detail, the work done in the early (conceptual) design process.
It provides a ‘route map’ to guide a student from the initial project brief to the validated
‘baseline’ aircraft layout. The early part of the chapter includes sections that deal with
‘defining and understanding the problem’, ‘collecting useful information’ and ‘setting
the aircraft requirements’. This is followed by sections that show how the initial aircraft
configuration is produced. Finally, there are sections illustrating how the initial aircraft
layout can be refined using constraint analysis and trade-off studies. The chapter ends
with a description of the ‘aircraft type specification’. This report is commonly used to
collate all the available data about the aircraft. This is important as the full geometrical
description and data will be needed in the detailed design process that follows.
Chapter 3 introduces the seven project studies that follow (Chapters 4 to 10). It
describes each of the studies and provides a format for the sequence of work to be
followed in some of the studies. The design studies are not sequential, although the
earlier ones are shown in slightly more detail. It is possible to read any of the studies
separately, so a short description of each is presented.
Chapters 4 to 10 inclusive contain each of the project studies. The projects are selected
from different aeronautical applications (general aviation, civil transports, military
aircraft) and range from small to heavy aircraft. For conciseness of presentation the
detailed calculations done to support the final designs have not been included in these
chapters but the essential input values are given so that students can perform their
own analysis. The projects are mainly based on work done by students on aeronautical
engineering degree courses. One of the studies is from industrial work and some have
been undertaken for entry to design competitions. Each study has been selected to
illustrate a different aspect of preliminary design and to illustrate the varied nature of
aircraft conceptual design.
The final chapter (11) offers guidance on student design work. It presents a set of
questions to guide students in successfully completing an aircraft design project. It
includes some observations about working in groups. Help is also given on the writing
of technical reports and making technical presentations.

Engineering units of measurement
Experience in running design projects has shown that students become confused by
the units used to define parameters in aeronautics. Some detailed definitions and conversions
are contained in Appendix A at the end of the book and a quick résumé is
given here.
Many different systems of measurement are used throughout the world but two have
become most common in aeronautical engineering. In the US the now inappropriately
named ‘British’ system (foot, pound and second) is widely used. In the UK and over
most of Europe, System International (SI) (metres, newton and second) units are standard.
It is advised that students only work in one system. Confusion (and disaster) can
occur if they are mixed. The results of the design analysis can be quoted in both types
of unit by applying standard conversions. The conversions below are typical:
1 inch = 25.4 mm
1 sq. ft = 0.0929 sq. m
1 US gal = 3.785 litres
1 US gal = 0.833 Imp. gal
1 statute mile = 1.609 km
1 ft/s = 0.305 m/s
1 knot = 1.69 ft/s
1 pound force = 4.448 newtons
1 horsepower = 745.7 watts
1 foot = 0.305 metres
1 cu. ft = 28.32 litres
1 Imp. gal = 4.546 litres
1 litre = 0.001 cubic metres
1 nautical mile = 1.852 km
1 knot = 0.516 m/s
1 knot = 1.151 mph
1 pound mass = 0.454 kilogram
1 horsepower = 550 ft lb/s
To avoid confusing pilots and air traffic control, some international standardization of
units has had to be accepted. These include:
Aircraft altitude – feet Aircraft forward speed – knots∗
Aircraft range – nautical miles Climb rate – feet per minute
(∗ Be extra careful with the definition of units used for aircraft speed as pilots like to use
airspeed in IAS (indicated airspeed as shown on their flight instruments) and engineers
like TAS (true airspeed, the speed relative to the ambient air)).
Fortunately throughout the world, the International Standard Atmosphere (ISA)
has been adopted as the definition of atmospheric conditions. ISA charts and data
can be found in most design textbooks. In this book, which is aimed at a worldwide
readership, where possible both SI and ‘British’ units have been quoted. Our apologies
if this confuses the text in places.




Design methodology

The start of the design process requires the recognition of a ‘need’. This normally comes
from a ‘project brief’ or a ‘request for proposals (RFP)’. Such documents may come
from various sources:
• Established or potential customers.
• Government defence agencies.
• Analysis of the market and the corresponding trends from aircraft demand.
• Development of an existing product (e.g. aircraft stretch or engine change).
• Exploitation of new technologies and other innovations from research and
development.
It is essential to understand at the start of the study where the project originated and to
recognise what external factors are influential to the design before the design process
is started.
At the end of the design process, the design team will have fully specified their design
configuration and released all the drawings to the manufacturers. In reality, the design
process never ends as the designers have responsibility for the aircraft throughout its
operational life. This entails the issue of modifications that are found essential during
service and any repairs and maintenance instructions that are necessary to keep the
aircraft in an airworthy condition.
The design method to be followed from the start of the project to the nominal end can
be considered to fall into three main phases. These phases are illustrated in Figure 1.1.
The preliminary phase (sometimes called the conceptual design stage) starts with the
project brief and ends when the designers have found and refined a feasible baseline
design layout. In some industrial organisations, this phase is referred to as the ‘feasibility
study’. At the end of the preliminary design phase, a document is produced which
contains a summary of the technical and geometric details known about the baseline
design. This forms the initial draft of a document that will be subsequently revised
to contain a thorough description of the aircraft. This is known as the aircraft ‘Type
Specification’.
The next phase (project design) takes the aircraft configuration defined towards
the end of the preliminary design phase and involves conducting detailed analysis to
improve the technical confidence in the design. Wind tunnel tests and computational
fluid dynamic analysis are used to refine the aerodynamic shape of the aircraft. Finite
element analysis is used to understand the structural integrity. Stability and control
analysis and simulations will be used to appreciate the flying characteristics. Mass and
balance estimations will be performed in increasingly fine detail. Operational factors
(cost, maintenance and marketing) and manufacturing processes will be investigated


to determine what effects these may have on the final design layout. All these investigations
will be done so that the company will be able to take a decision to ‘proceed
to manufacture’. To do this requires knowledge that the aircraft and its novel features
will perform as expected and will be capable of being manufactured in the timescales
envisaged. The project design phase ends when either this decision has been taken or
when the project is cancelled.
The third phase of the design process (detail design) starts when a decision to build
the aircraft has been taken. In this phase, all the details of the aircraft are translated
into drawings, manufacturing instructions and supply requests (subcontractor agreements
and purchase orders). Progressively, throughout this phase, these instructions
are released to the manufacturers.
Clearly, as the design progresses from the early stages of preliminary design to the
detail and manufacturing phases the number of people working on the project increases
rapidly. In a large company only a handful of people (perhaps as few as 20) will be
involved at the start of the project but towards the end of the manufacturing phase
several thousand people may be employed. With this build-up of effort, the expenditure
on the project also escalates as indicated by the curved arrow on Figure 1.1.
Some researchers1 have demonstrated graphically the interaction between the cost
expended on the project, the knowledge acquired about the design and the resulting
reduction in the design freedom as the project matures. Figure 1.2 shows a typical
distribution. These researchers have argued for a more analytical understanding of the
requirement definition phase. They argue that this results in an increased understanding
of the effects of design requirements on the overall design process. This is shown
on Figure 1.2 as process II, compared to the conventional methods, process I. Understanding
these issues will increase design flexibility, albeit at a slight increase in initial
expenditure. Such analytical processes are particularly significant in military, multirole,
and international projects. In such case, fixing requirements too firmly and too
early, when little is known about the effects of such constraints, may have considerable
cost implications.
Much of the early work on the project is involved with the guarantee of technical
competence and efficiency of the design. This ensures that late changes to the design


layout are avoided or, at best, reduced. Such changes are expensive and may delay the
completion of the project. Managers are eager to validate the design to a high degree
of confidence during the preliminary and project phases. A natural consequence of this
policy is the progressive ‘freezing’ of the design configuration as the project matures.
In the early preliminary design stages any changes can (and are encouraged to) be
considered, yet towards the end of the project design phase only minor geometrical
and system modifications will be allowed. If the aircraft is not ‘good’ (well engineered)
by this stage then the project and possibly the whole company will be in difficulty.
Within the context described above, the preliminary design phase presents a significant
undertaking in the success of the project and ultimately of the company.
Design project work, as taught at most universities, concentrates on the preliminary
phase of the design process. The project brief, or request for proposal, is often used to
define the design problem. Alternatively, the problem may originate as a design topic
in a student competition sponsored by industry, a government agency, or a technical
society. Or the design project may be proposed locally by a professor or a team of
students. Such design project assignments range from highly detailed lists of design
objectives and performance requirements to rather vague calls for a ‘new and better’
replacement for existing aircraft. In some cases student teams may even be asked to
develop their own design objectives under the guidance of their design professor.
To better reflect the design atmosphere in an industry environment, design classes at
most universities involve teams of students rather than individuals. The use of multidisciplinary
design teams employing students from different engineering disciplines is
being encouraged by industry and accreditation agencies.
The preliminary design process presented in this text is appropriate to both the individual
and the team design approach although most of the cases presented in later
chapters involved teams of design students. While, at first thought, it may appear that
the team approach to design will reduce the individual workload, this may not be so.

The interpersonal dynamics of working in a team requires extra effort. However, this
greatly enhances the design experience and adds team communications, management
and interpersonnel interaction to the technical knowledge gained from the project work.
It is normal in team design projects to have all students conduct individual initial
assessments of the design requirements, study comparable aircraft, make initial estimates
for the size of their aircraft and produce an initial concept sketch. The full team will
then begin its task by examining these individual concepts and assessing their merits
as part of their team concept selection process. This will parallel the development of
a team management plan and project timeline. At this time, the group will allocate
various portions of the conceptual design process to individuals or small groups on
the team.
At this point in this chapter, a word needs to be said about the role of the computer
in the design process. It is natural that students, whose everyday lives are filled with
computer usage for everything from interpersonal communication to the solution of
complex engineering problems, should believe that the aircraft design process is one in
which they need only to enter the operational requirements into some supercomputer
and wait for the final design report to come out of the printer (Figure 1.3).
Indeed, there are many computer software packages available that claim to be ‘aircraft
design programs’ of one sort or another. It is not surprising that students, who have
read about new aircraft being ‘designed entirely on the computer’ in industry, believe
that they will be doing the same. They object to wasting time conducting all of the
basic analyses and studies recommended in this text, and feel that their time would
be much better spent searching for a student version of an all-encompassing aircraft
design code. They believe that this must be available from Airbus or Boeing if only they
can find the right person or web address.
While both simple aircraft ‘design’ codes and massive aerospace industry CAD
programs do exist and do play important roles, they have not yet replaced the basic processes
outlined in this text. Simple software packages which are often available freely at
various locations on the Internet, or with many modern aeronautical engineering texts,
can be useful in the specialist design tasks if one understands the assumptions and limitations
implicit in their analysis. Many of these are simple computer codes based on





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