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Module 1: The Phases of Aircraft Design

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Aircraft Design Process

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Lecture No – 03
Design Stages
These particular stages are actually the same as what we saw in the previous clip regarding the
phases in aircraft design it is just a different way of looking at the same thing. So the first stage
of aircraft design is the conceptual design stage which involves only 1% of the total effort
involved in aircraft design. In this there are few activities we do, we look at competing concept
and we evaluate them.
Because we want to ensure that the requirements that drive the design are understood and
appreciated. We establish the performance goals to ensure that these requirements are met and to
confirm that the design will work and in the end after a comparative analysis of various
computing concepts we shortlist a preferred concept or concepts one or two concepts for future
evaluation.
And at this point of time we should have a fairly good idea of what our aircraft will look like?
The next stage is the Preliminary Design stage which roughly compasses, encompasses sorry!
The next stage is the Preliminary Design stage which roughly encompasses nearly 9% of the
total effort and time spent in the design and in this stage we do refined sizing and refine sizing
required some optimization little bit of optimization may be needed also in the previous stage,
because to arrive at which configuration is the best.
We do need to have some kind of an objective which has to be optimized, so optimization is used
also in the previous stage but here it is at a far larger level and then we examine and verify our
preferred concept or concepts using detail testing in the Wind Tunnel or structural analysis and
we might have to do modifications and amendments in the design that we had taken up
essentially to ensure that we are on the right track and the requirements are met.
And also at this place we need to get the detailed cost estimates of our design and then we come
to the last stage in our design which is the detailed design stage. Here 90% of the effort is spent
in the design process and in this process what we do is we do detail drawings and detailed CAD
of various parts that go into the aircraft the various sub-assemblies the various components
etcetera, the word component here should not be confused with the word component used earlier
for the major assemblies okay?
But and here you look at the details of the flight control system you go for sizing of various
components in detail, you again go for the detailed performance estimation and during this
process you do that check on the various systems and components and also remember that in this
particular stage only minor changes are permitted and now we start looking at what would be the
effort involved in certification activity that we will have to undertake.
So summing up one way of looking at the 3 stages in aircraft design, the names are the same
Conceptual, Preliminary and Detailed in this particular perspective and it is very important to the
students that in our classroom course of aircraft design we might be able to do only about 1%
which is the conceptual design stage and may be a small part of the second stage of the
Preliminary design.
It is rarely possible for a classroom environment to deal with detailed design and, the activity in
the classroom therefore will be limited only to the conceptual design and a very small part of the Preliminary design.
Lecture No – 04
Phases in Aircraft Design
The first phase in aircraft design is called as the conceptual design. This particular phase focuses
on the method of the aircraft design; in this phase our main objective is to establish the feasibility
of the concept. So when the requirements are specified by the customer and of course, there are
some unspecified by the customer, but applicable airworthiness requirements? There are several
possible configurations and several possible arrangements, which can , There are several possible
arrangements, which can meet the requirement.

So all these concepts have to be investigated for their feasibility and once the feasibility is
established, we need to shortlist a few of them and then maybe take a call on which of them
seems to be the most feasible. But the most important task that you do in aircraft conceptual
design is to identify the requirements that drive the design. It is important to sensitize the
students and maybe by giving some examples that sometimes the requirements specified by the
customer may be quite arbitrary in nature.

In many cases the customer may not really know what the customer wants. They may think they
know what they wants but they may actually not know what they want, partly because we are
talking about something that will actually come after another 10 to 15 years, so they are a bit
uncertain. So what many customers do is they look at the best features of existing aircraft, they
may scale it up by a small amount and they combine that.

So they want the aircraft that can fly the fastest and also take off in the shortest distance and also
have the least fuel fraction and also carry the maximum payload, it happens sometimes like that.
They may not know themselves, what is the repercussion of specifying these requirements on the
design? So, it is the designer’s job to carry out initial estimates and sensitize the customer that
look, you have given some requirements.

And you may not know but these are the requirements because of which the aircraft has become
three times larger in the wing area or two times heavier than what could have been and then the
customer says, wow, I had no idea, so maybe the customer will relax the requirement. In my
own experience, there have been several cases where I have done some calculations for the
customer and I have told the customer that, this is the aircraft and they say why is it so large?

And then we say but if you relax that requirement, it will become very small and they said I
never knew about it. I will illustrate this through some examples in another part of this course,
but we have to sensitize the students at this point, that the principal task according to me in
conceptual design is not to come up with those very lovely sketches of the aircraft, but to really
identify the requirements that drive the design.

And to have the one-on-one with the customer that these requirements are essential and hence
that design has to be accepted. Once we do this, it is important to carry out initial sizing and
layout. The word initial sizing in aircraft design has a very special meaning and I think the
students need to be sensitized about this particular word. Initial sizing in aircraft design stands
for the first estimate of the gross weight, design gross weight of the aircraft based on the
specified requirements and the operational constraints.

The design gross weight of the aircraft it may be noted is the weight of the aircraft just before it
is about to start the design mission or the mission for which it has been designed. We will look at
the initial sizing procedure very soon in a separate clip. The layout essentially looks at the
relative location of various major assemblies. So we need to carry out initial sizing and we need
to carry out the layout that means we need to work out how the aircraft will look like.

And also not very apparent sometimes but most essential is that we need to estimate the masses
of the various components, good estimates about the likely performance and very important, the
cost and not just the cost of producing the aircraft, but the cost over its own life cycle, these
aspects will be elaborated more in a separate clip. A point of clarification here for the students,
the word component in this particular slide does not stand for what we understand as the
individual component of any system.

The word component in aircraft design actually stands for major assemblies. So for example;
wing, fuselage, tail, landing gear, nacelles these are the components. So when I say component
masses, I do not mean the mass of each strip of metal that goes on the aircraft, the mass of each
item that we put on the aircraft, this is the mass of the major assemblies of the aircraft. The next
step is the preliminary design in which we try to arrive at a more firm estimate of the numbers
related to the aircraft that we are designing.

In conceptual design also we have some numbers but those numbers are not yet fixed, they are a
bit fuzzy. They are not exact, they are indicative at the most. In the preliminary design we need
to actually arrive at a little bit more accurate value, not the final value, but much more accurate
value of these numbers. But then there is a question here, when does the conceptual design stop
and where does the preliminary design start?

So that is very important, the preliminary design starts or you can also say that the conceptual
design stops when we freeze the configuration. That means, that among the various
configurations that were taken up for possible investigation or they were taken up sorry! Among
the various configuration that we have taken up as the possible candidates to meet these

requirements, maybe one or perhaps very few, two three would have to be taken to the next step.
But in most cases, in the conceptual design phase, we zoom into now one configuration by
freezing.

So you look at all possible configurations quite extensively, evaluate them to the extent you can
within the time and the resources available and then you come up to a decision. And the design
there you bored normally decides and finalises one configuration which would then be subjected
to all further work. The reason for that is the preliminary design exercise is a very time
consuming and expensive exercise.

And it may not be possible for any design organization to do this exercise for many, many cases.
Because it involves number crunching, it involves arriving at more accurate estimates. So
generally, we freeze one configuration perhaps two and then take it further for preliminary
investigation. What we do in this particular stage is to ensure that what we have finalized is
actually practical? As an example, we might have decided in conceptual design that the aircraft is
going to have a retractable landing gear.

Because, that landing gear is going to create a tremendous amount of drag during the cruising
flight or in the non take off on landing phases when it is needed to be launched by function,
during climb, cruise, descent we do not want a landing gear to be out, so this is what we decided
in the conceptual phase. But in the preliminary phase we have to now decide, okay which
landing gear does the main wheel go forward or backward is retraction, do we have to bend these
things in the process of retraction because of the space constraints.

Is it really practical to have the landing gear at a place where we have decided it to be? So we
ensure the practicality, similarly we may decide that we will go for fowler falls. But will you
have them in many components or will you have it in multiple elements or will it be the single
element, how will it be actually performing? How will it be actually moving, what would be the
flap track location, its dimensions etcetera, which motor will be used or which system will be
used, those things are decided in the preliminary design stage.

We also develop several mechanical and structural concepts which are then going to be used to
analyze the aircraft and to arrive at these numbers more accurately and we also have to develop
the test and the analytical base. So the large amount of wind tunnel testing and the large amount
of FEM analysis, all that takes place in this particular phase. Some amount of wind tunnel testing
and analysis will also be needed in the conceptual phase.

But it would be of a preliminary nature, very detailed testing, very detailed analysis is done
normally in the preliminary design and at the end of this particular phase, we actually have a
fairly good idea about the aircraft and its various major assemblies, how they will be located,
what will they way, what will be their basic configuration and then we come to a stage whether
either we decide that, no this particular aircraft as we have envisaged so far is not practical, is not
going to really serve the purpose so we either abandon it or we taken a decision that yes,

What we have arrived at a particular stage is a good candidate, which is worthy of the next stage
which is to be taken up for the fabrication and if that fabrication of the working prototype is to
test it out? So at that stage that detailed design stage starts, which is essentially the nuts and bolts
of design. This is the place where actually each of these components, which we have finalized, is
going to be taken up for very detailed analysis and design.

This is a place where we look at; this is the phase where we start developing the tooling and the
fabrication process that will be followed not only for the prototype but also for the full scale
series production aircraft. We test major items, we test the wing for failure, we apply loads
which are expected and see that it is able to take the expected limit load, we also test items like
landing gear, we look for retraction test, we look at the drop test, and we do detail testing maybe
separation of stores from a military aircraft.

All these items are tested and then finally we arrive at a very detailed estimate of the weight of
various parts of the aircraft and a very detailed estimate of its performance, because now we
almost know everything about this aircraft. So summing up the phases in the aircraft design
consists of three basic steps, the conceptual steps, the preliminary stage and the detailed stage.