Introduction to Aircraft Design
What is Aircraft Design?
Aircraft design essentially consists of arriving at layout and configuration which can address two things. One, It should meet the customer specifications and the airworthiness requirements and second, it should be less expensive and easy to produce in large quantities. The second point
is the one that actually decides to a large extent the choice of materials, the choice of manufacturing processes.
And even sometimes it decides the choice of the shape that we use, the choice of the systems that we decide to provide and also all kinds of decisions regarding the facilities to be included in the aircraft. Now, let us have a look at the two key requirements for which a design is normally attempted. The first of these are the customer specifications, obviously no activity of design can be done completely in isolation with the prospective users and the stakeholders.
So, for an aircraft design typically we look at the market requirements which are essentially for the civilian aircraft or aircraft to be used by civilians and there could be operational requirements. These are more applicable for the military aircraft. For civil aircraft especially, the
design requirements are also driven by the forecast of the economic situation in the future. It is
important to realize that what we are designing today will actually see the action perhaps 15 years later.
Because, a typical aircraft design cycle may last for about 3 to 5 years, but bringing the aircraft into service would probably be about 10 years from today. So, right now the designer needs to have a fairly good idea about what would be the expected economical situation in the future for a civil aircraft, and for military aircraft, we have to have a good idea about the likely operational scenario in which our aircraft is going to operate.
This is a very difficult task and hence this is something which is not very easy. So, when students normally come up with the various kinds of design requirements, it is important for us (faculty members) to sensitize them. The requirements which are going to be driving the design or the requirements for which the aircraft will be designed are actually not very easy to obtain. In a class room we can obtain or list any requirement, maybe from the top of our head.
But in real life, getting these requirements right is a very big decision and an important task and all aircraft manufacturing companies they have a division that does forecast of the situation in the future and based on those forecast they designed requirements are drawn up. The airlines and the stakeholders are consulted and they are the ones who actually sit together and come up with a broad list of specifications, which are then slowly narrowed down to more and more finer
So, students are sometimes not able to appreciate the importance of this particular task.
Similarly, design of an aircraft is also driven by the technological developments that are taking
place or have taken place in the recent past. We are on aware that there are many projects which are taken up for what is called as technology demonstration? In short, we call it as TD projects. If
you look at the US Air Force or US Department of defense, all the aircraft that start with the X series starting from X1, X2 to X15 etcetera.
They are all experimental category aircraft and they are all aircraft which have been taken up essentially to illustrate or to establish the working of a particular technology which may or may not find immediate application. But once the technology has been developed and tested It is then
available for the future. So, once a technology is matured and available it becomes a de facto
requirement that such a technology will be definitely provided in the aircraft.
For example, any modern civil aircraft people will assume that it will have a good fly by wire
control system. Because fly by wire control technology has now been matured and very well
established. Similarly, use of composite material, provision of glass cockpit are some examples
of technologies which were considered to be futuristic in the past. But they are now considered as de facto and hence the customer specifications would include these.
So, we have to be aware about what is happening in the technological world and be sensitive to the developments in those fields so that we can understand its implications in the customer
specifications. The other requirements which are specified by the airworthiness or the regulatory
bodies, they also have to keep in mind, many a times the students are not aware of this particular
requirement and several decisions, several constraints are imposed in aircraft design, simply
because it has to be eventually certified. In the case of military aircraft there are separate
requirements examples, the agencies which do these requirements. In India, we have CEMILAC, center for military aircraft airworthiness. They set out the requirements applicable to the defense or military aircraft and we have the DGCA the directorate general of civil aviation which actually sets out the requirements for the civil aircraft.
Both of these agencies may sometimes borrow the requirements from other agencies in the world who have already established these requirements. For example, the DGCA may take
requirements from the federal aviation regulations or the JAR requirements Joint Airworthiness
Requirements from Europe. So, the regulatory body of each sovereign nation has the right to
decide what requirements are applicable and binding in the aircraft operating in their domain.
Hence, it is very important for us to have a good handle about the airworthiness and regulatory requirements. We will take up a special capsule focused on the requirements and there we will elaborate in detail about how these requirements directly impact the design. Let us understand
the basis for the provision of airworthiness requirements. The first point to be kept in mind is that these requirements are generally driven by past experience.
Some of the requirements may not have any analytical or rigorous mathematical basis. They
come from experience and they may come or they may be specified based on what has been
learned in the past due to a scrutiny of the possible causes of the accidents and the incidents that
took place. The fundamental aim of regulatory requirements or airworthiness requirements is to
ensure safety in operation.
Safety in aircraft design is paramount and it is an over reaching requirement above all other requirements and airworthiness regulations and regulatory requirements actually ensure that the aircraft is safe to operate. No wonder the accident rates of aircraft, if you look at per capita per operation, they are the best compared to any other known transportation system. You have much more likelihood some estimates say that you are nine times more probable to die in a road
accident involving a two wheeler compared to a commercial airliner.
Also, The requirements are aimed at ensuring, uniformity and standardization in reporting of the data.
Many a times the customers can actually be fooled or cheated by claims made by the aircraft manufacturing agencies about the performance of the aircraft. One example of a requirement which can be considered as a potential area of misleading information would be let us say the
So, for example, how would one define takeoff distance? Now this is something that I think we should ask in the class when we are dealing with the students. We should ask them, for example, how would you define takeoff distance? In most cases the student would say the takeoff distance
would be the distance on the ground from the point where the aircraft starts moving in the takeoff operation till it leaves the ground or till the nose wheel lifts off.
Now this could be one definition of takeoff, but it can be very misleading. For example, if there are two manufacturers, manufacturer A and manufacturer B and they are both competing for supplying an aircraft to operate from let us say a small airport. Manufacturer A might say that my aircraft can have a takeoff as defined by this particular requirement, which I just now stated
that is from the starting point to the point where it leaves the ground. It can do that in less than one kilometer. But when you actually operate the aircraft you might realize that at the end of one
kilometer the rate of climb of the aircraft could be as low as let us say one foot per minute or a
few centimeters per minute. Now effectively it has left the ground but there is going to be a huge
chance of a disaster unless this airport is in the middle of nowhere, which is rarely the case.
In all modern airports eventually is not today there are going to be construction of various buildings nearby or other obstacles, maybe man-made, maybe natural and hence you need to have a descent rate of climb after you take off. So, the takeoff distance definition actually includes what is called as an obstacle height and this obstacle height requirement ensure that no one is offering an aircraft that can leave the ground very quickly but not have sufficient angle and rate of climb after leaving the ground.
Lecture 2: Aircraft Design Process
We let us look now at the aircraft design process in which we will reinforce what we have discussed regarding the aircraft design phases, and will look at generally how the aircraft comes Here the whole activity is divided into three basic steps the steps of configuration development detailed design and after that, once the first prototype is fabricated you have the product support
activity. So, is the same thing expressed in s different way and this particular image is credited to
Dr. John McMasters who was a very distinguished research aerodynamicist and technical fellow
And according to him based on his huge experience in Boeing, you can notice that each of these
steps involve some kind of back and forth, for example; once you complete the detailed design of
the aircraft and once you do the flight testing and operations during the service of the aircraft
there are some growth variations there are some improvements, for example that you know you
have Boeing 737 which has come in dash 200, 300, 400, dash 500, 600.
And now they are probably running out of numbers because they already have dash 900, so all
these are growth versions which involve a little bit of design effort, may be needed in incorporating these changes and then finally you have retirement from service and you have an archival process.
For the military aircraft you know one normally looks only at mission focused aircraft design, I am going to qualify this statement very soon and that is the whole purpose of this particular slide, that typically the missions are the ones that are available the mission requirements are specified
which tell the designer what this aircraft needs to do, and the design team then carries out a conceptual design.
To inform, the customer, what do we think it will look like and if the customer generally is okay with that, then we you know we carry out more detail analysis where we look at the sizing performance or optimization, weights, aerodynamics propulsion, etcetera and here we look at what the equations and the procedures are teaching us and then we go back and forth till we finalize.
And there is a very interesting observation that there is no uniqueness in the aircraft design there are as many designs which are possible to meet given mission requirements. And the regulatory requirements as the number of designers are available okay, so some will be better than the other
based on which criteria you follow the source of this information has come from, Dr. Bill
Mason's notes from Virginia Tech where he teaches an Aircraft design course.
I am going to talk about Dr. Bill Mason's design data repository and there you will get a lot of sources which can be used by you to teach the students, however, one should keep in mind that today the main driver of the whole design process is cost. Cost is everything in today's environment because that is becoming the driving criteria for most of the activities that are taken
place in the field of aircraft design.
So it is very, very important to have a Handle on the cost right in the beginning of the design process, this is something that we need to emphasize with the students, that you can do a very great aircraft design very good looking aircraft design but unless it is operating cost unless it is life cycle cost is less it is not going to eventually find favor.
I will harp on this aspect in more detail in another clip professor Scott Eberhardt had spent some time at IIT Bombay as a visiting professor and during his discussions, he presented this particular slide where it shows that the whole process of the conceptual and preliminary design of the aircraft actually goes into a loop at various stages. You have an inner loop where we have trade studies and testing where systems manufacturing, controls, propulsion, structures, aerodynamics software.
All of them are playing a role to ensure that the musts which are the design requirements and objectives which are the wants are attacked and he says that if you properly pose a problem, then you have solved half the problem does it make the design requirement of objectives if not then
you proceed if not then you reject, and also notice that this entire process is encompassed inside
and outer ring which is limited by the constraints from resource from the other external factors
like the regulatory environment and also the integration.
And every design that you do should be sensitive to changes what happens if the requirements
change because requirements are not going to be sacrosanct and fixed for the entire period
remember the aircraft design process is going to take time and the conceiving of the aircraft, I
mean from the point time you conceive it till the time you actually make it maybe 10, 15 years
time and during that time the scenario in the world will change.
The operating scenario will change if you have made some estimate but that may not remain, so requirements will change assumptions will change the constraints will change the design has to be sensitive enough to ensure that it can handle these change, okay?
Let us look at the aircraft development process from another angle is the same thing, I am just trying to reiterate essentially this material you can present it to the students the way you feel is
most appropriate the way you feel it will be able to clarify the concepts in the best fashion to the
students, this is one more way of looking at the aircraft development process although this
particular process is an evolutionary process.
(Refer Slide Time: 07:05)
Now Daniel Raymer is a very celebrated author whose book on aircraft design is a very popular
book on aircraft design. In his textbook he has given, this particular sequential procedure for aircraft design, he mentions that requirements specified by the customers or by the agencies that regulate the technological developments available and the concepts which can be investigated
together come up result in the layout and then you do the initial sizing.
Then you look at aerodynamics propulsion mass sizing etcetera, etcetera, there is a whole
process that he has laid out. However, in real life this whole process happens simultaneously, we
have reached a stage here in analysis today when we do concurrent engineering and aircraft design is a very good example of concurrent engineering where all the disciplines that have a role to play they actually work or they are analyzed together.
Because if you do it sequentially then you will not lead to an optimal solution, okay, you have to go lot of back and forth instead, why not do it together and today we have tools like Multidisciplinary design optimization, MDO for short which allow us to look the whole process at one shot, and that is a very important message to be conveyed to the students that do not look
at aircraft design as a simple element working in isolation.
And also when you work as a team you should not say this is the aerodynamics group, this is the structural group this group will work something and then pass on the information to the next one
no that is old story, the new story is that all of them work together and do it concurrently and that is why aircraft design is such a complicated and an exciting process because there is so much room for looking at the interactions and looking at how everything affects everything else, okay.
into being, so let us look at the process followed typically in conceiving a new civil aircraft.