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Module 1: Aircraft Configuration Design

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Issues in Airliner Design

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Lecture – 13
Airliners

Let us take a look at airliners now, the aircraft which are normally used by passengers like you and me for travelling long distances in flight. Even in airliners we can broadly define them into 3 categories; short range, medium range and long range.

So, in the short range airliners, we normally start from about 1000 kilometers where the long range regions are ending and we go up to 6500 kilometers. Most of these aircraft operate with a cruise Mach number between 0.75 to 0.8 they fly at height of approximately 10 kilometers
or 35000 feet above the Earth. Now in this particular category the popular aircraft that we see in India are the Airbus A 320 and the Boeing 737 as you can see in this particular picture.

So the aircraft which I show you together are the ones which are going to compete in the market also. You can see they look very similar, although one has come from the Airbus’s table and the other from the Boeing’s table, but it is very difficult just to make out just by looking at the aircraft which company has made it.

The next category is the medium range airliners which start at 6500 kilometers and goes up to 10000 kilometers. The cruise Mach number does not increase too much. It goes from approximately 0.80 to 0.86 and they may fly at a slightly higher altitude of around 12 kilometers. Obviously, the altitude that you fly is limited by the instruction from the air traffic controller okay.

So, you cannot just choose the altitude you fly, but the capability is built in the aircraft. In this category of medium range airliners, you see Boeing 767 and Airbus A-330 competing in the market and in this case the same airline is operating both aircraft types okay.

Then you have the category of long range airliners which go from around 10000 kilometers to 17000 kilometers. The Mach number range and the cruise altitude remains the same as the case of the medium range airliners. In this category, we see many aircraft and two of them which are directly head to head in the market are the Boeing 787 Dreamliner and the Airbus
A-380 okay. So, this is how the airliners are divided.

Alright, so when we look at airliners we have seen that the 2 companies Airbus and Boeing are competing head to head in the many sectors. In fact, in all the 3 categories which I showed you short range, medium range, long range airliners, we had at least one aircraft in the Airbus table competing with the Boeings table. This particular figure gives you a typical
landscape of operation of these aircraft.

So you can see from around a 1000 kilometers nautical miles to around 10000, the aircraft that you see in the red square in this figure are from Boeings table and the aircraft that you see from blue rhombus are from the Airbus table and you notice that roughly everywhere there is going to be a competition. Let me show and you can see there are cluster as I showed
you earlier. This is one cluster, this is another cluster and this is the third cluster.
They are the three cluster which I told you about and then you can notice that there are some gaps. Here is one gap, here is another gap, this is another gap, this is another gap. So new aircraft could actually be designed to fill these gaps okay, maybe there is a gap here also, maybe there is a gap here also, there is a gap here also, there is a gap here also, provided
there is a requirement for aircraft to fill in that particular gap.

So, in the class room exercise one can always say okay design something in this particular box and that can become a theoretical problem, but there has to be a real-life requirement for an aircraft to travel 9000 kilometers, but only 250 passengers. Interestingly, a new aircraft that came in the market was the Boeing 787-8 which is marked here, and this aircraft with
7300 nautical miles and a capacity of 240.

Now in our exercise that we do in the future regarding the aircraft design calculation, we are going to do our evaluation based on this particular aircraft okay. So, I will try to get rid of all these voids and gaps which are there and I will just limit our attention to the basic aircraft. So this particular aircraft Boeing 787-8 for no specific reason is going to become our base line aircraft for analysis okay with 240 passengers and 7300 nautical miles range.

This will be the base line value and we will investigate variants which are 280 passengers, 260 passengers, 220 passengers and 20 passengers and we will also investigate the variants which are 6300 nautical miles to 8300 nautical miles. So, you can notice that the intersection of all of these lines they represent a matrix of you know aircraft requirements which could be
addressed. Thanks for your attention. We will now move to the next section.

Lecture – 14

Key Issues in Design of Airliners

Let us look at some key issues in the design of airliners okay. First of all, the wing of a transport aircraft or an airliner is subject to large amount of optimization okay and why is it so many people wonder, but all airliners whether they are from Boeing or Airbus, they look very much alike because both of them have been subjected to the same amount of optimization, similar requirements and similar objective.

So independently, they have come up with solution which have arrived from various are very similar. There are certain variances of course and it will be interesting for students to actually be given as exercise to identify totally different approaches and totally different solutions to meet the same requirements, but in transport aircraft of this particular type you will find most of them have certain common features. What are these features?

The wings are swept in most of these cases, but the range in which the quarter chords sweep that you will observe will be between 35 and 38 degrees mostly okay. Now some of them might use supercritical aerofoils which allow you to obtain the benefits at a lower sweep angle okay and also when you sweep less you can get more efficient flaps and lower wing
weight. So, this is one thing.

So most of the aircraft in this category airliners which fly near transonic speeds, Mach number of around 0.8 they are going to have swept wings with a sweep in a very small band or supercritical aerofoils allowing it a little bit lesser sweep. The sizing of the fuselage is actually not a very difficult task. So fuselage is basically going to be almost like a cylindrical
body with some nose and some tail.

The nose is required to put the cockpit and to put some instrumentation and some features in the front and the tail is required essentially to accommodate things like the APU, some amount of baggage maybe and adequate length and conical taper to allow the distancing of the tails okay to provide sufficient moment arms. So the fuselage design is subjected to
volume and for passenger and cargo whatever the volume required.

And what we want to do is we want to make it as small as possible to reduce the cruise drag. When we come to the powerplant all of them have generally a high bypass ratio turbofan aircraft because these give you lower SFCs and lower noise levels and also lower emissions, especially the Nox emissions, and easy maintenance okay.

One example I would like to show you. This has been taken again from the booklet of Professor John Fielding, about how some advanced features have been provided in a transport aircraft to address particular requirements. So this aircraft for example is one of the aircraft

which has come with two member crew. This is now standard, but when this aircraft appeared in the market, it was one of the first aircraft that came up.

Before this we had aircraft like Boeing 747 older versions which had a pilot, a copilot and a flight test engineer or navigator. So the cost of the crew has come up because the workload is low. So you have a low workload cockpit, so you can manage with 2 people and this was possible by extensive avionics integration. This is an example of how technological
developments drive the requirements.

Earlier people were happy to, customers were happy to have the requirement of 3 or 4 people in the cockpit but now it is only 2. This aircraft has got a centre of gravity management systems, it allows it to fly optimally at various conditions and there is a flight envelope protection and load alleviation by automatic deflection of the control surfaces to handle the gust loading and this is possible because of the extended fly-through-computer system.

On the structure side, the designers have gone for use of new materials and processes okay, composites and superplastic forming and diffusion bonding and also aluminium-lithium alloys. On the power plant side, they have been installed with the CFM56 engine which gives you the, so it is an old engine which was on Airbus A320. It has been enhanced to power the
A-340. We will come again to this aircraft in the future because there are certain other additional features that I will discuss.

Now let us look at some trends in the design of airliners, especially let us look at families and commonalities. Now just like we have families in our own personal life, we also have families in aircraft design.

Now what you do in families is basically you design a baseline aircraft and then you meet the changing market requirements or provide a competition to another existing aircraft by stretching or shrinking the aircraft okay. So what you mean by stretching and shrinking, basically by inserting and removing the fuselage plugs, you could also do it by changing the
engine or by increasing the thrust of the same engine to meet a r requirement.

There are many, many examples of this in the Airbus and the Boeing stable which we will see very shortly. The aim of this exercise is to ensure that every aircraft which the competition makes available there is an equivalent or similar aircraft in your stable, so you give the airline a choice that if you want to beat that aircraft in the competition, here is our solution to that
particular problem.

You can see for example if you look at the large twin market okay, twin engine aircraft you can see, for example you have 767-200 which was for this particular range and this particular capacity and it has been stretched and again you know and again. Similarly, you know you have this family.

Similarly, you have another family of the Boeing 767 family where you are just reducing the range by or I would say the other way you are increasing the capacity by putting you know, sorry you are changing the market requirements. Let us look at this in more details as you go ahead.

Let us look at the Airbus A-320 family. This is a very popular family worldwide. It started with Airbus A-320 the base aircraft which came in 1984 okay, right. It had 5700 kilometers range and it has a capacity of around 150 passengers. Then in 1988, 4 years later the A-321
was made available with a larger capacity but a 100 nautical miles smaller range. After that we had the A-319 which was a best version as far as the range is concerned.

But a shrunk version as far as the capacity is concerned, 6800 kilometers and 124 passengers. This is one way of fighting the long range regional aircraft and then here is another example of A-318 which came in 1995, here the range was reduced to 6000 kilometers and the capacity was brought down to 107 kilometers. So the same basic aircraft is serving 4 different markets by using the family concept.

Similarly, you have the triple 7 family chart where you go from you know one aircraft to the other and then this one to the other and then there is a kind of fallout and there is also a fallout aircraft. So you can see 4 different markets, I would say 5 different markets are being addressed. So one aircraft is actually able to handle such a large range of capacity and range
by stretching and shrinking.

There are also some commonalities in airliner design and this is very interesting. Interestingly, you will notice that in the Boeing stable the Boeing 707 fuselage cross section continues to be used for further aircraft with minimal change. In the case of Airbus, we use the same wing for the two aircraft, the twin engine Airbus A-330 and four engine Airbus 340,
they both have almost the same identical wing.

This leads to lower manufacturing cost and commonality in service okay and in operation there are commonalities possible using what is called as a cross crew qualification in the Airbus aircraft okay.

You can see for example here that you know there is a aircraft called A340 all varieties within a day, the pilot can convert to flying A330 and from 330 to A340 it takes just 3 days.

These are large aircraft. Now if you have a pilot qualified for 340 and 330 and you want that pilot to operate smaller aircraft, it is a matter of just 11 or 15 days you can convert them okay and then 380s they are same, will also have a similar operational advantage.

Boeing also allows okay. Now commonality is available only between Boeing 757 and 767 seven five seven and seven six seven. So from 777 to 787 you can go in 5 days, but notice here from a non-Boeing aircraft, basically it means an Airbus aircraft to 787 you require 21
days of training. So commonality is also there in both the companies.

Here is a cockpit or the flight deck of Boeing 787 okay, a very modern cockpit and a similar configuration you will see now in the future Boeing aircraft.

And here is an example of commonality. In fact if I draw a line here, on the left hand side you have A330, on the right hand side you have A340 one of the versions. Both of them are having a common wing platform okay and one side you see a single engine because it is twin engine, on here you see two engines because it is four engine, but they have a common cockpit. They have a common cockpit, common fuselage, common landing gear, common empennage or tail, common systems and common wing.

The only difference is in the fuel piping, you know the piping that will go the fuel to the engine here, but for that it is all common. And from Airbus A340-300 to Airbus A 340-200 you have 8 frames shorter and you get when you workup to meet the market okay.

This commonality does not come without a cost. It is good to have commonality, but here is one assessment by Professor Scott Eberhardt about the drawbacks of commonality, the result is non-optimal design.

For example, if you look at the deliveries of Airbus A330 and 340 as compared to 767 and 777 you notice that 330 sold quite well. Any aircraft that sells around 400 units typically just is able to recover its market. This is for a new aircraft of course, 330 and 340 are not new aircrafts, but you notice that 767 and 777 have around as on to 2012, they had around 1000
aircrafts being sold compared to only around 850 and 400 for 330. So common wing is good but too heavy and it resulted in an inferior airplane okay.

Look at the loss in sales. What you saw earlier was loss in numbers, this is the loss in sales.
So compared to 767 and 777, 340 have around ten 100 billion dollars loss in the sales and consequently we noticed that there is no new order. So commonality is good, but commonality can lead to serious financial considerations also. Thanks a lot.