 Today we start the fourth capsule of this course in which the first lecture, the seventh lecture is on aerofoils or airfoils. This particular content has been essentially created by Shishir Damani, a summer intern who came and worked during this summer. Let us first have a very quick overview on what we are going to see in this lecture. First of all we will understand what an aerofoil is although many of us know about it but still we will go for a formal introduction to aerofoil. Then we look at the terminologies and nomenclature for a typical aerofoil. We look at some historical information about how aerofoils came about and how they were developed and then we move on to the bulk of the lecture which will be on the various types of aerofoils. So there is a list of aerofoils mentioned there, we will look at each category and try to understand the features and the requirement for each of these type and finally we look at some modern developments towards the end. So what is an aerofoil? This is basically a shape or a profile but then there are two spellings for aerofoil. Some people call it aerofoil, some call it aerofoil. The difference between the two is only the British English and the American English. In the American English we say aerofoil and in the British English we say aerofoil. They are the same. So basically an aerofoil is a cross section of the wing which is normal to the span. The span of the wing is essentially the lateral length of the aircraft. So the distance from one tip to the other tip is called as the wing span and if you take a cross section normal to that, now the span can also be like this, sweep forward or like this, sweep back. Whatever be the case if you take a cross section perpendicular or normal to the span you get a shape that shape is called as an aerofoil and there are different types of shapes which are different aerofoils which are prevalent. So let us look at the terminology. The meaning of the word terminology is description of various elements, various features and what they are called. What are their names? Because we will use these names and it is important that everybody understands what these names mean. So for that we have a short video clip. Typical aerofoil such as a wing from the side, several design characteristics become obvious. You can see that there is a difference in the curvatures or camber of the upper and lower surfaces of the wing. The camber of the upper surface is more pronounced than that of the lower surface which is usually somewhat flat. The two ends of the aerofoil profile also differ in appearance. The end which faces forward in flight is called the leading edge and is rounded. The other end called the trailing edge is narrow and tapered towards the rear. The cord line is a reference line drawn from the center of the leading edge straight through the wing to the trailing edge. The distance from this cord line to the upper and lower surfaces of the wing shows the magnitude of the upper and lower camber at any point. Another reference line drawn from the leading edge to the trailing edge is the main camber line. This main line is equidistant at all points from the upper and lower surfaces. I will pause it here so that I can give you some time to look at the various nomenclature items. You have an upper surface and a lower surface. You have a leading edge and a trailing edge. The leading edge is the one that is facing the oncoming wind. There is a line, a theoretical line, a straight line joining the leading edge and the trailing edge and as you can see that line can actually go outside the airfoil such as in the rear bottom portion of this one that is why we say it is a reference line and that is called as a cord. But you can also draw a line which is the blue line in this figure and that blue line is equidistant from the top and the bottom surface all along the length of the airfoil. So the vertical distance above the camber line to the top surface and the distance from the camber line to the bottom surface at the same location is going to be the same. So this particular line is called as the mean camber or the camber line and the maximum camber is the maximum distance between the camber line, the blue line in this figure and the cord line, the red dotted line in this figure and there will be one specific place at which this maximum camber will be located that is a very important point. It is called as the point of location of maximum camber. So as we move this point forward and backward the airfoil characteristics will change, the behavior of the airfoil will change. Further you can also from the cord line you can actually go perpendicular to the cord line along the length of the airfoil. But now in this case for example the distance between the cord line and the top surface and the cord line and the bottom surface at the same point it may not be the same. For instance we can see that in this case in most cases the distance from the cord line to the top surface is more than that from the cord line to the bottom surface. So this total distance is called as the thickness. The thickness will also become maximum at some point. That maximum thickness expressed as a percentage of the cord length is called as the thickness of the airfoil 15%, 18%, 20% and again the location of that maximum thickness may not be the same as the location of the maximum cord that also plays a very important role in the behavior. But not all airfoils have to be like this. This is a typical airfoil but today we will see some very interesting and very different airfoils about which mostly we do not see in textbooks. So I will once again repeat that the students who are attending this course you are expected to be going through the corresponding chapter that I am covering in the class from the or the main reference book by Anderson. You are supposed to do that as a self study, you are supposed to read it yourself and in the classroom we are going to try to reinforce some of the important points and also look at points which may not be explained. And of course as you all know there are student teams who are going to do the assignment on gathering more information on the topics which I cover in the class with more detail and adding it as their assignment. So this is a three level process of learning. The first one is what you hear in the class and then it is uploaded. The second one is what is there in the textbook which is a self study and the third one is through the step study folder that I upload for each capsule. And finally at the end of the course we will have content available. Very soon I am going to upload the assignments which other students have given. So let us see just like we all have families, Aerofoils also belong to particular families. Some families of Aerofoils are very well known. For example we have all heard about this NACA series, there is a NACA 4 family, there is a NACA 5 family, 6 family, 7, 8 etc. etc. But there are many other families about which perhaps you may not know and this is not the exhaustive list. There could be other families of Aerofoils which may be there and whichever team gets this assignment they would be requested to go and locate the detail about those families, new families or maybe elaborate on the families which are mentioned here but not explained. So because of limited time we will look at only a few families. Obviously I will not look at the NACA family because every textbook, every website, every source describes the NACA series so we will not touch that okay. We will just tell you that you look at this textbook, there is a chapter 5.20 on historical note and there these families are explained quite beautifully. We will look at some new families or families about which you do not know. Does anybody know about the Ceri nomenclature or the Ceri family, do you know what is Ceri solar energy research institute okay. So these are Aerofoils which have been developed especially for solar energy okay. So this is the agency that looks at alternative power generation systems, alternative systems and they have classified special Aerofoils for wind turbine blades. So for a wind turbine the purpose of a wind turbine is different from a wing. The purpose of the rotor of a wind turbine is to extract energy from the ambient air even at low speeds okay and also to ensure that as the speed changes we do not get too much of variation. So these Aerofoils have been designed based on these requirements in mind. So you have a thin Aerofoil family and a thick Aerofoil family, the thin Aerofoil family is used for the medium blades or small size blades which are going to rotate. The large blades are going to go going to rotate and when they rotate because of their size, their span they will generate a lot of power. So there they have gone for thick Aerofoil families okay. So more details will be available later on. I just want you to understand what are the characteristic features provided in these particular Aerofoil okay. So for example you have locations in the front or near the root where you would like to have power produced even at lower winds or medium winds okay. So the cross section at the location near the hub belongs to a particular type as well as at the tips you are very far away from the root anyway and by virtue of 2 pi r effect, the rotation effect itself you are generating power. So we would like to control the peak power that is produced by the rotor. So there there are different types of Aerofoils. So as you can see there are thicker Aerofoils towards the root and thinner Aerofoils towards the tip. So using various Ceri family members along the span you can custom design a blade for a turbine and this is a huge area of research and in our department also we have some of my colleagues who specialize in design of wind turbine blades as well as in the fabrication and testing okay right. I want to also introduce you to a very interesting family which is popular with the people who do Aero modeling. They are called as Klein Fogelman Aerofoils based on the person who suggested. So they were devised around 50 years ago and they are interestingly Aerofoils with steps okay. We are all conditioned to understand that Aerofoils should be smooth but here we have intentionally provided steps one step or two steps okay. So we need to understand what these are and that is a very nice video which shows the aerodynamics of these Aerofoils. Can we introduce the audio which is a utility Aerofoil very popular on scratch built planes. This Aerofoil is known to have a higher stall resistance than a conventional Aerofoil. Some speculations on the mechanisms behind this Aerofoil suggest that a small trapped vortex forms behind this step which somehow increases the lift being generated. As we can see in this test however the area behind the step forms a pocket of air which appears to be stationary with no vortex present. When the Aerofoil stalls an air flow separation occurs the pocket quickly disintegrates. At lower angles of attack we can see a significant amount of turbulence being created in the wake far more than observed with the regular Aerofoil. So this is a very special Aerofoil series mostly designed for model aircraft. They will have a very poor lift to drag ratio because drag will be quite high and lift is going to be relatively less. But as I heard and also you must have heard these Aerofoils are much better at higher angle of attacks they are able to withstand and the reason for that is the mechanism of creation of a vortex bubble in the region behind the step. So these are very common for remotely controlled planes also we have not seen many aircraft flying with these kind of Aerofoils. But if you look on the internet if you look at the forums which talk about RC planes apparently these are very easy to make it is like layering up material Balsa slabs can be simply layered and stuck with little bit of rounding. So it is very easy to make and there are many series as you can see there are some which are having two steps on the top one on the bottom like the KFM 8 series there are some which have got curvature also. So this is one very interesting series and then you have KFM 11 which has got not one but 1, 2, 3, 4 steps on the top surface and the bottom surface is flat. So the mechanism of generation of lift and the mechanism the aerodynamics of this particular configuring of this particular layout is such that it is helpful in a certain class of aircraft mostly for remotely controlled okay. Alright let us look at another interesting series called as the Gottingen Aerofoils which were created in a university in Germany mainly through wind tunnel testing because in 1910 we did not have CFD available to us. So these were very painstakingly developed through minute and very careful wind tunnel testing okay. So they also have 1 digit, 2 digit, 3 digit numbers okay. So this is one cross section, Gottingen 622 aerofoil we observe here that the bottom is almost flat, slight curvature in the front and the top one is having a good curvature. Now what is meant by this one series 1 digit, 2 digit, 3 digit. So this is something I want you to find out and upload on Moodle. So you are going to search for this kind of aerofoils, you are going to tell us what are these 1 digit, 2 digit, 3 digit numbers and what is the relationship. So for example Gottingen 622 does the number 6, 2 and 2 mean something okay just like in the NACA series you know that each number means something. So do they mean something here all those details I would like you to upload on the Moodle grid. Moving on to another family called as the Epler aerofoil family. Now this is an aerofoil family which has been designed using mathematics using what is called as a conformal transformation or conformal mapping. So what is conformal mapping something has to conform or remain the same. The base and the map surface. So in this case what we do is we consider a domain which is a simple rectangular domain. Here we see in the top half of the figure you see a simple grid with perpendicular lines horizontal and vertical. On the bottom the same grid has been transformed using a mathematical transformation okay. So y is equal to a sin theta x is equal to or you know something like that some kind of a transformation can be used to create something. So each point or each intersection in the bottom grid corresponds to a particular point in the top grid and the transformation happens through a function f. But notice that the perpendicularity at the junctions is still maintained okay to a large extent the perpendicularity is maintained. So this is the meaning of conformal. So using some mathematical expressions you have a transformation available but the functions are such that they preserve the local angles. So using these kind of expressions. Now when the initial studies of aerodynamics started there was a huge contribution by mathematicians. So when we were students for example there used to be a subject called as wing theory and this there used to be Zhukovsky aerofoil and some other aerofoils which are very theoretical okay practically you do not see these aerofoils exactly. Because for example if you take a classical transformation you will find that a Zhukovsky aerofoil has got a cusp at the trailing edge. It is very difficult to make a cusp at a trailing edge in a practical aerofoil. So these are theoretical aerofoils but they were suggested or proposed by mathematicians to do a analytical study of aerodynamics. So for details there is a YouTube link which I will put here and you can have a more look at it. The practical application of such aerofoils is fairly limited okay except for certain applications but basically they are meant for some theoretical optimization. So here is one aerofoil which is an Epler 1211 aerofoil again you will tell us what these numbers mean and what the categories are okay. So then there are so many so many other families we will not spend too much time on other families.