 Let us just go and look at little bit on historical development, so how did we get aerofoils and were they there right from the beginning, okay. So there is a very short video which tries to capture approximately 100 years of historical developments in aviation, so I think it is a very interesting video, so maybe you should just watch it, it is a brief history of aviation, 33 hours, okay. And then of course after this there have been many, many, many, many new developments, okay. So Sir George Cayley also flew some gliders quite many years ago and he was the first person who began looking at aerofoils and coming up with the idea of giving them what is called as camber, although the all aerofoils did not have camber during that time. So Phillips was the first person to talk about optimizing the shape of the aerofoils to achieve a particular mission and later on we had so many other people who studied various types of aerofoils, two names stand out here, the one of Otto Lillianthal and Octavo Shenute, they have contributed immensely to aviation, so this is the first glider which was flown by Sir George Cayley, it is designed by Sir George Cayley, okay. You can see it is being towed by a cable, so it is being launched by a cable, where are the aerofoils, there are no aerofoils here, what you see is just flat cloth placed over a structure but given some kind of a shape and actually it occupies a particular shape based on the wings. So then we had Lillianthal as I said a very famous aerodynamicist I would call him or glider designer who tried out various things including ornithopters which tried to mimic the flight of the birds, once again no aerofoils, only flat plates but curved and twisted at various places. So then even the Wright brothers also developed some gliders, okay, this is one of the gliders that they developed, again if you see the aerofoils are not there, these are again flat parchment sheets with little bit of rounding in the front, the focus mostly is on making a structure which can withhold a person and the control system, okay and which side does the aircraft fly, does it fly from your right to left or from left to right, what do you think? So how many of you say that it flies from your right to left, raise your hands, few people but most of you feel it goes from left to right, okay that is because you are conditioned by the thinking that the tail should come on the back, okay, this one is from Wright brothers who put tail in the front, so this flies from right to left, alright. So Wilbur tested the concept of wing warping or wing twisting, let us see what this concept is, it is a small example, it is a small demonstrator, it just shows the basic structure of the box type wing that were designed by them, look at the wing tips, they are twisting, so it will be nice if somebody can build a model of this one, this wing warping mechanism and instead of using a hand, you can use a small remotely controlled servo to twist it and to show the working of the wing. So in 1901 we had a glider and these gliders basically because they fly slowly and not as efficient as today's gliders, this tended to have a very large wing area, so they became very difficult to handle and because of that there were many deaths and many accidents because people were not able to manage or control these gliders. One more reason is the forward elevator which becomes very sensitive to the control, so according to the designers of these gliders, the Wright brothers, they were interested in controlling it proactively, they were very good in flying but they were not interested in stability, they were more interested in control. So the last capsule of this course is going to be on stability and control, so there I will elaborate this point little bit further. So because they were using forward elevator, there were many last minute escapes, several times they came close to crashing or came close to getting majorly injured because it is very difficult to control. So the aerofoils used by Wright brothers were essentially thin aerofoils or just flat sheets with some curvature in the front and they did a lot of wind tunnel testing at low, one of the special contributions of Wright brothers is the use of a crude wind tunnel to test the working of their own designs. So they did low speed wind tunnel testing in their own workshop and the results that they got in the beginning were very misleading and there is a very interesting story in which the Wright brothers did some internal testing and they found that the data that they are getting is totally different from what is published by other people. So and then how they actually figured out, in fact they wrote a letter saying that our results do not match with your results, we see that the flow remains attached at low speeds in the wind tunnel but in reality when we fly we do not find the flow to be completely attached. So the wind tunnel testing was misleading, one reason for this was the transition strips for absentee. So therefore they were not able to replicate the same conditions. These are pictures of some World War I aircraft and interestingly these are in the side view so you can look at each aircraft and you will find that the aerofoil is basically a flat plate which is some curvature. So all World War I aircraft they had an aerofoil which was of this particular type. So the ideas like providing camber, rounding, thickness they came, they were known but they were not implemented at that time. So you can see for example this World War I plane, if you look at the tip you can see get a rough idea about the aerofoil cross section. So notice it is essentially a flat thing. So then we look at types of aerofoils. We look at what kind of aerofoils are there. So flat plate is also an aerofoil and flat plate can also generate lift. It is not necessary for you to have curvature or rounding. In the next class I am going to talk about the dynamics of creating lift and there I will explain to you how a flat plate can also create lift. And here is proof, here is a CFD picture of a flat plate and you can see there is a difference in the pressure. The blue zone is very visible on the top of the aerofoil. So you can make out that there is a pressure difference. The pressure at the top is lower so there is going to be upward 4. Then the question is why not use flat plates. Flat plates can make you fly. I have built some small chuck gliders and I fly them using flat plates. I do a demo in many places and before me so many people have flown aircraft. So then the question is why not use flat plates? Why go for curvature, aerofoil shaping or do you think what is the need? When you can manage with a flat plate then why should you go for such complicated things like providing a profile or a shape. So let us see who can answer this question. Why not use flat plate? So the problem is not to generate the lift but the problem is to sustain lift at angles. So this is a question which many people ask, flat plates cannot produce lift, the answer is no they can produce lift but there is a problem. So as you can see here is an example of how flat plate can produce lift, they can produce lift but why do not we use them so the answer will be available through a small video. The first test I did was with the unprofiled wing. You can see that the wing experiences air flow separation at a fairly low angle of attack. Air flow separation is the generic term given to the occurrence where the boundary layer or the layer of air closest to the surface of the wing has a velocity of close to zero. This is visible in the form of turbulence and eddies as you can see here. This represents the wing at stall. You can also see the oscillating trailing vortices characteristic of a wing at stall here and here. When we look closely we can see a small trailing vortex present even when the wing is at a low angle of attack. The first test I did was with the unprofiled wing. You can see that the wing experiences air flow separation at a fairly low angle of attack. Air flow separation is the generic term given to the occurrence where the boundary layer or the layer of air closest to the surface of the wing has a velocity of close to zero. This is visible in the form of turbulence and eddies as you can see here. This represents the wing at stall. You can also see the oscillating trailing vortices characteristic of a wing at stall here and here. When we look closely we can see a small trailing vortex present even when the wing is at a low angle of attack. So the main problem as Ritu rightly mentioned is that at a particular angle a flat plate is sufficiently good. You may just have to round the leading edge to avoid immediate stagnation pressure creation of the air because a flat plate will have some finite thickness. So for that thickness and that thickness portion in the front the flow will come to a rest. So if you just round it slightly or give it some kind of a shape in the front and in the back it becomes very easy. So if you are always aware that the flow will be coming to you only at say 3 degree angle you can make a very optimally shaped flat plate for that angle and it will give you very good lift. This is what you see in the ceiling fan. What do you see in the ceiling fan? Do you see aerofoils in the ceiling fan? We see a flat plate with some bend. That bend is basically to ensure that the air is consistently and strongly pushed down because you do not want the fan to lift up but you want the air to come down on you. So you give you take a flat plate give it some shape towards the back that is all. So if I want to operate a fan at a particular angle continuously I can manage with a flat plate. But in an aircraft as your speed changes you have to align your aircraft with the wind at different angles and that is where a flat plate will be immediately a problem. So the initial aerofoils that people used were flat plates with a small curvature in the front. So that at a range of angles you still do not have a flow separation and that is how we got the wings of the aircraft during the first world war and other low speed applications. But soon people realize that it would not be enough we have to do something more to get a better information. So the answer is this if the leading edge is sharp there will be easy flow separation as we saw in the video also quickly a vortex will be formed and that is going to consume energy from the free stream so you will have more drag and the angle at which you lose lift or the stalling angle become very small. Therefore if you want to stay up in the air you need to have more powerful engine because drag is going to be larger. So then people said okay let us go for rounding and let us go for chambering or shaping the aerofoil. So there is a hypothesis here that the cambered aerofoil will perform better. Now let us just investigate let us see what are the conditions under which this hypothesis can be true. So first we should define what is the cambered aerofoil and aerofoil in which the camber line and the thickness line are not identical because there is a presence of a camber. So these are the terminology which we have already seen. So airflow speed and static pressure both of these are going to generate the lifting force as we will see in the next one. Now if you provide camber or curvature you change the velocity acceleration you change the increase in the you result or cause increase in the velocity and hence more changes in the pressure but this is true only if the flow is laminar to some extent it is true even in turbulent flow but mainly it is true when you have undisturbed flow and most of the wings are designed with this concept that you provide a curvature so that the ambient air comes in and because of curvature it accelerates as it accelerates you can expect lower pressure and therefore you can get lift. So the efficiency of the aerofoil increases if you provide camber but in certain circumstances this may not work as we will see sometime in the future. So here is a discussion about the effect of camber on the lift. So there is a curve called as a lift curve which is shown there on the x axis you have the angle at which the aerofoil faces the free stream or the angle of attack and the y axis we have the lift coefficient or a measure of the lift and rate. So if you have a medium speed aerofoil then it is good to have a minor camber or a mild camber because you will produce more lift due to more acceleration but also you will have a smooth stalling characteristics shown by the gradual reduction in the lift beyond the angle. Now if you have a high speed if you design something for high speed then if you provide too much of camber you can have a problem. So in this case you will start getting a sharp because if you provide too much camber and too much rounding for a high speed aerofoil very soon the flow will accelerate to numbers that you do not want it to exceed. Then if you have a low speed aerofoil or if you have a low speed requirement. So this is an example of the aerofoil which are seen in the engine components in the compressor and the turbines and the rotors there you go for thin cambered aerofoil. So if you are going for a paraglider something that glides for a very long distance. So in a paraglider you have a very thick aerofoil with less curvature with a reasonable curvature. So depending on the application that you have in mind the shape of the aerofoil that is suitable for that application keeps changing. So one has to have an information about now how do you determine which shape is better for which application it is determined either by experience or by wind tunnel testing for a particular application. So in the tutorial session that we conduct we will do some experiments and try out and see how these parameters change. So if there is a focus on the lift over drag ratio improvement not just increasing lift but increasing lift over drag then you need to have better aerofoil. If you just want to have more lift you may take an aerofoil which will give you more lift but there may be very bad drag. So the focus so now the drag is the basic key to it is the key aerodynamic characteristic and that has to be kept as low as possible.