 Then skin friction drag, skin friction drag as is mentioned there is created by the movement of air flow near the skin of the aircraft. Let us have a very nice look at a look at a nice video on how it is done. Aircraft moving through the air is having to move through trillions of air particles. Here is the skin of the aircraft and here are a load of air particles. These air particles are too far away to be affected by the aircraft but these three will be. In this very slowed down film, you can see that as the aircraft moves though the air, air particles close to the skin are absorbing some energy from the aircraft and are accelerating to the aircraft speed. Air particles close to the aircraft move closer to the same speed than those that are further away. This absorption of energy by trillions of air particles is what causes skin friction drag. Obviously, the picture would look like this to start with. The aircraft moving through the air is having to move through trillions of air particles. So as the aircraft moves, the particles which are near its surface are going to be dragged with it. They are going to consume energy and that is the skin friction. So we define this by a skin friction coefficient which is basically a ratio of shear stress by the dynamic pressure. So it is a function of Reynolds number because Reynolds number is present in half row v square, Mach number because v is affected by Mach number and also the nature of the boundary layer whether it is laminar, turbulent. Okay. Then we have interference drag. It is present in all situations where two bodies are brought together. So you have drag of a wing, some x units, drag of the fuselage y units, when you bring y and x together, the drag will not be x plus y but maybe 1.1 times x plus y. So this 10% extra is the interference drag because generally there is a negative interference between body A and body B and vice versa, generally. But here again people say by very careful design you could have positive interference or you may have less negative interference. So in general interference drag is always going to be present. You can minimize it but you cannot say that I have completely removed it. You can see here because of interference between the fuselage and the wing at the root you see a lot of turbulence generated and that turbulence is the one that is creating condensation of the water vapor. You do not see that much condensation happening towards the wing tips. It is more at the roots wing tips are also doing something but the concentration is more towards the root. It only shows that at the roots there is a larger mixing happening. That means there is larger interference. So this is a visual indication of the fact that interference drag is more when there are interfering bodies in the presence. Also notice between the flaps and the spoilers in the previous video or just before you saw that there is a large disturbance created there that also creates an interference drag. So if you have a small angle between the fuselage and the wing the interference drag is going to be lower similarly if you bring the landing gear nearer to the wing there will be higher interference. So there are certain suggestions or hints available for how to locate the two bodies relatively. So whenever you do a course on aircraft design you will see that there are many many ways in which you can attempt to reuse the induced drag sorry the interference drag. Most common way is by proper contouring of the body proper shaping of the body such as streamline shape. So notice how they have put a streamlined cover on the landing gear and even the wing root junction especially if you focus attention on this particular place it is beautifully streamlined and this particular so this streamlining is normally done only through wind tunnel testing it is very difficult to do this numerically although some people do it but using CFD analysis to do streamlining is not very elegant because how many cases will you run. So wind tunnel testing is normally used notice how we have streamlined the cowling or the covering over the engine and there is a nice beautiful intake for the air to come in it is not a very sharp or harsh intake especially the intake behind the spinner or behind the center of the so this intake if you see this one this one they are all streamlined so that the induced biaside drag is reduced. Notice here how aircrafts body is contoured so all these things are done especially I am very interested to show you the covering of the landing gear all these are there for making it look beautiful and also to reduce the drag because of interference and form drag. Look at the contouring of the engine or the nacelle but you see on the back there are some serrations ok. So now why are these serrations provided does anybody know this is a engine of Boeing 7777 I think and it is also there in 787 yes what those are chevrons ok they are used to reduce the noise correct so this particular configuration or shape of the rear of the nozzle is a chevron nozzle and the aim of this is to reduce the ground level noise so this is Boeing 787 nacelle right ok. You can see wherever there are interference of two particles we normally provide some kind of a fairing and the additional weight and the cost of fairing is worthwhile because it reduces the interference drag by a very large amount similarly covers and cowlings are fairing you can see flap truck fairings. So these members have been given a particular shape essentially to reduce the drag we need to have a flap track so that the flaps can move but if you leave them exposed they will create drag so you normally flare them and also the tail cone so what is inside this particular area normally what do we have inside this APU auxiliary power unit ok. So the tapering of the rear and the shaping is done essentially for reducing of the form drag of the fuselage and then we have also seen this creation of smart vertices you can generate the vertices and these can help you they can be many positions but this is still an area of research some aircraft already have them but large scale vertices large large scale water generators are not there. Another example of reducing the drag of the aircraft is to join them to the tail so structurally also you have a continuous structure it helps each other plus you have thinner wings smaller chord and high aspect ratio so it acts like some kind of a support and then in some of the modern designs where extremely large aspect ratio wings are being presented struck brazed wings have been proposed so there is a big study going on in Virginia Tech on strut based wing and a lot of analysis have been carried out to bring out how a strut brazed wing can be overall it can lead to a much better aircraft compared to a conventional one but then there is a problem you have to be careful about the interference drag of the struts if you do not design them properly like we saw in the previous example they can really cause massive interference. Another concept that is very common nowadays is blending wing body where is a fuselage and where is a wing we do not know both of them are blended together so you put all the payload and the passengers in the wide in the central root chord of the wing and you blend the whole thing with the aircraft so this particularly this is basically what is being suggested for the next generation commercial aircraft okay so wave drag now wave drag is something that we have seen in the last lecture so I will not spend too much time on wave drag basically it is there because of the shock waves okay and we have seen this same picture last time one way of reducing the wave drag is to sweep the wings backward and forward this is an example okay so let us see a swept wing in action we saw a video last time also. Then I mentioned to you about the kukumann carrots or kukumann bumps which are this so called anti-shock body and this particular body is there to reduce the drag at transonic speeds so these are intention this is not like use a flap track and do it this is something completely different you can also see them in the design of concord aircraft okay so with this I come to the end.