 Let us look at some other effect, aspect ratio itself that already we have seen. So you can see that as the aspect ratio is increased the slope of the lift curve. So cl versus the wing induced coefficient is reducing. So this is another way. So when you make larger aspect ratio wing you have a thinner wing or you have a more slender wing. The wing vertices are weaker because you are going further and further away so the induced drag comes down. So these are some of the ways in which you can address the induced drag. So what do you do for angle of attack? Do you reduce it or increase it? To reduce the induced drag, reduce angle of attack you will have higher cl for the same lift. So that will create more or less induced drag. Answer me. If you reduce angle of attack, what happens to cl also reduces, correct. So if you increase air speed cl reduces. If you increase aspect ratio then also there is a reduction. So these are the ways in which you can do it. So let us see one example of a high aspect ratio plane but this is a remotely controlled plane. Normally you do not see RC plane is such a large aspect ratio. But remember that if you make a wing very slender and very thin then structurally you have to put extra efforts to make it stiff and rigid, okay. It is not coming at no cost. The other way of induced drag management would be to proactively work on the strength of the vortex. So one way is to put tip plates. The one on your left, the one with the blue end is called as a Horner wing tip. It was given by Sigmund Horner. And the one on the right is just some kind of a device to create methods to reduce the wing tip. The other way is winglets which is very common and most of the modern aircraft now you will see with winglets. In fact there are some people who say that winglets can actually be designed in such a way that they can give you negative drag. Now that is a questionable claim because that goes against the laws of physics. You have airstream coming from front. How can you do something just by putting a tip so that the it gives you forward force? But there are people who say that by very careful design of a wing tip you can actually reduce induced drag so much that it is almost like giving it some additional thrust. There are some people who have done very interesting things also on wing tip which I will show you. A pressure air travelling beneath the wing curls around the wing tip and into the low pressure air travelling over the wing. It causes a vortex or mini tornado that creates drag. The fitted winglets form a barrier breaking up the vortex and the drag. The pressure air travelling beneath the wing curls around the wing tip and into the low pressure air travelling over the wing it causes a vortex or mini tornado that creates drag. the fitted winglets form a barrier breaking up the vortex and the drag. So, this is how winglets work, this is another interesting way, this is an attempt to copy the working of the birds, if you ever look at the tips of a bird you find they are like the fingers, they are not flat, they always open up slightly and they keep on manipulating them. So, they are proactively, they have used evolution to learn how much to bend in what way, what frequency. So, some people have attempted this by putting these kind of grids and notice that the angle of this is controllable by the pilots. So, there is a claim that you can reduce the induced drag by 60% by using such kind of configuration. So, people have mostly attempted either in gliders or in RC planes, but at least I do not know of any actual aircraft, powered aircraft which contains tips like this and which is now commercially available. So, there is a claim, now we have to see and we have to find literature. This is another interesting way, instead of having a tip just create some kind of a spheroid. So, you eliminate, now there are two wingtips which are joined together. This has been used in many aircraft. As you can see there are practical aircraft with passengers which have a very unique kind of wingtip called as a spheroid tip. So, more about this can be uploaded by people on the model page, some innovative ways or other interesting ways to reduce induced drag. For example, there are people who say you can put a small turbine on the back of the wingtips and that turbine can generate power actually using the air which is curling. Okay, let us look at parasite drag. Parasite drag is a direct result. The air resistance is the airplane flies through the air. There are three types of parasite drag. Form drag, inner force drag, and skin friction drag. Form drag results from the turbulence created as the air tries to flow around the aircraft. Aircraft with larger cross sections will have higher drag than thinner, more streamlined designs. Other items like the landing gear and the antennas in the aircraft will also create form drag. Inner force drag occurs in locations over the aircraft where different surfaces meet. For example, where the wing is attached to the fuselage. Skin friction drag is caused by the wealth and perfections of an airplane's surface. A good example of this, where the rivets located in the airplane's skin. These bumps disrupt the air flow from otherwise flowing smoothly along the surface. Parasite drag is a direct result. So you just saw that any object that disrupts the flow of air by virtue of its presence, that creates what is called as a parasite drag. It has three components. The form drag because of the form or the shape or the frontal area of the body which creates a difference in the pressure on the upper and lower surface or behind and forward of the body. So that is what is form drag. Then you have skin friction drag and you also have interference drag. So all these three together, they are not at all connected with generation of lift. Even if you have no lift, you will still have these three drags. So we call them as parasite drag. And this drag increases as the air speed increases. In general, if you fly slow at a high angle of attack or if you will have higher form drag, but when you fly fast at low angle of attack, at that time the rubbing of the air on the surface could give you more. So it depends on which of these terms is more in magnitude. So generally at high speeds, parasite drag is far, far more than induced drag because at high speed CL is low. When CL is low, CL square by pi A is low. So high speed aircraft are more concerned about parasitic drag reduction, low speed aircraft are more concerned about the induced drag reduction. So all aircraft in which you see special type of tip devices. Now when you say high speed, it does not mean Mach number 0.7 or high speed basically means very high speed. Actually beyond Mach number 0.9 or 1, 1.5, those aircraft, they do not bother to put any wing tip devices because they say induced drag is not going to be our main problem. We are more worried about wave drag, skin friction drag and parasite drag. So form drag, very high form drag because of landing gear when it is extended. Very high form drag because of antennae, beacons or anything projecting out. Also very high form drag because of objects which are obstructing the flow such as wings struts. Now tell me, why do you think in this aircraft that you see here, they have used these wing struts? What are the wing struts doing? Why are they there? Anybody here? Yes. Sir my name is Vishnu. Yes. I think that these tricks are provided to give support to the wing because the wing seems to be very clean. So because of the bending moments it cannot sustain the moves. Okay. Now let us look at this argument further. What do you notice about the wing? It is mounted below on the bottom. We call this as low wing. So why do you have low wing in this aircraft? What do you think? The hint is that black strip that you see on the wing root. So why do you think this aircraft has a low wing? Any guesses? Take a mic, mention your name. No, I do not think you can connect propeller power with high wing or low wing. There is no connection. I can give you many examples of propeller powered aircraft which are high wing. Donia 228 is propeller. What is it? The wing would be exactly. No. No, no, no. Exactly. There are many aircraft which have propeller and which are high wing or mid wing or low wing. So propeller location is not. I told you the hint is this black strip. Yes. It seems that it provides some provision for the pilot to step on on the end of the plane. Exactly. So the black strip is an area where you put your step, walk over the wing and get inside the cockpit. So that you do not depend on any ground based facility to enter the aircraft, a trolley or a stair. So therefore the wing is on the bottom. Now you could mount the strut on the bottom or on the top to support it. On the bottom it would have been better because that would have given you the strut in compression. Here the strut will be also in compression because the wing is going to bend up. But when the load comes on a wing with strut on the bottom, the strut is in extension. So this is actually bad because compression is not good because there can be buckling. It is a very slender strut. So you are actually creating a tendency of buckling. But there is no place on the bottom because the wing is on the bottom. So therefore if you have to put strut, you have to do this, you have to put it on the top. You can minimize the tendency of buckling by joining the member sideways. So it is a compromise solution, not a very good solution because the top surface of the aircraft is where the maximum lift is generated and there you are putting up these messy things. So it is not a very good solution, not elegant, does not look neat, creates a lot of drag. So there will be very high form drag. But perhaps the designers have found that this is the best solution given the circumstances. Then form drag can also be because of the disruption. So by presence of an object, the flow becomes turbulent. It could be a protruding rivet or it could be something else. So you can also have form drag because of the presence of some surfaces. We have already seen this in the first lecture. Just after you land because they create extremely high parasite drag. But that is intentional because at this point we do not want lift. We want to spoil or kill the lift. So you create high parasite drag by projecting some surfaces almost perpendicular to the airstream. So as the cross sectional area of the aircraft increases, the form drag will increase. So from 747 to F-16 to Cessna 172, that is a reduction in cross sectional area, just my virtue of the size and therefore you have a lower form drag.