 So now I want to show you the results of a practical experiment. So as I mentioned to you, during this summer we had some students who came to do internship. But before them we had two students who came and worked with us on their undergraduate project. These students came from an institute called IIST. One of them is sitting right here, so she did his BTEC project along with his friend. And in their BTEC project they worked on a very interesting problem. They worked on the design of a glider which I used to go to schools and colleges to demonstrate principle of flight. So his friend Vishwas Moli and Sushil they worked on these two. The other one was launching by hand is going to bring a lot of variability, correct? Every time I throw it will be a bit different because my angle is not fixed. So they designed a launcher and a glider. The glider that they designed was basically a very simple that I will show you now. So after they left, I am sure you are wondering, after they left we did some further experiments using their glider and their launcher. So I want to now present to you the results of those experiments. There are two experiments here. Another experiment is what is the effect of change in the center of gravity of the aircraft on the flight characteristics of a glider. But this is not a hand launch glider, this is a launcher launch glider. So it will have much better variability, so the relative changes will be less. The other experiment is effect of angle of attack of launch on flight, okay? So let us see. So this is the glider that was made. You have a top view and a bottom view. In the bottom view, we have drawn some lines. These lines are the location of some payload that we have put to adjust the center of gravity. So this aircraft will have some center of gravity location on its own because of its geometry. But if you want to play with the CG, you have to add weight. I cannot delete weight because then I will have to destroy my glider. I can only do CG change by adding weight, so some screws have been added. So the first position of center of gravity, on the left hand side you will see the position of the payload or the screw. On the right hand side you will see what is the CG location because of that. So this is position number one which was slightly ahead of the aerodynamic center, okay? So let us see, position number two, we move the payload little bit behind, CG has gone slightly ahead of the aerodynamic center, third position the CG is at aerodynamic center, fourth one CG is behind aerodynamic center and the fifth one is horrible, the CG is now behind the neutral point. You can see, the CG is behind the wing. Now I do not know where neutral point is and nor can I find out, I told you I cannot find out neutral point, but I can find out the CG. So by putting the weight at the fuselage on the rear side, we ensure that the CG is so much behind that it is behind the wing. I know that normally the neutral point is you know 35-40 percent, it is now 100 percent at behind the wing. So I am very sure that the neutral point has been exceeded, okay? So now I want to show you the effect, okay? So this one, the payload is here, the CG is here and let us see the flight. This is in slow motion, okay, it is in slow motion because it flies very fast. So if I do not make it slow, you will not be able to really see how the aircraft behaves. So can you comment on the flight? Is it flying straight? Yes or no? No or yes? You cannot say, I would say it flies straight, straight, straight, straight, straight and then here it starts going down. So initial flight path is fairly straight and then is the aircraft flying stably? Yes? Where is the disturbance? Stable unstable is only if you disturb it, did I disturb it? I did not disturb it, correct? So if you did not disturb an aircraft, you cannot say it is stable. So right now we cannot comment on stability, we can only comment on whether the flight is good or not. Now good or bad is a relative term, this is the first video you are seeing. So hang on, comment at the end of the fifth flight, this is the first flight. So what do we observe? The plane leaves, the launcher is not visible, we have hidden it out of the camera and we have put this black background for increase. We also have a software, an open source software through which we can track this object. I will show you the result of that little bit ahead. Enough of the flight with first CG location, this is the tracked or recovered trajectory from the software. So as I said from a location around 20 from the wall which was the starting position, it flies straight up to around 320, sorry up to around 270 or so. This is not meters or inches, this is just some frames. And then it starts going down. So its range will be approximately 600 in this figure, right approximately 600 by projecting it. We do not know but approximately 600, alright. Now let us see what happens if I, in number 2 I have moved the CG slightly behind, okay. It is little bit ahead of the aerodynamic center. I am telling you because of, this is post-facto information, you do not know right now. But we just moved CG behind, is it better or is it worse than the previous one? There is a slight amount of role introduced, do not worry about it. This class is not about lateral stability or rolling stability, this class is about longitudinal stability. So is this flight better or worse? We have moved the CG slightly behind but still ahead of aerodynamic center, worse. Why do you say it is worse? I think it is better. What is the measurement of better or worse? Range, not how it flies. We are competing for maximum, did I say if your aircraft goes like this and hits the pillar there is no mark? Did I say that? No. Your aim is to travel maximum distance and hit the pillar. So I think it is better. How do you know it is better? Not by looking at the video but by trajectory. So now what we see is, it was earlier flying almost straight and then down. It will go down, it is a glider. It cannot go up unless some gust comes in, there is no gust here. So this particular flight is actually slightly better, let us not argue. You may say it is worse, I accept it, if you say better I accept it but I found it better simply because we got little bit more range in this, okay. Because see it is going up slightly, so whatever goes up will also come down and will go longer. So you got slightly more range, okay. Now let us go to location number 3. In location number 3, the CG is now at aerodynamic center. What do you say now, better? So if you are designing your glider and competing for maximum range, what will you do? One or two or three? Three? Yes. You can see by that trajectory also that the aircraft, it now climbs further up, goes down. The range we got was more in this case, okay. You can see surely more than 600, although I wish I would have captured it. See what happens is when you capture with the camera, there is a software. So it captures that to some point and then if there are some table or something else, it loses, so we will lose it. So this is the data, this is actual data of our captured trajectory, okay. Let us see what happens if we move the CG further behind to position number 4. Behind the center, behind the quarter chord, this is not good. The range was not larger in this case. You can see less than 600, okay. Now we take the worst case which is when the neutral point has been crossed. So the aircraft should actually be unstable in this case, but here stability and stability is not going to really matter. What will matter is what angle the aircraft takes or what angle the aircraft trims into when you launch it. So although you get very high initial climb, but the range is not highest. So this is what happens. You can imagine now, it is like the tail is heavy. So when you throw the aircraft, angle is increasing and the angle increases, drag is more. So when drag is more, with the same energy, you are not able to travel much longer distance. So you go like this and then you fall down. So the range in fact was the least in this case, 500. So in short, here is a comparison of all the ranges with the various CG positions. So you can see that these black dots which represent CG at 3, which was we know at aerodynamic center, gave us the maximum range. So this is a experimental proof of what we learnt through a toy airplane from Scott Aberhardt's book. And this is my friends a hint to you to get, now the difference will be, now all of you will put the CG at quarter chord. So now who will win the one which has got better aerodynamics? The one which has got better repeatability, so that you do not disturb. Remember we will take average of two throws, so after one throw if it breaks gone. So you have to, I will not take the, actually I will not take the best of two throws. No. I want the design to be robust. So if your plane breaks after first throw, then you have only one reading. If your plane performs bad after second throw, it is actually a bad design. Some people will say, we will break the plane after first throw, because where you get negative marks, it is up to you, you take a call, alright? This is our conclusion. Okay, now why did this happen? There is no stability in this calculation, because there is no disturbance. So it is not to do with stability, so what is it to do with? If you have listened carefully, and if you have listened carefully to what I have shown you today, then you should be able to answer. So why is it so that at position number 3, CG at number 3 position, we had the maximum range. Let me help you by saying, apart from the center of gravity location, what was the difference in the flights of each aircraft? Remember, one minute, remember, we have attached the aircraft to the launcher and released it. Now tell me, yes, what is your comment? Okay, he says that the angle of attack of the aircraft during the flights was different. How many of you agree? There are three people here, three musketeers, only two there. So others are thinking the angle of attack is same in all flights, why is it same? Because the weight is same and the launch angle is same, so the angle of attack is same. Is it different or is it same? Is different, okay. So you say it is different. How many of you say that the angle of attack of the aircraft in the flights is the same? The logic is very simple, the angle of attack will, because lift is equal to weight in all of them. Assuming the launch speeds are same, which I expect because it is a very good launcher. So and the angle is same, the angle at which the launcher has been launched, we have not touched it, we have locked it. So they are all going at the same angle from the launcher, they are all same weight, because the same weight is moved backward and forward, CG has moved but the W is same. So why should the angle be changing? L is equal to half rho V square SCL, why should the angle change? So why is the angle of attack changing? Behind neutral point, correct. So basically the aircraft is the same but the angle at which it trims itself when left in flight is automatic because moments have to balance. So the moments will balance at a higher angle of attack when the CG is far behind. So therefore the alpha at which they are flying is not the same. So is the V same? The V is also not same because V will have to adjust automatically to keep lift equal to weight because angle is changing. So although this video recording has been showed you in slow motion, the speeds are not the same, they are slightly different. So now at which angle of attack do you get the best range? The angle of attack at which L by D is maximum, so that is what is happening. So this is a graph of the L by D versus alpha, okay. So at alpha 1 we had some angle of some L by D, at alpha 2 we had some L by D, at alpha 3 we had the best L by D, at alpha 4 we had less, alpha 5 was the worst. So that is why at 3 I get the maximum range. So on the left hand side we have the flight profiles, on the right side we have the corresponding alpha positions, yes, no, no, no, it is on the launch angle. The launch angle is same for all. The line that you are seeing is the line for CL by CD max for various alpha. The red line in the figure is the value of CL by CD for this aircraft for various alphas. Because the aircraft will automatically because of center of gravity orient with the flight path at an angle at which the moments are cancelled or balanced. So it will fly at this condition throughout. Now you cannot see the flight, you cannot see the arrows of flight direction and the angle of attack. So that is why you cannot see. But the angle between them is the constant in the entire flight. Once the aircraft leaves the launcher, look at the flight number 5. What happened? When you launched it, it became like this. Now it went like this only. So if you draw a line which is the trajectory and if you draw a tangent to the flight path you will find the angle is the same. So what is happening is the aircraft is flying at different angle of attacks at which L by Ds are different. So the same aircraft is having a different range. So now let us look at the neutral point. So now I want to show you the effect of stability. So far it is no stability. So first position, now I introduce vertical gust which is the disturbance. Now you have to see the response of the aircraft after the gust and then decide which is more stable. So the same flight, the first one and now what we have done is we have put a small disturbance. We have put a small fan. So you can see behind the fan there are a lot of response. In fact here the response is happening even before the flight slightly because this fan is not giving me just a clear vertical gust, it is not giving me a stepped gust. The fan is actually going to give me a band, a fan of air. So the effect will be highest just above the fan at this point. So the aircraft responds beyond that, remember this is a slow motion recording. So once the aircraft crosses the fan, now look at the response. This is when the CG is at number one position. Let us see CG, now I have skipped two, I am just trying to save some time. So at number three, without gust, with gust you can now see the difference in the stability. So it responded but responded little bit less, see. Now the air hits it, okay. Let us see, so there is a difference in the flight. This arrow, vertical arrow shows the place where the fan is placed. So it shows you that behind that there is a slight dip and increase in the flight. Okay, let us look now at position number four. First without the gust, now with the gust. The gust acts now, aircraft responds to it. So now what is happening is unfortunately these experiments we have to do them a little bit with more care because you are actually going so much above the fan that its effect is getting depleted. But still you see the reaction. There is a continuous increase even here because this fan is not going to give only a straight line like this, it is going to give a band of air. So I think if you see the response of the aircraft, it starts falling here. So I would assume that this area is being affected by the fan. And let us see now the response when you are behind. So now this aircraft is unstable because it is where you go. The moment you throw angle of attack become very large and then it immediately starts descending. But when you put vertical gust, let us see what happens. I would say not much effect because the air is not able to reach. But if you see the data that we captured. So instead of coming down it is actually going little bit, the angle at which it is falling is reducing slightly. So these are experiments we tried. Then we have flights at different angle of attacks. So I will show you quickly. Now here we have given the alpha, yes, yes, most stable position, yeah, no least, it will be the least. So response should increase, yes. The effect of the gust should be more. Unfortunately it is flying so much above that by the time you reach there, the numerical value of the gust is not the same. I want to show you also the effect of angle of attack, very briefly. Now you can see the launcher also. This is zero angle of attack. So therefore you saw it went exactly straight in the beginning. These are results taken by you, right. These are the recordings by him and his friend. You will see them also. Now alpha at 4 degrees, 8 degrees, range is lower, 12 degrees, L by D is going to be maximum at one particular angle. That is not 8 degree or 12 degree but at a lower degree, approximately 3 to 4 degree. So that is where you will get the maximum range. So if you compare the flights, this is from their paper. If you compare the flights you find that in this case the maximum range came for actually 0 degree but the next angle we had was only 4 degree. So maybe at 3 degree or something like that, maybe we could have got a good one and you see a horizontal line on the ground that is because of the sliding on the ground of the glider. After falling it slides so that gets recorded, alright. So that is all I had for stability and control.