 The main problem is that in the transportation system it has to be reliable because people plan their journey in advance, you cannot say we will go if we can go. Now this is excellent when you have adventure or when you have a situation where people say okay no problem we are here as tourists, we would like to go around these places. If the ship is available and flying we will use it. If it does not, it does not matter, we have other options to do. So for such applications, airships are being used and they can be used. Where the dispatch and reliability is not a major issue, for adventure, for pleasure, for aerial sightseeing when possible they are being used. But another reason why they are not used is that they tend to be very expensive from the total operating cost point of view. Fuel cost is the small component of operating cost. You should have an infrastructure, you should have the trained crew for operating it, maintaining it okay and unless you invest a large amount of money, this is a very niche business. In all niche businesses, things go very expensive okay but that does not mean that they are useless. We will discuss in this course and I hope to convince you that for certain niche applications they are the best and hence people are designing them now or looking at bringing in airships or aerostats for those applications okay. So we have looked a lot at the drawbacks but the advantages of aerostatic lift fuel efficiency and less complex mechanisms, these are making it very easy for students to build airships and fly them okay. When you make a flying vehicle and some of you are experiencing making aerial flying vehicles or aerial robots right, it is very common that when we learn aeromodelling or when we learn how to make an aircraft, it is very common to spend 8 hours in making a vehicle and 8 minutes in breaking it because it is very difficult to control this rogue force called thrust and unless you properly control this force, it can really take you immediately away and other aerial vehicles, they need certain minimum lift generation just to overcome the weight and then we will use the balance to provide the forward or the other directional motion. So if you look at students who want to make an aerial flying vehicle very soon, we have got the feedback from students that we tried quadrotors, we tried UAVs, we tried, we found that making a balloon is easier. The balloon is forgetful, if a balloon is flown and if it hits the wall it bounces back, it does not break okay but try that with a quadrotor okay, very soon you will find that unless you get it perfectly balanced, you will not be able to even start, that does not mean we should not make quadrotors, every aerial vehicle has its own role, quadrotors have fantastic abilities which airships do not have. So we are not saying that this is bad, that is not bad, we are just saying this is one more thing available for you to play with. So this line, less complex mechanisms is a very important line for novice aerospace engineers who want to make a small flying system with all its limitations but they want to make something that flies. And the challenge that you have to overcome in making a small airship which can withstand even small disturbances is really a very exciting one okay, three types of systems are normally considered, there are hybrids also, so you will have combinations of these and we will cover that in the last capsule of this course but for the moment let us assume that there are three basic vehicle, three basic systems, extreme right is hot air balloon which is simply hot air. So you generate buoyancy by using the fact that when air is hot its density becomes less compared to the ambient air. So either you can cool the air around you, that is very expensive and very difficult. What is easy is you create a bag which is gas proof or air proof and the air inside can be contained heated and hence you get buoyancy. So you go up but that is all you can do, you can go up and then the wind will take over. So with the hot air balloon you can have fun, you can have adventure, you cannot plan your journey, your journey will depend upon the wind conditions, your destination and speed is a function of wind conditions. Some people have tried to make controllable balloons etc but they become very complicated. So this is the most basic LTA system and we will explain with this system also but as you can understand it has got limitations in its capability. Go to the middle of the slide, you come across a system which is actually a kind of, you know it is a very confusing thing. It is an aerospace system which remains stationary, hence the name is Aerostat. This is very much contrary to common perception, how can an aircraft remain stationary? But this aircraft is considered to be good and well designed if it remains stationary. If an aerostat moves, we are unhappy with it, we do not want it to move, we want it to remain stationary, we want it to automatically align with the wind and maybe drift slightly but remain stationary. So what we do is we arrest one of the degrees of freedom of the balloon. So you can assume it to be an aerospace system which is tethered to the ground. So all of us who have played with balloons as children, we have played with the most basic aerostat, a string attached to a balloon which contains gas is an aerostat. The difference is that the envelope of the aerostat is designed in such a way that it has best possible aerodynamic behaviour. So please tell me, what kind of aerodynamic behaviour would you like to have in a good aerostat envelope? What aerodynamic features would you like to have? Do you want to have it or no? So that is dynamics. You are even pitching convented dynamics, I am talking about aerostatics, yes that is dynamics. Alignment requires motion, talk about only statics, will you be happy with positive lift? Will you be happy with some amount of lift? No, I want maximum lift, I would like to have maximum lift possible from a given shape. So now which shape will give you maximum lift? Which shape will give you maximum lift or aerostatic lift, elliptical is one answer but I do not think elliptical is right, it will be spherical because a sphere is a geometrical body which has got the least surface area for a given volume. So if weight of the system is proportional to square unit area of the envelope which is true then the best envelope shape will be spherical from the weight point of view but a spherical envelope need not have the best aerodynamic characteristics, it may not be the best to align, it may not be the best shape to prevent yaw and roll or pitch. So therefore you have to give it an aerodynamic shape so that the drag is minimized. So an aerostat envelope is shaped so that the drag is minimized, on the backside you can see there are these fins which are given so that it can align with the ambient wind because if it is not allowing with the wind it will have more drag, if it aligns with the wind it will be facing with the least possible frontal area and hence it will have less drag. So your question is that if it is a stationary system which would have more drag so that it does not move your argument is true for a flying vehicle which is untethered. If in a balloon you have more drag what will happen is it will actually go like this. So you do not want the balloon to be near the ground, you want the balloon to be up in the air at the height you want. So there is something called as blow by which is the lateral motion of the balloon because of drag it has to be the least and for that we need low drag shape. And then on the extreme left we have a system called airship which is not which is untethered. It has got an aerodynamic shape to give you low drag but it has got these control surfaces on the back. It also has some portion of the control surfaces are fixed for example in this figure if you can see the hashed member are fixed and this white thing is the moving rudder. So you need stability also and control. We will discuss this when we come to stability and control but additional feature is there is also a power plant or an engine in this which gives it forward motion. So this is like a proper three axis control with propulsion system as against an aerostat which has a tether on the ground as against a balloon which has no propulsion, no tether and no directional control at all. So mostly we will discuss aerostats and airships because they are the ones which can be used for some serious or useful scientific or commercial work. However, hot air balloons make a lot of money. So for commercial purposes hot air balloons are also very good. So we are not dismissing them but we are saying that their capability is limited because they are at the mercy of the wind. So hot air balloon is a very exciting system and the amount of LTA technology that you need to know to make a good hot air balloon and to make it fly well is phenomenal. So all the LTA experts actually are hot air balloon enthusiasts normally because they can implement their knowledge and their expertise live on a system. In an airship like any flying aircraft they are the same four forces. Now first thing I want to ask all the aerospace engineers here, is there something fundamentally wrong in this picture? We are showing an airship okay, I have to remember logo is not a wrong thing, it is good. Shape also seems to be alright and yes there are four surfaces, two vertical, two horizontal but there is something wrong, fundamentally wrong in the way this figure has been drawn. What is wrong with the drag? Right, there is one point that the direction of drag will be always along the ambient wind. So this may not be at a zero angle of attack, that is acceptable, what else? So if I assume that there is an airship in equilibrium with zero angle of attack, suppose I assume that then lift will be equal to weight and thrust will be equal to drag. So the four forces will be in balance as shown but still there is something fundamentally wrong. What is it? Okay. What is the real scenario? They may not be aligned. What about thrust and drag? Correct, so may not be. So there is no need for lift to be exactly opposite to weight and drag to be exactly opposite to thrust. That is what is wrong in this picture, that the point of action of these forces need not be exactly at the same, it is a representative picture which just shows that there are four forces, two of them are cancelling each other in the vertical direction and two in the horizontal direction. Now out of these four forces, two of them are natural and two of them are man-made.