 create or discover, I do not know, a gas which has got density lower than hydrogen. If we can, we will bring a revolution to LTA systems. So this is the area of research which I would urge people to look into. Can we think of creating either naturally discovering, probably it is not there that is why we have not discovered it so far but we do not know. But can we not think of doing something so that the density of the gas reduces? So practical limitation today is hydrogen but hydrogen somebody said is highly combustible. So let us say we do not want to put a combustible gas, now what do we do? The next best gas is helium, what is wrong with helium? It is very scarce, where is, you know how is helium produced or how is it created? Small distillation of air, yes I have with me in the lab a small helium leak detector which essentially does an analysis of ambient air and tells you how much helium is available. If I bring that instrument here and know around, it will show presence of helium but 10 to the power minus 5, very very small amount. So if I take the entire air in this room and pass it through a special filter, I may get a very small amount of helium, the whole process is very expensive and it is not commercially viable to recover helium from atmosphere, the presence is in traces. So how is helium produced? So what happens? So this is the first question on Moodle for everybody. I want you to tell me where is helium available, what is the cost of helium, which country has maximum amount of helium reserves and the toughest question is in India where is helium being produced, not purchased and sold, that way I can give you so many phone numbers. Naturally occurring helium, where is it being mined in India and let us say if you want to, if you want to fly, so get me a commercial price for helium and tell me for example how many reserves are available, what is the expected quantity of helium, so let us do some study about availability of helium because helium is our principal LTA gas, we are forgetting, now what about hydrogen, is there any shortage of hydrogen, can it be easily produced, is it easily available? It is easily available, everybody who does welding normally has a hydrogen cylinder, party balloons, you can create hydrogen from water also. So if you can do something to hydrogen to make it non-condustable and also not lose its density, that is another great discovery but it is not easy. So maybe somebody can search and tell me if there are efforts going on in this direction to create less combustible or uncom combustible hydrogen, ADRD, okay, so find out and tell me what ADRD is doing, what that, alright. So this is the first thing that I want you to do, now here is a slide which tells you about the components of an airship, so like an aircraft we have control surfaces but in the aircraft we normally see only one vertical tail, normally we see one vertical tail on the top and we see a conventional horizontal tail, here also we see horizontal tail but here we also see a tail on the bottom. So this is another interesting question that I want you to think about, why is it so that in aircraft you can manage with one vertical tail and two horizontal tails and nothing below or why you do not put anything below, in airships on the other hand almost always you will see four tails and interestingly in case you want to have only one vertical tail in airships we put it below not above, so that too I want you to figure out that on a single finned airship, single vertical finned airship why do we see vertical tail below not above and first of all why do we see if possible people put two of them, so that is a second question on model. Now the envelope of the airship is equivalent to the wing of the aircraft, the main lift producing system and the gondola or the body which is below the envelope is equivalent to the fuselage of the aircraft where you put the passengers, payload etc. But interestingly the gondola is much smaller in size compared to the wing and we see the opposite in aircraft, generally the fuselage is bigger than the wing is relatively smaller or of the same size. Here the gondola is much much smaller than the actual envelope. Alright there are 4 types of airships from the structural configuration and I will not spend more time here because we will have a special section on structural configuration of airships but just to tell you very briefly that these rigid airships were the things that you saw mostly in the past they had a rigid framework, totally rigid framework. On the other extreme is a hot air airship, this is a hybrid balloon. So this is a hot air balloon but its envelope has been shaped like an airship envelope. So below the balloon you have the same heating elements and this balloon gets inflated with hot air and you also have integrated on that the vertical and horizontal surfaces which are also filled with hot air and you can fly and you can sell KitKat, make money by I know a Russian pilot who says I like to fly hot air airships and KitKat pays for it or a beer company pays for it because they just put the ads on the balloon and the revenue which they get is used for their own flying. So this is like a very intelligent modification to a hot air balloon. Then on the bottom left we have a non rigid airship, this is a airship which has got no moving parts or sorry I am sorry it has got no structural members inside. This is an airship which has got no structural members inside. So the entire thing can collapse and be filled into a bag. So last summer I went to Brazil to spend 2 months in a company and I took an airship envelope with me in the suitcase which was a non rigid type just fold it pack it in the bag and take it. It was 12 and a half kilograms, the airship is 8 meters in length when inflated and it can give around 15 kg payload capacity but it can pack into my suitcase because it is non rigid and then we have semi rigid which is a combination of both or a cross between rigid and non rigid. Some part of it, so there is an internal framework structure but the envelope is flexible. In the rigid you have an external framework and then you have inside gas bags. Here there are no gas bags, this envelope is a gas bag but there is a structure inside. So as I said we will discuss about it more when we come to the next configuration. I think this is historical facts and we will come to that later. So we have come to the end of today's class, it is already 12.30. Maybe I can take a few questions that you may have based on the discussions we had so far or based on what you heard so far. So are there any points that you would like to get clarified? If you have a few minutes before we wind up. Yes, there is a question here, yes. The question that I have been asked is what is the maximum altitude to which these airships can go? The maximum altitude to which airships generally fly is around may be 11,000 feet, sorry 11,000 feet that is the typical maximum altitude at which they can fly. Please understand that in an airship there is no benefit in flying higher. In an aircraft the efficiency of the propulsion system becomes more efficient when you go to higher altitudes. In case of airships there is no advantage because most airships are using either an IC engine or some of them use gas turbine but there is no advantage in really going much higher. And generally airships are made to be seen because their bodies used to advertise something. So normally airships fly at around 1500 to 2000 feet above the ground level and there is no benefit in making them like helicopters and they lose payload capacity as they go higher and higher that is another reason. So the loss in payload capacity with altitude is very dramatic in airships that is why they do not fly very high, yes but the ability to carry payload will sharply fall at high altitude. So you may be flying very smoothly but no payload and nobody can see you. If something goes wrong then you have great time coming down. So that is why the whole purpose is not to fly very high. So above the ground level they are generally 2000 to 31 feet not more. Unless there are regulatory requirements unless they are made to fly a little bit higher by the air traffic control they would rather fly as low as possible. But clear from the ground turbulence, the turbulence from the ground is up to around 500 meters that is around 1700 feet. So that is why they fly just above the ground boundary layer of the earth, just above that where the weather is fairly steady, anything else? So what happens if you have an aerostat and you just release it, it keeps on rising and after a certain point of time does it stay there or does the envelope break? That is a good question. If there is a breakaway aerostat that is what it is called, an aerostat which has got a tether break. So what will happen is that it will start rising up, why will it rise up? Because in general the total lift produced by the envelope of an aerostat is kept slightly more 15 percent or so more than the weight to ensure that the rope remains tight so that it remains more or less vertical. So when that tether is cut, it will become a free flying balloon, it will rise up. As it rises up, the ambient air density will fall whereas the density inside will remain almost the same or the same. So the delta rho will increase so the envelope will get getting stretched. And time will come when the envelope cannot take it so it will tear. So we have done some studies on mathematical modeling of breakaway aerostats. We will cover that as part of the course towards the, in the modern trends, we will show you some results of our studies on breakaway aerostats. So what do you think will happen? So let me first explain to you that an airship or aerostat envelope is not a high pressure balloon. The pressure inside is only slightly more than outside. Typical values are around 500 Newton per square meter, higher. So C level pressure is 101325 and the delta P is only 500, just to maintain the shape and of course we will have some calculations, we have to determine this number. There are kinds of loads coming on the aerostat envelope, we calculate this particular number. But it is not very high. So therefore the tendency of the gas inside to gush out is not so high. But if there is a hole on the top, then being rather than air gas, it will tend to go out and it will go out. However studies have shown that because the pressure is not so much more than atmospheric, the rate at which gas pushes out is not very large. Interestingly if the hole is on the bottom side, then what will happen simply is that the outside air will come inside because the gas wants to push the envelope up inside. If the hole is created, some gas will come out. Very soon you will find that ambient air will start going inside and it will make the airship slightly heavy. But to answer your question specifically, there have been instances of envelope tear during flight. The time taken for the balloon to come down has been 3 hours, 2 and a half hours. Last for you to do, recently there was a German airship accident in which the pilot was able to steer the airship with the puncture envelope away from the human territory and unfortunately he died because in the crash landing there was some fire, not because of hydrogen. The fire was because of the engine and the gasoline in the engine spilled out and got fire, not the envelope, envelope was helium because legally speaking or regulatory bodies do not permit use of hydrogen for any human carrying airship. So today you cannot make an airship and fly with human beings on board either pilot or passengers or both with hydrogen, it is legally not permitted today. So in this case there was helium, there was a panel envelope tear but people were saved, the airship caught fire and the pilot lost his life but it took quite amount of time. There is a YouTube video which shows it slowly coming down, then getting little bit neutrally buoyant and then drifting by wind again coming down. So it is not catastrophic, it is not catastrophic. We had one experience interestingly in IIT Bombay where we were flying a small RC airship and you know a remote controller has got an aerial, a telescopic aerial which comes out. So while starting the engine, the pilot was holding this by mistake, his aerial hit the propeller tip and the tip of the aerial broke and it went through the envelope inside but we did not notice it, airship was flown, it began flying then my son was on the field with me, he was in class 7 that time, he observed that it is a bit flabby, why it is so baggy he said. So we got it down and then we discovered that there is a hole in the bottom and then we found that tip and then we did a rewind of the video and we could see the antenna hitting and there was a sound and the tip broke off. So hole from the bottom no problem or not much problem. From the top, yes and one more thing I want to tell you, in a recent study on the modeling of dynamics of aerostat, there was a question asked that as the gas comes out, there is going to be a thrust created. So will that affect the dynamics of the airship? So our conclusion was that the thrust which is creating is only 2 percent of magnitude of the total and the plots of the trajectory with and without this considered are all identical. So to answer your question, operationally there are measures available to recover an airship even when the envelope is and finally I will show you a video of our own testing in Gymkhana ground in which we put fire, a high-level field balloon to see how it how it explodes and that video has become viral all over the world now because I will show you. You can draw your conclusions when you see the video. I think on that note we can close.