 So, with this we look at we have finished looking at all the variations in the atmosphere. Now we look at very important aspect and this is one of the limitations of airships which we have to understand. Tell me how do you decide the amount of volume that you want to give in a balloon air? Let us say you are now designing an airship, very soon you will start looking at airship design. You are given some operating requirements, you have to design the airship. So, when you design the airship the first thing they will ask you is what is the envelope volume, right. Then you have to also decide what is the balloon air volume. First question I want to ask is, is provision of balloon air compulsory in an airship? Can you manage to handle the situation without providing a balloon air? Is it something that you cannot live without? What do you feel? So, recall what is the purpose of giving a balloon air? Can somebody help me with the answers? To control the lift, which lift static or gross, gross lift you cannot control. Gross lift is simply a function of the weight of the gas displaced, get weight of the air displaced which remains fixed, which remains fixed if the amount of, if the shape of the envelope remains fixed, the weight of the, the weight of the gas. Now, if you, if your ambient temperature or pressure changes then the weight changes but at a given condition you cannot. So it is a net lift, not the gross lift, okay, be very careful about these terms now onwards. So do you use a balloon air for changing the net lift? Is that the purpose of balloon air? To prevent over stressing of the envelope because of the difference in pressure. Now, when will this over pressure occur due to height is one reason, okay. Can you experience an ambient pressure drop due to any other reason? That does not change. Weather changes do not change the ambient pressure that much. They do not. Do they, does it change? Have you ever heard of ambient pressure drop or an ambient pressure increase? Across the weather front is a very macro analysis for an airship which is going to be very small compared to the big weather front. You will rarely encounter a situation where there is a change in the ambient pressure. What can happen is that you take this airship from Mumbai to Los Angeles and due to some peculiar weather situation there may be a slight change but generally ambient pressure at sea level does not change because ambient pressure is mainly because of the weight of all the air acting above the earth, which is roughly the same everywhere, okay, okay. Earth is oblique, so there is a difference at the equator and the poles. If you go into that detail, yes but generally we ignore it. So the principal cause for stressing of the envelope will be because of change in the altitude as the airship goes up. Now you have just not done a calculation for change in the delta P at a height of 1 kilometer. So what was the pressure that you encountered as 1 kilometer, ambient air pressure and at sea level 1013. So what is the percentage change tell me? No, no that is in the inflation fraction. Percentage change in the pressure of the ambient air 11.3 percent, okay. So the ambient pressure has fallen down by 11 percent from sea level to high altitude, 1 kilometer. Now, suppose we assume linear variation from 0 to 1 kilometer, drop in the ambient air pressure or percentage drop in the ambient air pressure. So if I restrict my flight to 500 meters it will be 5 percent. So if I make a remotely controlled airship and I only fly it to, how much, how I will you fly an RC airship? What is the typical range of a remote control? What you say Sandeep? You have flown so many UAVs, 1.5 kilometers, it can be, okay. But will you fly an airship at a distance of 1.5 kilometer vertically up? Typically remotely controlled airships are flown to be seen. They have to be seen because they carry a logo or yes you may like to put a camera or some system to go high. So we do not normally fly them beyond 500 meters also, normally not. 100 meters is also quite a large height for a typical RC to fly a small airship. So from sea level to 100 meters height or 500 meters height, how much delta P do you expect? Not much. So therefore, the envelope can probably stretch slightly and take care. It will not reach those excruciatingly painful pressures that it will tear, okay. So to take care of delta P effect, you require a ballonet only and only if you go above around half kilometer, 1 kilometer, etc. This is one important observation. What is the second reason to have a ballonet? Pitch control. So you can have two of them and then you can use them for pitch control, agreed? Any other reason? But if you look at now a ballonet operating at sea level and let us say we have two of them, how much of gas will actually go out and will you really get too much of a pitch? Well, you might. You might. I do not want to argue that there will be no effect, there will be. But there might be better ways of getting pitch control for a low altitude airship because a ballonet is a very complicated system. Why is it complicated? Because it has to automatically detect the delta P and start either sucking air or throwing air depending on the delta P range. So it is not easy to make a ballonet. This is making a ballonet for an airship is perhaps one of the most complicated tasks in an LTA system. So therefore, you can live without a ballonet if your delta edge is not very high. But how will you take care of expansion of the envelope because of let us say super heat or suspension of the airship in the atmosphere for a long time due to which the air gets heated, really heated and then because it is hot it expands. So read the notes, there is one section in the notes which explains how you can live without a ballonet for low altitude airships or aerostats. So I want you to understand that these are the kind of things I expect you to know before the quiz which are explained in the notes.