 So, to take care of static equilibrium especially to make an airship statically heavy or you want to make it statically buoyant, we resolve to what is called as ballasting. Ballasting basically, ballasting is nothing but use of weights to adjust the static heaviness and this is summarized as SH from now on so static heaviness will be called as SH from now on. So, there are 3 types of ballast which is normally available on airships. One ballast is called as the ballast which can be discarded that is enable ballast. This is something that helps balance during propulsion failure but you do not throw it when you have propulsion failure, if you throw it becomes light then it will not come down, you have no propulsion system. So, this jettisonable ballast is there to ensure that you are heavier than air but in case everything is fine and now you have come into land or you are coming into land. Let us say the flight is getting over and the pilot is coming into land. At that time it is better to have static heaviness. So commonsense says during takeoff I would like to be statically light, during landing I want to be statically heavy, during cruise it is best to be statically neutral. So now the airship is coming into land. If I do not reduce the weight, now how do I reduce the weight, if I do not increase the weight. So, how do you increase the weight, we will see. There are many, many methods, usually we use water because if something has to be thrown it cannot be stones or it cannot be anything heavy which can hit people. So therefore, the jettisonable ballast is always water. Then we have ballast which can be removed, which can be removed. This is adjustable. This is basically used to compensate for the people who will be in the airship, but when you park it they are not there. For example, during flight the airship will have a pilot, co-pilot, generally there is a there is a air traffic regulation that beyond a particular capacity 9 passengers to be precise you need to have 2 pilots in the cockpit, a pilot and a co-pilot. So, any airship with more than 9 seats will have pilot and co-pilot. They may also have a flight crew, if it is a passenger airship, they may, they might be a flight attendant or a flight crew member plus when you actually travel in an airship you carry fuel, you carry payload, etc., but when you park the airship on the ground you do not carry, you do not put, you do not ask the pilots to stay there. Please do not get out because it will become neutrally in less buoyant. So, please stay in this cockpit. You cannot do that. So, what you do is, as the pilot is going away, as the material on the airship is going away, you replace that with some weights. So, these weights they compensate for change in the buoyancy because something has been removed. So, equivalent amount has to be put up. Now, what would be the desired requirements for these weights? Just like I said, the reasonable ballast should be something that does not hurt people when it is thrown. So, water is fairly heavy, but it may not hurt people as we throw them. So, for the removal ballast, what would be the desired requirements? If you are designing a removal ballast system, what would you put? Yes, Chetan? Wheels at the bottom. So, that is basically meant for ground motion, ground movement. So, are you saying you will attach the wheels and make it heavy? Agreed. So, in some cases, you may have, but then you need someone to run behind the airship, which is a dynamically moving system and you have to run behind the airship and there should be a hook somewhere and you have to go and attach this hook. Not an easy task. So, we do not, we have not seen people using wheels, etc., to be attached as the removal ballast. This is something you will put inside the cockpit, let us say, inside the gondola. Yes, Chetan? Yes, I mean. I would like to have multiple units of a small unit of it, like sandbags or something. So, I can, one thing is that it is modular, so you can quickly just throw them and if they are of 1 kg each, let us say, then we can very easily match, you know, that sort of thing. That is exactly the case. So, usually sandbags are used. Usually we use sandbags, heavy sand, which occupies less volume for the given weight, because if I use a lightweight object, it will occupy more space, it will be difficult to remove. So, we use sandbags, so depending on the airship's size, when we fly our small remotely controlled airship, we have bags of 100 grams, 50 grams, 200 grams, 500 grams. So, we just use directly, we put bags when the airship is moved and then we balance it. But in passenger carrying airship, there could be 100 kilograms sandbag, 200 kilograms sandbag, 50 kilograms, etc., up to down to 1 kilogram, half kilogram, a few of them for the finer balance. So, this is another kind of ballast. The third type of ballast is permanent ballast. This is not something that is removed. Now, why would you like to have permanent ballast? Because when you build the airship, the customer may want you to install a particular equipment, so you install it and you can carefully design such that when this instrument is installed, the CG is below the CB, so you adjust the location. Out of some time, the customer says, I have now one more camera or my camera has changed or there is another equipment of 350 kg which has to be installed and you cannot say hang it on the balloon, it has to be carried, these are heavy items, you cannot just put them anywhere and from the point of view of operational use, they may be needed to be at a particular place. So, with that your CG will go for a 6, so if you have for instance heavy items added on the back side, you will have a ship with nose up, now you cannot fly with that kind of an airship, nose up flight is not possible, there will be very high angle of attack and hence there will be a substantial increase in the drive coefficient. So what we do is, we trim the airship to a desired angle by putting dead weight at particular places called as the ballasting bays. Now for the ballast is basically there to cancel the moments, so the best place to put the ballast is the farthest away from centre of gravity, farthest away from the nose, from the CG would be the nose or the tail. So what we do is, we always build some kind of a pocket or some kind of a container, either at the extreme nose or behind. Now normally our experience has shown that CG normally moves back than desired. So by and large experience shows that ballast is to be carried more in the nose side or in the front side. But there could be a situation where the ballast is needed on the rear side, so there should be provision for ballast both in the extreme front and extreme back. So ballast which is provided to create or to move the CG to a location that is desirable and is not removed as never touched unless there is an imbalance that is called as a permanent ballast. The permanent ballast is also used in aircraft, all aircraft have permanent ballast. In fact mid 27 I remember used to have around 1995 kg of permanent ballast, mounted in the nose because of modifications and changes over the years, it is dead weight. But if you can use, suppose you are given a requirement to mount a new payload and that matches with or is somewhat comparable to the weight of the ballast needed and if you can put it at that location, great then you can say fine my payload capacity is not compromised. The ballast also is the payload but this is an ideal situation which may not happen. Now there is a term called as way off which is very very important as far as LTA systems is concerned. A way off basically means what you do to trim the vehicle to the condition at which you want to operate. So just before each take off and remember this is each take off, what we have to do is we have to adjust the ballast for maintaining the required static heaviness. So first you make it neutrally buoyant by adjusting the removal ballast. Now the airship is neutrally buoyant. So that you add the geosammable ballast usually water tanks are filled up to provide the required static heaviness. So when you fly the airship there is a permissible static heaviness or a recommended static heaviness which the manufacturer tells you. So in some airships it is 500 kilograms. So an airship with 13-14 passengers would have a static heaviness of maybe even more than 500 kilograms. So that much weight you carry in the form of a geosammable ballast. Now during flight what you do during flight is the pilots during flight, during level flight because remember airship flights are normally for very very long endurance. They normally have short flights. So flights of 10 hours, 15 hours, 17 hours are routine or normally in airships. And the consumption of fuel also is slow not as rapid as that in a large transport aircraft. So you have to keep on looking at the rate of descent during the flight and you have to keep on see if I trim the aircraft that means if I balance out all the moments and if I start flying the aircraft or the airship does it starts descending slowly. So one has to maintain that. Secondly during daily maintenance this is a dynamically changing vehicle. No envelope material is perfectly gas proof especially if you look at hydrogen it is such a nasty, nasty gas smallest molecule. So there are stories of people saying that it has escaped from steel containers. People think steel is very safe way to store things. The gas can go out from steel containers because the molecule is very small. So what about this LTF fabric? So LTF fabric can only contain the gas to some extent. In real life now we will have a special lecture on airship envelope materials where we will discuss the properties of materials basically the rough ballpark value of leakage of LTA gas from a good airship is around 1 litre per square meter per day. So over so many days there will be some gas loss and that will affect the lifting capacity. So if you want to make it statically light it will not happen because the gas has gone out. So then we will have to go for top up if you want to weigh off, weigh off means balancing. If you want to achieve neutral buoyancy for an airship which has been flying for let us say 6 months you may have to top up the gas, you may have to push in more gas and then acquire the required vain features. So these are different things compared to aircraft. In aircraft you do not have these problems. In airships you have these very strange situations. Of course we know that as the airship flies there will be loss of weight due to fuel. Sir as we are flying, if we are flying for 15 hours the circuitry is large and we will face a severe loss of the gas. Yes. Correct. So now due to many reasons, one because of flying for 15 hours nonstop, the other because we are flying and fuel is getting consumed, the weight of the system which was statically heavy will become statically neutral or even light. So 15 hours ago you were statically heavy, 15 hours from the flight now you are statically light. So what do you do? That is what we are going to look at now. We have to have some mechanism of recovery of the weight that we are lost because of the fuel. So this is a very imaginative area and here you will see people have done very interesting things. So let us look at what people have done, what are the pluses and minuses of reach and then you can suggest some better methods of doing it. So because of the weight loss during flight, there will be a limit imposed on the statical heaviness and static lightness with which the airship will be allowed to operate. There should never be a situation that during flight you achieve a situation of static lightness and you cannot do anything about it after that. So therefore you can never come down. Because there is a power plant failure you are stuck in air. So one has to avoid that situation. So basically we define certain maximum static heaviness for takeoff and certain maximum static landing, static lightness for landing. So we need to calculate during the whole flight how much static light we will become. It cannot allow below a particular number because then it will become unsafe for you to operate. So now for improving the range and endurance because suppose you start becoming statically light below the permissible limit, the only option is please go and land and make yourself heavy by correcting something. So that means you are limiting your range. Just like we studied in twin engine aircraft operations over the sea there are the limits on what routes you can take because of the one-engine inoperative criteria. So there are limits on operation. Similarly, in every airship there will be a limit because of the permissible static heaviness, static lightness. What will happen, tell me? What will happen if you are more statically heavy than permitted during takeoff? It will not take off. So how does it take off when it is statically heavy? Propulsion. Propulsion. So it is statically heavy. So if you start the engines, maybe you start giving it some kind of a thrust vectoring if it is available, so you leave the ground. And then you start moving forward. As you move forward, there will be a component of the force acting. So that component will overcome the static heaviness. So you will be able to become neutrally buoyant and then you can fly. What will happen if your airship is less or I should say more statically light compared to maximum permissible? You cannot come down because the thrust produced by the tilting of the engines is not able to overcome the static lightness. So you are struggling to come down but you cannot come down because the net vertical force is taking you up. Therefore there is a limit. So what we can do is can we do something in flight? We have 15, 20, 30, 40 hours of flight. Can something be done to collect weight during the flight so that as the fuel is consumed and the airship becomes light, we kind of compensate that by collecting weight while going. Now obviously you cannot create mass in mid-air that is against the basic principles of physics. So you have to do very interesting things.