 So, now let us start with our concepts of aerostatics. Because we are going to use aerostatics as our primary lift generation tool. Therefore, it is important for us to know what are the important gross basic concepts. Now the gross static lift or LG as I will call it from now on is nothing but the buoyancy which is what you saw. So the upward force experienced by this dust bin bag which is an LTA system by virtue of displacement of air with magnitude equal to weight of the air displaced that is the vertical buoyancy force. And there is a center of buoyancy just like we have center of gravity, center of mass we have a center of buoyancy that is the center of the gross static lift. And this particular center will be the center of the mass of the displaced air. Now in this particular case the hot air or the LTA gas hot air it had covered the entire envelope. So, therefore the center of the hot air bag and the center of buoyancy will be the same but that may not be the case in many systems as we will see. So, remember do not get confused between center of gravity which is the mass center and center of buoyancy which is the buoyancy center generally it is vertically below center of gravity. So, if you have no propulsion that means no engine force and if the air is still then obviously the center of the volume but there could be differences is not it? There could be a situation where you mount some equipment such that the center of gravity moves forward. Let us say if I mount a camera on this airship the center of gravity will move slightly forward center of buoyancy will remain same. So in general we try to bring the center of gravity below center of buoyancy but it need not be the case that is very important point to note. So that was gross static lift now we also but we are not concerned about gross static lift what you are concerned is the net static lift. Now the net static lift is going to be the gross static lift minus the self weight of the lifting gas because the lifting gas will have its own mass correct. So this is important for us because this represents the lifting capacity of the gas. We may have a very very large gross static lift but if the gas weight itself is very large of course it has to be less than air but still let us say it is almost equal to air for example hot air. Hot air the mass of hot air is not very very different from the mass of cold air it is slightly less because we know that the temperature density reduces but not so much therefore the lifting capacity of the net static lift of air hot air is far far below that of helium or hydrogen. So this is the real McCoy this is what we are really interested in. Now the weight of the lifting gas you might say anyway it is gas what is the problem it is not a small thing for example if you look at Zeppelin NT which has got around 8000 meter cube of envelope the weight of the lifting gas is more than a ton 1.39 Kg so it is not a negligible number. In calculation suppose you say let us ignore the weight of the gas because it is an LTI gas you will end up with an error of this huge magnitude. So be very careful and also do not do double accounting for gas. So some people what they do they will say lifting capacity is so much and then minus self weight and then there also be minus gas weight this is a very common mistake people do. Gas weight is already accounted for in the net lifting capacity so do not do a double deduction of the gas. Now although we are looking at aerostatics we also have to understand what is dynamic lift as far as LTS systems are concerned. Now any form of lift that is generated in an LTS system other than because of buoyancy it could be because of the engines by tilting the engines it could be because of the aerodynamic force acting on the body because it is a bluff body this bluff body when it flies at some angle of attack it will definitely generate lift drag also I am not saying that LOD will be very high but I am always sure that there will be some lift at particular angles of attack and again acting over such a large body such a large area it will not be negligible it will be there. Now do not forget tails tails are horizontal surfaces they are like small wings and on big airships the size of the tail may be bigger than that of many aircraft wings they will generate lift and that is a vertical force although it is used for balancing but the force is vertical so in the equilibrium it will give you vertical component so that is also very important and the other thing is by either tilting the engines now there also could be one situation you have an airship which is flying at an angle of attack of let us say 5 degrees now the engine is also at an angle of attack of 5 degrees it has a thrust vector there will be a vertical component. So the vertical component of thrust vector although it is a direct force we include that in the dynamic lift this is only accounting principles where do we put it we put it as dynamic lift strictly speaking even if the airship is stationary and the angle is 5 degrees for the propeller there will be some vertical force. So one can catch and say why you call it dynamic it should be called as thrust component in static lift we are just using the nomenclature here so anything other than aerostatics is dynamic lift where we look at this particular class of vehicles very soon called as hybrid airships now they are a cross between aircraft and airships in these cases the dynamic lift can be very large in magnitude it can be of the order of 40 now there is no there is no hard and fast definition of hybrid airship but it is generally considered that an airship that has dynamic lift component more than for around 40% it starts getting classified as a hybrid airship and if it is 60-70% it is definitely a hybrid. So you can see there are 2 types of course we will study this in more detail down the line but there are 2 principle types of hybrid airships one is called as dynastats. So it is interesting dynastat dynastat is dynamic moving stat is stationary. So dynastats are hybrid airships which generate lift either because of the presence of a wing like structure so this airship the one in the centre you can see there are the wings one here and one here these wings are attached to the semi rigid airship structure this is the company called as Ohio airships and they are trying to build this particular airship for transporting large amount of cargo and they say that for that application a hybrid airship such as this is much better than a classical airship. So please visit their web page have a look I will show you the link very soon and see what they are doing but practically what they were able to do so far is only make a small airship which can carry the pilot so they have made a small demonstrator. The second one on the extreme right is the airship from Lockheed Martin called as P791. This is the only hybrid airship which has actually been built and which was fabricated against an order by the US military and first of all the order has been cancelled that is because of financial reasons because of the economics it is not because of anything wrong with this particular airship but it is there there is a prototype which is flying actual flying prototype so these are dynastats. Now in case of the P791 the shape of the envelope is a double or a triple bubble and this shape generates more dynamic lift as it starts moving then we have VEPA stats which are I would say airships plus helicopters in which you augment the lifting capacity of the airship by attaching rotors to it. So there are many concepts single rotor, double rotor the one which actually flew and was tested is the Pysarchy airship unfortunately there is an accident of this airship during the flight testing so it caught fire and interestingly in the same place New Jersey where Zeppelin NNT caught fire sorry not Zeppelin NNT but the Hindenburg caught fire but still the concept was proven that it is a concept which can be used for heavy lift. So more than this we will not discuss now because as I said we have a separate section on hybrid LTA systems at that time we will discuss these in more detail right. Let us look at now the static equilibrium of LTA vehicles. Now a system is in equilibrium when forces and moments are balanced and a static equilibrium for an LTA system occurs when the forces are in balance so when its weight equals the net static lift that means the entire lifting capacity is consumed in overcoming the weight and nothing is remaining. So if they are equal we say it is neutrally buoyant or it is in static equilibrium. If on the other hand the weight is more and the net static lift is less then the vehicle has a tendency of going down so this vehicle will be considered to be statically heavy. Statically heavy means the weight is more than the net static lift and then you can have an aircraft or an LTA system which is statically light in which case the net static lift is more than the weight and that is why if you release it it will start moving up till it reaches a situation of static equilibrium. Now my question is why will an LTA system which is statically light I can understand why it starts going up because there is an imbalance in force and if that force overcomes the drive or the resistance it will go up but why does it reach a situation of static equilibrium is it always the case that it will reach static equilibrium as it goes up or there can be something else happening before it reaches static equilibrium. So what do you say will a statically light LTA system when released and starts moving up will it always reach a state of static equilibrium always okay why do you say that okay. So you are saying that as this system starts ascending the altitude increases hence the air temperature and hence air density will reduce. So a state will come when the density of the air is so less that the difference between the density of the air and the gas inside will vanish it will be equal to the self weight right okay but before that something else can happen before that the envelope can tear because of the pressure difference. So it is not always the case if that is the case any balloon which I have left as a party in a party would always be there up okay but I do not see my balloons up in the air none of them have survived because they have blown or they have torn before they were able to attain static equilibrium correct. So you are assuming that the structural strength of the fabric will always overcome the pressure difference but it will not happen in many cases there will be a tear before static equilibrium but in some cases it can happen it can happen in some cases right do you agree okay. So it is not that always there will be static equilibrium before that other things can happen. Now my question to you is would you like an airship to be in static equilibrium or statically heavy or statically light you have to choose one of these three and you have to raise your hand and then we will see what most people think you are an airship designer would you prefer their ship to be in category 1, 2 or 3 and you must give me some reasoning also not because what came to my mind I like odd numbers I like even numbers I do not like that kind of an answer let me say yes option 2 you would like their ship to be statically heavy okay whatever logic okay. So you are saying that you would like their ship to statically heavy always so that you can generate additional it by some means but if there is a problem then how do you bring it down okay this is one good reasoning any other difference opinion. Statically light okay because I have two important parameters in mind the first is economy and the second is safety in terms of safety I understand the second one seems safer okay but what we could do is one have a propulsion system and second have a ballooning both of them can help us bring it down and in terms of economy since it produces lift without any other mechanism it will definitely fly for a longer time and consume lesser fuel as such and yeah in terms of safety we have two different mechanisms so if one fails the other can take it. Okay you have mentioned a term called ballooning which I have not covered yet I will cover it today so after that what we will do we will reuse it this answer after we discussed about this concept of ballooning okay anybody for yeah Amiya. Statically light would not be fuel because at a constant altitude you will need constant thrust force as opposed to a static equilibrium okay so that will consume fuel okay so your point is that if you want to maintain the altitude but why do you want to maintain the altitude. Because generally in any kind of flight we reach a specific altitude and then we carry out the yeah but why what is the necessity. Okay so you are saying that operationally an airship is used for surveillance yes one of the big uses of airships is surveillance and you would prefer to have the same altitude if you are doing surveillance even for tourism for tourism there is no harm there is a slight increase in altitude but you forgot that if you fly an airship in the controlled air space there will be air traffic controllers also who will be monitoring its flight. And this is a much slower animal compared to other aircraft so it will be in the space for much longer time it is more important from safety of the ATC I mean from the point of view of air traffic control that the ATC has a mental picture of where the airship is. If the airship continuously increases the altitude or decreases the altitude okay the decrease may not be a problem because they normally fly at low altitude so if they fly lower there is nothing else competing with them for the airspace but near a major airport for example if you have an airship flying and there are helicopter operating so yes keeping the same altitude is desirable from many operational considerations yes. So for the third category is it possible that after it has taken off and it has attained a certain altitude it attains the static equilibrium position then you do not need any other mechanism. Correct so that is what we will see now what would be desirable is a complete control on lightness or heaviness if the pilot can have a system by which you can make it statically light when needed make it statically heavy when needed and make it neutrally buoyant when needed then it will be the most ideal situation. So I would design the airship to be statically heavy because in case all systems fail only gravity is my friend to bring it down to earth but I need not make it always statically heavy because I can make it heavy by putting extra weight so the airship will not be statically heavy it can be flown statically heavy okay so let us see what kind of systems you have so airships are normally flown with static heaviness the reason is the flight control system is improved if you have a statically heavy system plus the ground handling is also improved because if a system is statically light it will keep bobbing up as soon as the wind comes a small lift gets generated because of the dynamic forces because it is statically light or even neutral it will start going up so then that will be more load on the mast the mooring mast which we will study in more detail in the future and the thing is that you can always generate lift by flying little bit faster so if you are able to overcome if you are if you are able to if you make the aircraft or the airship slightly heavier than what the lift is available from static forces alone and if you start flying you get more dynamic lift so you can overcome the static heaviness by generating dynamic lift by flying at an angle or even by tilting the propellers if you can if you have available engines or available or some other thrust vectoring mechanism so this is the preferred option to fly an airship now the lift shortfall can be met and you can adjust it to take care of a fuel come because as you consume fuel during flight you become lighter so now something has to be done we will see what is to be done and what people have done now when you actually more the airship see there is a there is a big difference between ground handling and mooring ground handling basically means she is coming into land or about to take off at that time there are people on the ground which are holding it with ropes or a system which is holding it with ropes during that time I wanted to be statically heavy but if I want to leave it then I do not want it to be very heavy because then it is going to rub on the ground the landing gear will rub on the ground some systems will be loaded so what I can do I can do kiting that is on the mast you make it statically slightly light so now she is going to go up in the air so there will be no rubbing of wheels on the ground so the airship is actually above the ground maybe only 1 meter above the ground but above the ground and even if there is a wind disturbance it will go slightly above the ground 1 meter will become 3 meters 2 meters but there will be no rubbing on the ground this is called as kiting or like flying a kite so when you moor it with static when you are mooring it you make it statically light to allow kiting when you do ground handling you want it statically heavy and arrow stats on the other hand you cannot have them statically heavy or even neutral because if they are statically neutral that means they will go to a height and maintain that height but there is no force acting on it in the vertical direction so what will happen to the rope below it will become loose okay now a small wind disturbance it will start oscillating up and down then your surveillance camera will start getting disturbed pictures so for the arrow stat I want it to be statically light that means the lift should be always more than what is needed to overcome the weight now the rope will be under tension and that tension can be handled by the rope by careful design the airship will remain fairly the arrow stat will remain fairly stationary so from operational point of view you have to be very careful about what equilibrium position you like to provide to the airship okay any questions on this so let us go ahead now if you look at hot air airships and gas balloons now you want them to ascend right like just like the balloon that we saw it was statically light that is why it went up so for hot airships and balloons or gas balloons remember a hot air airship is basically a hot air balloon that means it has got continuous heating but it has a propulsive system and a gas balloon is basically a closed balloon so the static lightness is what is needed because they we have to make them ascend we want them to go up but now if you want to bring them down so what do you do if you want to bring a hot air balloon down you made it statically light so it went up now the fun is over you want to go back home stop stop heating correct stop heating very soon it will become cool just like our balloon came down when we stopped heating or actually it came down after it got cold air as it went up but during the process I mean during this time when you stop heating and the slowly it cools it will drift away that is you have to accept it so a hot air balloon cannot be expected to come back to the same place where you launched it only by fluke what can happen it can go up and go on this side and then the you stop and then the wind bring it back and bring it down that just fluke so hot air balloon enthusiasts they are used to travel long distances away from the launch site but for an air ship you need to have a facility that you should be able to come back to the place where you took off from