 Let us look at the gondola now, gondola as you know is the box in which we keep the payload and all the other items all the equipment that goes as aeronics if they are not directly mounted on the envelope they are put in the gondola. So gondola can be made from a lightweight corrugated plastic sheet for an indoor airship and the best way to attach it to the envelope would be through a series of ribbons which can be wrapped over the envelope. This is the simplest method or you could actually constructed from some lightweight structure for example in our case we have constructed the gondola from cane. You can see that there is a motor mounted on the gondola in the front actually the gondola has been kept at 90 degree angle. So material choices are different you can use as DPE sheet, you can use taro foam sheet anything that is lightweight but that can carry the loads of the items and provide sufficient volume and location. So the material which we recommend would be either a rigid corrugated plastic sheet it should be spacious to accommodate the various avionics items and the payload. Now on the gondola we actually mount the motors so in some cases if it is a corrugated plastic sheet you can just cut a slot and put it there or you could use various kinds of contraptions to attach it is all up to you. So in our case we have put two brushless DC motors one is for the forward thrust the other is for the downward force which gives you the augmented lift and as I mentioned flexible attachment using ribbons and velcro strips or it could be the ribbons. This allows us flexibility because you can locate the gondola forward and backward to match the CG with the center of buoyancy so it is very easy that way. And also it becomes very easy to assemble and disassemble remember we wanted to quickly assemble and disassemble this airship within about 60 minutes when we did when we go to a place for a demonstration. Looking at fin and stabilizer sizing fins are required on the airship on the back for providing adequate amount of stability and the control surfaces are needed if you want to deflect them to create aerodynamic forces. But for indoor airships actually the rudders and elevators may not really work very well. So what we do in indoor airships normally is that we provide a small motor called as a yaw motor we mount it on the fin usually the bottom fin for ease in excess. We mount it on the fin so that you can get direct side force and this allows you to do very quick maneuvers and if you remember there was a requirement to have spot turns. So that is why it is important to have a yaw motor. Sizing of fins actually might require fairly detailed stability calculations but you know for an initial indoor airship you may not necessarily have the information or the need to do such detailed analysis. You could just use information based on the survey of literature of existing remotely controlled airships. So what we have done is we have looked at several indoor remotely controlled airships indoor and outdoor both and based on that we based on the data we arrived at some area ratios aspect ratio all these parameters in terms of the fin geometry. So you can just use it to get a good estimate for the initial for the fin. So we recommend for SDP fins high density polystyrene fins in a plus configuration and you know we can mount the motor on the bottom fin. So you notice that if you have a heavier bottom fin then you have better static stability but what are these slits meant for? Can you just think about why are we putting slits in the envelope actually by cutting slits in the fin we are going to make it weaker at that location. So then why are we doing it? Think about it. The reason is that air has to pass through them in order to provide the side force. If I install a motor with the propeller on the side of a fin and if I do not give space for the air to go through then how will it actually create the side force. So to give enough space for the air to go through and hence create thrust because thrust is created by pushing the air and the reaction gives you thrust. We will not be able to do that unless we have slits. Let us look at the mass breakdown. So if you look at Avionics mass estimate we have just gone or we recommend that for indoor airship you purely go by information which is available. So the graph of thrust versus weight for typical motors is shown here. There is a company called Ternigy which has got a series of electrical motors and we recommend you use these motors for your indoor airship. You can see that for various thrust values there is a various weight. Similarly the number of cells which are required the lithium polymer battery cells which are required they range from 2 to 3 to 4 to 5 even mind and 10 in some cases. So as far as the motor parameters are concerned one parameter that is important is how much current the motor will draw which is a function of the thrust that the motor produces. And the second parameter is the propeller diameter based on the thrust. And remember that we have to increase the current by 20% because the current will be required not only by the motor but also by the electronic speed controller. So that we should increase the current by 20% here. Similarly we did study of what is the mass of typical cells as a function of their capacity measured in milli ampere hours. So it is a fairly straight line as you can see based on the data for 2 cell batteries you can get an estimate for the mass which varies from about 20 grams for very very small batteries to about 500 grams for an 8000 milli ampere hour battery. Similarly you can go for 3 cell battery similarly you can go for a 3 cell battery which is going to have higher masses. So using the equations listed in this figure you can estimate the mass of the battery as a function of the capacity and the capacity will come from the duration and the current that you have to carry. So if we just summarize the design process basically you have to give some inputs to the process which include the payload that you have to carry in this case was 250 grams. The velocity of the maximum speed of the airship typically 2 meters per second or 5 meters per second operating altitude depends on the location at which you are flying you if you are at a high altitude you have to give a higher altitude otherwise you can give it at a low altitude and endurance which we wanted to be minimum 15 minutes. And what do you get as an output from the methodology you get envelope size if you choose the envelope shape and when you choose the envelope you can get the petal layout and all the required avionics mass breakdown and specifications. And remember friends that airships are buoyant systems and therefore handling them is not a straightforward thing you need to have some ground handling support equipment along with an airship because you cannot just leave them on the ground like you can do it for a quadcopter or for a UAV because gravity will just keep it on the ground in our case it will tend to go up. So therefore you need to have a mast as you can see when you work on the airship whether you are installing some item or whether you are installing the payload someone has to hold the airship. Now it is very difficult to tell that okay you guys hold the airship till we work so there has to be a mast you can see in the nose there is a mast. So here is a example of a small indoor mast that you need to build this one is a mast developed by two interns who came to our laboratory Sayedan Bharatwaj Utsav Bharatwaj and you know they the one on the left is the CAD model that Utsav had created and one on the right is the fabricated mast okay. Now this particular work masks designed for indoor as well as outdoor airships it appeared as a paper in the Journal of Institution of Engineers and I am very happy to share that in the year 2017 this particular paper won the President of India Prize for the best paper in this journal I am very happy and very proud about it. So you can see this is how you attach the airship to the mast and the attachment is done through what is called as nose battens. So in this case you can see that the nose of the airship we have created we have made a small structure this is also made up of cane light weight this is a very important requirement for airships nose battens why do we need the nose battens we need structural strength in the front we also need some mechanism to connect the airship envelope which is a flexible material to the mooring mast which is a rigid material. So for that some kind of a nose battens structure has to be done the one on the left is the small indoor remotely controlled airship the one on the right is actually an outdoor airship where we went for a slightly different kind of a layout but whatever you may do you do need some structure in the front of the airship to give it structural strength and also to connect it to the mast and there is one more purpose of this particular mast. So you can see in this airship which was our second airship way back in 2002-2003 we did not put any nose battens. So therefore I would like to show you what happens to an airship when you fly it without a nose battens. So you can see this is flying right now the wind speeds are quite low I would request you to focus the attention on the nose of the envelope and very soon you will start seeing what happens when you fly an airship without any nose battens. You can notice that the nose is imploding that means the pressure of the dynamic pressure half into rho into V square of the ambient air that dynamic pressure acts on to the nose and if it is more than the internal pressure then the nose tends to go inside. This is called as nose implosion and this nose implosion makes it a flat plate like a structure very high dry and therefore it is not a desirable feature in case of airships. So thank you so much and I hope you have got an idea about how to size an indoor airship. In the tutorials you will be able to do these calculations step by step and be able to understand how we can do it for a particular case. Thank you.