 Today we will look at the various options available for propulsion systems on an airship. So this presentation has been made with some inputs by an intern called Agnit Mukhopadhyay who had come over to LTA lab. So first a question about what exactly is propulsion? Can somebody help me start the discussion Pratik? What is propulsion? Something that helps you propel is using the same word and what is meant by propelling? Yes? Thrust providing. So you have got a new word called thrust, simpler language. You answer in a way that there is no need for me to ask you further questions. Simple language. Can't you simplify this further? That's exactly the point. Propulsion basically is some way of creating force and the purpose of this force is it should result in this booty body moving. It could be forward, it could be backwards, okay. It could be upwards and even downwards. For downward motion you might use gravity itself but propulsion need not be overcoming gravity. Propulsion overcomes a drag. So any means of producing a force by which a body can move forward. So then there are two sub-elements. One is that you necessarily have to produce a force and that force has to overcome the energy of the body or the resistance offered by the body and allow the body to move forward, move ahead. Now as far as airships are concerned what is the relevance of propulsion? This is very simple. Basically by providing a source of propulsion you can give direction to a body, you can take it to a particular direction or allow it to move in a direction but you can also use it to give it some stability because it is a force and that force can also be used to give some stability. So can you give me an example of how a propulsive force can be used to give stability or control to an airship. So I will once again come back and ask you what is meant by stability? I think we have discussed this in the class, right? So if you understand the meaning of stability, it could be static or dynamic. We are not differentiating now. Then if you are able to give stability or if you know what is meant by stability then how can you use a propulsive device to give stability? So you can use it to overcome the moments or the imbalanced moments but the principal function of propulsion will always remain to provide the propulsive force. So let us look at the various types of propulsion systems which have been used on airships. We will go back into history slightly as we always do and then we will quickly move forward to the current and then I will project to use some things about the future. The simplest mechanism of providing propulsive force to an airship is to use a propeller or an air screw as it is called. So what is the working principle of a propeller? How does a propeller work or how does it give you the forward, the propulsive force? Let us see if somebody can start the discussion, very good. It sucks in air and throws it behind. So very good. The blade is designed in such a fashion that it sucks air and throws it back and therefore across the propeller there is a change in momentum. So if it throws air backwards, the air throws it forward by the Newton's third law so it will create propulsive force. Now let us get little bit deeper. So obviously our aim would be to arrive at the largest possible propulsive force for the minimum possible fuel consumption or energy consumption, correct? You will size a propeller or a propulsive system. In such a way that you are able to generate the required amount of force with least amount of consumption of fuel, am I right? Everybody agrees? In other words, you need to have a very efficient conversion. And one important parameter would be the efficiency of the power plant. So you can call it as power input and power output to the power plant. And then the next thing is this energy which the power plant is generating, we need to extract it using the propeller. So you may have a very efficient engine but the propeller may be very poor or very inefficient therefore you will get less power. A large amount of power that the engine generates may not be usable by you, it will be lost. So there is a mechanical efficiency or propulsive efficiency of the system and then there is the efficiency of the propeller. So let us first look at only the propeller. So I want you to tell me what aspects of the propeller can improve its propulsive efficiency but only propeller right now. One answer we already got is the shape of the blades, shape of the blades, that is the AFL cross section of the blades. What are the other parameters about the propeller that improve its efficiency, yes please? Weight of the propeller. So what do you want it to be, you want it to be lower or high? But how does the weight on the, like it depends on the requirement he says. So I am talking about the propulsive efficiency of the propeller. How will the weight play a role in that? If I make the propeller 2 kilograms lighter, will it affect the propulsive efficiency? It may affect the aircraft weight. Agreed, it will reduce the aircraft weight, that will reduce the engine weight, that may reduce the thrust required but that is a third other effect. If the blade is too light, then it will also take length, it will not affect the aircraft weight. Okay, structural. Okay, well agreed, very nice. If the blade is very light, then it will start deforming under the loads and that will reduce its efficiency, okay. So rigidity of the blade should be sufficient so that it can withstand the forces acting on it, agreed? Any other thing about the propeller? Yes. Diameter, pitch and material. So one is the blade shape, the other is the diameter. So should diameter be more or less? No, we are doing it for a given power plant. Now I am saying here is a power plant attached to it a propeller. Will you go for high dia or lower dia for propulsive efficiency? Does diameter affect propulsive efficiency? Some people are saying no. See these are very basic questions about aerospace propulsion, very basic issues about aerospace propulsion. So I would encourage you to go and read about it. I do not wish to spend too much time here in teaching basic propulsion but I just want to sum up by saying that propeller diameter, propeller RPM, propeller blade angles and the angle variation along the span as you can see in these two propellers. They are all going to affect the propulsive efficiency but with the best possible design what is the maximum eta p that you can expect to get? What do you think? 0.7 to 0.8 is a very reasonable limit. By very careful design you may even go up to 0.9 in some cases but rarely beyond that and difficult to sustain it beyond that. So let look at some historical information. The first airship that flew with a propulsive device was by Henry Gifford and that airship used a steam engine, 2.2 kilowatt steam engine. So why did it use a steam engine? Because steam engines were the only reliable source of propulsive force available at that time. But very heavy, they are very heavy for airship applications but still Henry Gifford managed to fly an airship so you can see it is a nice simple tapered envelope and suspended below that is the gondola where he is standing with some control lever and a propeller. Even the propeller is also a flat plate kind of a thing with an angle, it is not a contoured propeller it is a flat plate kind of a thing with an angle and you can notice that there is a long exhaust pipe so that the steam can go below. So that the steam does not start creating problems to the person flying it. We will see little bit more about steam and there is a very interesting experiment about which I will talk to you. So this is the history. Interestingly contrary to popular belief the use of electrical motors on airships was attempted before the use of IC engines and this is something which is not very commonly known. So there were two French brothers and they made the first electric airship but as you can see there are these two big buckets lying here, these two huge buckets. What are these meant for? No, I thought they are the batteries that is a ballast, that is a ballast. Remember we need to carry disposable ballast for buoyancy control. So these are water buckets containing ballast and one of them contains fuel and the batteries are here, these huge racks, they are the batteries. And you can see we have one jogger sitting here setting all the connections, electrician and then there are the two brothers and this is the electrical motor with a huge propeller with a kind of a pulley or wheel drive. So the first attempt to use other than steam was electrical on airships and then the first person to fly an airship with the IC engine was the Brazilian Alberto Santos Dumont. He also has a record, he has 2-3 records, first record that he has is the first person to encircle the Eiffel tower using an airship, so a demonstration of controllability of the airships. At that time aircraft were struggling to fly around the Eiffel tower because the maneuverability needed was much larger or better than what aircraft could actually offer at that point of time. So he said look we have a more maneuverable system called as an airship which can fly around the Eiffel tower. It sounds so silly now an airship is more maneuverable than an aircraft at that time. So but he was the first person and to make it very maneuverable he went for a very lightweight engine and that was the IC engine. So the user IC engines with this particular historical airship it became almost a standard after this. So people did attempt other systems but IC engines became propulsion of choice. So broadly speaking if you leave out some unconventional engines which we will talk today, steam engine, electrical engine and IC engine are the 3 basic types of engines which have been attempted on airships.