 Let us now move on to the tail plane layout. Now, when we come to the tail plane layout, we have many letters of the English alphabet appearing as the possible candidates. But before we can go ahead, let us look at what are the key requirements from a tail plane. The horizontal tail is provided to serve two main missions. One is to provide trim of the aircraft. So, there is going to be a wing pitching moment and that wing pitching moment has to be cancelled out by providing the adequate trim. Then you require it for stability and stability in the pitching motion. And finally, we want it for control or to do certain tasks. For example, during takeoff, we would like the nose wheel to lift off from the ground when the pilot makes the aircraft travel beyond a particular speed called as the takeoff rotation speed. When you have low speed flight with flaps down, there is a huge amount of pitching moment which is created generally nose down. And this pitching moment has to then be counted by the deflection of the horizontal tail. And also we need the presence of a tail horizontal tail for additional moments created during transonic maneuvering. As far as the vertical tail is concerned, a vertical tail is provided again from the same three considerations of trim stability and control. But as far as the trimming is concerned, in case one engine fails in a multiple engine aircraft, the vertical tail is required to give the correct correcting yawing moment so that the aircraft can be flown safely. Also, whenever we have a single engine aircraft nose mounted, then there is going to be a swirl of air in one particular direction depending on the direction or rotation of the propeller. So, that is going to give some kind of a sideward moment and to cancel that moment, the vertical tail is mounted at a slight offset angle to ensure that this single engine asymmetry is taken care. The vertical tail is also needed for stability in the yawing motion and to provide adequate damping for the aircraft to overcome the Dutch roll. Dutch roll is a classical behaviour of the aircraft which is because of the coupling between the yawing and the rolling motions of the aircraft. When aircraft typically has a very good directional stability, but poor lateral stability, then we end up with a phenomena called Dutch roll in which the wing oscillates between roll, pitch and little bit of yaw. We also need a vertical tail to provide the required moments to control the aircraft. We need to control the aircraft if you have an engine outflight at low speeds, at that time we need control the aircraft. There is a specified requirement for some roll rate which the aircraft should have and also we need to recover from the spin. The vertical tail plays a very important role in spin recovery of the aircraft. So, the horizontal and the vertical tails together called as the tail plane or the empanage are required to meet the three basic requirements of trim, stability and control. Now, let us see what kind of options do we have. The most common configuration that you see is called as the conventional configuration. And in this configuration you have a dedicated single vertical tail of adequate size and there are two horizontal tails which are all three of them are attached to the rear of the fuselage. So, this one is the conventional tail and nearly you know 3 out of 4 aircraft that you will see nearly would have this kind of a tail configuration. But then there are some variations. Those variations normally come when this part of the fuselage is not available for mounting the tail or it is being used for mounting something else more important. So, we will now have a look at some of the unconventional or not so common tail configurations. And as I mentioned there are going to be many alphabets of the English the first alphabet is the t-tail. So, in the t-tail you essentially move the horizontal tail vertically upwards. So, the horizontal tail is mounted at the top of the vertical tail that is the t-tail. So, because you have something at the end of the vertical tail it creates an end plate effect. And that end plate effect essentially gives you better horizontal better vertical tail more effective vertical tail. So, maybe you can reduce its length. Secondly, the horizontal tail has now moved away from the centre of the fuselage. Therefore, it is away from the prop wash and the wing wash wing down wash during normal flights at reasonably low angle of attack. And such a configuration is normally provided only because you want to mount engines on the tail. And if you mount the engine on the tail then you know t-tail is a configuration which allows you to mount the engines on the tail without necessarily creating hot spots on the horizontal tail. But there are problems the t-tail is heavier structurally it is not as efficient as a conventional tail and at higher angle of attacks when the wing wake is actually much larger and it is actually subjecting. So, in a normal configuration as you can see here in the figure the tail is completely away from the wash of the wing. So, that is a good thing. But when you come to a situation where you have a very high angle of attack flight and the wake of the wing is quite large. Unfortunately, the surface which is helpful in overcoming from this situation that is the tail is completely submerged and this phenomena is called as the deep stall condition. T-tails suffer from the deep stall problem. Let us compare a t-tail versus the conventional tail. The t-tail looks faster because it is far away it will be less prone to foreign object damage from the engine and the landing gear. And as I said it allows rear mounted engines but there are problems heavy and also it is bad for maintenance. The t-tail is actually far away. So, therefore it is difficult to reach and hence it is not a very preferred option from the maintenance crew. In fact, there is one accident that took place with Alaska's MD-83s in which a jack screw of the horizontal tail got jammed. And one of the reasons why that particular accident took place is that it had a t-tail configuration and the maintenance crew did not actually reach out to that particular part for lubrication because it was a very cold wintery night and you know in such situations there is always a lapse people would like to cut corners. So, there was a maintenance problem because of which the aircraft crashed and there was heavy damage. Here are some aircraft popular aircraft which have a kind of a t-tail configuration. It is a very popular configuration amongst gliders. One of the most efficient gliders available today is the ETA glider. You can see how much the wing is flexing in the flight and that configuration has a t-tail. Piper Tomahawk is another famous aircraft which has a t-tail and there are other two aircraft the military F-101 Voodoo and the Canadair RJ. They all have a t-tail configuration. In most cases, this particular requirement is driven by the need to have for example, you can see there is an engine here. There is an engine here on the rear. There is an engine here on the fuselage and because of this there is a requirement for a t-tail. There is a cruciform tail. It is a compromise between a conventional tail and the t-tail. So, it is the horizontal tail which is not as high as a t-tail but it is also not exactly at the junction of the fuselage and the vertical tail. So, it is little bit up. You can see it is little bit up, little bit above and you can see here it is right in the center. So, this configuration generally is arrived keeping in mind the angle at which the aircraft would be normally flying in the trimmed flight. During that condition we keep the tail away from the horizontal tail junction. The horizontal tail is kept away and at very high angles of attacks again the wing would be out of the, the tail would be out of the wake of the wing. Twin-tail is very common in military aircraft. Generally a twin-tail is given if you would like to reduce the height of the tail. So, instead of one big tail you can put two tails and also it helps in spin resistance but it gives you higher structural weight. So, many, many military aircraft especially fighters you can see especially those which have very high requirements on their you know performance especially in turning flight they go for a twin-tail configuration. Then we also have triple-tail and edge tails lockheat configuration is an example of a triple-tail configuration and this is a need driven by the requirement to fit the aircraft vertically into the hangers which are available. The horizontal, the edge tail has undisturbed flow on the vertical tail at high angle of attack. This is the edge tail configuration in the P38. So, what you do literally is you move away the vertical tail from the wake of the engine and also you provide double tail configuration so that the size of the tail can be smaller and when you have the tail at the ends of the horizontal tail then you start creating an end plate effect of the horizontal tail. So, the horizontal tail becomes more efficient and can be made smaller. There is also configuration called as a butterfly tail or a v-tail. The most prominent example of this is F117A. In the butterfly tail rather than having three surfaces one vertical tail two horizontal tail tails we get rid of them and have only two surfaces. So, we lead to reduction of one tail surface. This gives you lower interference drag it gives you lower weight but there is a problem about complexity because now both the tails are going to be deflected or used partly as a vertical tail and partly as the horizontal tail. So, you require a very complicated mixing unit to provide the required deflections. So, Beechcraft Bonanza which is a very popular reactor of general aviation it began its design life with a butterfly tail configuration but then there were many accidents and many people lost their lives and hence you know the Beechcraft company decided to modify their design and go back to the typical conventional tail configuration. So, the spate of accidents that took place in a Beechcraft Bonanza forced the company to convert the design from a v-tail or a butterfly tail to a conventional tail. Here are a few other aircraft which have a butterfly tail. The most prominent among them is the Global Hawk and also there are a few other aircraft which are not so popular not so well known but yes they have gone for this particular tail configuration Sirus Vision SF-50 and Eclipse 400 you can see they want to nestle one engine on the top of the fuselage and because of that by providing a butterfly tail you are clearing the engine clearing the tails from the hot exhaust of the jet engine. So, in short we have many configurational possibilities and layout options available for a designer you have to take a choice based on your own requirements and your own vision of what is most suitable. Thank you very much.