 Alright, so now we already saw that changing the angle of attack of flight which the pilot does can make you fly level at various velocities. So the lift that you need should be equal to weight and that is a function of only half rho v square s and cl that is there in level flight. So as the value of v reduces that means you want to maintain lift equal to weight at low velocities. Then if you cannot do anything else because if the altitude remains same then the density remains same, the aircraft is same so the s remains same. So the only option you have is your lift will come down but if you do not want the lift to come down because you want to maintain level flight you have to go for a higher c l. In other words you must change the angle okay. So the angle of attack at which you fly is a function of the velocity at which you are flying for level flight. So the aircraft is going to fly at a lower angle of attack at higher speeds sorry yeah and at a higher angle of attack for lower speed but you know that there is a limitation you cannot keep on increasing the angle of attack because you will hit the stalling value. So that is how limits are imposed. You cannot fly lower than a particular speed because you will stall. So whether you have power available more than power required does not matter you cannot fly because you will not be having lift equal to weight. Remember for level flight both conditions have to be met you have to have lift also equal to weight and thrust also equal to drag. So you may have enough thrust with you at low speeds to overcome the drag but you may not have the capacity to maintain level flight okay. So now let us go inside the cockpit and let us have a look at how pilots are going to do this. The pilots do not have any angle of attack meter they do not have any way of knowing which angle we should fly. They fly with reference. So let us see how they do it okay. So essentially for the pilot you have a cockpit which gives you a view outside and in the outside zone there is something called as a horizon. This horizon should be maintained horizontal because if the horizon is changing it means you are not flying level you are flying at angle you are rolling and then there is a nose in the aircraft now I am talking about small aircraft something like the aircraft used by general aviation aircraft small aircraft they will have some kind of a reference point and the aircraft say the nose or there is one more reference point called as a spinner. What is meant by a spinner do you know see in front of the propeller you always have a small conical projection that is called as a spinner. So that also becomes something like it houses the propeller assembly and you give a nice aerodynamic shaped it because it is the first thing that faces the flow. So the spinner is a fixed item it spins with the propeller so it remains relatively at a fixed location so that can also be my reference point. For the pilot study level flight will occur when you maintain a constant gap between a reference point on the aircraft say the spinner or the nose and a reference point in the field of view or in the horizon let us say there is a there is a building or a church which is quite far away so if you maintain that gap you are flying level so let us see a short video taken from a pilot training manual on how they do it it is a very interesting video. Straight and level flight in order to fly the aircraft straight we first of all select a reference point towards which will fly we keep the wings level with the horizon using aileron we keep the aircraft in balance using the rudder if the ball went to the right right rudder will be required ball to the left left rudder will be required to bring the ball back to the center to fly the aircraft level we need first of all to select the level flight power. Remember when you are saying level flight you also do not want to have a flight like this it should be straight and level straight study level technically speaking a flight which is like this is also straight and level but that is not what we are looking at so therefore the cockpit has an instrument called as a turn and bank indicator where there is a small black colored ball which is floating in a fluid and if the aircraft goes banking then the ball goes outside a small socket. So the pilot brings the ball back in the socket by controlling the ailerons. Similarly there is a problem with the yaw actually they use the rudder. Similarly there is a you will see that very soon they will show you a video in flight. Combined with the correct level flight attitude and that is the distance between the spinner and the horizon now as long as the power is set correctly and the attitude is set correctly and the aircraft is in trim the aircraft will maintain level flight which can be confirmed by reference with the altimeter. So there are three things you have to maintain the correct power the correct attitude so that the gap between a fixed object here and in this horizon remains constant and also you have to trim the aircraft correctly but now my question is many people don't understand the meaning of trimming the aircraft. What do you mean by trimming the aircraft or when can we assume the aircraft to be in a trimmed condition can someone define when is the aircraft trimmed. When the center of gravity is slightly shifted then we just vary the horizontal at the tail horizontal stabilizer by some angle and that is known as trim to make the aircraft. See what you are saying is what an engineer does to the aircraft so that it can be trimmed I do not deny what you say and that is only one thing that is only a horizontal trim or trimming in pitch you do it by adjusting the tail my question is what is trim when do we say that the aircraft is in a trimmed condition yeah maybe can you answer. So when all the control surfaces neutral position is maintained neutral position correct so that means okay now let me go one step further when is the neutral position in an aircraft maintained. So before flight it is not referred to zero there is on the flight requirement control surfaces set at some adjustments some adjustments may be up or down so that initial position has to be maintained. So again the same thing see you are talking from the ingenious point of view what do you do to trim the aircraft is you adjust the control surfaces to some position okay but my question is not what you do but when does the pilot feel that the aircraft is trimmed. When he removes the stick they should not be any when he leaves the strict condition correct. That is important in a stick free condition there should be no net movement that is what I am looking for in a stick free condition there should be no net movement if the pilot has to apply some control forces to ensure that there is no movement it is not trimmed it is untrimmed condition you are forcibly adjusting it but so you have a trim wheel in the aircraft you have trim tabs behind all that is there but when everything is done you fly hands free the aircraft flies without any net yawing moment net pitching moment net rolling moment unless there are weather disturbances then of course they will come okay but if it is a stable aircraft which we will see after a few lectures it will almost come back. So from the pilot's point of view trimming means removing all imbalances which may be there great. So now let us see inside the cockpit now. Okay we are now going to look at straight level flight let us divide this into two first of all level flight to fly the aircraft level two requirements first the level flight power correctly set second the level flight attitude you can see where the horizon is cutting through the wind stream that is the position in this aircraft for level flight if I were to select slightly too low a nose attitude you can see the horizon climbing in the wind stream the nose pointing towards the ground and we begin to lose height if I have slightly too high a nose attitude the converse happen the nose pointing towards the ground and we begin to this instrument is the vertical speed indicator which we saw in the instrument and it is not stationary it is showing some vertical speed which means the aircraft is going in this case it is going down okay so that should not happen in level flight lose height if I have slightly too high a nose attitude the converse happens the aircraft begins to climb it is climbing so neither this attitude nor that too low a nose attitude is correct because we know that at this power setting that is the correct level flight attitude all right let us now talk about straight flight to fly the aircraft straight again two requirements the first one to keep the wings level it is no use having the wings like that were turned so we got to keep the wings level to the horizon and the second requirement is to keep the aircraft in balance that little black ball needs there you see now if you just focus your attention on this instrument here okay this instrument shows the aircraft as a reference and these two dashes are the horizontal line and there is this ball which is visible here as slowly moving out why is it moving out because the aircraft is not level it is slightly banked towards the right side so for the pilot it is difficult to orient in the cockpit which is horizontal so the pilot looks at this particular instrument specifically looks at the two ticks at the end of the instrument and the reference line of the aircraft they should be in the same line or visually appealing will be there is a small ball here it should be centered so the pilot will simply do the controls so that the ball is centered you will see that now be kept central there bring it firm that we're flying straight select a reference point and if you look ahead I'll just slow the nose so you can see it I've got a beautiful chimney in view and as long as that chimney remains in my 12 o'clock position I know that I'm flying straight further confirmed of course by looking in the directional gyro indicator to confirm that we are maintaining the heading we require so in a nutshell that's flying straight and level okay so for the pilot flying straight and level is little bit different from the foreign engineer alright so what are the objectives of level flight you have to establish a few things and then you have to maintain first is you establish the correct amount of power so that your power required is equal to power available you are at the right altitude and you maintain that okay and then you trim or balance the aircraft so after you establish then you have to maintain it because as a function of time you cannot have sinking or raising so for that you have to do three things one is look outside check the reference on the aircraft and in the horizon and maintain a constant gap between them and look at the instruments in the cockpit which are going to help you so now suppose you fly in the nighttime and you want to fly now can you imagine what would be the requirement for say a military pilot sir in every case the horizon may not be state it might be in there is anyone right so in that case how the pilot could that's a good question I wish you had asked me this question so now let us see what other people think his point is very much valid number one the horizon may not be visible nighttime no horizon secondly the horizon may not be so see it depends on how you define horizon so horizon is defined as that line which is remaining almost flat and constant in the view so either you have a horizon or you don't have it cannot pilot can't know sir if the pilot can see something that is horizontal and remaining horizontal that is the horizon if the pilot cannot then the pilot flies by instrument that exactly what I was saying nighttime flight flight over an ocean where the horizon is very far away or may not be available to you flight in against a black background there is no horizon now in those conditions the pilot flies by instruments because the instruments in the cockpit are independent of the horizon there is a turn on bank indicator there is a climb into VSI vertical speed indicator so you look at the vertical speed indicator there should be no vertical speed visible the ball should be in the center that means you are flying horizontal and level look at the altimeter that should not be changing that's all that's the only thing available but can you think of a situation what could be the requirement for a military pilot to maintain steady horizontal flight in which scenario do you expect this to happen for a military pilot pitch darkness no horizon can you think of a scenario in other words why is it important for us to study and to know about study horizontal flight so most of the armament that you launch especially when you are launching precision bombs which have to be guided it is desirable that there is no great change in the acceleration of the aircraft otherwise there have to be more corrections made in the aiming systems so study level flight is a very useful requirement even for military pilots not only for transport pilots ok just to move on and to increase the scope slightly let us look at now helicopters they also maintain study level flight yes or no we see them flying across our campus regularly generally they are in study level flight in a helicopter it is very important that the angle of the blade is continuously changed as it goes across otherwise what will happen is when the blade is advancing there is some air coming from the front ok so the advancing blade is going to get more relative velocity compared to the retreating blade so therefore across the rotation of the blade of a helicopter the angle has to be changed continuously so in the retreating position or when in the in the rearward motion the angle is going to come down in the forward motion the angle is going to go up and this is going to happen in the entire cycle so this particular change in the angle which is cyclic is obtained by the swash plate mechanism you can read about it yourself and the collective is the collective or the total one but now suppose I want to move the helicopter forward then I cannot have the same angle in all the blades collectively because that will give only vertical motion now what I want to do is I want to tilt the vector forward so for that I have to give a cyclic pitch so there is a cyclic pitch lever with that the angle of attack is changed cyclically so that the net force is forward and the helicopter can move forward that is how the helicopters function ok now look at the balance of forces in the helicopter it is similar to that in the aircraft you have a drag force acting on the main rotor that is dm because it is a rotating blade you have a drag force acting on the body which is d and you have a drag force acting on the tail rotor which is t so the thrust that you produce by tilting the rotor blades forward should be equal to dt plus dm plus d and the lift that you produce again rotor blades produce lift they have to be such that lift is equal to weight and now you may also again have moments because they may not be balanced at the same point so then the same trimming mechanisms have to be brought into place in some cases the flat tail on the back that you see here this tail this tail actually helps it helps in creating this balance that is why we have a small horizontal tail fixed on a helicopter so that the moment can be continuously balanced but further trimming can be obtained by adjusting yeah this particular diagram I have already explained to you few minutes ago this shows the working of the helicopter so this you can bought at leisure okay the last thing we will do today is to look at what happens to the thrust and the power required and available as you change the altitude of the aircraft so the conditions do not remain same at all altitude so this is just a small explanation very simple explanation so at low altitude at sea level you have many higher density of air so you have many more molecules of air available right at a higher level density becomes less so let us see the effect so as the height increases as the air density reduces and as the so the paradise right drag also reduces at higher altitude you have lower value of air density because density is lower the parasite drag is going to be lower so at sea level if you have a wing which has got some particular relative flow you have some induced drag an arrow which is because it is being pushed back at the higher altitude you will find that you have to fly at a larger angle because density is less so what you have with you only is density and velocity the two things are available to you to change so let us see the effect so as the height increases the density reduces lift reduces so therefore you have to make a higher angle of attack to balance the same weight and therefore the induced drag is going to go up because you are going to be at a higher angle of attack so if you look at now what happens at various altitudes so you are at some altitude 5,000 meters 10,000 meters so notice how the lines are changing the thrust available line is coming down and the thrust required line is going up and tilting to the right why does it tilt to the right for that you have to do the numerical calculations which we will do in the tutorial probably to know how it changes so the net important point is that the gap between the power available the blue line for a thrust available blue line and the thrust required black line this gap keeps reducing and the velocity at which this gap is maximum also keeps changing so at sea level you have that point at a higher speed you have that point 10,000 15,000 so the numerical value of velocity at which the difference between the thrust available and thrust required is maximum increases or shifts as the altitude increases so you have to fly faster and faster to be at the maximum gap so now what will happen is can you imagine a time will come or an altitude will come at which these two lines will be intersecting only at one point so that velocity is the velocity that you need to fly if you want to fly at a particular altitude and that is the ceiling after that the velocity after that the power thrust required will be more than thrust available so you cannot now the same thing we look at in the power pattern now so in the power pattern also if you see the power available is reducing in the book by Anderson the power available was shown as a straight line and I said it is a simplification this is the real story is not a straight line it is a slightly curved line which we ignore and make it assumption that it is a straight line you can see now that a similar story is happening and once again the velocity at which the power gap is maximum is also increasing slightly so once again the time will come when the power available equal to power required you cannot go above that okay so this is just to tell you how the power curve changes so one simple approximation will be that the power required at an altitude is the power required at sea level times the root of the density ratio this is a simple approximation and many engines follow this particular rate also and once again from the same textbook I have copied and included here charts which are similar to what we saw so you can see that at lower altitude this is the power available this is the power required the maximum occurs at this particular speed at higher altitude sorry at sea level this is the value so at sea level you have a velocity and at altitude you have a velocity so the maximum velocity at which you fly it is a function of the difference between the power available and power required and the same thing for propeller aircraft okay now this is something which I want to leave it to you for self-study actually we already discussed it this is the ceiling calculation okay.