 Now, let us go for air speed measurement. Now, for the air speed measurement we use a system which is called as a pitot static system in respect to a scientist called pitot who was very much active in pneumatics. So, this is a system that is used to measure the aircraft's air speed. We will see in detail how the system works, but in principle you can see that there is a tube which is the one on the bottom, the L-shaped tube. You take the ram air or the ambient air, you bring it to rest ascentropically, connect it and then there is also a static pressure. So, let us see how the system works and then we will be able to talk about it in more detail. The pitot static system is an essential component that powers three vital flight instruments. The pitot static tube contains the blaffle plate, the pitot tube, the static chamber and the pressure chamber. These are mechanical instruments. The hole in the front of the pitot tube is used to measure the ram air pressure that powers the speed indicator. The static port is just a small hole on the outside of the plane that measures the outside air pressure. They are often found on the side of the fuselage or on the back of the pitot tube. The pitot tube's ram air only pushes the air speed indicator side of them. The air does not flow past the speed indicator. Static port pressure is specced through internal tubing with bellows, expand and contract that powers both the vertical speed indicator and the altimeter. We will study about them later. They indicate both climbs, descents and change in altitudes. Both the static port and pitot tube are prone to multiple failures. The pitot static system must be inspected every 24 months. This 24 months is just an indication. Actually, it is inspected more frequently. So, the voice in the video was that of Siddharth Joshi who actually made this presentation. So, we saw that the system consists of a pitot static tube which measures actually it has an intake which directly measures the total pressure and the static pressure comes from the side of the aircraft through the static holes which we will see very soon. So, the difference of them can be shown as the dynamic pressure or half rho v square and therefore you can get the value of v. The other two instruments the vertical speed indicator and the altimeter work only on the static port because they are only going to measure the parameters based on the static pressure with changes. So, this is how the pitot static system works. So, there is a pitot tube, there is a static port, there are instruments and there is a ordinary static port for removing the errors or any blockings because of any blockage or any error there could be mismatch. So, there are backup. So, you can see in this particular example the pitot tube is mounted on the fuselage. It could be in the nose, it could be a boom coming out in the front with the pitot static tube. There are many, many locations and the location is decided based on different aircraft to remove an error which we will study about called as the position error. Basically, we want it to be in substantially undisturbed flow. We want it to be like that. So, ram air pressure enters the tube and it prevents ice from blocking the air inlet to drain hole etc. We have already seen this, this is a simple explanation and I am putting this in the presentation merely to allow you to use it for self study. I think the system is very standard. So, this is a photograph of a static port which is on the side of the fuselage. So, these are small air inlets which are on the aircraft side and they are the ones which are conveying the static pressure to this particular system. Alternatively, you also need to have some other static ports to ensure that if there is a blockage you do not get any false readings. So, there could be static ports on the other side of the fuselage. There could be static ports at some other place but the important point to be kept in mind is that there should not be a location of static port which causes errors. It should actually be in such a place where it is perfectly perpendicular to the oncoming free stream. So, you could put it on the side of the pitot static tube itself which is done most of the cases or you could put it at some other locations on the aircraft. So, where are they located? So, you can see here these are examples of the main and the backup static ports on one transport aircraft and this is another example of the location of static port and these are actually signposted very clearly. So, these are signposted. So, there is a pitot static system which actually involves these three instruments. Variometer is basically the indication of the change in the height. Let us look at the air speed indicator first. It is a differential pressure system which measures both dynamic air pressure and static pressure. So, the dynamic pressure is converted. So, total pressure it is wrong. The total pressure comes from the pitot tube and the static comes from the static port and the difference is going to push. So, interestingly the air does not go through the instrumentation. Yes, that is the question. My codes are perpendicular to the air flow. Then do not you think when air will flow the local pressure just above the static hole will get down. So, we will get ball city. Why do you think so? Sir, because according to Bernoulli principle when the velocity dynamic head is high then we get the static flow. So, when I say perpendicular to the flow what I mean is that if the air is flowing like this I put it like this. So, when I put it like this dynamic pressure is not conveyed to that port. But sir, local pressure will get reduced. Why? There is a stream. Let us say there is a stream line. On that stream line you have a flow of velocity v. So, if I put the probe along the flow direction opposing it then I am going to bring it to rest ascentropically or maybe it just goes through. Then I can understand. But if there is a flow stream this way and the port is this way then it is going to measure only static pressure. How will Bernoulli principle make any difference on this? Actually, I thought that the total energy must be constant of that pressure head plus dynamic head must be constant. Right. So, if dynamic head is rising then the. Why is it rising? Because there is a velocity of the air. So, the dynamic head is there. Yeah. So, there is a dynamic head it is not rising. The dynamic head say as I said you locate these instruments at a place where the velocity is roughly equal to the free stream velocity. That means the velocity is constant. If I put it at a place where there is a bulge in the aircraft then I understand your point that because there is a bulge in the aircraft the local velocity there may not be equal to free stream velocity. Then there can be errors. But the location of the static port is at those places on the aircraft where we do not expect too much change in the ambient velocity. There will be a change. See the presence of any body definitely will affect but how much. So, instead of putting it right let us say below behind the propeller then it is a wrong reading because the propeller is going to give some dynamic pressure. Or if I put it in the wake region where there is a reverse flow or if I put it at some other place where the flow is separated. So, I put it in a location where I expect the flow to be attached. I expect the flow to be maybe slightly more than the ambient or almost equal to ambient and undisturbed. So, the direction is such. So, that is why on the fuselage sides below the cockpit not very much ahead not too much behind. Good. Any other questions anybody has? Yes, Mike please. Sir, myself Venkat sir. Yes. Sir, the boundary layer effect will show some effect on static pressure. Definitely it will. So, the thing is this either you keep it projected out like I showed you some examples where there is a lateral gap between the fuselage and the pitot static tube. The purpose of that is as long as you clear the boundary layer you will not be. So, the boundary layer is going to slowly build up. So, that is why the location of the static force is such that we do not expect it is normally located at the point before the expected transition point. So, there will be boundary layer but it will be perhaps laminar or the disturbance because of that will be minimal but still we will correct for it. We will not take that direct pressure reading we are always doing a compensation which I will talk about. Sir, I want to add a point to his point. Yes. Sir, let us consider an aircraft is flying at some altitude, fixed altitude. Yes. And as the aircraft speed increases, the dynamic pressure increases. Yes. So, there may be chance of decreasing static pressure and we may get wrong altitude value. No, no, no, no. If the dynamic pressure is increasing, why should static pressure decrease? Because I mean as per Bernoulli principle the velocity increases, the static pressure decreases. I mean as per a constant pressure. Is it really true? Is it a long? Do you agree with this? So, let me see if somebody can clarify this point. Yeah, take the mic please. Sir, the error in his assumption is because the it is not the fluid that is flowing. So, it is not its energy that is we have to consider. The relative velocity dynamic head that is the fluid is getting is because of the motion of the aircraft. So, the relative velocity just we cannot assume that because the velocity is increasing the static pressure is decreasing. The pressure of the air is constant. It is the speed of the aircraft that is increasing and thereby there is an increase in the. Okay. Anybody else would like to add? Yes. My name is Devan Sharma. Yes. So, I think in this case when aircraft is increasing its speed, so the flow is not steady. So, we cannot apply Bernoulli principle in this case. We need to account for the energy change also. Now, why do you think the flow will become unsteady? Is it going to a flow is unsteady when there is a time component to flow? So, this unsteadiness is a is present. Now, I do not agree with you the flow can be still steady when I am in a let us say I am in a cruising flight, I maintain a velocity and then I accelerate. It does not mean that the flow is unsteady. Unsteadiness is present in a flow only when there is a time related component of the flow. Okay. So, it is not because the flow is steady or unsteady and anyway in unsteady flow, we cannot apply Bernoulli principle. So, there is no question of saying that the flow is unsteady and hence Bernoulli states something Bernoulli cannot be applied. So, unsteadiness is not the reason or not the not the justification. Okay. Think about anybody you have also had a point just next to you there is a person. My name is Amok. Yes. So, we are placing the pitter tubes at positions where there is like like where the speed of the aircraft is not affecting the wind speed outside the aircraft. So, even if we are accelerating the stream velocity of the wind is not changing. Therefore, I do not think there will be any change in static pressure. I would say do not try to bring in any Bernoulli principle here. Okay. Do not think that because we have studied Bernoulli principle, we have to put it everywhere. This is a very simple thing now. You are locating as far as possible these sensors in the area where the conditions are equivalent to the free stream conditions. So, the free stream has some static pressure simply because of the altitude and the free stream has static pressure or dynamic pressure because of its velocity. So, as long as I do not mess up with them, I will not be getting too many errors in the instrumentation. Okay. Shall we go ahead? Convinced. Okay. Yes. Hello. Yes. Sir, my name is Vidushi. Yes, Vidushi. My question is that will it differ with the speed of the aircraft? Will it change? You mean to say that which pressure? Static pressure or dynamic pressure? Static pressure. Static pressure is a function only of the altitude. So, what is static pressure? At that altitude because of the weight of the atmosphere above it, what is the load acting on an element above and below and we showed in the first class I think or in the second class. First class was just course introduction. In our first lecture on atmosphere, we showed that the static pressure basically is a function only of location in the atmosphere. So, if an aircraft is flying at any speed, now if you bring in supersonic flow, shock waves, shock wave, boundary line interaction then things might change slightly, but let us keep it away. Let us say we are flying from Mach number 0.1 to 0.2 to 0.3. Static pressure will have no change. The only change will be the dynamic pressure. Okay. Yeah. Mike. Sir, my name is Virgil and my doubt is that when we say pressure altitude, then we look at the reading of the pressure and we compare with the standard atmosphere and we say that we are at this altitude. So, when we read the pressure reading, will it be a total pressure or static pressure or dynamic pressure? What do you think? I think total pressure. Why? Why are we bringing in velocity effect? Total pressure is dynamic plus static. Yes. Why are we bringing in the velocity factor into consideration? We want to measure the pressure in the, we want to find out what is the pressure that aircraft is facing and that pressure is faced by an aircraft under the ISA table at so and so altitude. So, when we, yeah, so altitude, I will show you very soon, altitude measurement. Altitude measurement is only using static pressure. Okay. It is just static pressure. We will not use total pressure for altitude measurement. Okay. So, let us now see how this air speed indicator works. So, very short video. Okay. So, it is very simple. As the speed of the air coming in that pitot static tube or a pitot tube keeps on increasing, the bellows are going to expand more because they are under higher pressure and there will be a static pressure acting opposing to it. So, the difference will be the dynamic pressure. So, this is how the ASI work. So, you have one line, you have static pressure from inside and outside the diaphragm, they cancel out. Okay. So, the change in the air speed is shown by the needle. Okay. So, now we have to look at corrections because whatever is measured by the instrument, one should not blindly believe it. Okay. If the aircraft is flying at a reasonable speed and you know it is flying because you can see things going past you and the air speed indicator shows 0. It does not mean that the speed is 0. Okay. Can it happen in a flight? Do you think it can happen in a flight that the aircraft is flying and the indicator shows 0? What could be the reason? Under what condition can you have? Yes? Yes. So, will it show infinite speed or zero speed? Why? If the opening is blocked, the opening is blocked, so what will the instrument read? Where is the momentum transferring to the instrument? No, the opening is blocked. So, there is no sensing now. The opening is blocked. So, if the opening is blocked then the pressure converges how much? Okay. At what height do we have zero pressure? Where? In space. So, it will show that you are in space because it does not get any pressure. Okay. So, when can you see it is zero? Yes. When will you see the pressure is zero? When the reading is zero? No, not infinite pressure. It will show zero pressure. Where is pressure zero in atmosphere? Where is the pressure highest in the atmosphere? So, if you go a pressure reduces, reduces, reduces, reduces, where do you get zero pressure? Correct. So, that means if the indicator is blocked, it should show infinity, right? It should show that you are in outer space where there is no pressure. The instrument does not know whether it is sensing or whether there is a mistake. There will be some air inside the instrument. So, that air is stationary and now the intake is blocked. So, the pressure acting on the bellows by a stationary fluid in a tube is zero. Some pressure. Why would it have a pressure? Pressure from the fluid comes either because of motion or because of the altitude. Now, you have a tube containing air and you block it. No, you try it out. Take a tube, okay. Put a pressure gauge and just block. The pressure will be zero and pressure is zero at infinity altitude in that atmosphere. So, it measures the pressure when it is changing. No, no, no, not that. It measures the pressure which is conveyed to it. If you remove the block, suddenly it will get some pressure because now the static pressure is going to be conveyed to the instrument. So, if you remove the block, it will show the correct pressure and that correct pressure will move the instrument to the correct value, okay. So, now the question is when will it show zero? Yes, anybody can answer or you want to do it on modal? Yes, what do you think? When will the instrumentation show zero speed? So, my name is Namya. I think when the wind speed is the same as the speed of the airplane, there is a net difference between the speed which shows zero, velocity, air speed. What do you think? Are we measuring relative speed in the instrument or the real total or the actual speed? I am asking the question to everybody in the class. It is a good point. Do you think if the speed, so do you think the instrument is showing you the true air speed or the relative air speed? You answer yourself. You are saying that if the aircraft is flying at a speed v and the wind is coming from the front at speed v opposite direction, then the instrument should read zero. But the instrument, in the instrumentation did we ever say measure the speed of the aircraft, measure speed of the wind, do the cancellation and then display? We did not. So, what we are measuring is only the ambient wind speed. So, think about it.