 Welcome to the second lecture of capsule number 3 or the sixth lecture in this particular course. Today we are going to look at measurement pressure and air speed. So the outline of today's presentation is going to be we start first with pressure measurements then we move on to measurement of air speed and once we study about air speed measurement we also need to know that there are different types of air speeds. So we look at the differences between those speeds. We will also look at two measurements one of altitude and the other of the rate of climb and descent and then we will move on to errors in the pneumatic measurements because most of the instrumentation in the aircraft so far has been based on pneumatics. We then look at how there are errors in compressible flows and what kind of compensations are provided and finally we look at the future about usage of non-pneumatic instruments for measuring pressure as well as speed as well as other parameters like angle of attack. Okay, so let us start with pressure measurement essentially what we need is a mechanism or system on the aircraft that can sense changes in pressure but the question that immediately behoves us is which pressure are we measuring? Is it the stagnation pressure or the pressure when the fluid is brought to rest ascentropically or is it the static pressure or dynamic pressure? So the question can be answered by saying let us measure both of them because both of them are of some use to us we will see very soon. So the measurements made such that the flow is not disturbed is called as the static pressure measurement and in static pressure measurement we have to be passive. In both the cases of course one should not disturb the flow field as far as possible to get a true reading in static pressure measurement we are not interested in changing the parameters of the flow. In stagnation pressure measurement we need we want to measure it when you bring the flow to rest but this rest but bringing the fluid to rest has to be done ascentropically. So what is ascentropically? Do you remember we discussed this in one of our previous lectures? So can somebody help me? What is meant by bringing flow to rest ascentropically? So what happens in ascentropic flow or what does not happen in ascentropic flow? No heat is exchanged, no heat is lost or gained would like to add something take the mic mention your name first. Sir my name is Sathya Murayar and sorry in ascentropic process entropy should be 0. Entropy should not change. So in real life can we have an ascentropic process no it is very difficult to have a physical process which is perfectly ascentropic but if the losses are minimized if the heat gain and loss is almost negligible if the entropy change is almost negligible then we can say that it is ascentropic. So why is it required to do ascentropically yes if you have some interesting point you can discuss in the class as I mentioned in my first lecture I prefer discussion and interaction. So I would like you to elaborate that that is a very very valid point so you elaborate. What you are saying is the process that they have described is not ascentropic but adiabatic. Okay. So can somebody answer yes please take a mic there is a mic somewhere there in the middle yes okay reversible reversible not it is reversible but then what about entropy change is entropy change permitted in adiabatic flow what about ascentropic flow reversible adiabatic flow is ascentropic can we say that okay. So we just go ahead basically we want to bring it to us adiabatically so in the static pressure measurement what we normally do is there is a flow and you see there is one L shaped pipe but that is not our concern right now in the static pressure measurement we have another pipe or a probe but that is flush with the inner edge of the tube. So therefore it is not protruding inside and the pressure that it will measure is called as a static pressure okay the one on your left is a tube which is going to probe inside which is going to disturb the flow and that is going to measure the total pressure okay. Similarly on the right hand side we see a similar thing you have a total pressure connection and then on the side you have a static pressure connection and on the top you have a bent stem which has a static pressure port on the sides and in the front it takes the incoming flow head on we have the total pressure port. So both these pressures are important to us and we need to measure both of them okay. So let us see how the pressure typing is done this is just a close up of the same figure it shows you that the tube that is used can be integrated to the pipe partially but the probe where you do the measurement has to be absolutely flush with the pipe even a very small projection can lead to losses and errors and that is going to be a problem. So this is a small hole which is drilled normal to the surface and on the bottom one is connected to the pressure measurement instrument. If you want to do it in a free flow that means external flow that one was internal flow in external flow you create these holes equally spaced if possible on all along the periphery of the tube which is projecting into the flow direction. Notice that the front of the flue tube could be closed or open for static pressure measurement we do not want to worry about the air coming in the front but in most cases as we will see very soon we couple these two together okay. So P-static probe is inserted without disturbing the flow streamlines and if you look at stagnation pressure measurement the most commonly used instrument is called as a pitot static tube. The last T is silent it is called as a pitot static tube and this actually is used for measurement of both I am sorry we are talking about pitot tube right now. So pitot tube does not have any static port it is just a pitot tube basically is to measure total or stagnation pressure is very simple just take a tube round the edges in the front so that it does not create any sharp discontinuities and bend it perpendicular to the stem you get a tube which can be aligned to the flow and it measures total pressure. So the fluid is this is rated isentropically to rest now how do you confirm that the fluid is brought to rest isentropically in such a tube why are the isentropic requirements or how are the isentropic requirements met how can you be so sure that this particular method of bringing to rest is isentropic in nature can someone ponder over it and tell suppose we challenge and say no this is not isentropic then how will you justify because if we do not stop it isentropically then we are not making a true measurement. So one requirement is it has to be perfectly parallel to the flow so this is something I will leave it to you for Moodle homework people are very active in Moodle in this class I am very happy they are posting things on Moodle but from now on we have to look at posting quality material. So when you post material on Moodle remember we are not playing a game of fastest finger first on Aikmanaka on Manaka Krodpathy that you simply go google the term whatever first link you get copy and paste it that is not the intention I do not want to check how soon you can do a simple google search that is not the intention the intention of asking you to go on Moodle is a self-study aspect of this course I would like you to ponder before you post you can post youtube videos because that improves clarity even you see I also use a lot of youtube videos to improve the clarity that is not a problem but the problem is like there was a post about rivets and I have given my comments on that on the Moodle page similarly do not simply put something because it has to be put the quality of the post that you put is going to be evaluated not the number we want to see how much processing you do are you able to get some interesting material which is not hitherto available something simply from a textbook unless it is mentioned that okay I want derivation of something to be shown then it is okay you can go to a textbook you can copy just give the reference and paste it but merely putting something just for number game is not our intention so now we have a very valid question we have to now convince ourselves by arguments and by some kind of explanation why a pitot tube which consists of a simple bent tube 90 degree bent tube immersed in flow provided it is kept perfectly aligned to the flow direction why is it and how is it sure that the flow is brought to rest isentropically okay argue out in a technical manner and then we will learn the question that I want to now talk about is the instruments are for the people who work on instrumentation etc but the pilot is the one who actually sees on board so the question is how does the pilot see the pressure the pilot sees the pressure basically by an instrument in the olden times they used to be dials where needles used to move such as the one that we are seeing this is a boredant tube but there could be other ways of also showing it to the pilot you can show it digitally also you can create the same display using electronics okay so what we have nowadays is called as a glass cockpit by glass cockpit we mean that the instrumentation or the display that you see on the aircraft are not physical or mechanical instruments these are computer simulations of instruments so these are basically most aircraft nowadays have what is called as a multifunctional display MFDs there will be four of them typically and any of them can be made anything by programming so if there is a pilot and copilot and there is a problem with the copilot okay I am just saying the whole instrumentation can be shifted to the left hand side and vice versa so these kind of facilities are available in a combat aircraft for example you may have a person sitting behind many combat aircraft have two member crew so typically the person sitting behind will be doing the navigation mission planning weapon release etc and the one in the front is going to basic airmanship or flying suppose the person on the back seat has a problem suppose that person faints the display can be transferred to the front so that the pilot can do all the actions it is not that now I cannot do anything because I cannot see so that is the beauty of multifunction displays but let us first go into what is currently available the physical instrumentation system this is a very typical bourdon tube which has got this bourdon tube which deflects based on the pressure which is coming I will soon show you a video so the fluid it could be water it could be air in aircraft normally we use air measurements so the air will enter from there and that is going to basically lead to so this is the open end on the bottom and the open end is fixed in place it is held inside the instrumentation the other end is the closed end to which we connect some kind of bulk ranks and levers so this is free to move so the pressure of the fluid enters and the tube under reaction to that starts to straighten as it straightens then the tube will recoil and as the tube recoils the pointer there is going to move the screen and this pointer movement will be seen by the pilot in the display so this is the most basic instrument and it has to be calibrated on ground based on the pressure expected at a particular condition should be replicated so one application would be engine oil pressure gauge the other application could be hydraulic pressure gauge the next one could be de-icing boot pressure gauge and the fourth one could be oxygen time so any place where fluids are used in aircraft under pressure you can use the fluid pressure itself to communicate to the pilot what is the magnitude of the pressure by moving the screen so what is meant by de-ice anybody knows what is meant by de-icing and what is meant by de-icing boot there are two questions here yes okay okay there is a small correction one need not have to go to high altitude for de-icing even at sea level at very cold conditions in many countries you can have icing icing basically takes place when there is presence of ice okay so yeah mostly when aircraft are flying at high altitude temperature drops so the icing builds up but you can have icing even at ground level at the airport before takeoff so whenever there is collection of ice we need to have a system which is meant to remove it called as de-icing system so what is meant by de-ice boot anyone yes the mic can go there so de-icing we know but what is meant by de-icing boot that is right thank you so there are many ways of de-icing one of the method is called as a pneumatic de-icing you could do electrical de-icing by sending a heat element some current and that melts the ice but one very simple way is the mechanical method or the pneumatic method where there are certain rubber based contraptions typically near the quarter chord of the wing upper and lower areas so they start vibrating or they inflate and deflate which breaks the ice so the inflation and deflation of that will require some fluid and that fluid will have some pressure and this tells the pilot so if the de-ice boot pressure gauge shows 0 it means the de-icing system is not working okay there is something called as a minimum equipment list or MEL when you fly an aircraft so if you do not expect icing conditions during the flight you might be allowed to fly without the de-icing system working okay it is not that every system of the aircraft has to always work in every flight but if you are flying in what is called as a known icing conditions then you will not be allowed to fly if the de-icing boot so because it will be in the minimum equipment list so when the pilot does a pre-flight check and sees de-icing good pressure is 0 he or she checks is it permitted to operate if it is okay known we fly similarly oxygen tank pressure why do we need oxygen in the aircraft oxygen system is required because when you go to high altitude for passengers as well as for the pilot you need oxygen in the case of a situation where there is a complete loss of pressure and ambient air will not be breathable similarly hydraulic pressure similarly engine oil pressure so these fluids they have exert pressure and that is used in the golden tube okay then there is another system called as a bellows so bellows basically are a collection of diaphragm chambers which are joined together something like the spring that you can see you can see there is an there is a diagram of the bellows so the pressure enters from the bottom the spring is basically going to hold the bellows together when there is no pressure when the pressure acts the pressure forces against the spring so the pressure force is acting on the spring and it is expanding the spring it is a constrained spring okay so either it is a compressed shape spring which will expand because of the acting of the pressure and the movement of the side walls that is the bellows they are correlated with the change in the pressure and at the end you have a pointer linked to a scale so as the bellows move you will be able to get a reading there is a short video showing how the bellows work bellow gauge contains an elastic element that is spring that is a convoluted unit that expands and contracts actually which changes in pressure the spring embellished gauges is made of brass phosphor bronze stainless steel or other metal that is suitable for the intended purpose of the gauge obviously depending on the amount of pressure you are expecting and the size limitations in the instrumentation you have to choose an appropriate material for the spring and the material of the spring should be such that it can come back it does not have any hysteresis so that when you extend it contract it thousand times it does not have any permanent deformation so that is why we have to go very carefully about the movement of the bellows can be converted into linear displacement this displacement can be converted in terms of pressure as flow of pressure increases a pointer starts rotating on the dial which indicates the pressure of the flow and to make this measurement precise the scale of the dial is divided in many sectors after some time the pointer assumes a new position on the pressure calibrated scale on the dial to indicate the applied pressure directly and thus pressure is measured on the dial baggage okay so this is very simple in fact this instrument is much simpler than even the modern gauge because for the modern gauge you have to have provision for the displacement of the gauge okay here there is no displacement it is only the spring which is getting compressed or extended okay so this is the information regarding pressure measurement