 Last class we had stopped at turbojet engines with after burner and water methanol injection we this we understood how these things work right and why thrust augmentation is needed now let us look at where all are these turbojets currently being applied okay where do you think they are applied then they are not so much in use in fighter aircraft very very few fighter aircrafts use it so they are basically used for cruise missiles and then supersonic transport the supersonic transport was the concord that was using it okay and very few military aircrafts use it most military aircrafts currently use a low bypass turbofan we look at what a low bypass turbofan is and why they use that a little later okay now we have understood how thrust is developed right in a turbojet and we have described we have derived that equation all that we have done now what is the Mach number range of a turbojet what range of speeds can it go up to any idea it is it goes up to 3 sometimes okay Mach number of 3 and it can go from 0 to Mach number 3 altitude it is known to go to very high altitudes is not a problem the Mach number is not a problem so we should have stopped here technically right it fulfills all our requirements of flying faster and higher so why are there other engines okay one of the key things here is that the specific fuel consumption of turbojets is very high and therefore people started to look at whether there can be an alternative in terms of reduction of SFC okay now the next generation of aircraft engines was the turboprop engines the idea behind a turboprop is very simple people had piston engine plus propeller earlier now they had a turbojet now in a turbojet the turbine produces sufficient power to just run the compressor and there is scope for extraction of more work from the fluid which we would in a turbojet allow to go through the nozzle so that you get the required thrust now what you can also do is not let it go through the nozzle not let it expand through the nozzle but take out as much work as you can in the turbine itself okay then you have a lot more of shaft power which you can use similar to what a piston engine was doing and connect a propeller in front okay that is what is the turboprop engine so if you look at it if you look at the turbojet engine as a box let us say this is the turbojet engine then you attach a propeller in front you get your turboprop engine okay that is the essential idea of a turboprop engine and if you take a look at this figure here you will see that this entire thing is a turbojet engine right with a centrifugal compressor and you have a turbojet engine now if you attach a propeller to in front of it then it becomes a turboprop engine the power to drive both the compressor as well as the propeller should come now from the turbine that is you should extract sufficient work out of the fluid so that it runs both the compressor and the propeller okay so that is a turbo prop engine it is possible in a turboprop engine to distribute the power obtained from the nozzle and the power obtained from the propeller optimally okay but if all the power is supplied only to the propeller then it becomes what is known as a turbo shaft engine all the expansion of hot gases then it becomes a turbo shaft engine turbo shaft engines are used in helicopters right so these are now the performance of this turbo prop engine is in between that of a turbojet and a piston engine plus propeller okay so it is more more fuel efficient than turbo jets and it can fly at higher altitude and speed compared to a piston engine plus propeller typical altitudes at which turboprops can fly are 5 to 8 kilometers and speed is around 750 kilometers per hour okay yeah okay good question see let me look at the thrust equation that we derived in the last class now if you remember our thrust equation this was our thrust equation now as I said this portion is larger compared to this portion this portion is a very small point now if you look at this portion you can look at it as mass flow rate into velocity differential if we assume f to be very small compared to 1 f is typically around 0.3 0.3 to 0.1 so if you assume this then you can rewrite and if you assume that exit pressure is equal to ambient pressure then this goes to 0 and you can rewrite your thrust equation as so mass flow rate into velocity differential right between the flight velocity to the exit velocity of the jet fine this is how thrust is obtained now there are two ways of getting the same thrust here one is if you get the same thrust by increasing this part right that is one way or you can get to the same thrust by increasing the mass flow rate through the engine okay now what is done in a turbojet is that you increase the velocity differential V e is much greater than V a and therefore you get a high thrust now you can get the same thrust by having a smaller V e- V a but a larger mass flow rate in a few classes from now we will be showing why if V e- V a is large you will get a lower efficiency we have to discuss something known as propulsive efficiency which we will talk about a little later in the course and then I will be able to show you why if this is large you will get a lower efficiency okay so what is done in a turbo prop engine is to get to the same thrust you are now pushing through a larger mass flow rate but with a smaller velocity increment remember the velocity increment is something similar to what we discussed in piston engine plus propeller so you have V a and V e here and this differential is not large but a large mass flow rate is passing through the fan and therefore you get the high thrust okay and not through the a large portion of this therefore it becomes more efficient okay but turbo prop engines also have a limitation the limitation is they cannot go beyond a forward speed of something around 0.7 Mach number although this is more efficient there is a limitation that you cannot go beyond 0.7 Mach number now this limitation comes about because of what happens at the fan tip okay now you want it move out to be more efficient so you want a larger propeller so that you pass through a larger mass flow rate through it right so if you increase the diameter now you are increasing the diameter and the rpm that it rotates at is typically around 2500 rpm the propeller rotates it if you are increasing the diameter then what happens to the speed at the tip the rotational speed that also increases right so in addition to a forward component it has two components one VA and one is you so the combination of these two which is the actual velocity that the blade tip sees might exceed Mach number one when the forward speeds are around somewhere around 0.7 okay when that happens there is a shock structure that gets formed at the propeller tip and that reduces the efficiency of the propeller very much and therefore you will not be able to get the required thrust okay so therefore you cannot operate it beyond this forward speed so what is done is you do not want to reduce this diameter because then your mass flow rate through it will be small so there is a limitation that you cannot go beyond 0.7 Mach number okay now we had taken a look at how the piston engine plus propeller performs like if you remember in the previous class two classes back I looked at what are all the numbers SFC and other things right if you remember what is the definition of SFC we had defined SFC for a turbojet engine for a turbo prop engine it is slightly different it is m.f per unit power that is produced okay so the unit will be what will be the unit if you have this in watts what will be the unit what is what per second what second so either you can leave it as kg per second that or you can put it as okay so this is the SFC for definition for the essential idea is we want to compare the power that is developed by the engine and not look at what is the final thrust that is delivered the final thrust that is delivered is a function of the propeller characteristics so we would want to take it out and look at what is the SFC without the propeller being involved okay and in the last class I had defined what is SFC for a turbojet engine okay now in addition it is the shaft power it is not the power generated by the turbine but it is the power that is available that is power generated by the turbine minus the power consumed by the compressor and what is available for doing useful work okay now there are also other parameters that we had looked at that is power to weight ratio okay now what does power to weight ratio tell you it should be higher or lower what is desirable if it is higher than the weight of the engine that you need to carry to deliver the same power will be smaller okay and power to volume is it tells you what is the size and therefore a higher value would be desirable because it reduces drag okay fine now let us look at the comparative stats between a turbo prop engine and a piston engine plus propeller what I put together in this table is the ones in the black are piston engine plus propeller and the ones in red are turbo prop now notice that for the same power level we can compare 2600 and 2000 nearly the same power level look at the mass of the engine that is required it is a dramatic reduction right and therefore consequently you have the power to weight ratio of the power to mass ratio is something like 7.5 for a turbo prop whereas it is around 2 for a piston engine plus propeller so it makes this power to wait it reduce the weight of the engine dramatically now if you look at power to volume right it is a very small number for piston engine plus propeller around 0.3 whereas it is around 6 maximum in the case of turbo prop engine so it not only reduces the weight of the engine it also reduces the size of the engine so power to weight and power to volume of turbo prop are much more superior to that of piston engine plus propeller and that is the reason why you would not find piston engine plus propeller operating in a large number of applications these days they are very much restricted to either a unmanned aircraft or 2 to 4 seater aircrafts and agricultural aircrafts okay and most of the large passenger applications has been taken over by turbo props okay because the drag is less as well as the weight is much more smaller why does this how does this happen why is it that you are seeing this we go from one to the other and you reduce magic why do you think is this happening what is your reasoning week now also reduction gear was there through part of it is correct the other part that you are missing is if you remember that we derived for the power of the piston engine plus propeller you will discover that it is directly proportional to the RPM right and I told you that if you look at Enfield bullet and the current day pulsar other vehicles you will find that the RPM is higher and therefore the power to weight will be much more right and a very similar thing happens here the RPMs of gas turbine engines are much higher than RPMs of piston engine plus propeller okay it is typically in the range of 20,000 and if you go for a smaller micro turbines it will be in the few lack RPMs go to few lack RPMs very large gas turbines operate at a reasonable RPM of around 10000 so it is much larger than typical IC engine RPMs and therefore you find that these two are smaller in addition it is a larger mass flow rate that is going through the engines right and you can add more fuel and therefore you find this advantage being there and as I said because of this phenomenal advantage the IC engine or the piston engine plus propeller lost out and you are only left with turboprops and turboprops have an application and you have turboprop engines being used in large military and civilian transport this is mostly restricted to cargo okay large military aircrafts anything comes to mind India recently acquired something from the U.S. C-130s right they acquired very recently a large number of aircrafts something similar is Antonov very large aircrafts for military transport and civilian cargo transport only because if you are looking at cargo it does not matter to cargo that it has to sit for a large number of hours in the plane but if you are looking at passenger transport because you have this restriction on forward speed of point seven the time taken will be much larger for longer haul flights right so therefore you tend to use turboprop engine in medium medium range passenger transport so you won't find it across continents but inside countries and side continents you will find this for passenger transport right okay now the next generation of engines came out of turboprops primarily to address this limitation okay this was the limitation with turboprops SFC wise it is very good but I cannot go beyond point seven Mach number so we want to have the best of both words can we do with good SFC as well as go to higher Mach numbers what was available was only turbojets at that time which was very high SFC so the next generation of engines that came about was what is known as turbofan engines now what was the problem with turboprop engines it was only point seven Mach number and that restriction was because at the blade tip right now if we assume this is this box is the basic turbojet engine and this was our turbo prop engine the trouble was that because it is rotating at high RPMs and you have a large diameter at the blade tip the Mach number was exceeding one and the forward speeds was point seven so the idea was why not reduce forward speeds ahead of this fan and a simple way to do it is put a diffuser in front of it right if you put a diffuser in front of it what happens is irrespective of the flight Mach number you can control what is the flow speed upstream of the propeller or the fan right so that is the essential idea of a turbofan engine so you have a part of the flow that is going through the fan and going through the this is known as the bypass duct and this is known as the core so you have one part of the flow that is going through the core and the remaining going through the bypass duct okay. If you look at this figure here you have what is a turbofan engine notice that you have an intake upstream of this and you have fan blades that are mounted on the same shaft as the turbine blades and the low low pressure compressor and the high pressure compressor stages are mounted on a different shaft that is connected to the high pressure turbine here high pressure turbine here right so the advantage with this kind of an engine is called two spool engine because there are two shafts concentric shafts the advantage of this two spool engine is you can control the rpm with which you rotate different parts of the compressor the low speed compressor can be operated at a lower rpm and the high speed compressor or the high pressure compressor can be operated at a higher rpm okay and that is the advantage of this and you have this fan here and there is a nozzle that and the flow goes out through this essentially this is something similar to the concept that we discussed earlier that thrust large thrust can be obtained either with an increase in mass flow rate or an increase in velocity differential what is done here is again a large portion of the mass flow rate goes through the fan and the velocity differential is very small there right there is more like a propeller and therefore the velocity differential is small so you get better performance typically the fan pressure ratios are around 1.4 to 2.2 and with this arrangement you can go up to very high Mach numbers right and still obtain the same with a lower SFC turbojet engine okay and which is why turbojets are hardly being used in any military aircrafts as well as civilian aircrafts right if you look at civilian aircrafts I had said earlier in the class that they mostly use a turbofan engine this is the reason now you can define parameter called a which is known as bypass ratio a is equal to air passing through to the air passing through for engine okay so what is a for a turbojet engine plane turbojet engine there is no bypass duct right as I shown here this is the bypass duct a part of the flow passes through the bypass duct part of it flows through the core engine core engine is still the turbojet engine right if it is only a plane turbojet there is no flow passing through the bypass duct and therefore a is equal to 0 for turbojet engines now you might ask me why is this that you still insist that velocities are very small velocity differential is very small right a large mass flow rate with a smaller velocity differential is what I said you can have this nozzle also choke the pressure ratios allow you for that and yet I say that this is so the reason is if you look at this flow that is going through the bypass there is no heat addition anywhere right and therefore the temperatures of the gases is lower so if it is choked even then we know that right so if the temperature is a lower then the velocities will also be lower a large portion of the flow is going through with a lower velocity a small portion of the flow is going through with a larger velocity overall you still have a very low velocity differential and that is how this produces higher SFC okay and typically alpha for military aircrafts will it be large or small alpha values for military aircraft okay you are right if you look at any military aircraft most military aircraft do not have their engines on the wings okay their engines are fitted into the body they are in the body itself right because you do not want a large drag whereas civilian aircrafts you will hardly find anything engine being fitted to the body engine is always outside okay so military aircrafts the engine is fitted into the body and you would like a smaller alpha because that will mean smaller frontal area also okay so typical values are between 0.321 and alpha for civilian aircrafts is around 6 you can use a large alpha here because the engines are no longer fitted on to the body it is outside and the typical Mach numbers that some of the civilian transport aircrafts look at is around 0.8 to 0.85 they are not very high Mach numbers they do not go beyond the speed of sound okay so therefore you can look at a large value of alpha in fact G uses even larger value of alpha G engines go for typically G 90 engine has a alpha of around 9 which is very large the reason they give for this is that if you look at the drag component of the engine for the nasal drag as it is called and the drag component coming from the main body of the aircraft the drag component coming from the nasal is very very small compared to the main body of the aircraft and they say that it is only in the second digit that it matters so you can go in for a larger frontal area here right alpha increasing means you are going in for a larger frontal area they say if you increase alpha you are going to increase you are going to decrease SSC right so therefore that is the advantage that they are looking at although frontal area is increased drag is increased they say it is not too much compared to the overall drag whereas the advantage that you get in terms of SSC is much much more so they go in for this alpha of around 9 so what we need to remember is alpha higher means larger frontal area larger drag for the same like we are fitting as simple fan in front of the turbo jet if we remove that fan and we run the turbo jet is turbo jet has more thrust for turbo fan when we work out some more details I will be able to show that fan will have much larger thrust for the same if you look at for the same fuel that is consumed same amount of fuel that is consumed but the turbo fan will have a larger thrust which is which will which will be obvious in terms of SFC so if you look at the data that I put together here in addition to what we had in turbo jets there is another additional parameter alpha coming in here right this is alpha and you see that alpha ranges between 4.9 to 2.4 to even smaller smaller ones are for military aircrafts there is an additional advantage in a military aircraft military aircrafts do not want to use turbo jets because there is a other advantage that you can get with turbo fan low bypass turbo fan any idea what that is no yes that is not a lot of it is another advantage that is yeah that is a by-product I would say more to do with propulsion let me tell you that the advantage is look at this if you in most civilian engines you do not mix the two streams right whereas in a military aircraft you can mix the two streams and pass it through a same nozzle okay what does that mean your additional air that is coming in into the afterburner right and that additional air you can add additional fuel and burn it so the increase in the afterburner thrust that you can get with the afterburner on is much more with a low bypass turbo fan and that is a significant advantage in a military aircraft you would want this you would want a large ratio of thrust when the afterburner is on to when it is not on so and therefore military aircrafts have been tending to use a low bypass ratio turbo fan for this reason also okay now coming back to this table here if you look at all these engines the top few are military aircrafts and the bottom ones are civilian aircrafts G EC FM 60 Pratt & Whitney engines okay and this is the Russian engine notice that SFC here is different from the SFC that you saw in terms in the with regards to turbojet okay and if you compare SFCs fans are typically between 16 to 24 Newton second whereas it was around 30 30 what was it 31 to 36 milligrams turbojet so there is a difference in SFC it is almost half the SFC that you get with turbojets and therefore it is quite obvious that you see these in civilian aircrafts more and the other advantage that I talked about for its use in military aircrafts is that you can get a higher afterburner thrust okay but you would also probably notice here the thrust to weight ratio would be lower compared to a turbojet engine right and so also the thrust to volume it is a larger volume larger larger frontal area it has so the thrust to volume and the thrust to weight are different from the turbojet engine it is larger than turbojet engine okay and what are the applications of this where do you think these are applied civilian civilian transport okay we will stop here and continue in the next class in the next class we will look at what are ramjets and how did they come about and further go into what is cramjet okay thank you.