Hey jet people welcome to the shop at Jet City Turbines. And it’s time for jet questions 90. The 90th video in the series of your questions answered. So it’s got a looong list and there is kind of an index. So if you’ve got a question look there first, it’s probably have been answered. We have some different questions this month. Week – month – week. Week. OK. So let’s just get started. Let’s see. The first one is by Clifton Jamerson. And he says: “I am diesel. I am a diesel for 47 years.” I guess he is a diesel mechanic or technician. Or diesel something. “I never worked on jet engine”. “I have had a diesel engine runaway one time. It was an old V-6 53-series Detroit Diesel.” His question is: “Can a jet engine runaway?”. It’s a very interesting question. Can it runaway? Yeah. I’ve never heard of it happening. I guess it could. Let’s go through the details involved. Most of the air taken into the engine is not used for combustion, it’s diverted around the outside of the combustor liner. We are going to take a look at one of these – those – this thing. These. In just a second. So the combustion air goes down through the fuel nozzle, around the fuel nozzle hole, and it is burnt with fuel. The rest of the air is leaked from the outside, because there is greater pressure outside than inside and it flows inward. There is enough air to burn more fuel but the way fuel systems are designed it very very difficult to … it’s not just gonna runaway. The fuel control of a gas turbine engine is much more precisely and carefully cntrolled than the fuel going into a piston engine. Just because the consequences of it literally getting out of control are so much more disastrous so they make it so that is just can’t happen. Usually when the engine is running at full power the fuel pumps are almost at their limit. It might have a little bit of extra capacity for adjustment purposes but there is not the ability to pump massive quantities of fuel into the engine. However, if the air going into the jet engine is of a flammable mixture, the right proportion of, let’s say, propane or gasoline vapors or natural gas and air. It would be compressed and introduced into the combustion area and it would catch fire but than that flame would travel out through the front of the compressor and everywhere. It would…the engine would no longer function. So it would not speed up, It would just…everything would catch fire. So … yeah. I don’t think a jet engine can runaway but I can always be proven wrong. One of the ways that is dangerous to put flammable things into a jet engine is if it is running, or even if it’s not running, especially if it’s running sometimes a jet engine would be cleaned with methanol. If you spray enough methanol into the intake when it’s running, that methanol would catch fire somehow. And if you spray a lot in, I can see that, you know, increasing the speed. However you are not gonna water wash or rinse an engine with methanol at maximum speed. You are gonna rinse it at idle at best. Than I think you are supposed to do when the engine is motoring but not running. So for an engine running away is highly unlikely but I am sure, it can happen. If you took a garden hose of gasoline and spray it into the intake of a jet engine it probably wouldn’t be very good to do, I wouldn’t recommend it. Anyway. So very unlikely. But a very interesting question that makes you think. Makes you think. Cool. Thanks for that, Clifton. Now. Bill ??? asks: “And how do jet engines start on an actual plane like a fighter jet?” “I do not believe they connect the place to a huge starter like the one in the video”. “So how do they start jet engines like in a confined space like the one in fighter jet or in a Apache helicopter. Well, our little green start cart and the yellow one before that, in my first videos, is an aircraft start cart. So there are fittings on aircraft to take an air hose, most of the older ones anyway, to start the plane with external power. It’s more convenient to not have to need a start cart so some aircraft, modern aircraft, and larger aircraft, have an on-board APU which is a lot like the engine in our start cart which supplies air to start the engines. Certain aircraft like the F-86 Sabre are started with electricity, it’s got an electric start on the engine and there is a plug-in on the side of the plane, right there, right there, you are plugging in and you have to start the plane. The plane has on-board batteries and they are not sufficient to turn the engine quick enough to get it going. They do not have enough, just not enough capacity to the batteries, the batteries are for running the communications and navigation stuff and all that kind of stuff. So the Sabre definitely needs external power to start, there is a lot of different ways to do it. But, an airliner is usually electric start, the ones that I’ve seen. For they have air starters on their engines, the big engines, most planes that large have an APU which again is just like our start cart. And a ducting towards the starter on the engine, there is an air starter on the engine or an electric starter, depending on, depending on but if it’s air, there is ducting from the APU all the way through the plane to the engine, to the starter. And pilot flips a switch, and the air… does APU, supplies the air, starts that engine, that engine can supply bleed air. To the manifold that leads to the starter of the other engines or the APU can start the other engines. There is always different ways to do it. This is one of the things what aircraft has more than one way to do almost everything. So, it’s a good thing. Cool. OK. We are gonna take a look at this combustor liner right here. Because I get asked all the time, not this time but before, and haven’t really answered it, is how does the cooling air get diverted to do what is does. Is there a separate compressor or is there a valve? How does that happen, how do you split the air flow between combustor air and cooling air? And you will see it’s very simple. Hang on. This is an annular combustor. And it’s inside an outer combustor case. And from a different engine we’re going to look at an outer combustor case. We’ll see how complicated it is. This is one right here. It’s a big piece of pipe but is really like a big annular chamber. It’s very well made but there’s really nothing to it. It’s meant to contain the air pressure around the combustor and this one has two holes right there, those are for the igniters, those holes look like drains. It was blocked off, those ones are of an unknown… unknown purpose to me. That is where the hot air is taken off for the anti-icing and those two holes here, those are for the igniter plugs, see. Pretty complicated. Okay. A great deal of air is blasted into there and that air encounters this. Lets not block the light here. Ok, so air is rushing this way, is going to get to this solid surface and go around it. This surface and go around, and just go around and some of it is, that’s where the fuel nozzle goes, some of it’s going to get down a little circle, circular hole and travel into the combustion chamber. Now, all this other stuff, other air is blasting on the outside and it’s got little holes to go in and it gets in whenever it can. And the holes get progressively bigger and then it ends. Ok, now it’s upside down and the air comes out here. All the flame and all the combustion takes place inside this thing. And all the little holes everywhere are where cooling air, or the rest of the air, can leak in. By the time we get to my fingers right there. The cooling air is mixed with the combustion
air and it’s all one thing. And I like to call that working air. It’s the hot combustion gases that go into the turbine so the way that the air is diverted to be cooling air is by the metal structure that this thing is. The combustor liner and by tuning the diameter and the number and the shape you can control how much air gets in, where it gets in and all that stuff. So that’s how you control the amount of air that mixes with the fuel and the amount of air that mixes with what was once the air and fuel and is now flame. That’s a combustion liner. This is an annular combustor that looks a lot like a series, a circle of individual combustor liners so it’s always … there’s an evolution of shape from individual cans like those to a pure annular combustor like the LM2500 which was featured in, I think it was your questions or jet questions 56 or something like that, and this is kind of an intermediate looking thing. So, one of the questions this month, this week, this day is by Nobeltnium. He asks: “Why are there little holes on the combustion chamber?” Pretty big question. And let’s take a look at the cutaway in front of the shop to answer that question. Ok, we’re here at the cutaway in front of the shop and we’re going to see why you need to be more specific. Why are the little holes near the fuel nozzles or near the combustors? Let’s see. Those little holes. Those little holes. Those little holes. These little holes. Those little holes. This little hole. That little hole. These little holes. Those little holes. Those little holes. This little hole. How about these little holes? That little hole? You see? That little hole. That little hole. There you go. Just try to be more specific! Oh, and what about these little holes? If you can’t be more specific, at least put a second by second time signature on the video related to the question you’re asking. Otherwise nothing means nothing. Nothing means nothing! Yeah, look at that, there’s a lot of little holes everywhere. Ok, along the compressor, go to the front, past the actuating rings, looking on the intake. There we go, and some Speys in the background. Himjan asks: “Excuse me if the question is answered before, why do you need to separate fuel lines to start and to run?” Well, if you look at my “fuel nozzles 3” or “fuel nozzles 4”, I’m not really sure which one, I can’t remember. It shows the different patterns for starting and for running. For starting you need a nice fine atomized wide spray pattern to distribute flammable stuff everywhere so it’ll catch fire. And then, when you are running, you need to pump massive amounts of fuel in. And so the two orifices are, well, different sizes. And on an old fuel system like this, this is an Orenda fuel nozzle, the same thing that’s in that guy. Okay, see. That there’s a connections here. It is continuous all the way through and they supply this little line that goes into a tiny orifice in the center of the fuel nozzle and they to supply a fine spray suitable for catching fire. Which is what you want. Independent of this connection, this goes all the way around the engine in one circle and it’s supplied by one supply line from the fuel control. This one, and although it looks like they intersect there but they don’t, it just looks like it. This is supplied by its own individual fuel line from the fuel distributor, field distribution block and supplies this large line and it introduces fuel from this larger annulus right there. The outer one and allows much more fuel to be introduced. So you can burn lots of it and make your jet fighter go almost 600 miles an hour. Let’s take a look at the fuel distributor because it’s quite interesting. We’ve seen all this before but it’s always fun to revisit, isn’t it? Hang on, there we go, you see how there are six individual lines leaving that and one, big one going in. That’s the fuel distribution block and each of those lines goes to its own fuel nozzle. This is what’s called the secondary line or secondary circuit. The primaries are for starting, secondary is that what you call the main running line and they come to each fuel nozzle. There’s another one. And Seth Jensen asks, based on, uh, yeah. The “Rotor balance job on the J-44”. He says: “This is the first time I’ve seen a centrifugal compressor on your channel. Any chance you could explain how they compress air as compared to an axial?” Ok. I will, but we’re going to use your imagination. No diagrams. An axial compressor has 10 to 17 stages which is a lot of stages because each stage is not very … doesn’t have a very high compression ratio. Can raise … it can compress the air by about 10 percent maybe 15 percent which is not very much. Whereas a centrifugal compressor, because it has more of a … it’s not a positive displacement device but it has more of a hold on the air, it contains it better. It’s almost positive displacement but it’s not. It can compress air up to 9 times. 900 percent so 900 percent versus 20 percent is a big difference. Which is up why centrifugal compressor is usually one stage, sometimes there’s two but usually one. So the air comes in the front of a big impeller, they call it, wheel, and it gets … starts to turn and as it travels rearward, the impeller is shaped like you saw in the J-44 video. It’s not here anymore because I shipped it back to Darren. And as it travels outward, it experiences increasing amounts of centrifugal or centripetal force. It’s flung outward and the compressor works best when it’s turning really fast And basically it savagely flings the air to the edge. And the wheel is usually as big as it can be made until it wants to fly apart at the operating speed. So you want to get the air rushing outward as hard as possible and as it speeds up, the pathway gets smaller. That does not do any compression at all. None, whatsoever. It just means that because the air is going so fast it has to. It has to pass through a smaller, the same amount of air going quickly goes through a small hole as going slowly through a big hole. That’s why the passage gets smaller. The pressure at the intake and the pressure at the outlet of the impeller is exactly the same. Doesn’t change at all. Nothing. Now at the edge of the impeller, if we turn it facing you, now it’s facing you. We change from … we change axis or whatever. The way we would draw things, I might have to draw this. No I’m not going to draw it. Ok, so your air is … it’s coming at me and it’s rushing out to the side now in a very small height and width of a passage. All the way around so it’s a complete circle. And it goes into diverging … diverging pathways. And because the air slows down in that diverging pathway it gets compressed, hugely. Now I know you have difficulty with wondering how air rushing into an opening up cavity can be compressed, it seems crazy. “It shouldn’t, it should be the opposite.” Well it’s not. Subsonic aerodynamics are very counterintuitive. If you look at the Bernoulli equation, the energy, the total energy of the flowing airstream stays the same. So the speed is traded for pressure. And I’m not going to try to prove it to you. You can do research on your own. That’s how a centrifugal compressor works. So, very interesting question. I’m glad I didn’t have to draw diagrams for that. There’s lots of really cool animations and diagrams on the Internet that I have nothing to do with but I endorse entirely. Have a look at them and all I can talk about for Bernoulli’s equation is read as many sources as you can until it finally makes sense to you. It’s really hard to wrap your head around subsonic aerodynamics and then when things go supersonic it all changes. So try not to get confused and I can’t explain it any better than that. Really good question, thanks Jeff. Okay. Now, all this stuff around us has something to do with the last question. Last question is by Jangle2007. When a customer send you a turbine engine, what is generally, typically wrong with it? And what work does the customer ask you to perform on it? Is the customer request as simple as: “See engine in close please, restore it to working condition at any cost”. Sometimes. Or does a more nuanced conversation take place with customer involving the type of work involved, expected cost in parts and labor, etc. I understand that some of your work involves transforming an aircraft engine into ground-based turbine unit. Do some engines sent to you by customers fail a cost-benefit, a cost-benefit analysis in which case you notify the customer wholly macro buddy? I realized that the question may verge into proprietary company information. I’m not asking for company secrets. Okay. Yes to all of the the above. They’re expensive. They’re hugely expensive so they can cost millions of dollars to overhaul. And if one, if there’s one problem the customer may find that in a borescope inspection or a visual external inspection and they’ll ship it to us to say: “Can you fix that?” And we’ll go: “Yes”. Then we take the engine apart, we find other things like: “Hey that’s cracked, that should be replaced, it’s going to go” or: “This is worn out”. And then the owner usually says: “Okay, fix it”. Because you’re not going to approve one repair job with a damaged part and say: “Just put it back together with the damaged part”. Knowing that damaged parts going to cause failure. So each engine is a specific case, each operator has different requirements. If you’re flying the thing, if it’s in a military fighter jet or especially if it’s in a passenger airliner, which we don’t deal with. There are very stringent and expensive requirements to be met with. Every single component and every step of the way and it can cost a huge amount of money. If you have an industrial version of that engine and you say: “It sucked in a coke can and it broke something in the compressor, can you just fix it? Just get it running.” We can do that and then we’ll notice things saying that: “Well this looks really worn, this looks like it needs to be replaced”. And the owner will go: “No, don’t bother”. And we’ll go: “Okay, we’re not guaranteeing anything”. Or they’ll say: “Good idea, let’s increase the safety and performance of this engine and do that”. And then other times they’ll say: “Well, you know it’s old and worn out and rather than spend millions of dollars making it new again we’re just going to retire it”. So it’s like cars. Every car has its own story, every engine has its own story. There’s a lot more leeway when it’s an industrial engine and the owner literally can do whatever they want with it. With certain aircraft, military aircraft, they have high standards but they also are not bound by FAA regulations. They have their own. And if it’s an FAA controlled situation like a commercial airliner you have to do what has to be done. There’s no ifs, ands or buts and it’s going to be expensive. We do not do that work because in a nutshell, and the short way of explaining it, massive amounts of paperwork in legalese. And we would need several more managers just to handle all the red tape, let’s call it, so we don’t do that. And there’s a lot of job to do. There is a lot of work in the industrial sector and in the experimental aircraft and military aircraft market and that’s what we stick to. But really the answer to your question is all of the above and none of the above. So, there you go. Good question Jangle2007. Oh, this is an example of how we transport an engine in a slightly non-standard way. So, this engine is finished it’s an overhaul. It’s ready to be shipped to the customer. This particular cart trolley, trailer, thingy, majig, whatever it is, it’s designed to be used on an airbase only on flat ground. It has no suspension. Got brakes and steering but no suspension. And it is a universal kind of trailer. It can handle different engines but it’s meant to be towed around on an airbase so the engine was shipped to us like this because there was no engine container. So now that it’s finished it’s mounted on it by the engine mounts to the trailer here. It’s wrapped in bubble wrap and then shrink wrap. Not shrink wrap. What is this stuff? Plastic wrap, several layers. And then it’s going to be on a truck on these tires. The trailer will be hooked to the trailer. The trailer of the truck will be lashed to this thing not over the engine and then it’s going to be tarped. So what we’ve done, because the truck drivers request this and they love it when we do this because it saves a lot of time, is we put softeners on all the sharp corners. We got foam here, foam there and here and in the back in the corner. So that the truck drivers’ tarp doesn’t get holes born in it. This is going to be on the truck for a couple of days and so it’s going to be protected from the weather. But it’s a bit of an unconventional way to ship things because this is not … it’s not a shipping container. It’s a transport stand for an airbase. Usually they’re inside a metal can which provides complete protection from the weather. This is going to be fine for up to the week it’s going to be. It’s wrapped in a not exactly waterproof fashion because we’ve got these struts that we had to wrap around. But it’s going to protect it from most contamination and then all of this is going to be under a tarp so it’s going to be pretty good. That’s one of the ways we ship an engine. And there you go. Well, funny you should ask that, as a bonus question. I might be a little bit behind the times here but I just looked at “Star Wars, The Force Awakens” and I was amazed. Was funny how in Star Trek, a lot of engine stuff that we deal with was in the background in the engineering section. And sure enough, when Han Solo steps onto the Millennium Falcon and he’s got three of these in his pocket. What the hell are they? What are those things? You tell me. This one slightly different. We’re going to take a close look at this. It’s pretty hard to tell because actually the movies are not that sharp, on my computer anyway. Just the previous I was looking at three of those right there. Hang on. What’s this thing? This is … read that. Why, of course it’s a Vickers VB3497. What it is, it’s a magnetic chip detector plug and got two o-rings on because it fits into a hole and then these two things lock it in place. That’s a magnet and I’m not sure of these little tap, these little whatever they are, tabs. Things are on the ones in Han Solo’s pocket and some of them have the o-rings removed and it looks like one of them has only one o-ring groove and the magnet is smaller. Because each engine is different, this is from a Rolls-Royce Avon, it looks familiar doesn’t it? I think it does. Ok, if you’re the prop guy for “Star Wars, The Force Awakens”, I want to know if that’s what this is. Otherwise anybody else has an idea … I think that’s what it is. There are probably Han Solo’s whatever these things are supposed to be in his vest. They are from a different engine maybe, slightly bigger diameter, slightly shorter magnet. But I am … it’s my opinion that this is what they are, magnetic chip detector, Rolls-Royce Avon. This particular one, your basic Vickers VB3497 MCD plug. So, from the shop at jet city, thanks for watching.