Yes, every dimension of a possible repair is laid out in very specific detail for every stage in the manual. One specification is that the radius of the final blend must be at least 8 times the depth of the initial defect.
Every dimension of every possible repair to a blade or vane is laid out with strict limits in the overhaul manual, and are strictly followed in aircraft maintenance, so there are no judgement calls. But with industrial engines, sometimes spare parts are not immediately available, or sometimes the owner wants to repair a little past what are aircraft rules. Carefully blended blades work just fine, even if much larger than specified amounts of material are removed
We do have all the repair manuals and follow the procedures regarding dimensions of repairs. Most of the blending we do is reconditioning the leading edges. We have a large inventory of spare blades, so damaged ones are removed and replaced. Our engines are not as high profile, nor as powerful as your giant turbofans. The resources aimed at maintaining perfection in their rotating assemblies is very impressive !
A properly repaired and blended blade is "servicable", as is a new blade. No distinction, and no limit to how many you can use. The airflow is marginally less, but the blades are designed with a bit of a "bonus:... I've seen compressors with severely blended and worn blades run just fine.
Hi I work for Rolls Royce and inspect the large wide chord fan blades for all Trent jet engines, I've found you videos interesting but have a question, our tolerances even on the massive fan blades for the Trent 1000 are very tight, when you refurbish a compressor or turbine blade, how do you control it dimensionally ? Do you have drawings of the blades which applied when they were new? We also check the leading edge radii with a v-cam projector at 10 times magnification
Exactly, and the heat of the TIG would change the composition of the leading edge weakening the blade. Very informative AgentJayZ, I finally now can mathematically prove why rounding something out or drilling a hole prevents further fatigue cracking. Love it. Many thanks.
There are specified radii for every stage of blade in the manual. You remove the minimum material possible, which is a compromise with maximizing the radius. Titanium Nitride is very hard... it's that gold coating on drill bits and such. Titanium is a relatively soft, tough, light metal. The titanium alloy used in these blades is stiffer, stronger, and slightly harder than pure titanium.
1) the better the repair, the less of a loss is caused in airfoil function. Ideally no there is no reduction in efficiency 2) The primary goal of blending is to reduce areas of stress concentration, which reduces the chance of crack formation. A secondary goal is to avoid destroying the aerodynamic function of the blade. So you remove the nick, and try to restore the airfoil shape.
Some people specialize in blending blades. Apprentices are used and coached until they get good. Machines that can do this work would be extremely expensive to develop, and I'm not aware of any. It is expensive, but then for a fan on an airliner engine...the engines are worth over 10 mil, and the planes over 100 mil, so expensive is a relative term. Not every blade needs the repair, and every repair is different.
One of the reaasons I like to check for two or more separate sources, is because many textbooks are full of mistakes. Could be my sources. For my purposes I only care about the relationship, so I don't need an equation with an equals sign. I just need a relationship with a (sideways 8 with the head cut off) sign...!
There are definite limits set in the repair manuals on what can and can't be repaired. Stators do have slightly more lenient repair limits. The forces inside a compressor are incredibly violent, many times more than in the most destructive tornado, and things behave in mysterious ways. Objects can bounce around in a compressor, severely damaging several whole stages of blades, yet one stage in the middel of the damage might remain untouched. How does that happen? Good question!
31:30 Trying to understand the formula I would argue the following: When r becomes large, S_a approaches S_n, which means for any given damage to the blade of length L, taking away all the material of the blade around it down to length L in a straight line (making the radius infinitely large) reduces the stresses to normal again but results in changes to the aerodynamics of the leading edge. Making r=4L (the radius r is four times as large as the length L of the deficiency) results in twice the actual stress S_a, because making the ratio L/r=1/4 results in 1/2 when taking the square root, so twice that yields S_a=S_n(1+1)=2S_n. Therefore taking your example of L=.125 in and blending to r=4L=.5 in results in S_a=2S_n.
Not that I know of. It is a very important job, and every blade is different. It is physically tough on the hands, and if done for more than a few hours, it becomes very boring. I blended blades for 10 months straight, and I'm still recovering from the experience .
Fantastic! I only include subjects in my videos that I have experience with. If I don't know for sure, I won't try to BS my way around it. Thanks for trusting me !
Great video. Highly technical, but I don't think you can appreciate the sound of a jet engine starting up until you know the tedious grunt work that has gone into it. So thanks for sharing that.
Dude... you are crazy! I appreciate that you like this stuff, but it's really not entertainment for the masses. Especially this one. It's so long and, although a detailed technical training video... most people don't want that. I'm amazed that you and everytbody else even watched the whole thing. Thanks.. but just remember, the fact you even watched this video makes you just a little "eccentric". !
Those marks are holes or divets in the surface...you could fill them in by welding, then grind back to surface level, then heat treat the blade to restore properties... a hundred thousand times, once for each pit... and then after all that, do you want to put it on your 250 million dollar plane...? Usually the choice is to spend the few tens of thousands of dollars a new blade costs, because in this shape they are what's known as "beyond economical repair".
This is an interesting and very slow process of refurbishing blades. I currently work for GE-Aviation Engine Services we repair jet engine components such as blades, nozzles, hangers on engines such as GE90-115B, CF6 series, CFM, F414 Millitary and many others. This method would kill any sort of profits in my opinion. lol
I love watching these videos, Im currently in school for my A&P certifications and these videos are by far one of the best references I have for understanding the material in a more practical application. Thanks!
You sir, are a true professional. Not only do you have a true understanding of your tools and work but you also look at the theory behind the work. In this case your look at crack propagation. People like you make the world turn and I strive to do also. As an educational tool I think this video is second to none in describing the work and justification for radiusing sharp crack tips. I think all your videos are excellent but this one in particular I would like to use, with your permission, as educational material for my students
I don't know why you do these videos Jay but they are incredibly engaging and instructional. You would make an outstanding shop teacher, though I'm sure you're getting paid far more for actually doing the work.
Well, I started out for fun, then out of a sense of duty... I should share how lucky I am to work with so much cool stuff, eh? But recently, I've been wondering if it's because I thrive on the abuse I get.... ...only kidding!
Experiencing a lot for what goes behind the scene for regular maintenance....I learned a lot today. Its been over a week i am going through all your videos with tonnes of Engg. and workshop experience.....where was this channel up till now.....anyway thanks for uploading these type of videos and also waiting eagerly for future videos..........
a labor of "love[?]. i am sure WE all greatly appreciate the time you take do do these videos AFTER a full days work. you could be bringing down your lap times,, but, instead, we gain knowledge. thanks jay!! and, please say you don't clamp a serviceable blade across the airfoil in a vise.
I personally vote for Agent JayZ to produce a full-size movie (just add another half an hour) and he can easily conquer cinemas around the world with his charming narrative and bits of fun math :)
I absolutely love these educational videos mate. It's fascinating to see the level of detail and precision engineering and maintenance. But I have some advice for you, or anyone else who might come across this. Take it as you may. I learnt this day one, (probably not day one, but you get the idea 😉), of my fitting and turning apprenticeship back in 1981. Never never ever ever use gloves of any type/kind when using a grinding wheel. No excuses EVER. Unless you think that a couple less digits, or more, might make you more........... hugable.
At 35.00: To reduce Sa we need to keep L/r as small as possible. We can do that by increasing r since we do not have control over L, so we round off at the end of crack by drilling a hole (hope I got that right). Outstanding explanation. Hats off to you AgentJayZ you really make engineering seem like maths and maths like fun. Great.
I just wanted to say thanks for putting out these videos. I use videos like these to supplement my teaching and it seems that whatever I search for turbine related, I end up finding something you posted! Thanks again and I appreciate the effort you put into these!
You can rest assured, that the vise was for convenience, and only as tight as needed to keep the blade from moving in its aluminum jaws... but you are correct in thinking this is not the way a servicable blade is repaired. All the holding for "real" blades is done by hand. Although I have seen MTU techs blending turbofan blades in wooden-jawed vises...
Its cool and not banned!! and what I meant was, in India the maintenance department for jet engines are less compared to American countries and this is not a secret, as an Aeronautical Engineer its very useful to watch your videos. I appreciate you for your useful videos! and I would like to know about water injection in jet engines and how it doesn't affects during operation?
Hi Jay, another great video. I calculated, if one of them takes 5 min it will 250 hrs. this is in text books and was able to see that the surface is not a large surface as before. But they need repairs anyway, if all blades have to be replaced, it will be a high dollar repair. Thank ou for these videos! Hello from Panama city, Panama, it is very hot in here almost all day! I'm missing California, Los Angeles and Norwalk, places where I lived at. Remember to have some fun sometimes!
Well, I really can't argue with this. I mean, I completely see where you are coming from, and it seems as though you have a legitimate source, and yet these are two different equations for the same phenomena being taught to engineers and technicians alike. When I get the time though, I'm going to see if I can't sort this problem out with my materials science teacher. I'll definitely let you know of the outcome.
Jay I want to thank you for all the time you take out of your day to make these videos. I had an turbine overhaul & maintenance exam today and I passed with flying colors, he was very impressed with my performance . The knowledge you possess has been a blessing to many. Thanks again and happy holidays
I;ve just discovered that a guy i'm working with is hugely into model aircraft and has 2 jet powered ones, along with about 15-20 petrol and electric powered ones.......let the turbine fun begin! :-)
I cannot say how many time my instructor at college yelled at me for using a file on other aircraft parts, we always had to use a deburring tool on aluminium parts not a file or anything else, and i could never get the darn thing right i always had additional nicks in the finished product.... in the end to get a passing grade i did what you did on the blades. Obviously I would not have done that in a real life situation, but in my college class i was fed up of getting B's and C's for panel work! I work in computers now - in case anyone was scared i worked for something important!
Simon Chappel Why wouldn't you do what I showed you to? I showed you the right way, as directed by the engine manufacturers Rolls Royce and GE, and based on years of experience repairing compressor blades for jet engines. Your instructor sounds like he is teaching without knowing.
AgentJayZ This was not for blades, they never actually let us do much on the (none working) engines we had aside from the odd bit of wire locking and occasional item removal this was for regular paneling since stress fractures still have a similar issue on a pressure cabin, personally I saw nothing wrong with your method for most all of it, and i was much more comfortable doing similar, but it was the requirement of the course, I was just quick at cheating then acting like I used the stupid tool, I am glad my con-job was good enough to pass that section without them being any the wiser.
First question:no the roughness of the leading edge has (perhaps susrprisingly) very little effect on airflow. Second question... no, compressor stall occurs when the effective angle of attack of the airfoil is too large, for a variety of reasons. IF the rough texture was to reduce efficiency of the airfoil. that would reduce the tendency to stall.
Structures, by J.E. Gordon Ch. 4 P. 67 Also, I did a quick check of an independent source...ahem... Wikipedia, look for fracture of materials... In your equation, instead of adding the nominal to the calculated, you are doubling the calculated. I humbly submit that this is an error.
Jay. Why couldn't I have had you as my materials science teacher? Excellent, excellent video on stress concentrations. I had to load up my e book on materials to verify the equation and have to say, I couldn't have explained it anywhere near as well as you did. What I like about you is that you not only know how to work on turbines, but you know the theory very well (even though you may not claim to). This video is officially endorsed by a wanna be engineer as accurate and authoritative :)
Haha! You are correct. What is important here is the relationship. But for my sake as an engineer it still bothers me that there is a discrepancy. I'll get it figured out.
+Robert Housedorf Only a few of my videos are what I would call instructional, useful in a training program. I call those ones Jet Tech, and I think this may just be my best crap ever...
+AgentJayZ Jay, every one of your crap is a truck load of diamonds of information for the rest of us. I can't thank you enough for all the videos you've posted. They are so friggin informative, entertaining and most of all, so well presented, that even I can understand what's going on - and that's a quite tall order I tell you... :D
Once again a fantastic video, thank you! I do have some questions though. Is there a limit to how many refurbished blades you can use per stage? Like you said it will never be close to a new blade, but is there any significant performance drop when the blade gets blended? By that i mean does the blended edge create any abnormal airflow or drag that reduces the overall efficiency of the engine in any meaningful way? Keep posting these Jet Tech videos they are hugely interesting!
I have not laughed that hard at something for a long time. Just calm down, and just remember, wherever the word math appears we replace it with fun. Very well put. You are an excellent teacher. just sayin'
As my brother was a mould maker, he needed to polish the surfaces of the moulds to a very smooth level. I made for him a small belt sander which was about half an inch wide and it had a steel backing where for a length of abut nine inches it was backed by a steel plate so that we could grind " straight parts" I wonder if such a small belt sander would help you. Any type of rotary grinders as you said are not to be used, but a narrow fine grit belt sander which is backed along a part of the belt would indeed help in preparation of the leading edge.
You have mentioned your reasons for not using power tools (grinders, sanders, etc.) when reconditioning. Perhaps you are familiar with soft grinding wheels, if not, they are basically Scotch Brite pads shaped into wheels used on a bench grinder. Very slow material removal, very nice finish, easy control. I was wondering if you ever tried soft grinding wheels on blades and what was the result?
I've used a thing called a unitized wheel, which is close to what you describe. I still think that power tools are a great way to wreck things really fast, and that the skills you need are best gained by doing everything by hand for the first while.
Really interesting video. I have one question though; how do you deal with balancing of the rotor after removing material from potentially every rotor blade? I imagine there may be some uneven weight distribution? Does the loose fitting of the blades help balancing this out?
The amount of material removed by an allowable repair is less than can be measured by our scales, which read in 0.1 gm increments. Usually a rotor has its blades blended in situ when there is less than a half dozen blades affected with minor damage. Many times only one. If the damage is more severe, or spread across more blades, the rotor is disassembled, damaged blades are replaced or repaired, and reassembled. Then it is balanced.
basically filling it would just cover the stress ridge. On the molecular level you have to think of metal like wood it has a grain. It was explained in mathematical terms not so good if you're a laymen.
On the CF6 fan blade at 37:50 - Can you use anything like shot peening or bead-blasting in a cabinet to eliminate the surface pitting? I can understand that you don't need to do this for an industrial conversion, but it seems like sanding a concave surface is even more, well "fun" on top of the leading edges.
I was wondering about you on these last days...it's quite some time i don't see a video from Jay i thought,and then wow! 44 minutes of video! Superb as always! I always hated maths but when you put it in your videos i can understand it almost immediately,i wish my math professor was like you XD Thanks again for sharing your knowledge with us Jay!
Wow, I love your channel. Great tutorials and demonstrations. With compressor bades. After blending the nicks, could you use a speed controlled air driven orbital sander with a p100 sanding disks to remove the pitting and p400 grade to finish off as this would only warm the metal not heat up to a glow?
You could do that, with grit levels of 220 and 320 or 400. Power tools require great skill and control, and still allow you to wreck things very quickly.
Very interesting. Given the engineering design and manufacturing precision of compressor blades, I am surprised that a pitted blade can be repaired at all, and that one can do so starting with a file. Intuitively, right or wrong, I has presumed that any attempt to repair a blade leading edge would unacceptably change the blade's aerodynamic properties. I note that video at 40:50 shows a titanium blade with damage beyond repairable limits. Chapter 26,389: What I learned today.....
Seems like when you're filing off the leading edge past the nick you're removing the lever arm almost altogether, which would reduce the stress to...nominal? Or maybe I didn't understand.
That was excellent to watch. Oddly, it's a job I think I'd like to do. I was wondering what sorts of lines you draw when deciding a blade isn't worth keeping. I assume it's different for each engine/stage/type of damage. Are stators less critical since they're not under centrifugal stress? I'm really curious how the trailing edge of a blade is damaged like the ones you show. All I can think of is something else breaking off and bouncing around the inside. Do you know the story of those blades?
To be honest... the pits probably help performance because it creates an air boundary layer. This is why golf balls have dimples, the boundary layer allows the golf ball to float through the air.
+Wapn Perfo The performance is what was designed, without the help of impact damage. The purpose of removing surface defects is to eliminate the stress concentrations cause by their very existence. It's all explained in the video. You should watch it.
I could see it causing stress... But my observation is purely scientific. I have designed an air intake for an automobile and placed small dimples on the inner runners. This creates an air boundary layer which helps laminar air flow by keeping air flow above the surface rather then the surface causing friction.
? It's not self evaluation it's scientific fact. The reason a golf ball has dimples is because it creates an air boundary layer and they fly further. They figured this out when golfers who used old balls, originally with no dimples, could hit further. The more the marks on the ball the further they could hit. I would say to you... That your lack of self evaluation is the problem with this principal. You are acting like an old golf player hitting balls without any marks.
+Wapn Perfo This video is about stress concentrations in metal compressor blades, and how to reduce them. I don't know how we got to the point where golf balls prove you are a scientist, but I am exiting this discussion. Thanks for watching.
Hey Jay, Another great vid. The profile of any aerodynamic surface is can be sensitive to slight facets, tool marks, and repairs. Have you ever found yourself saying "yes I can remove all stress concentrations, though the blade wont produce acceptable aerodynamic forces for smooth operation?" And if so, at what point do you recognize that. Is it a judgment call?
Wouldn't working the blade manually like that create hot spots when the engine is running, sort of like in a car engine when a piston has a different profile, it heats up and melts through under boost
Great detail! While I may never rebuild an engine myself, it certainly is impressive knowing the detail you guys go to. A question on the stainless, since they have to absorb vibration, minor impact, and insane forces, is a stainless spring steel used? 17-4 PH or 17-7 PH for instance. Then again... CMSX alloys are being developed... though, they are extremely cost prohibitive... Probably only in the newest generations of engines.
JayZ, is it typical to find an experienced jet-engine mechanic blending blades or do you usually hire apprentices for such work? Are there machines capable of quality blending work or is it usually done by hand? I only ask because it would seem incredibly expensive to pay mechanic wages for the blending of 1000 blades.
preempting that you may have answer this later in the vid (I'm at 32.40) but a question popped into my head. By removing some of the leading edge of the blade are you chaging (significantly) the mass of the blade? And thereby possibly unbalancing the engine? Which from previous videos is something that is super important right? thanks for the vid :)
ANOTHER cool video! :) Your formula nicely illustrates why things quickly go to hell once an actual fracture develops and the radius suddenly becomes very, very small. Do leading edge impacts ever manage to cause outright fractures in the material? How many times have you ripped up your thumb? ;)
Hi Jay ! I asked myself: The blades are loosing a little bit weight. so must you arrange them new in the turbine ? I mean that the turbine don't turn noncircular ? Are these blades all repair by hand ? (sry for my english)
