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I've recently found a channel on Youtube where a jet engine technician goes into great detail about the work he does on maintaining and repairing jet engines

In this particular video (and also an older prequel to it), he demonstrates removing nicks from compressor blade leading edges and subsequently smoothing out the leading edge. One thing that caught my attention was the tools he used: files and 100- and 400-grit sandpaper. He basically went over the leading edge by hand, removing a small amount of material in the process.

With the tools and the method, there is obviously no precision.

Is it actually acceptable to modify compressor blades that way? To what extent does it affect the operation and efficiency of the engine, if, say, some compressor blades are missing a millimetre off their chord, or have lost their twist by a few degrees?

I guess a related question is - what precision are compressor blades designed and built to?


Thanks to ymb1 for pointing out a related question, but I'm more interested in a quantification or analysis of the impact to the engine performance. It seems that the linked question explains the term blending (which is mentioned in the video as well), but did not go into details about performance impact.

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    $\begingroup$ He is a professional turbine tech and rebuilds jet engines for a living, so I suspect he knows what he is doing. $\endgroup$ – zeta-band Aug 30 '17 at 15:09
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    $\begingroup$ I never thought of this, but now I'm curious also. I've heard that the blades are carefully balanced by manufacturer, so eyeballing it seems to be a crude method that could lead to vibration, unless he's pulling it off and carefully weighing each blade that is sanded to make sure its within tolerance. Its probably not significant after one pass, but after several passes with the sand paper eventually the material removed could add up. I'm sure there are some A&Ps on here that can answer this. :) $\endgroup$ – Devil07 Aug 30 '17 at 15:12
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    $\begingroup$ I would agree with @Devil07 - even the nick itself could throw the compressor fan (or any other spinning engine component) out of balance, possibly setting up destructive vibrations. Again, there's an A&P somewhere who will come along and set us all straight. $\endgroup$ – FreeMan Aug 30 '17 at 15:17
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    $\begingroup$ @zeta-band I don't doubt that he knows exactly what he's doing. He makes a point of emphasising that he's just a technician, but he definitely understands the principles and intricacies. I guess the more appropriate phrasing would have been, are compressors designed for such deviations in tolerances (assuming such deviations are even significant in the first place). $\endgroup$ – aerobot Aug 30 '17 at 15:52
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    $\begingroup$ @Devil07 To add on to Dave's comment, the channel has some videos of the technician demonstrating and explaining compressor and turbine balancing. I expect they'll do balancing after doing stuff that could affect the balance, before they sign off on the repair or maintenance. The channel is a real treasure trove of information. $\endgroup$ – aerobot Aug 30 '17 at 15:56
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Yes, this is not only acceptable, it is required by the engine maintenance manual. The tools might be more sophisticated in other shops, but the basic concept of removing material by hand is the same. The process is called "blending". The impact of sand or dirt on the blades causes little nicks and scratches. Those sharp or rough areas create what is known as a "stress concentration". If left as-is, the nicks will eventually grow into cracks (metal fatigue). Smoothing out the nicks removes the stress concentration and prevents the crack from forming. There is an impact on the performance/efficiency, but that's better than having your blades crack. From my experience, the performance impact of what is shown in the video is actually quite small. As a rough guess I would say 0.01% - 0.1% efficiency loss. Much larger nicks can be successfully blended.

Edit: Regarding how I came up with that efficiency number. Two thoughts:

First, The compressor shown in the video is a 10 stage compressor, and as a rough estimate, each stage probably has about 30 - 50 blades. I didn't try to count exactly but let's just call it 400 blades. It is a reasonable assumption that each blade does the same amount of work. In reality some stages will be a little more heavily loaded than others, but it's fairly close. So if you removed 1 entire blade, as a rough order of magnitude estimate, you would expect that to be about 1/400 = 0.25% efficiency loss. The amount of material removed is clearly much less than an entire blade, so the efficiency loss should be less than 0.25%.

Second, I have seen compressors that had much much larger nicks be blended out and have on the order of 0.1% - 1% efficiency loss.

So between the two of those ideas, I made an educated guess on the efficiency number. Could I be off by a little bit? Absolutely. But I'm probably at least close.

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    $\begingroup$ Since you put forward a figure for the efficiency loss, may I ask how the figure came about? I have no idea how it might be quantified from real-world performance, but I imagine it would be possible to run CFD simulations and arrive at the relative degree of compressor pressure loss, for example. $\endgroup$ – aerobot Aug 31 '17 at 13:09
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The process normally used to alter the shape / remove material is called linishing and it is used in both manufacturing and maintenance but do not attempt it yourself! Only a professional should even consider hand linishing a blade.

The shape is typically controlled to within at least 0.1 mm* during manufacturing, though it can be even lower. There some highly experienced people who can actually achieve this level of accuracy, but more commonly robots are used to get a more consistent shape and the shape is verified by measurement machines after, something I'd be surprised if the person in your linked video did.

If you distorted the shape of the leading edge too much from the intended design it can have a significant effect on the performance as it can cause early flow separation making the blade less efficient. There is also the effect off stress caused by notches on aerofoils to consider.

While of course an off design shape is probably better than the flat/distorted shape from a notch, it is still a job for a professional. Your life literally depends on that engine working properly and it would cost a fortune to replace if you messed up. So why would you want an amateur maintaining a critical component?

As for any efficiency loss, that is a something we could only guess at without some data from an engine manufacturer to back it up, therefore it would be quite meaningless.


*To give you an idea of the accuracy achieved during manufacturing, here is an example of a GE machine that verifies blade shape. It has a published accuracy of 0.001", which isn't bad but there are machines capable of a fifth of that. You typically want your machine accuracy to be 10 times better than than the accuracy of what you're trying to measure.

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