I'm not sure that they are painted but it looks like they are. What are they for?
More images:
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$\begingroup$ It looks like they end a bit for the edge of the blade. Could they be for judging the wear of the blade, i.e. something like "if the line touches the edge, the blade needs to be replaced?" $\endgroup$– Jörg W MittagCommented Dec 21, 2020 at 11:41
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3$\begingroup$ @JörgWMittag most definatly not wear indicators. If your fan blade rubbed enough to wear that much you have serios problems. Hard to see if they are 3D or not in these pictures, but I would guess either some sort of alignment mark for manufacturing if they are 2D, or a turbulator of some kind if they are 3D. $\endgroup$– XRFCommented Dec 22, 2020 at 3:10
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$\begingroup$ This is what I love about aviation: every single piece has a meaning and serves a purpose. Always. If those lines were on a car, everybody would assume they are just decoration. $\endgroup$– PerlDuckCommented Dec 22, 2020 at 13:34
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1$\begingroup$ I just sent an E-Mail to the E-mail addresses that found in these two pages of Rolls Royce company. Hope to get answer.: rolls-royce.com/products-and-services/civil-aerospace/… and rolls-royce.com/products-and-services/civil-aerospace/airlines/… $\endgroup$– RohCommented Dec 22, 2020 at 14:06
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2 Answers
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— airliners.net; cropped
In the photo above (taken in 2011, four years before the Pearl 15 first ran), that feature can also be seen; that engine is in the same family that includes the Pearl 15, the BR700 family.
— flickr.com; click image for a closer look
In the high-resolution Pearl 15 photo above, that feature is seen to protrude slightly above the blade's surface as if it's glued on. The location, protrusion, length, and being attached by high-strength epoxy, match an invention for fan blade boundary layer control by Rolls Royce Deutschland (the makers of the BR700 family):
Here, a feature (90) has been located on the high-speed (low-pressure) side (52) of a turbofan fan blade (40). The feature (90) is located toward the leading edge (55) of the fan blade (40) and extends from the tip (43) of the blade, inward. In this application, the feature is constituted by grit bonded to the blade using a high strength epoxy.
— U.S. Patent No. 7,878,759. 1 Feb. 2011. [Filed in 2004.]
Also, there is a possibility that device above can also be used for a new vibration monitoring and control system, wherein the device is made to be magnetically detectable, which is something R-R have mentioned very briefly in a press release:
... new-generation Engine Health Monitoring System that introduces advanced vibration detection ...
This is based on a newer (2015) R-R patent.
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$\begingroup$ "the feature is constituted by grit bonded to the blade" either poor translation to English, or "constituted" doesn't mean what I think it means. $\endgroup$– FreeManCommented Sep 23, 2021 at 17:50
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$\begingroup$ @FreeMan: I thought it was machine translation, but after checking again, it's not, at least, Google's. $\endgroup$– user14897Commented Sep 23, 2021 at 17:52
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Expanding on the answer from user14897, these roughness strips work like the dimples on golf balls. They trip the boundary layer from laminar to turbulent, which helps keep the flow attached. A turbulent boundary layer means more skin friction, but the added flow speed and momentum close to the surface helps it overcome an adverse pressure gradient and stay attached.
Usually turbulent flow means higher drag than laminar flow, but separated flow causes much higher drag than attached flow, be it laminar or turbulent.
