Hi.
If the OEM doesn't object to blade tape then it can be applied. but the guys at Bell need to be in the loop,
The first photo is liquid impact damage, by the looks of it. The tip photo is showing abrasion from particulates, not liquid... The TE wear is characteristic abrasion as well, which could pass for the photos of the NH 90 blade after 20 minutes of high hover over sand.
EPS goes back a long way, to the mid-60s at least, but good reading is Weigel on the tradeoffs and Calvert on the reason why it is a PITA to have tape on a rotor. Normally, anyway. Martin and Co, give an idea of what was attempted and why in the way of VG's, and also that conventional ones didn't work.
The phot above is St Gobain material we developed for use on wind turbines, having got tired of 3M 8663HS, 8672 and similar failure modes. The HDPE material has major issues when you rotate that, and it took about 2 years to sort that out. That is not a rotor, that was my T28B N242J, and it was s good liquid impact testbed, at least on the ground. driving it into hard rain airborne would drown you. We ran that for about 5 hours at 2000RPM, which gives an 1140 RPM prop speed, R-1820-86 RGB is 0.5625:1. for a tip speed of 766FPS. that's more than the hover tip speed, but only a modest advance ratio tip speed, about 50kts... The nice thing about the HDPE is that it abrades like your steel or nickel leading edge is doing. As it does, the shiny leading edge goes dull, and it is possible to determine the amount of wear you are gaining, and then do a field swap out, at least the STC we did permitted that by the pilot in the field. The tip angle requires a molded section which requires a former to be made and the vortex design to be incorporated. It is similar to the CMRB which was flown for a quick investigation in 2013? on the HH3 and SK61, using Franks blade which had a tip similar to yours. The step between the metal leading edge is causing an erosion of the resin matrix behind, which is what happens with a blade where there is any step between the leading edge and the composite. same as a GE-90 Fan blade.
Calvert, M., & Wong, T. (2012). Aerodynamic Impacts of Helicopter Blade Erosion Coatings. 30th AIAA Applied Aerodynamics Conference, 1–13. https://doi.org/10.2514/6.2012-2914
Calvert and co show what happens at the aft edge of the tape, and also due to the aft step, note that there is a tendency for the vortex structure to lift the TE of the tape, so sealant of the TE of the tape edge with DP190 or whatever the OEM recommends is worth the effort, it will lift otherwise or did in out STC development. My STC, SVR-516 went on the R22s only, and we withdrew from PMA of that as we went playing with the B737 STC for dealing with the slat TE step effect. Chat to the Safari guys and they can comment on what we did, it may suit your blades, it avoids the issue that Calvert was alluding to.
Weigel, W. D. (Kaman A. (1996). Advanced Rotor Blade Erosion Protection System.
Weigel gives the background on tape, and the last half of his paper covers the issues of liquid impact, which is entertaining. The takehome is that in the collapse of a droplet, the surface acts as a near-perfect reflector, and above 100m/sec, there is a hypersonic shock formed with a little bitty plasma jet normal to the surface, and that is the damage that is in the LE photo at the top.
Martin, P., Wilson, J., Berry, J., & Wong, T. (2008). Passive Control of Compressible Dynamic Stall. 26th AIAA Applied Aerodynamics Conference, 1–33.
Heine, B., Mulleners, K., Joubert, G., & Raffel, M. (2011). Dynamic stall control by passive disturbance generators. AIAA Journal, 1–14.
Coder, J. G. (2010). CFD Investigation of Unsteady Rotorcraft Airfoil Aerodynamics (Issue August). Pennsylvania State.
If you owners and operators of the 505 want to talk to your tech reps, they are welcome to talk to Shawn Coyle or Greg Lynch, and if they ask me, I am happy to design a CVG EPS section for the blade. I am just not interested in doing STCs with flakey investors.
Aliphatic TPU's such as 8663HS etc are still good materials, the annoyance is that they screw up the aerodynamics and the properties of the material make forming it a PITA. The bad side is that they do not show that they are approaching failure in normal use. The abrasion is straightforward, the problem of concern is the fatiguing of the elastomeric bonds from the liquid impacts, which result in no observable damage until the material collapses and opens up a pocket. On your UH60 that is not a big deal, On a BK 117 or MBB 105, that will remove your fillings. On a semi-rigid, you will get vibration as the tracking goes out. That is itself one of the main reasons that putting tape on a TRB needs some caution, as in, don't unless the OEM approved it, which is not often the case.
Weigel goes into some depth in the reasons that TPUs work in abrading conditions but mainly it is because it is a thermoplastic, the kinetic impact of a particle causes local melting which freezes rapidly leaving the material mass in place but slightly dislodged. With liquid impact, the surface flexibility dissipates the kinetic energy in the shock wave formation within the material, which is OK for a while but causes the elastomer bonds to be fatigued over time, and then they fail rapidly, without warning. A 100m/sec impact gives a Mach 6 shock roughly which gives the plasma jet.. that's at half of your tip speed in the hover.
SG2010 CVG on an R44B test
Liquid impact fatigue failure of TPU elastomer cross-links. The initial failure is at the lower (front) edge, the real damage is from the turbulent flow over the initial failure. failure in a test during a severe thunderstorm heavy downpour, after 18 minutes of liquid impact.
PS: for military types, the approval and method of EPS application is given in TB-1-15615-351-23 (currently rev 3) If its there, that is the approval. For RHC, it requires an STC, and there are 2 STC,s mine and the Airwolf one. EPS is generally a Part 43 Appendix A, para 10??? item, but if you are on commercial ops, then you need to have a policy in the Maintenance Policy Manual that states what may be permitted by the maintainer.