Blade paint erosion question

[slowrotor]

Blade leading edge erosion is a function of blade speed mostly I think. The inner blade has less erosion of course.
My question is: At what radius of a blade does paint start to erode?

or, what blade speed in feet per second does not require a metal leading edge erosion protector?


I know the VS-300 had fabric blades and this worked at the low tip speed of around 370fps but rain caused erosion.

Any thoughts would be appreciated.

[Graviman]

I would do some digging for papers in AHI or SAE on that one, slow. It's a "how long is a piece of string?" type of question dependant on design environment and intended life. Another way of looking at the problem is that i bet Robbie wished they had been more conservative on leading edge erosion. Unless it compromised performance i would design in an edge strip.

Mart

[bladepitch]

i read on a previous thread that it might be an idea to take care of the leading paint edge some how, where it erodes to minimise it.

Do i have to do this?

does/would the engineer do this at 100 hrlys?

do i need to do it at all?

BP

[slowrotor]

Mart,
I think downwash velocity is a major factor in blade erosion, along with type of soil/sand etc.
For a tip speed of 400fps and 14fps downwash, I don't think a metal erosion strip would be needed.
Thanks

Bladepitch--- check for service bulletins

[NickLappos]

The basic problem is that the speed of a sand particle becomes an awesome abrader when the tips of the rotor are considered. Most blades have a hard metal erosion strip bonded to the blade at the last 35% of span or so, where the tip speed is worst. The strips are stainless steel or nickel.

These strips still ebrade, so the US Army uses a clear tape that covers the strips, and lasts about 20 to 50 hours, before it has to be replaced.

[Cyclic Hotline]

Go to your local aircraft parts supplier and buy some epoxy prop paint.

The best, cheap fix for erosion. Won't stop it, but you can see how fast it is coming off, and restore it in no time flat.

The field fix in ultra rainy locations is the Stainless Steel tape you can buy from JC Whitney - the best tape ever produced. None of this is approved, recommended or endorsed, just happens to work superbly well! :ok:

[Rigga]

Dont paint your Blades! Or put any sort of metalic Tape on them!

Blades are really sensitive things, unlike Props, and you may place a great deal more paint on one blade, than on the other(s), and cause a sizeable imbalance creating more damage than mere erosion does.

If you are worried get Blade Erosion Tape as mentioned above, its not great, but its cheap enough, and (if approved for your Type) your mechanic can put the same amount on all blades without too much hassle.

The hardness of your paint also comes into the erosion equation. Cleaning your blades regularly might help you keep an eye on what is happening to them. (and a spot of polish on the top surface really helps generate lift and preserve some of the paint!)

[bladepitch]

slowrotor

about half an hour after i posted i recieved the faxes from my chiefy with them. i felt like a tool. cheers

[rotornut]

If you are worried get Blade Erosion Tape as mentioned above, its not great, but its cheap enough, and (if approved for your Type)
Make sure it's approved for your type!
A few years ago I was with a outfit that had a Hughes 300.The leading edges of the blades showed signs of erosion so the engineer put a NON type approved blade erosion tape on them.
The machine performed normally and the tape seemed to work well. However, a few days later I flew through a rain squall. After I landed the machine I noticed the tape was shattered but I didn't think anything of it as the performance seemed about the same. However, shortly after that I picked up a passenger who asked me to hover about 200 ft. over part of a dense forest. The machine started to lose RRPM. I desperately pumped the collective but it wouldn't stay in the air. Fortunately I was able to put it down in a swamp and suffered no damage - except to my ego. Later on the engineer removed the tape and performance was back to normal. It was a very cheap lesson!

[slowrotor]

I am not a fan of tape for that reason.
That is why I posted the question about paint erosion.
An easily applied, sand resistant, rubbery paint would be a better solution I think.

[Rigga]

Once again, If you're going to use paint - DON'T do it yourself.
If it is approved for your Type, get an expert to apply it. He will apply it in a measured and uniform way and possibly need to rebalance your blades afterwards.
The touble with paint is that it too erodes and may not do so in a regular way, thus starting a rotor imbalance. At least with blade tape you can remove the remaining tape to restore balance.

In the end - If it aint broke, don't fix it!

I'm away for a week now.

[fdr]

Interesting thread...

I would add the following information. First off, disclosure; I am involved in certification of EPS, erosion protection systems, and have conducted research flight test in the area for about 4 years, on an ongoing program.

All things erode, some more than others.

Conducting standardised abrasion testing per ASTM, TPU's lose around 4mg of material. Ceramic is about 5 times that amount, but can fracture, and stainless steel is about 50 times that rate of material loss. Without boring anyone, the process of abrasion, erosion and particularly liquid impact is highly dependent on the laminates material properties.

The rate of any erosion is dependent on the kinetic energy of the particulate, impact angle, and boundary layer conditions. The outcome is nearly a direct relationship to the local velocity squared, ie highly tip loaded wear rate... 1/2 span = 1/4 rate of wear of tip. The location of the primary wear is dependent on the AoA of the section, and the section design, but in general is going to be highest at the area of the leading Kutta Condition.

The manner that the erosion occurs is dependent on the material properties, and having spent some time with samples under high magnification, the reason that TPU wear is remarkably low is found within the microscopes field of view. Without getting into details, the behaviour of the material subject to a local heat flux is an important factor.

Comment has been made as to the behaviour of a tape in rain... liquid impact studies show that damage to a surface is dependent on the peak accelerations that occur, and also whether critical values of acceleration occur in the development of internal shockwaves in the droplet. The erosion of hard materials is far higher than a compliant surface in general. The case of TPU penetration is not a failure of the material per se, it is a failure of design and/or installation, that the laminate has an unvented void in the region of high local impact velocity and impingement angle, which results in a fracturing of the material as it is unable to deflect adequately within the time of the impact. If there is no void, there is no damage to TPU by liquid impact. Aluminium, Steel and Ceramic are damaged by the high velocity jet impingement in the region of high local velocity and near normal impact angles, exhibiting cratering for the metals, and micro-fracturing for ceramics.

On the application of tapes or other solutions, the take home point is that lift/drag is affected by the application of paint or tape; a paint edge on the upper LE in the LBL will increase drag by around 2% from a clean blade, and reduce AoA capability of the blade, resulting in an earlier stall of the blade. Placing a tape in the area results in around 4% increase in drag, and a greater loss of AoA capability. Operationally, this increases auto-rotational sink rate by a nearly equivalent margin, reduces the time to go from normal NR to stall NR in a power loss, and reduces the rate of acceleration of NR when attempting to recover the NR in the flare or following a low NR event. These are not good things... in fact, unless you are complying with the manufacturer's approved and tested installation you are a test pilot, and are not in an aircraft that conforms with it's TCDS, i.e., you are experimental, and your insurer has a legitimate beef on any coverage.

Currently there are very few manufacturer's that provide any approval for application of material to their blades other than painting in accordance with their procedures. Putting a spray kote over the blade is in the realm of an unapproved procedure and modification. The US Army has a procedure that permits application of paint or tape to their blades, as does the USN. Other than that, there are 3 STC's for specific types around the globe to date, using specific materials which are authorised by the STC for that purpose. The application of the material requires that material to be approved and manufactured by PMA; there are 2 PMA approved organisations for the application of erosion protection tapes, neither of them being 3M. 3M has an approval for the application of a thermoformed boot to a radome for a particular TPU material only.

"Commercial..." - we are developing a series of STC's extending on the initial STC for the RHC that avoids the problems of using a tape on the blade in relation to adverse performance effects. We have applied a conformal vortex generator (cVG) into the element design, which is a unique VG, that does not suffer from centrifugal (radial) pumping, which generally precludes placing a VG on a rotating component. We manufacture under an APMA approval to the STC approved design, and also provide non certified experimental use products. The cVG uniquely achieves a reduction in drag on the blade, not just at high AOA as a VG would do on a non rotating blade, but at low AOA. In testing we demonstrate up to 9+/-1% reduction in drag at zero collective, although the final STC for the R-22 is a compromise that achieves only around 5% due to the relatively uncontrolled paint finish on that blade by the OEM. Blade stall NR is reduced by around 5%NR, and auto sink rate is reduced by approximately the same amount as the torque reduction from the clean finished blade. While these changes improve the H-V and IGE/OGE hover, the STC does not change any performance in the POH, however for any airframe that was not approved by the OEM for operation with the mass addition of EPS, then we include a new technical limit to MAX NR POWER OFF to maintain radial loads at the level tested by the manufacturer. (Due to this fact, it is a mandatory requirement that any installation of our product must show a torque reduction in the zero collective, controls neutral condition, relative to baseline performance, to ensure that the original performance of the aircraft is met or exceeded while maintaining loads within original tested case). Contrary to a comment previously made on this forum by other parties in the industry, any (conventional) transverse step in the surface of the LBL, ie paint step or tape edge, degrades performance in all aspects. It is also the case that any mass addition increases the radial load of the blade, (the comment was that no increase in loads occurs from putting tape on a blade tip... utter nonsense mathematically, physically, and in measured outcomes).

For an example, adding 60gms of tape at the tip of a small low inertia rotor around D=25-26', NR 500-530RPM, (around 1.8 x 0.1m x 375um) adds around 0.4% to the root load when at the MAX POWER OFF NR.

The dynamic effect of the cVG is a reduction of peak to peak cyclic amplitude of strain in the order of 30%. While not tested, it is expected that any transverse step from paint or linear tape edge will increase the peak to peak amplitudes. The cVG improves stall, and has been tested to 1.23VNE at RPM 3% below MIN Auto NR, whereas a paint step or linear tape edge measurably increases stall NR, and will result in a reduction in the advance ratio for achieving RBS.

PS... If you are not under experimental category, and following a test card with appropriate risk management practices, comply with the Manufacturer's POH and AMM, and ALL LIMITATIONS. There are very few rotor head designs that will accept operating outside of the limitations without consequences. This is particularly true for rigid rotor systems and others which do not relieve root bending moments. The limiting factor may not be the one you are thinking of either, the TR may end up with excessive loads in certain cases, which your pre test analysis needs to evaluate, even if your MR is not challenged. The fact that it is "standard practice" to undertake a demonstration does not make it legal or even good practice.


regards,

FDR.

FLIGHTLOG PTY LTD
EDGE AERODYNAMIX INC
EDGE EXPERIMENTAL LTD

[Saint Jack]

Sorry guys, I can't help agreeing with 'Graviman's' analogy with "how long is a piece of string?" and that 'slowrotor's' original post is simply a statement and not a question. Main- and tail-rotor blade erosion falls into the category of fair-wear-and-tear. However, as we all know, this can be exacerbated by local conditions - particularly dry sandy areas.

Take good care of your blades (wash daily if necessary) and they'll give good service. In my experience, blade tape, both polyurethane and metal, can be helpful but very often the effort outweighs the advantages. DO NOT attempt to counter erosion by local paint touch-up.

[blakmax]

Guys

I am aware that there are ADs for some types of blades requiring repaints to supposedly prevent blade disbonding (incorrectly termed delamination). The theory is that the adhesive in the bond between the skin and the spar is eroded sufficiently to cause disbonding. There are also STCs for applying tapes to prevent this erosion of the adhesive.

I have grave doubts that these approaches will be effective. From the evidence I have seen on a number of blades, the failures I have seen are occurring at the interface between the adhesive and the metal. This can not be due to erosion. If an adhesive bond is eroded sufficiently to cause bond failure, then the failure will be cohesion, in other words the adhesive will fracture, leaving some adhesive on both surfaces. It will not cause interfacial (adhesion) failure which results in all the adhesive being left on one surface, or else mixed-mode failure with some adhesion and some cohesion failure.

Interfacial failure is typically related to hydration of the chemical bonds at the interface, and this is a chemical process, not a mechanical process such as erosion.

Having said that, if your aircraft type does not experience disbonding, there may be merit in applying tapes. (I stress that the tapes and paint as well will not prevent interfacial disbonding.) I have seen some of fdr's work and I am impressed by the level of research he has done. I am also impressed by the results he is achieving with his tape system. Not only does it reduce erosion, it improves blade performance. Keep up the good work fdr.

Regards

blakmax