Electrolisis Chroe Moly against Aluminum

rotorhed

Newbie
Joined
Apr 21, 2008
Messages
77
Location
Hill, NH.
Aircraft
open frame movified Bensen
Total Flight Time
over 400 hours
Alright Guys,
I found some 2x2x 1/8" wall chrome moly square tubing. If I use it for the spacers in the cheek plates of an RAF, what kind of damaging electrolic effect are we talking about? One guy thinks it's really a serious matter of catastrphic failure, another tells me that yea, in a hundred years or so. My own experience with another ship showed no ill affects, maybe because I painted the steel? I understand that powder coating won't work as an insulator, I planned on painting them once sized and fitted white so that if any cracks occur, dirt and dust makes them show up easily. I know this to be true from when I was a helicopter operator/owner/pilot. I plan on using the chrome moly spacers in an attempt to stop them from cracking. The factory wants them replaced every 50 hours, pre and post flight inspections. I'm pretty thorough but if the factory mandates it then there's a fix for it. If anyone has any ideas to insulate the steel from the aluminum, maybe a thin layer of plastic, maybe painting plus some plastic. I'm open to ideas. Using chromo moly will result is a stronger mast less prone to the spacers cracking when aluminum is used. I know another RAF owner who has 3-400 hours on his, but also confirmed the cracking problem. Merry Christmas, John
 
I think you're talking about galvanic corrosion. You don't want continuity between dissimilar metals and you should seal the union of them so water, soap, etc. can't get between them. Once the parts are fitted and drilled they can be painted with polyurethane and allowed to cure. When you're ready to assemble, you could paint the mating surface of the spacer with the same paint which will seal the joint. You need to be sure those 4130 spacers will not overly stiffen that area or some other parts may start to crack and they may be more important than those spacers.
 
The aviation industry uses jointing compound, commonly referred to as Yak ****, sold as JC5A jointing compound or equivalent. It is designed to prevent galvanic/dissimilar metal corrosion.

I would also recommend painting all the parts before final assembly.

Once you try to use it you will know why the techies refer to it as Yak ****. :yo:
 
You will need to isolate the mating surfaces with paint or primer . The joint must be tight and not move or the paint barrier will rub off. We use epoxy primer here at work to isolate dissimilar metals. 6061 has pretty good corrosion Resistance . Some of the more stronger alloys (7075) are very prone to corrosion and need to be protected from dissimilar metals.
 
Mylar shipping tape applied to surface, then trimmed with a razor blade. A 'gasket'

One caution with Mylar.....it will work okay in a lightly stressed joint, but in a highly stress joint, the Mylar can act like a bearing and allow movement. Bedding or jointing compound, or wet poly paint will stick the parts together and help to prevent movement and liquid wicking.
 
Rotorhed, I strongly recommend that you do not exchange the the sq aluminum tube in the upper mast with 4130. That mast must flex or the rotor system will not function properly. I believe you will end up with a Strong 2X shake. John K.
 
Good Ideas Ed,
Lots of ways to skin this cat but I never thought of urethane, maybe some epoxy paint which has some give to it. Thanks, Merry Christmas, John
 
Karl,
That's an even better idea than the last one. It is obvious that I am not alone with this dilemma since a product exists to fix this problem. If you know where to get it, please send me a private note. Merry Christmas, John
 
I love you Guys,

Even ones I may not agree with. We must all remember that it is not in the Experimental catagory because everything, every procedure is cast in stone. I'm not an engineer, just a lowly helicopter pilot with a gyro sign off. My original B8-M was stock. I modified it so much that it was no longer a bensen, it was a CHR-1, the only thing left stock was the rudder, wheels, pedals and keel, everything else was changed, lengthened, CG changes, Thrust line changes and after all that I hung it and it happen to fall withing safe accepted standards (can'[t remember what it was at this moment), and I flew the heck out of this little ship. I suppose the moral that is you can make just about anythig fly properly. The change I am making is one of metal type and this comes with consequences, you have all provided me with great ideas to think about before I actually complete this component. Regarding the issue of it being too strong resulted in more sick shake, I believe some flexing will still occur, even if I used bolts instead of rivets, and I have yet to fly any gyro and/or helicopter that didn't have some stick shake. Aside from my background with helicopters, I have been my own test pilot and feel confident that if something isn't right, I have enough experience to know that and to correct whatever problem I have. Stick shake is the least of my concerns at this point. So, I thank you all and have a Merry Christmas, if someone would like to see some pics of what I have to work with, send me an email at [email protected]. Thanks again for your input. John
 
What if you just used 2X2 blocks of · UHMW Plastic (Ultra High Molecular Weight plastic) in place of the aluminum sq. tube.
do you think it would have enough give to keep the mast flex needed?
or would it also make it to ridged?
 
Now there's a thought,
However since the holes are already drilled in the cheek plates I think it would take a better drill press to make sure all is perfectly aligned for the drill to make it perfectly out the other drilled plate. That would also eliminate rivets necessitating use of bolts. I'm not worried about the mast becoming too rigid, just looking for something better than the aluminum. aluminum is brittle, if it flexes as much as people say I can see why they would crack. I know steel can take it better, and if anything, longer before any cracking occurs. With every hole I drilled making these plates I inserted the correct bolt/rivet. The outcome is an extremely tight fitting set of plates, so tight that I had to thread one of the half inch bolts through. I thought of slightly reaming one hole, but excess play/slop will lead to more flexing than I desire.
Too bad the plastic isn't square tube and at least as strong or stronger than aluminum. Thanks for the idea. I have many ideas at this point, the chrome moly is less than $15, plates cost me $30. RAF wants almost $400 for these items and you still have to drill everything out, the holes are only prick punched from the factory.
 
In the UK we use JC5A or Duralac.

I have been told these are difficult to obtain in the USA.

Mastinox is available and I would probably recommend Mastinox 6856H as it does not fully harden making dis-assembly at a later date easier.

From the website.

Direct contact between different alloys can lead to galvanic corrosion at the joint.
In assemblies which are subject to shock and vibration, this corrosion is exacerbated by the presence of humidity. Jointing compounds are therefore
required which both inhibit corrosion and at the same time, are effective insulators.
Their remarkable flexibility minimises any risk of cracking or separation resulting from ageing.

The choice of the correct grade of Mastinox can be
based on two criteria.
1. Whether the joint is likely to be separated at any time or whether it will remain permanently assembled.
2. Temperature resistance required.
a. Permanent assemblies choose a version of Mastinox which cures
in contact with air Mastinox C627B (part number 2047/3600) which is very flexible and contains corrosion inhibiting pigments. It has very good resistance
to ageing and to temperatures between 100- 120°C.
b. Separable assemblies: choose a non-hardening version Mastinox 6856H (1914/3600) which resists temperatures up to 115-120°C.
Mastinox 6856K (1913/3600) - use in preference to Mastinox 6856H where temperatures are likely to exceed 150-160°C Mastinox D40 (1640/3600) - use where temperatrues are likely to reach 200°C.

You can get it here.


http://www.westernamspec.com/Inventory/DoSearch.asp
 
At this point what I plan to do is get some of that stuff and apply it once tha part is made and fitted, furthermore I will insert a thin sheet of plastic between the cheekplates and the blocks and once in, trim the excess with a razor. This will satisfy me, and I think all who whave contributed to this discussion. I don't feel it will make the mast too stiff, the thing shouldn't be flexing all over the place anyway. On my modified Bensen, when the engine failed an d the less suitable landing zone resulted in the main rotor striking the ground and destroying the ship, the redundant mast of 2x1 broke completely (snapped)just above where the first clamp is (what the seat mounts to). One of our most experienced builder/pilot said they were meant to break away. Who knows. I really enjoyed the input from all, may contact a couple of you in the near future to locate some stuff. One of you out there gave me a source for the chrome moly and before that I couldn't find any 2x2. It's $13.20 a foot plus a little for shipping, how cheap does it get. I'll still continiously inspect this area because after all, it's an experiment, and maybe a small step forward to making this area less prone to failure. Merry Christmas everyone. I'm all alone for a few hours and will retreat to our shop and continue building.
 
Karl,
Can you tell me a litle bit about Mastinox?,,,How it is applied, with a brush, spray, dip? I got a quote for the stuff and it is very expensive. Looking forward to hearing from you, John
 
Karl,
Can you tell me a litle bit about Mastinox?,,,How it is applied, with a brush, spray, dip? I got a quote for the stuff and it is very expensive. Looking forward to hearing from you, John

Yes it can be very expensive, might be worth visiting your nearest maintenance shop to see if they will fill a camera film can (can you still get those ?) that should be more than enough for your requirements. Is there anywhere that does airframe manufacture or repairs, they will have cans of the stuff lying around. Just ask for jointing compound, it could be Mastinox, JC5A or even Duralac they all do the same thing.

Mastinox is about the consistency of a face cream, most engineers I know just use there fingers to spread it but it is probably recommended that you use a brush and gloves. It tends to get everywhere so don't use it wearing good clothes.
 
actual application

actual application

Good Morning Karl,
The actual application is for the 2x2x4" spacer blocks that fit between the cheekplates of an RAF. The factory uses 1/8" wall alumium tubing so common with gyros. The problem is that the factory reauires pre, and post flight inspection of these blocks as they have a tendency to crack and would like you to replace them everry 50 hours. I view this as a design flaw. Even the Bell 47 I had required no such low limit on its parts. So after speaking with my instructor, also an RAF builder, it was suggested that they be replaced with steel. Chrome moly being so light (like aluminum) the added strength of the steel can't hurt. Electrolosis is the problem and once the part is made, it will stay made, for a while anyway. I already have the cheekplates cut, drilled, fitted, but not painted or finished. I would like to complete the final fabrication of the cheekplates (fine sanding, painting, etc), install them and the rotorhead on the aircraft, then insert the treated spacers where they belong (further insulated by a small piece of plastic), drill them and rivet them in place where they will stay. I'll probably end up buying the mastinox myself. So when I get to the spacers, should I sandplast them clean, apply an etching primer, then the final coat of white paint before applying mastinox to the areas (the sides) that will contact the aluminum cheekplates? If this sounds like a good way to accomplish this then just tell me so. I really appreciate your help with this issue and you seem to be one of the few out here that knows what he's talking about. If you have read other sections abbout this modification, some people are dead against this, saying that everything muxt flex to having too much stick shake. I'm a commercial heli pilot and have yet to fly something that didn't have some kind of shake, even a fully articulated rotorhead has some feedback to the cyclic. I have a PRA meeting today so when you have a chance, I'd appreciate further guidance on what I plan to do. Have a good one and thanks again, John
 

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I have used it mostly during airframe repair and we would etch and prime the skins etc before applying a thin coat of jointing compound to the joint before rivetting then paint the top coat.

Landing gear mounting brackets etc usually were completely painted with top coat before assembly, the joint compound being applied over the paint.

We also applied it to structural bolts such as main spar bolts and lift strut attachments, Engine mount bolts, axle bolts.

I would recommend you discuss it with the manufacturer to ensure you get the right stuff, my only concern with your plan is the plastic gasket. The joint compound may act as a lubricant on the plastic.

Normally etching, primer/paint and joint compound is more than enough to prevent most problems.
 
mastinox

mastinox

Thanks Karl,
I didn't want to put that plastic in there anyway, just thought it would be another barrier to prevent electrolosis. Your method appears sound and I will probably go with it, just as soon as I can afford the stuff. 160 ml will cost over $300, hard to believe but true. At some point I'll have a completed pic for all to see. Many thanks, again, John
 
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