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Aluminium spar attach in carbon fiber (avoiding galvanic corrosion)

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billy

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Joined
Mar 25, 2010
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7
Location
Spain
Hello everyone

I am building a spar attach arrangement "similar" to that in the Dr-107 One Design, that I put a copy on the plans. There is the AN bolt that will go through aluminium bushings that will be held by the carbon fiber spar attach points. Since I am making a big chunck of carbon to hold the aluminium bushing I am looking for the max strength of the carbon be retained. Problem is the galvanic corrosion if aluminium gets in contact with carbon. In other parts, I have avoided by (as per recomended in this forun and aparently by Locked Martin) using the last layer of the layup with fiberglass. Here I can't since the carbon will be "machined" to recive the aluminium bushing.

I would love to ear what would be my best option to retain the maximun strength provided to the structere once mated with this options:

-one design plans. Machine the carbon with some tolerance and insert the aluminum and fill the gap with epoxy. Weakest part would be the epoxy lining....

-one design plus using flox (epoxy plus choped glass fiber). Here weakest part would be the flox and as you can see in the attached study can be 175% of what epoxy is.

-put primer on the part of the aluminium that would contact the carbon, then cover it with a thin layer of epoxy and machine tight the carbon. Sand a little the epoxy for good adhesion and insert it in the carbon using epoxy to permanently join the part. This would mantain max strength of the carbon (holding the aluminum part wich is weaker) and if there is a thin layer of primer and other of epoxy, in theory no contact in between carbon and aluminium.

Please let me know what you thing or if there is other way to bond aluminium to a machined part of carbon.

Thanks

Jorge
 

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TFF

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Apr 28, 2010
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Memphis, TN
I think a steel bushing would eliminate the worry and be a worthy weight gain to not jump through hoops. That assumes you can’t shrink the bushing.

Side note. If I was building a One Design, I would want 3 more ft of wingspan and at least another foot of fuselage. Exciting to watch, but doesn’t groove well.
 

billy

Member
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Mar 25, 2010
Messages
7
Location
Spain
Actually it is not a One Design, it is a Laser 300, soo, it does have those 3 ft of wingspan and more than a foot of fuselage 😁, plus 120 extra ponies! That is why I need the strength of the carbon.
 

wsimpso1

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Saline Michigan
The other way is big tubes as the pins in Garolite bushings that you bond in. This gets the diameter at bonding to the spars up to keep stresses down. Everything has to be checked for stresses... BoKu uses this to mount his sailplane wings and has covered it in posts on here.
 

BoKu

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The other way is big tubes as the pins in Garolite bushings that you bond in. This gets the diameter at bonding to the spars up to keep stresses down. Everything has to be checked for stresses... BoKu uses this to mount his sailplane wings and has covered it in posts on here.
Yeah, that's what we're doing to join the carbon RV wings to the aluminum stub spars, too. Each bronze bushing is potted into a slab of Garolite with an epoxy paste mix, and the Garolite slabs get integrated into the wing spar shear webs. For short bushings you can just butter both parts and mash them in. For the long bushings on the glider (~2" long) we inject the bonding paste into the gap under pressure. The Garolite has great bearing strength, and is non-conductive so galvanic corrosion isn't an issue.

For the places where we do bond metal parts into carbon structure, we just isolate the two with a wrap of fiberglass over the metal. That's what we do with the steel carrythroughs for the fore and aft lift connections on the glider.
 
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