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Correct grit to sand spars for rib bonding

10 posts in this topic

Posted

I have read in the Highlander manual that it is correct to use 100 grit emory cloth for this. My RR manual states no rougher than 320 grit. Now what has the Avid/Kitfox guys done? I am almost ready to build the wings and would like to know what others have done from their manuals. Just trying to figure out what the scratch limit is on these....before the spars are not safe. Seems like 320 would not offer too much bite for the epoxy....but unsure here.... :dunno:

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Posted

I have read in the Highlander manual that it is correct to use 100 grit emory cloth for this. My RR manual states no rougher than 320 grit. Now what has the Avid/Kitfox guys done? I am almost ready to build the wings and would like to know what others have done from their manuals. Just trying to figure out what the scratch limit is on these....before the spars are not safe. Seems like 320 would not offer too much bite for the epoxy....but unsure here.... :dunno:

I used 220 on my Skyraider,careful on the sides of cap strips or you will end up with a hell of a splinter in your hand. Randy

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Posted (edited)

I am OK with rough stuff on the ribs..thanks !...but to scuff...etch... prepare the spar area for bonding the rib is what I am asking. How rough can you get with sanding the spar to accept a good glue bond and not ruin it?

Edited by Tree top pilot

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Posted

I am OK with rough stuff on the ribs..thanks !...but to scuff...etch... prepare the spar area for bonding the rib is what I am asking. How rough can you get with sanding the spar to accept a good glue bond and not ruin it?

Not sure what grit It was but I used a fine emery cloth,I layed out where all the ribs were to be attached and then cut a piece of emery long enough to wrap clear around spar and worked it back and forth a few times.Probably 10 to 20 seconds on each location and thats about all it took to scuff them up and clean the oxidation off.

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Posted

My Kitfox Classic 4 manual from 1994 says: Use 220 grit or Red Scotch Bright to rough up a 1.5 inch band where the structural adhesive will contact the spars when the ribs are glued in place.

I used 240 grit emeroy cloth, I think because I couldn't find any 220 grit.

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Posted

I don't have my Avid manual in front of me, but I am holding my KF 4 manual. That says to use 220 grit to sand the ribs (not capstrips), as well as 220 grit sandpaper or Scotch-Briteâ„¢ pads to rough up the spars prior to gluing the ribs. As tjc noted, if you are going to use Scotch-Briteâ„¢ pads to clean aluminum spars, only use the maroon color pads (item #7447). They are manufactured with a very fine grade aluminum oxide grit abrasive. Do NOT use the gray pads (item #7448), as they contain silicone carbide grit abrasive. The silicone carbide could break down and stick in the aluminum causing corrosive stress points, very much like marking aluminum with a pencil (which is why we only use ball-point or felt tip pens when marking on bare aluminum).

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Posted

Here is a little something I flund on the net

> I'm in agreement about all the other comments about scratching, scoring and

> the

> like. A small defect like a scribed line forms a small stress riser,

> specially in

> thin materials. As the material undergoes load cycles the thin line always

> sees

> higher stress levels and exhibits more rapid work hardening. When the life

> cycle

> of the part gets near its end, this area will be the first to go.

>

> Here's a little concept to chew on about the pencil line on aluminum.

>

> As you said in a previous post pencil lead is usually made from a graphite

> clay

> composition.

> I think that refined clay is a relatively inert akaline material and shouldn't

> participate in deterioration, other than it does readily retain moisture.

>

> So, I looked again at the galvanic series.

> The American Boat and Yacht Council (ABYC) series lists graphite as the most

> passive (cathodic) material. Aluminum alloys are 4th from the most active

> (anodic).

> The series from the Corrosion Engineering Handbook at work lists graphite in

> 3rd

> place next to gold and platinum as the most passive, and aluminum alloys in

> 5th

> place as the most active.

>

> So, when we put the pencil line on the aluminum we have put two of the most

> incompatible materials in the series together!

>

> If we accept that putting these two materials together constitutes a galvanic

> cell, we can also imagine that the subsequent galvanic corrosion will result

> in a

> small fault line, just like the scribe mark and lead to the same accelerated

> failure.

>

> Putting some paint over the thing doesn't do a heck of a lot to slow it down.

> Water migrates thru most paints. This doesn't cause many problems when the

> parts

> are left in open air to breathe. Wrap them up and let them sit for long

> periods

> of time and the paint will blister.

> We just had that happen to a dozen spars shipped to England last August. They

> got

> wet along the way and sat till November. All of them had blisters due to

> moisture

> trapped in the coatings. This was really expensive high grade polyurethane

> paint,

> similar to that used on aircraft.

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Posted (edited)

So I guess this is a danged if you do- danged if you don't thing. Knowing these things indeed makes one wonder the service life of the spars. I will scuff the area to be bonded with emory cloth since the sand paper I have is Norton brand and unsure of the actual abrasive material. I also have red scotchbrite....using the scotchbrite would seem to not provide much bite into the aluminum. Will get hold of some fine emory cloth and investigate it.....One thing I noted from reading about the Belite UL is that they apply a strap around the spar and rivet it to the top and bottom of the rib while into the bonding process. Seems there many ways to perform the task and all seem to be safe and work well.....the service life is the thing that would be nice to know.....guess time will tell all.

Edited by Tree top pilot

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Posted

KF3 manual suggested wrapping bungee cords around spars to hold the ribs in place while the Hysol set up. I used strapping tape and a piece of scrap wood to make a garrotte that pulled the spars together tighter than bungees. The ribs were checked constantly and never moved. I also added straightedges from rib to rib to ensure the capstrips were level (the less sanding on them the better). To keep bare capstrips free of dirt, oily fingers and epoxy varnish while working, just cover the capstrips with the easy off blue painter's tape or green Frog tape. Don't use regular masking tape, it will leave a residue.

post-53-13253563101377_thumb.jpg

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Posted (edited)

Thanks for the tape tip! Nice looking build. I am ribstitching and not leaving the capstrips unsealed. It may be overkill for me to do that....but I would feel better if I did. Planning of sealing the ribs with West system 105 epoxy...2 coats sprayed on or applied by sponge. It looks like your drag struts are .500" diameter. Mine are the same with .035" wall. I did make 2 yeterday that are .625" diameter and .035" wall for the first unit in each bay. Not sure I will use them as the clearance hole in the rib will have to be made larger and get close to less than .200" rib truss structure in that area.

Picked up the emory cloth in town this morning....may have travel to Wallyworld {Wallmart} and pick up a few more cans of bungees. Very nervous about the 40 minute time allotment to get all the ribs dialed in...guess a bit of time in preparing and planning eliminates errors and shortcomings. I may try and build a wing Sunday....if everything else falls into place. The ribs, spars,and a very fresh {packaged in 8/11} 5 ld. Hysol kit at hand. Ordered from R.S Hughes Co. and were extremely friendly and willing to share knowledge about the adhesive being fresh. They stated that most of their aviation customers wanted the adhesive no more than 8 months old. Top notch service from them.....total shipped to me was $118 in 2 days ! Thanks for all the help......specially for me being somewhat of an outsider to the aircraft group. You guys are what makes this place great.

Edited by Tree top pilot

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