Sunday, June 26, 2022

Smooth Launch Lug Fillets?

Fin fillets are pretty easy. 
Deep launch lug glue fillets are another story.
   
Here's how the first wood glue pass looks after it has dried. You can see the air bubbles at the root.


A second fillet pass and the bubbles are getting smaller.


After that fillet has dried - 
Lightly run the back of a knife blade over the fillet and you might feel some dips. Add another fillet.
After two fillets of wood glue I switch over to the Titebond Quick & Thick glue. It is 3x thicker than standard Titebond original. There's still a small bubble hole - Add another layer of glue. Every additional layer uses less glue.

I roll off the ends with a Q-tip. This tapers the ends at the top and bottom of the lugs.


Here's a smooth fillet on my Orbital Transport Laboratory build.

When white undercoats are first set down you might notice some ridges on the sides of the fillets. After the paint dries, lightly sand the glue ridges down. 

On a model like the Orbital Transport I wouldn't even try to apply a glue fillet in the wing/tube joint. 
You would have to apply too many layers of glue to fill in all the bubbles in the fillet! That's a lot of extra weight on a glider. 
 

7 comments:

  1. Hi Chris,
    Talk about aggravating! I've given up on launch lug fillets for sometime now. Those air bubbles are pesky in that tight joint. After I glue in place and lightly smooth with creased sandpaper it seems to hold just fine. That's just my experience anyway...

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  2. Why wouldn't you just go with Quick and Thick all the way?

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    1. Hi Rob,
      It's just the way I do it!
      For me, the Quick and Thick is a filler.
      The regular wood glue has more water in the mix. I would think it would make a stronger initial bond. I do the same things on fin fillets. First fillet with regular wood glue, then use the Quick and Thick.

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  3. I glue with a small bead of Titebond then use 20 minute epoxy with micro balloons dabbed in with a steel toothpick then smoothed with a wipe of isopropyl alcohol on a finger cot. I taper the ends like Chris but I think his way is faster and less involved.

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    1. Hi Anonymous,
      I don't use epoxy unless I have to! I've seen many well formed epoxy fillets. For me, Epoxy is saved for the bigger builds.

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  4. What of the old tip to draw the point of a hobby knife through the fillet when wet to release the bubbles?

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    Replies
    1. Hi Bob,
      I've found that doesn't always work. I think it helps though. If air is trapped, bubbles still show up after the glue dries and shrinks up.

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