Bending balsa fins after they are attached to the body tube

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BDB

Absent Minded Professor
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The title says it all. Is it possible?

I’m working on a SR-71, and I worked hard to make sure the long wings were aligned and even at the rocket’s aft end, but, in so doing, they appear to have curved up at the front by ~1/16” so they don’t line up correctly with the nose cone. I suspect that I either have to eat this one or break the fins loose from the BT, but I’m open to suggestions.

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If you think you can do it, sand the sides down that are sticking out to match the nose cone, and build up the other side. Your choice of what to use though. Putty....thin wedges of balsa...then sand all nice and smooth. It is the path of least resistance as far as I can see.
 
Or carefully saw along the glue joint with a razor saw or exacto makes a keyhole style saw blade that works really well and mounts in their heavy duty handle.
 
did you put the nosecone on upside down? or, assemble it in one configuration, only to then re-assemble it correctly, and have misalignments.. done that before..
 
did you put the nosecone on upside down? or, assemble it in one configuration, only to then re-assemble it correctly, and have misalignments.. done that before..

I don't think that is the issue. The ridge on the NC runs right down the center, so the problem exists no matter which way it is turned.

The good thing about this build is that there will be shrouds covering these wings. I may ugly them up getting them straight, but the shroud should hide my hacks.
 
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The title says it all. Is it possible?

I’m working on a SR-71, and I worked hard to make sure the long wings were aligned and even at the rocket’s aft end, but, in so doing, they appear to have curved up at the front by ~1/16” so they don’t line up correctly with the nose cone. I suspect that I either have to eat this one or break the fins loose from the BT, but I’m open to suggestions.



View attachment 336309
View attachment 336310



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On the Titebond website they suggest softening the joint with heat from a heat gun or blow dryer, or steam from a steam iron. Once the glue softens, you can reposition and hold or clamp as it recools. I would add a new fillet at the area: https://www.titebond.com/frequently_asked_questions.aspx

Good luck!
 
The title says it all. Is it possible?

I’m working on a SR-71, and I worked hard to make sure the long wings were aligned and even at the rocket’s aft end, but, in so doing, they appear to have curved up at the front by ~1/16” so they don’t line up correctly with the nose cone. I suspect that I either have to eat this one or break the fins loose from the BT, but I’m open to suggestions.

Based on the pics, it looks like the fins are glued correctly, and straight, but the outboard portion of the balsa fin had started distorting?

If my understanding is accurate, I've had something similar happen on many of my balsa-finned rockets, to some extent or another. Sometimes also to plywood.
The only permanent preventive remedy is to laminate them, (and seal the exposed edges with CA), or paint them as soon as your are done gluing.

Otherwise, the moisture will warp them.
And even if you succeed with straightening them now, the moisture will warp them again in the near future.

For straightening, I've had partial success by soaking them with ammonia/Windex to soften balsa, then pressing fins into the desired shape between two pieces of wax paper.

I thought the fin looked 98% perfect afterwords, but by the time I got to painting, the curvature had partially returned. Not as bad as initially, but not as good as it looked right after I removed the fin from between the table and the glass top.


HTH,
a
 
To straighten balsa/bass/tung, get some craft popsicle sticks (the ones with the notches). Make a little box by interlocking 4 (or more). Make a duplicate for the top.
Put your first box on a flat surface, hose down the wood to saturate it, set it on the box with the grain perpendicular to the contact surface of the box, set the duplicate box on top (aligned the same way as the other), then set a weight on top of the duplicate box and leave it until the wood is dry.

You should be able to do this with 'already glued on' fins but be careful at the saturation step.
 
To straighten balsa/bass/tung, get some craft popsicle sticks (the ones with the notches). Make a little box by interlocking 4 (or more). Make a duplicate for the top.
Put your first box on a flat surface, hose down the wood to saturate it, set it on the box with the grain perpendicular to the contact surface of the box, set the duplicate box on top (aligned the same way as the other), then set a weight on top of the duplicate box and leave it until the wood is dry.

You should be able to do this with 'already glued on' fins but be careful at the saturation step.

Phil, what are you using to “hose down” the balsa? I was considering making a clamp of some sort and using ammonia to bend the balsa, though one of the previous posts made me nervous about it re-warping over time.


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Plain water. Once you've got it straightened, get some finish on it and that should help prevent re-warping
 
Phil, what are you using to “hose down” the balsa?

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Update: I decided to avoid the fire hose and simply reattach the first ~2.5” of the wings. I don’t have a hobby saw, so I just used a sharp exacto blade to separate the fronts of the wings from the BT.

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I then reattached them using the braces that came with the kit and applied pressure with water bottles.

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They came out great. It looks like the reattachment has counteracted the small warp in the wings. Now for some hefty filets, and the forthcoming shrouds will hide the issue completely.

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