Fin Alignment - What went wrong?

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Mr Rocket

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Working on my Loc Athena. Got the front fins attached perfectly with my fin alignment jig, but I read using straight bar stock was the best way to make sure that the front and rear fin sets are in perfect alignment, but it did not work for me. All of my fin tips are about 1/16" off from the front ones.
FinAlignment.jpg
Anyone know what happened, or more importantly, how can I fix this?
 
I've always used aluminum angle, not bar stock. I believe there's less chance of the angle twisting. Haven't had any problems, but I haven't built that many split fin rockets. In your case, how thick are your fins? If they're 1/8", I would suspect warpage. If they're not warped, then your bar stock must have twisted or some such. Are your forward fins TTW? If they're surface mounted, you might be able to remove them and reattach them using a piece of angle.

https://www.rocketryforum.com/showt...emi-Scale-Bullpup-Build&p=1659477#post1659477

https://www.rocketryforum.com/showt...uild-upscale-Estes-Shadow&p=407459#post407459
 
id also say using barstock and could also be only 1 clamp used on each fin. imo, its best to use 2 clamps per fin- one at leading edge and one at trailing edge of fin.

after struggling on some builds, i decided to build a fin alignment jig like the one in this thread:
https://www.rocketryforum.com/showthread.php?9859-Homemade-Fin-Alignment-Guide-Version-2

with a piece of material the thickness of the fin in between the 2 pieces of wood, i get perfect alignment every time. i built one for 1/8" fins and one for 1/4" fins.
 
If it’s any consolation, my Binder Tyrannosaur used to have perfectly aligned split split fins. (I used angled aluminum.) But after a crash and subsequent repair, one of them is ~1/4” out of whack. It still flies straight.


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Working on my Loc Athena. Got the front fins attached perfectly with my fin alignment jig, but I read using straight bar stock was the best way to make sure that the front and rear fin sets are in perfect alignment, but it did not work for me. All of my fin tips are about 1/16" off from the front ones.
View attachment 332912
Anyone know what happened, or more importantly, how can I fix this?

Was the epoxy fully cured before you attached the back fins?
 
I purposely did my Dark Star by hand, hoping small imperfections would accentuate fin whistle.

It does not seem to have any real effect, either on whistle or performance.
 
For me:
Forward (or rear, usually) fins fully cured & solid.
2 pieces of angle or pieces of metal / wood that you know are dead straight! One on each side of the fins. And running parallel to the body tube. Also, placed close to the body tube / glue joint.
2 clamps per fin, 4 total.. (affix the bars & clamps onto the already attached fin, then the 'to be added' fin slides in the the gap between the bars, then clamp once in place)
Once assembled & clamped, check with a straight-edge to ensure all aligned. Adjust as needed. Leave to cure / set for the night.
 
Ahhhhhh cripes. Leave as is, ditch any prospect of an onboard video camera and see if it turns out spin stabilized!:wink: If all the errors cancel out and it doesn't spin, you'll just have the drag lower the altitude and then you can
slap on a camera. I've had a few rockets that really didn't spin much under boost and made nice "natural" camera platforms until they finally died. Kurt
 
I would have thought twin angle aluminum would be more correct, bar can flex.

Thanks. You're probably right. I just did not think with only an inch between the fins it could flex that much, but I have been proven wrong.
 
I've always used aluminum angle, not bar stock. I believe there's less chance of the angle twisting. Haven't had any problems, but I haven't built that many split fin rockets.

You are probably right. They looked perfect while they were clamped, and when the clamps were removed, but a day after all the clamps were removed,
In your case, how thick are your fins? If they're 1/8", I would suspect warpage. If they're not warped, then your bar stock must have twisted or some such.

Yes, the fins are 1/8", but I have been working on this for a couple months (Yes, I know, slow build) so I don't understand why they would pick now to suddenly warp. I was thinking warping as well, but why?

Are your forward fins TTW? If they're surface mounted, you might be able to remove them and reattach them using a piece of angle.

Yes, front fins are TTW. I have the fins all clamped between 2 pieces of bar stock for a week now, and they are slightly better.



Wow, that is a big gap between the front and rear fins on that Bullpup. I can see why that would need angle and multiple clamps per fin
 
Was the epoxy fully cured before you attached the back fins?

The front fins were attached with 30 minute epoxy. Dried for a couple of days before I moved on to rear fins. Rear fins were put on with J-B Weld. 24 hour cure time before moving on to the next set. With all of the fins, I generously buttered the root edge of the fin tab. Inserted the fin through the fin slot, and then pulled a fillet in the epoxy that squeezed out.
 
I purposely did my Dark Star by hand, hoping small imperfections would accentuate fin whistle.

It does not seem to have any real effect, either on whistle or performance.

Good to know that I don't have to rename the rocket "Boomerang"
 
For me:
Forward (or rear, usually) fins fully cured & solid.
2 pieces of angle or pieces of metal / wood that you know are dead straight! One on each side of the fins. And running parallel to the body tube. Also, placed close to the body tube / glue joint.
2 clamps per fin, 4 total.. (affix the bars & clamps onto the already attached fin, then the 'to be added' fin slides in the the gap between the bars, then clamp once in place)
Once assembled & clamped, check with a straight-edge to ensure all aligned. Adjust as needed. Leave to cure / set for the night.

Sounds like a good procedure. I will definitely follow that on my next split-fin. Someday I would like to do a Sea Wolf... someday :wink:
 
On a recent The Rocketry Show podcast, N00b was talking about using Windex to straighten out warped balsa fins. Has anyone ever done it on birch plywood? Do you think it would work here?
 
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