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  1. #1
    Join Date
    23rd March 2013
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    384

    Fin Alignment - What went wrong?

    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.
    Click image for larger version. 

Name:	FinAlignment.jpg 
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    Anyone know what happened, or more importantly, how can I fix this?

    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  2. #2
    Join Date
    30th January 2016
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    I would have thought twin angle aluminum would be more correct, bar can flex.


  3. #3
    Join Date
    26th January 2010
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    Northern California
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    13,151
    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.

    http://www.rocketryforum.com/showthr...77#post1659477

    http://www.rocketryforum.com/showthr...459#post407459
    NAR 91107, Level 2

    I really, really hate bugs.

  4. #4
    Join Date
    25th February 2014
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    562
    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:
    http://www.rocketryforum.com/showthr...uide-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.

  5. #5
    Join Date
    22nd August 2015
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    Rhode Island
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    1,139
    If its 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.


    Sent from my iPhone using Rocketry Forum
    NAR #100940, RIMRA & CMASS
    L1 - 4/17/16, Tyrannosaur (by Binder Design), Loki H144
    L2 - 8/19/17, Terrordactyl (by Binder Design), CTI J250

  6. #6
    Join Date
    17th January 2011
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    Spring Green WI
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    Quote Originally Posted by Mr Rocket View Post
    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.
    Click image for larger version. 

Name:	FinAlignment.jpg 
Views:	171 
Size:	89.8 KB 
ID:	332912
    Anyone know what happened, or more importantly, how can I fix this?
    Was the epoxy fully cured before you attached the back fins?
    TRA 2225
    TWA
    QCRS
    WOOSH

  7. #7
    Join Date
    22nd September 2014
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    2,559
    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.
    - punishing the punishers, nuking the nukes -

  8. #8
    Join Date
    5th February 2009
    Location
    Montreal, Qweebec
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    3,401
    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.
    -paul

    NAR# 101258 - L1
    www.CRMRC.org
    I don't know the same things you don't know..

  9. #9
    Join Date
    26th November 2009
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    4,910
    Ahhhhhh cripes. Leave as is, ditch any prospect of an onboard video camera and see if it turns out spin stabilized! 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

  10. #10
    Join Date
    23rd March 2013
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    Akron, OH
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    384
    Quote Originally Posted by dhbarr View Post
    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.
    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  11. #11
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    23rd March 2013
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    Quote Originally Posted by qquake2k View Post
    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,
    Quote Originally Posted by qquake2k View Post
    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?

    Quote Originally Posted by qquake2k View Post
    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
    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  12. #12
    Join Date
    23rd March 2013
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    Quote Originally Posted by crossfire View Post
    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.
    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  13. #13
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    23rd March 2013
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    Quote Originally Posted by watermelonman View Post
    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"
    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  14. #14
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    23rd March 2013
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    Quote Originally Posted by dr wogz View Post
    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
    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

  15. #15
    Join Date
    23rd March 2013
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    Akron, OH
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    384
    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?

    Bill
    Member - MTMA
    Akron, Ohio

    "Rather than cursing the rose bush for having thorns, it is better to thank the thorn bush for having roses."

    Currently searching for that sweet spot between "Paralysis by Analysis" and "Extinct by using Instinct"

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