Non-Matching Fins and Performance

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Necromancer

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Any problems with flight if all the fins don't match perfectly in size / shape?

I cut my Nike Smoke fins today but, one of the fins (its a 2-piece X-brace design for a 10" Smoke) trailing edges is about 1/8 to 1/4" too short.

Its probably not a big deal, but its driving me freakin' crazy since i tend to be a perfectionist and trying to find some justification in not going out and spending another $50.00 on another 5x5 sheet of 3/8" birch plywood.

Thanks :mad:
 
:surprised: You did not cut perfectly matching fins?! Shame on you. Shame. SHAME!

You'll be fine. ;) The difference in aerodynamics is so subtle for 1/8 in. that it would be impossible to detect for a rocket not analyzed by NASA. And I have analyzed NASA rockets. :)
 
Thanks for the comments.
Yeah, thats was I figured, but i needed to here from someone else for justification.

I got that OCD thing going . . . where perfectionism is a disease! :D
 
I personally would make sure all my fins are perfect. Sure on small model rockets different size fins may not cause a problem. You're 10" Smoke is hardly a model rocket though and will see flight stresses that smaller birds won't. Sure, your rocket will probably be fine, but how hard is it to fix the fins? Trim the larger fin to the same size as the smaller one. This is important in simulation as well. You will be dealing with a huge rocket than you want simmed as accurately as possible, if you are only .25" off and you have 10" of fin you are losing 2.5 square inches of area. May not sound like much but if you are going 500mph it is.

Yeah, you will most likely be fine. I wouldnt take chances with a 150+ lb rocket. Its an easy fix, why not change it? Its a 10" rocket, its worth doing right.

-measuretwicecutonce
 
I think it will work fine. But if you want them to match exact - why not trim DOWN those that are larger?
 
I think it will work fine. But if you want them to match exact - why not trim DOWN those that are larger?

That was one of my first thoughts, however it wasn't as simple as it seemed and with additional cuts, i just have another chance of making a bad cut again. (With the size of fins, i'm limited to what i have to cut those fins (the entire piece if about 4 feet across from tip to tip)


I think i'll just leave it as is.

You will be dealing with a huge rocket than you want simmed as accurately as possible, if you are only .25" off and you have 10" of fin you are losing 2.5 square inches of area. May not sound like much but if you are going 500mph it is.

People have huge honkin' camera bays sticking out on their big birds, so i can't see how 2.5 sq inch difference on a fin is going to make a difference.

Plus, i just simmed the rocket removing one fin entirely (leaving 3 fins in a 0, 90deg, and 180deg fashion) and it flew fin (up to 20 degrees AOA) - at least in simulation.
 
Thanks for the comments.
Yeah, thats was I figured, but i needed to here from someone else for justification.

I got that OCD thing going . . . where perfectionism is a disease! :D

Yah, I hear ya. A damaged/chipped lower ( a 1/4" at best) fin while building my PML Endeavour turned into a new 4x36" slotted QT BT, 6 new G10 fins, an extended 54mm MMT(+ the 38mm KS to match) and a urethane tail cone to boot.

THEN I modified the lower fins to look better with the tailcone (kinda Thor-ish)

End result, a lot of money spent due to a damaged fin that probably would have ended up modified anyway.:lol: :(

My vote is plug the saw back in and even up the trailing edges. The aft end of the BT needs to get filled where the fin can slides in anyway right?
 
I personally would make sure all my fins are perfect. Sure on small model rockets different size fins may not cause a problem. You're 10" Smoke is hardly a model rocket though and will see flight stresses that smaller birds won't. Sure, your rocket will probably be fine, but how hard is it to fix the fins? Trim the larger fin to the same size as the smaller one. This is important in simulation as well. You will be dealing with a huge rocket than you want simmed as accurately as possible, if you are only .25" off and you have 10" of fin you are losing 2.5 square inches of area. May not sound like much but if you are going 500mph it is.

Yeah, you will most likely be fine. I wouldnt take chances with a 150+ lb rocket. Its an easy fix, why not change it? Its a 10" rocket, its worth doing right.

-measuretwicecutonce

Look at this, and then tell me that 2.5 square inches of fin area really matters:

https://www.rocketryphotography.com/rockets/imgFreezeFrame01.html

That one flies straight too. I saw it last weekend on an N2800 and a pair of K700s.
 
I pointed out in my post that my course of action, where it my rocket, would be to fix the fin. Given that it is easy to fix the fins since they are not installed in the rocket, it personally is a no-brainer to me that one would make them perfect if for no other reason than simply to make them perfect. I had no intention of a starting a fight with a bunch of people who saw a rocket fly once that worked with some obscure modification. Your credibility is unmatched in my book. Regardless, its an easy fix so why not change it? I don't care if you have an answer, it was a rhetorical question.
 
Its a moot point anyways. I was planning on "fixing" the fins, but saw that in (4) places the plies were delaminating? WTF? Don't know how that happened - don't remember seeing that yesterday.

Anyways, i bit the bullet and bought another 5x5 sheet of baltic birch from my local supplier.

All is not lost though. I can still use the old fins for bulkheads and other rocket fins, and i will also use the "bad" fins for testing out my new Fin Bezel jig using my router. (I had previously had a nice jig using a bench disc sander for bevels, but wasn't practical due to the large size of these fins)

Anyways, thanks for the comments!

This **** hobby is costing too much. And to make matters worse, i broke the collet on my new plunge router (never had that happen before with my other routers), so thats another $30.00 !!!! :(
 
Thanks for the comments.
Yeah, thats was I figured, but i needed to here from someone else for justification.

I got that OCD thing going . . . where perfectionism is a disease! :D

Perfectionism...don't you hate it? It takes me so long just to paint a freaking rocket.:mad:

I was going to suggest that you take small strips of wood and add it on to the smaller fin to increase its size and then glass over it for strength, but nevermind.
 
I pointed out in my post that my course of action, where it my rocket, would be to fix the fin. Given that it is easy to fix the fins since they are not installed in the rocket, it personally is a no-brainer to me that one would make them perfect if for no other reason than simply to make them perfect. I had no intention of a starting a fight with a bunch of people who saw a rocket fly once that worked with some obscure modification. Your credibility is unmatched in my book. Regardless, its an easy fix so why not change it? I don't care if you have an answer, it was a rhetorical question.

Oh, I'd probably fix it too. That's because I'm picky though, not because I think it actually matters.

Of course, I don't have anything on one guy in our club (he practically considers it a failed launch if he scratches the paint :lol: )
 
My grandpa always used to say that

'Good enough is good enough - perfect is
nothing but a pain in the @$$ and not
worth the effort to start with...'

Man was a genius...

Funny thing was that my dad was a 'perfectionist'
but grandpa's 'good enough' usually had him beat...
 
My grandpa always used to say that

'Good enough is good enough - perfect is
nothing but a pain in the @$$ and not
worth the effort to start with...'

Man was a genius...

Funny thing was that my dad was a 'perfectionist'
but grandpa's 'good enough' usually had him beat...

You got that right. I already live by the 30 foot paintjob rule, but knowing i put something on the rocket that could easily be fixed (as opposed to repainting), it just makes me feel so violated . . . :cry:

As my old friend Walt said (who is in his 80's), "It looks good from downtown Pittsburg"
 
My dad awalys said "It's good enough for the gal that I go with". And lately he added a room on his trailer house that was within 1/16" square. Heck im lucky that the house that I live in which was built in 1910 Is within 1/2" square. makes it a pain when putting up shelves without a level.

My paint jobs on my rockets look great when there up in the air about 500' or more.
 
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