Low Power Fin Fillets

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Weekends

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What is the rule of thumb for how thick a fin fillet should be? This is for a low power, balsa wood, outside fin fillet. Not using epoxy, just wood glue (white or yellow).

I was just putting the fillets on an Estes Skini Mini and the question popped in my head. I generally just use one or two applications (after smoothed out).

Thanks,
Weekends
 
I'd suspect that anything beyond 2 applications smoothed out would just be for looks if you want more of a rounded fillet look. That being said, I can't wait to take my 3" Black Brant X to Whitakers to let 'em all laugh at the huge honkin 1" thick fillets I poured onto that thing. :D
 
It depends on the size of your rocket, I'm assuming you want to know about all low power rockets and not just your Skini Mini.

If you have a very light rocket the fillets need not be so big, infact I find just one average sized fillet to be plenty as long as you do a good job.

If you have a larger rocket you obviously need bigger fillets because most will want a rocket to be undamaged even if it lands a little harder that it should, in this case I would say that two applications should be sufficient.

I have to agree with Kermie that anything more than two would more than likely be for looks.

HTH
 
Here is my rule of "thumb". (pun intended)

All of my filets on LPR models are with Elmer's white glue.

I always [ALWAYS] use my finger to form the filet! The smallest models get my pinky and the largets get my thumb. Use the same finger for all of the filets on a model.

The trick for consistancy is to lay a bead of glue at the filet point and run your finger from the front first (always from the front first!) and back to the trailing edge.

You must do this in one smooth move. Immediatly wipe your finger off on a paper towel. Then go on to the next filet...usually just two at a time.

sandman
 
I tend to have "dumb thumbs" and screw up Sand's finger technique. I use a small cylinder of some sort (spent 13mm motor? Drinking straw?) to smooth the fillet down the side.
 
I use Locktite four minute epoxy gel and the finger technique. The gel stays were put and dries quickly. I smooth it with a finger dipped in 91% isopropyl alcohol. It took a long time to learn that I really don't need THAT much of it. Tiny fillets on BT20 rockets take only a few grams of the stuff. It is also incredibly tough when cured.
 
One of the techniques I found worked best in helping to keep the fin attached is to put glue on the root edge first - let dry - using a hobby knife crosshatch the the surface where the root will go - sand down the BT so the gloss is taken off and using weldbond ( preferred over white glue) apply glue the fin root again and attach to the rocket. I know this will take some extra work - but you will be guaranteed a very strong fin joint
 
On my larger models, I use a plastic spoon to drag the fillets. Just start from the front and drag towards the back keeping the angle the same. I also lay down masking tape along the fins to provide a "limit line". With a little practice, my fillets are now perfectly smooth.
 
My daughter was reading this post and reminded me, I used to really gross her daughter out when she was very very young by pulling the dried Elmer's glue off of my fingers...It looks like I'm pulling my fingerprints off.

OMG!!! she just had her 31st birthday last Sunday...GAWD I'm OOOOOLD!!!

sandman
 
Originally posted by sandman
OMG!!! she just had her 31st birthday last Sunday...GAWD I'm OOOOOLD!!!

sandman

Well, I'm never one to start an argument..... :p
 
A note on forming fillets. It depends on the type of glue that you using as to whether or not you should pre-form the fillet to a concave shape. If you are using a glue with a volatile (evaporating) base, such as wood glues, you should _not_ preshape it. If you are using an epoxy, you should. The reason is this. With a volatile-based adhesive, the mass is reduced as teh glue dries. With wood glues, this is often 80%. If you pre-shape the fillet, 90% of the visible fillet actually provides no strength, since when it dries, it thins down to a layer too thin to provide actual strength. It is only in the very corner that you actuall get any gain. With a non-volatile adhesive, there is no shinkage.

I learned this when I learned correct window caulking technique from an engineer with a window manufacturing company. Many of the acrylic cauling compounds reduce by 50% to 60%. When appling a caulk, it is considered _critical_ to leave a large bead. This shrinks down to a concave fillet after a week or so, and provides the required strength.

If you want a wood glue fillet that is just in the corner, consider getting a glue syringe, and applying a convex bead that will shrink down to what you want, rather than a larger bead that actually does not provide the strength that is implied by the fin root surface that is covered.

I wrote an article on this for the Canadian Rocketry Association's magazine, "Earthrise" a year ago. If there is intrest, I can send a copy of the article by private response.

Leon Kemp
CAR S561 L3, RSO L1
 
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