Fin Attachment

When attaching standard balsa fins to a paper tube rocket, are reinforcement fillets really necessary if you attach the fins in the first place with a thick bead of CA?

To be safe, yes they are. They will add longevity and flight strength to your rocket. I would personally NEVER fly without fillets.

Fillets are usually built up along the fin-airframe junction to reduce drag; but I think their main purpose is to add strength to the glue joint. If you are using high impulse motors the fins might not hold up very well especially for a large number of flights. Some CA glues get brittle with age, so a fillet of another glue might help strengthen the fin body tube joint. If you are willing to risk loosing a fin and crashing your model rocket, try out the CA only attachment. If it works out then let us know so we can all learn from your experience.

If you are using a "thick bead" of CA aren't you making the fillet anyway? The fillet can be put in at the fin attachment step if your adhesive allows you to do this. I usually sand the glassine layer off the outside of the body tube at the fin attachment points so the glue can soak in and wet the cardboard and make for a stronger bond. I put fillets on at a later step so I can prevent the glue from running over the surface of the fin.

Bruce S. Levison, NAR #69055

you know, for smaller modrocs I think a properly attached fin with thin CA only would survive flights just fine, its the handling and motor extraction and primer sanding etc that seem to be why reinforcement is really needed on the small guys!

I tend to believe that if the fins are rear swept past the end of the rocket, the fillets help keep the fin on more than just a root attachment of CA. Alot of times, you'll see the body tube seperate and the fin comes off. The fillet helps keep that from happening.

Loopy

well, fillets certainly help strength but as an example, I have a number of smaller alpha size rockets that I only used thin ca to glue fins and they're going strong many C flights later.... haven't tried a D21 but I like these models and want them back! However, I have had chute failures or separations where it comes in backwards (like an alpha with the long swept back fins) and breaks a fins off and the other modrocs I've had similar failures with but where I used epoxy fillets, the tube just bends above the fins... so... I think flight stresses on smaller modrocs arent enough to break ca but flight failures and rough handling can... just my observations but you make me want to try a min diameter 18 and 24mm rockets surface mount fins ca only to see what happens!

Oops - Trogdor is right - that's what I meant to say in my last post. I'm not talking flight stresses - I'm talking about landing stresses.

Loopy

Originally posted by Trogdor
you know, for smaller modrocs I think a properly attached fin with thin CA only would survive flights just fine, its the handling and motor extraction and primer sanding etc that seem to be why reinforcement is really needed on the small guys!

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I almost never use the "thin" CA - I use the thick or slow stuff almost exclusively, except to put drops on kevlar or elastic knots to make sure they hold.