Strengthening attached fins

Hi all,

I'm just curious to know what would be a good way to strengthen up fins that have already been tacked on to a Thrustline Pumpkin Head build in progress?? I've been considering brushing on thinned-out PVA (I believe it's the same sort of stuff as Elmer's Glue-All). Or is using thin CA a better option??

Considering the Pumpkin Head has performance like a Goblin on 24mm motors, would it be a good idea to consider the use of epoxy fin fillets?? If so then what would be the ideal curing time for the sort of epoxy I'd need?? Otherwise would it be okay to stick with my trusty method of using PVA fillets??

Any suggestions and thoughts most welcome.

Cheers ...... Pete

Epoxy is usually the best bet for strength. The longer cure time epoxies tend to be the stronger ones. You can always apply some heat to shorten the cure time. Epoxy cures by chemical reaction. Heat it up and the reaction is faster. Cool it down and the cure time lengthens. There are limits. Heat it too much and it will crack and craze. Cooling too far down will kill the reaction and you will be left with a nasty goo. I use a chamber at about 150degF for general epoxy curing. Temperatures below 60degF will kill the reaction of many epoxies.

The trick I use for fillets is put the body tube on a BBQ rotisserie (sp?), apply the epoxy and let the assembly rotate for several hours under a 100W lamp. Careful that the fins are attached or they will flop around. I have had very good results with this method. The fillets smooth out very nicely and there is no gravity drooping. I will use 3M DP-460 or DP-4XL-EG since I use it at the shop and get it basically for free. For hobby type epoxies, a medium or thick type might work the best. Thinner varieties will run too fast while they are being applied. It's best to expirement. Don't worry about having extra epoxy around, it'll get used.

For stiffening fins, I use a sanding sealer. Other people have noted good results with laminating paper or Monokote over the fins. The most elaborate method is to "glass" the fins with fiberglass. Use a thin epoxy with the fiberglass rather than the polystyrene stuff. The classic fiberglass odor is eliminated and there isn't the issue with toxic fumes. You also need to use an epoxy that has a decent working time like 20 mins or so. Keep in mind that the larger the batch of epoxy you mix up, the shorter the pot life (working time). I just mixed up about 150 grams of West Systems 105 with 206 hardner to overcoat a pre-glassed 4" body tube and when the epoxy really started to kick off, it got very hot and remaining mix cracked and hardened very very fast. The coating on the body tube was thin and took several hours to cure. There are excellent glassing tips on the forum and on the web.

Ken Brown

Ok, epoxy may be the way to go. Any drawbacks or cons then as for using PVA/wood glue fillets? Will such fillets be able to handle the stresses imposed on the fins at peak acceleration??

Pete

Slow cure Epoxy and maybe a bit of Mill Fiber( 1/32 stands of fiberglass) mixed in. Along with a rough sanding around the glueing area. The Milled Fiber also lightens the epoxy weight a little.

I have used paper on fins for added strength 3 times now with good success. Even though its a 24mm build, I wouldn't see any advantage to using any exotic building methods like epoxy etc.

I think doing anything to a surface mounted fin other than white (or wood) glue and glue fillets is overkill, but that's just my experience so far. But paper does help out if the fin material was too soft or thin to really be used.

Sandy.

In all honesty, with balsa fins, wood glue is more than adequate. I have flown balsa fins attached with wood glue and strengthened with just a little brushed on thin CA to supersonic speeds. Epoxy will just add weight and be overkill.