Fin Attachment debate.

[flying_silverad]

I am designing a kit which I have put on the back burner but only because of the fins. The problem? I wonder if balsa fins glued to the outside (NO TTW) is asking too much for this particular rocket. So, what do you think guys? Here are the Specs so far:

Length-34"
Dia- BT80
Propulsion- 4 (yes 4) D12-5's
Recovery- 24 inch chute
Weight- 6.0oz.
Max Vel- 470 fps
Max Acceleration-870 fps

Right now my plans are to attach them the conventional way with yellow glue...maybe epoxy fillets. Here's a pic for you folks that like...pics!

Mike 9

 

[powderburner]

I am using intuition here (versus analysis), but I think you should be fine with plain balsa-n-glue without any TTW or other stuff. Looks like a 'sierra hotel' design!

Of course, the grain direction needs to be oriented along the fin leading edge, or along the 50 percent chord line. And a good solid root joint, plus extra application of glue for fillets.

The velocity that you present (470 fps) is nowhere near sonic, so the fins should have no excessive aerodynamic loading. Judging from your configuration picture, the fins look like they have a low aspect-ratio shape that will not flutter.

I am concerned about the "acceleration" value that you show. The units are those used for velocity? If the numerical value is indeed correct, then 870 feet per second per second is something like 27 g's----that is quite a bit, even for four D motors firing together. Time to re-check the numbers?

 

[Justy]

"KCee" would be a good guy to ask. He regularly flies an Estes Broadsword converted to take 4x 24mm. I don't know if he's used TTW fins on it or not, but whatever he's done to it, it flies great on 4x D12!

 

[el chubbo]

I cannot ever think of a time that I have built a model with TTW / aircraft ply construction and have been sorry. Yeah it's work and bother and labor and a bit of cost but the result is so much more resilient...

I avoid kits with balsa fins like the plauge - my 7-year-old has an affinity for breaking off fin tips....

 

[rbeckey]

Why not laminate the fins with cardstock and glue? It is an easy process that adds much strength. The attachment to the BT will be OK with good fillets.

 

[Austin]

Surface mount is ok and you are not pulling alot of speed, so it should be fine. On the lighter side, the Estes Cluster Bomb kit is only 12" of BT70 tubing, has TTW fins and for this reason, I have been able to push it up with a F24W...very cool launch btw. Never hurts to have a safety factor. I try to use TTW fin mounts for everything above D motors, just as a rule of thumb. It certainly can't hurt.

Carl

 

[hokkyokusei]

Originally posted by flying_silverad
I am designing a kit which I have put on the back burner but only because of the fins. The problem? I wonder if balsa fins glued to the outside (NO TTW) is asking too much for this particular rocket. So, what do you think guys? Here are the Specs so far:

I'd be more worried about strength on landing and during general handling than on launch, but if you get the grain direction right (as others have pointed out) it should be OK.

But with a four D cluster, there's plenty of internal room for TTW, so why not do it anyway? I find TTW fins so much easier to align properly.

 

[jetra2]

I personally probably would go with basswood fins on this particular rocket, just for the extra strength that basswood offers. Then a CyA attachment and epoxy fillets would do you good, I'd think!

Jason

 

[jflis]

that kit looks fine with conventional fin attachment. And pulling 27 g's is really nothing much in this field. Heck, the Flea on a C6 pulls (if i recall...) better than 50 g's

 

[cydermaster]

Originally posted by jflis
Heck, the Flea on a C6 pulls (if i recall...) better than 50 g's

Crikey!

No wonder mine stripped a fin on a C!
(original thread here)

 

[WiK]

Heck, the Flea on a C6 pulls (if i recall...) better than 50 g's

Whoa! Im gonna go put some epoxy fillets on mine then

 

[vjp]

My guess is that they'll be plenty strong enough for flight, BUT - 9 out of 10 times, fin damage happens upon landing, or during handling. With 4 empty "D" casings in the tail end on touchdown, a fin might snap off if the rocket's swinging the right (wrong) way at the moment of impact.

 

[wwattles]

Easy solutions:
1) BIG parachutes (usually require big launch areas with little winds)
2) Reinforced fins
3) Rear ejection models
4) Soft landing zones

Of course, this from a guy who is rebuilding the fin can on an Estes Phoenix because one of the reinforced fins broke off because of an early ejection that shredded the big chute.

WW

 

[Micromeister]

Flying:
every single one on my clustered upscales have standard attachment fins, some use expoxy rivits to the standard BT-80 and BT-101 body tubes. Upscale BT-80 OT same deal. your design should be just fine with standard attachment balsa fins, with epoxy or yellow glue fillets.
this BT-101 Goblin has 1/4" balsa fins epoxy rivited to the body tube. only support is the 1/8" lite ply 5 D12 cluster centering rings. the model has NO stuffer tube.

 

[rstaff3]

Great pic micro! What a flame

 

[Micromeister]

Thanks Dick:
For some reason I never took a static pic of the "Hobbie Goblin" just flight photos. I do love the smoke and fire from 5 D12's
PS flying-Silverad: in about a doz or so flights I've only cracked one fin, when the 36" hemi chute fowled, which caused a "nylon wad" landing.

 

[slim_t]

I think it would be fine.
But I know a way to find out. Build it, and fly it a few times.
Someone here has a tag that says something about 1 test result being better than 10 expert opinions.
Build it, fly it, post pics.

Tim

 

[flying_silverad]

Originally posted by Micromister
Thanks Dick:
For some reason I never took a static pic of the "Hobbie Goblin" just flight photos. I do love the smoke and fire from 5 D12's
PS flying-Silverad: in about a doz or so flights I've only cracked one fin, when the 36" hemi chute fowled, which caused a "nylon wad" landing.

Looks like I'll stick with the plan then. Ill consider basswood but it will just come down to cost. Thanks for the great pics and advice!!