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Keeping Fins On Minimum Diameter?

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DynaSoar

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I'm doing a 29mm minimum diameter bird and building it for max engine size. I'm estimating well over Mach. Being minimum diameter, I can't very well slot the tube. How else can I insure the fins won't pull loose? I though of gluing a thin slat to the bottom to give it more surface contact with the tube, but realized that only the slat would have more contact; the fin would still have the same contact area. Nor can I drill holes in the body for the epoxy to seep through and "rivet".

The tube is heavy paper (T52-H from BMS, a bit lighter than AT's motor tubes) and I'm planning on plywood fins. I could go with G10 fins, but don't see much benefit from that.

Drill a few small holes near the root and thread some thin material through them, which will then be embedded in the fillet material?

Drill the holes near the root and let the epoxy seep though those?

Glue a thin strip just above the root, which will be embedded in the fillet?

If fins tend to tear off due to sheer, maybe just the foreward portion needs the extra beefing up? Too bad there's no 29mm Acme fin can.

The root of the fin will not be completely flat. There's a motor retainer over the aft portion that the fins will have to have notches cut/sanded to fit. But this shouldn't change the problem or solution much.

Would CyA on the root with epoxy fillets be better than just epoxy?
 

DPatell

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I had good luck with just scuffing up the tube, tacking the fins on with CA, then small epoxy fillets. After that, a layer of 4oz fiberglass. I did this on my 24mm mach buster, and it had 1/8" balsa fins. With the glass it made for a very lite structure with maximum strength.
 

DynaSoar

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Originally posted by DPatell
I had good luck with just scuffing up the tube, tacking the fins on with CA, then small epoxy fillets. After that, a layer of 4oz fiberglass. I did this on my 24mm mach buster, and it had 1/8" balsa fins. With the glass it made for a very lite structure with maximum strength.
Did you glass the whole fin or just the fillet?
 

Matt

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i cut slots in the airframe, then slide a coupler inside with vaseline or wax to stop epoxy sticking.

add the fins into that slot and apply the epoxy. then fibreglass with a heavy cloth (3.5oz- 4oz).
once dry, take the couple out then it's all good and the motor fits fine
 

DynaSoar

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Originally posted by Matt
i cut slots in the airframe, then slide a coupler inside with vaseline or wax to stop epoxy sticking.

add the fins into that slot and apply the epoxy. then fibreglass with a heavy cloth (3.5oz- 4oz).
once dry, take the couple out then it's all good and the motor fits fine
That's a good idea, but a bit late here. I've already installed an externally mounted engine retainer.

Again, glass the entire fin, or just the joint?
 

hokkyokusei

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Glass from fin tip to fin tip.
Down the fin, over the fillet around the BT over the next fillet and up to the tip.
 

Justin Horne

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I guess since you've already got your tube, what im about to say is probably useless, but:

On PML's Min. Dia. kits, they dado-slot their tubes. This is just a slot that doesn't cut all the way through. This gets them straight, and it also probably helps in tying them down, so to speak. You might be able to do it to your tubes too.
 

DPatell

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I glass from fin tip to fin tip. Very strong!
 

Ryan S.

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glassing from tip to tip is the best idea, it makes the fin part of the airframe, and I could not imagine one breaking off, the fiberglass will break first, but the fin wont come off
 

powderburner

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I would recommend the 'thin strip in the root' idea.

Specifically, I would recommend checking out the balsa/basswood/spruce supply at your local hobbyshop, and looking for a triangular-shaped piece of spruce about 1/8 x 1/8 (or less). Add it to both sides of the fin roots, making sure you have good glue coverage underneath. Cover with a bit more glue or a thin epoxy. Keep in mind that the adhesive only does you any good (structurally speaking) if it placed between the root of the wooden fin (and it needs to be well-fitted too) and the outer wall of the BT. Any adhesive beyond that (like big fillets) is dead weight.

The grain of this root strip will be running chord-wise, which is not exactly the best. Preferably, the root strip should be cut ACROSS the grain so that when installed, the wood grain in the reinforcing fillet runs perpendicular to the BT and directly to the fin (which also has grain running spanwise, right?). However, cutting and shaping of such reinforcement strips is a PITA, so longitudinal strips made from spruce should still have good cross-grain strength. (Was that clear as mud?)

Adding glass (and epoxy) all over the rear end of the rocket will add too much weight (IMHO) and is unnecessary anyway. You don't need glass on the BT between the fins, and you don't need glass on the fin tips or mid-way out the span. If you must use glass, limit it to strips placed at the fin roots.
 

DPatell

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The thing with mach busters is that the motor is usually long enough that it HELPS with stability, rather than hurt it. The motor extends well past the CP in most cases.

The fiberglass root to root, if done right, is not heavy at all. It's actually very light. It also only adds a very thin layer of added drag.

This also allows you to use a weaker inner structure that is lighter, but gain the strength by making the fins and tube a single structure, as Ryan said.
 

GL-P

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Just glass tip to tip the fins. Slightly heavier balsa in a correct fin shape like 30-60-90 (looks similiar to a VB extreme) with a properly scuffed tube should be plenty!
 

Omega_D

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I made a 24mm Mach Buster using LOC MMT tubing, Estes balsa NC and 1/16" aircraft Pywood fins in the 30-60-90 shape mentioned above.

I attached my fins using 15min epoxy and small fillets making sure they were perfectly parallel to the body tube. I sanded the body tube very well before applying the fins. I sanded the leading and trailing edges to a knife edge.

Overall length was 18" and weighed ~3oz including streamer and shock cord. Don't forget to paint your rocket some flouresent color that's easy to spot and the use of tracking powder is highly recommended! I also stuck some chrome stickers on the fins.

I flew it twice on a AT G110-15, two awesome flights. With the chrome stickers on the fins it sparkled in the sun on the way down! The flouresent paint made it easy to spot on the ground.
 

GL-P

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I forgot some stuff. I've heard mylar is great for streamers and chutes because it glitters in the light. For me, red and bright orange work great for spotting. Get yourself Aerolab. It's free! Just google it and you should get some stuff. It does Cp up to Mach 7 or 8 and does drag profile graphs to that too! Great piece of software! It's got graphics and can do airfoils!
 

DynaSoar

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Originally posted by powderburner
I would recommend the 'thin strip in the root' idea.

Specifically, I would recommend checking out the balsa/basswood/spruce supply at your local hobbyshop, and looking for a triangular-shaped piece of spruce about 1/8 x 1/8 (or less).... (etc.)
Thanks, that does make sense. I was considering using 1/8" styrene angle bracket, attached at the root (1/8" basswood) with CyA, to add width to the root and to the area under the fillet (less likely to pull out).

Glass over whatever I use on the root makes more sense due to permeability. The epoxy going through every pore will become a structural point. Making use of the pores in wood for the same reason makes good sense. Glomming on a bunch of glass defeats the purpose of this bird, which is going to be near 1 caliber stable anyway. I'm just not convinced wood, paper and glue can't bust Mach without glass (OK, and urethane; I'm getting a PML urethane conical nose for it).

I'd thought of somehow permeating the wood with adhesive first. What do you think about soaking the area of the wood to be under the fillet with just resin? If enough of it soaked in, then when the mixed epoxy hit it later, the hardener could work on this stuff too. If the resin itself could be thinned with alcohol, which could be left to evaporate off before glueing, it might soak into the wood deeper, but not get weakened since the alcohol wouldn't be there when the hardener hit it.

Or am I just making it a lot harder than it needs to be?
 

rstaff3

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There are lots of ways to skin a cat. The commercial 'Machbuster' had a phenolic tube and small G10 fins which slipped into a slot in the near minimum diameter body tube. Because of the materials epoxy was required to attach them. Very simple. Of course I can't vouch for the fact that the fins stayed on after a G55 flight. :rolleyes: :cool:
 
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