Fin options

[MikeSModerator]

Food for thought. My 14X Swift was a ‘slow motion’ of fin flutter.
1/2″ plywood fins.

In was not even close to mach but the extra large fins fluttered
badly and tore the lower part of the rocket apart. If it had been
going any faster, there would be nothing left.

I know the Swift was an exteme example of fin size. Most do not
thing anything would have helped.

Mach fin flutter is scarrry. Most have seen this video but here
it is again.

http://www.youtube.com/watch?v=u-b_VDFWFDg

No rocket I have built is ready for this. Much to learn.

Definitely listen the the Guys that have been there.

[new2hprParticipant]

I’d recommend the G10, but shy away from the thinner stuff like 1/16, unless you’re very light or have a large chute.

The last flight of Excess in Moderation was perfect, other than when it touched down. One of the 1/16″ G10 fins (with 3 layers tip-to-tip) tore along the root edge, pulling it outside of the fillet. Definitely too much flex, but that was with a 5lb rocket (unloaded), with 4 38mm motor casings loading down it’s fat rear. Still need to rebuild.

Any recommendations on repair without making it a ton heavier still? This is 4″ LOC paper tube, 1/16″ G10 fins, 3 layers glass (heavy and light) tip to tip, and foamed fin can. Definitely messy to tear apart.

Thanks,
Ken

[edwardModerator]

I generally use 1/16″ G10 and a big parachute. My Bunny Thumper rocket was 5.5″, 15lbs loaded and I used a chute that gave me a descent rate of 7 fps. I even cart wheeled it a couple times and didn’t do any damage. No tip to tip. Instead of epoxy I used PL Premium polyurethane glue (from Lowes) to secure the fins. When the rocket finally met it’s demise in a flat spin, the PL was still bonded to the G10, but it peeled back the MMT.

Edward

[Steve JensenParticipant]

I’ve a PML Kit with the same thickness 1/16 inch (0.0625) G-10 fins. It’s 46 Oz empty. I have torn fins on rough landing though I have significantly increased the chute size.

A five pound bird will be tough on these, especially if there’s a cross wind upon landing.

My fix is to grind out the whole fin down to the motor tube with a dremel fixed with a tungsten grinding tool taking care not to ding up the motor tube. It takes less time than you’d think, if your tool is sharp. Since I foam my fin cans, all that’s left is a slot. Then puddle epoxy in the slot and slide in a brand new fin taking care to align it well. I usually leave the slot slightly unfilled with the fin in place. After it sets I replace the fillet filling the remaining slot volume at the same time.

I’ve seen people try to epoxy a torn fin with bad results. 2 fin rockets have stability issues 😉

I use GLR G-10 3/32 (0.09375), which meanders from .095 to .102 in thickness, on a 7 pound rocket (empty) with good results.

I too am still playing with fin thickness/weight issues. The video from Mike above was impressive. I shy away from wood on anything flying over H sized engines. I may be too paranoid…

[Mitch Smith]

I appreciate all the good info. I think I will stick with what I have now and go with slow speeds and lower altitudes until I get the dual deployment down and continue learning.

[Steve JensenParticipant]

“PL Premium polyurethane glue (from Lowes) to secure the fins”

You are using this to attach the fins internally? The MMT connection makes sense.

How about the internal fillets?

I foam my fin cans, but have been making rockets that are on the heavy side due to generous epoxy fillets on the inside. I’m starting to think this is overkill. The foam is polyurethane as is the glue you recommend. Seems like a great idea. They should stick like crazy.

Using the PL Premium polyurethane glue only in the fin can and then foaming it seems appealing. I’ll still use epoxy fillets on the outside of the main tube. Is this what you do?

[edwardModerator]

I assemble my motor mount and fins outside the rocket, then slide into the airframe. I use the PL Premium on the MMT to fin connection. I don’t do internal fillets to the airframe. Never have. I don’t know why. I don’t foam the fincan either. Never wanted to add weight to that end of the rocket. On the outside I’ll go over the gap between the airframe with some very very thick epoxy to form nice fillets. After that I’m done.

I have before run a 1/4″ to 3/8″ fillet of PL Premium and then put a strip of fiberglass over that onto the fin a bit so that it won’t show through the airframe. That really stiffens them up and gives the glass a radius vs. a sharp bend.

I build my rockets as light as possible and for a proper flight. I could build them to come in hot or a parachute malfunction and be okay, but why build to survive failure. I’ve had much more success building them for flight and concentrating on having all my events happen properly.

Edward

[Steve JensenParticipant]

As it differs with my approach. Your experience is invaluable.

I think I’m overbuilding, think 57 Chevy. I’m going to try something radically different along your lines and see how it goes.

This is interesting and a definite improvement in my build philosophy.

Taking 20% of my build weight is significant. I did a quick calculation and feel it’s an easy weight percent to drop.

[MikeSModerator]

Great info from all… Thanks

[Author]

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