Fin Flutter?

ok, EdwardW and I were having a discussion about putting an M in a 14lb, 4" rocket. I've attached a drawing of the fin with the dimensions, it's made out of 1/8th G10.
I've never launched a rocket of this size or speed before, so I'm unfamiliar with what I would have to deal with, but I wanted your opinions on whether or not this rocket would survive those speeds (possibly close to mach 2ish...maybe more like 1.7)

Any thoughts or debates you can throw out there would be swell.

(and don't worry, I told him I was gonna ask you guys, I'm not undermining him )

Go to www.lunar.org and then go to LUNAR clips and download the the last issue of the clips and there is an article in there by Steven Kendal on his L3 build of a PR Comp 4 a four inch in diameter rocket with an 75mm motor motor mount they're are some help tips in there. Here is the link click here and go the one that says "Mach-Up" My L3 Project.

cool, that's a great article.... very interesting read

Anyway, his fins are twice as thick as mine, and with a bigger span, so that doesn't help me much. I know how to make joints and fillets strong enough to withstand very strong motors, but I'm just not sure of flutter.

1/8" G10 is absurdly sturdy. You'd be fine with that, and if anything it's overkill. Definitely bevel the edges.

really? ya think so? well, the edged are already beveld, but I'd like to sharpen the angle...they're at about a 20 degree angle now...I'd like about 10.

Edward was saying I'd need fins almost twice that thick and tip to tip reinforcements..... and I always want to prepare for worst case scenario....

Twice as thick as 1/8" G10?????????

holy crap, those would be huge. I see no need for twice the thickness for a project like this, and all it would do is add weight where you least want it. I could maybe see going to 3/16" G10, but 1/8" plus tip to tip (t2t 5oz carbon would be awesome, but t2t glass would probably be fine) should survive no problem. G10 is VERY strong, and 1/4" G10 would simply be ridiculous.

If you really want certainty, ask Bruce Levinson to do an analysis in Finsim (or anyone else who has finsim). That will tell you exactly how fast those could take before fluttering.

Nate, I have a similar rocket that has fins very close to those you posted. Mine too are .125 G-10 with 2 layers of glass that goes 2/3rds tip to tip. I have flown it on up to N3000's with no problems. Just make sure that you really rough up the G-10 to give the epoxy a place to 'bite' and glassing tip to tip with a underlying layer or two of glass will help.
Tony

Cool guys thanks! you've given me much more confidence in these fins now.

I've also got a ton of 11oz uni-directional carbon fiber I'm gonna lay on this....maybe not all the way from tip to tip, but pretty close...then I might finish it up by covering it in a very light layer of fiberglass for a sanding and filling veil.

Just to clear something up: The Comp4 (as it's been called) has 3/16" fins , "not double the thickness" of 1/8" which would put them at 1/4" thick. I know because I own this kit.

Using 1/8" G 10 should be fine for you fin design.
Fin flutter is more of a problem when you have a large span (width) & a smaller root edge.


JD

ok, obviously I know that double 1/8th is 1/4 however, 3/16ths is only 1/16 from being double, so I figured it wasn't enough structural difference to be so specific in this case.

FYI; There is a Fin Flutter estimator Excel spreadsheet written by Duncan McDonald available for download from Info Central under Rocketry Design at:

https://www.info-central.org/finflutter.xls

It is worth reading Duncan's article about fin flutter at the Info Central site: https://www.info-central.org/design_finflutter.shtml

Bruce S. Levison, NAR #69055

I think that particular planform would be less susceptible to flutter than something like a swept-fin.

umm....I'm pretty sure that is a swept fin.

Or do you mean something more along the lines of the AT Mirage?