Help w/Fin Redesign On This Stubby 4" Kit...

[deandome]

I'm not one for build threads (WRITING them, not reading!), but I'm knee-deep into building THREE 54mm glass kits simultaneously; a 4" Wildman AGM-78 Standard ARM, a 3" Mach1 Jericho missile (from 'Iron Man') and the 4" Wildman Jart (ish) that is the subject of this thread. It's a Wildman-club special circa 2010-11, I say 'ish cuz I think the actual Jart kit he came out with had different/more swept fins.

I don't need KRAZY stuff, but I don't like plain boxy 3 or 4 FNC rockets, as this one looks/looked to be...so the plan over the last year or so was for me to cut my own fins from some nice 1/8" G10 stock I have laying around. I worked in an old tailcone I had, and that actually makes the original fins look a bit better...kinda Bullpuppyish. But I was also thinking that sweep downard to...or well BELOW...the bottom of the tailcone will help make it more stable...bullpups need a TON of nosweight, and I want to fly this on H-J motors without much concern other than figuring out the correct, non-obscene, amount of nose weight

I could be way off on this, but I think moving the fin's 'center-of aerodynamic-force-action-point' (CAFAP, trademark pending ) downward would effectively/aerodynamically elongate the rocket making it easier to stabilize. Normally I don't like the idea of fins extending below the rocket, as they get damaged on landing, but this fin stock is really effing thick, so I think it can handle it. I'm thinking about something as basic as an Estes Alpha design, Mosquito, big bertha, a mild cranked-kite like the Aerotech G-Force, or something like the cool rocket in this photo, but maybe pulled downward a bit more (right hand side):


Never having done this, I have a few questions:

• Does my logic about extending the fins downward to improve stability hold any merit?
• Do I need to keep the fin-span and/or the surface area about the same?
• What do YOU think would look cool on this stubby, tailconed bird?

Thanks!

 

[jmasterj]

Make a sim in OpenRocket and experiment!

 

[Zeta]

Yes Deandome, getting more surface area on the fins aft of the tail cone is a good idea for that rocket. Stability will improve. This one is a 4" dia. 18 inch long airframe with 9oz of weight in the nose. 38mm motor (H). 1.19 stability ratio.

 

[PSLimo]

Is that a CW 4" Lil Rascal? If so, I built one cropping the fins up. It only needed 1/2 lb in the nose and flew straight as an arrow. The 4X 12" diameter one next to it needed 5lbs in the nose.

 

[deandome]

I have rocksim, but a) creating custom fins is a PITA unless they're basic, and b) from what I've experienced and/or read, simulation software doesn't really work that well/accurately with 'SuperStubbies' like this (booster is 4"x12"!). I also read somewhere here in TRF that un-swept fins ala Nike Smoke, BDR Dragonfly don't work well on short/fat rockets, and my kit's fins aren't that far away from that

I'm more interested in if the IDEA of lengthening the fins will improve stability. Like when I purposefully built a 5" gizmo with reverse-swept fins; it spun like a pinwheel cuz I didn't add enough extra nose weight (flew great since I dumped like 3 lbs into ito tip of NC. The fins weighed the same, and CG wasn't THAT different compared to normal build...but it sure flew differently!

 

[jmasterj]

Perhaps it would be worth your time to look up how to better sim stability on stubby rockets. And fins with only four points are pretty easy to get close on.

Yes, swept fins will move the CP back.

 

[Zeta]

12 inch long air frame :

I 211 motor / stability 1.22 / 16 oz nose wt.

 

[teepot]

From looking at the picture of your rocket I would suggest larger fins. Greater surface area. Greater in span.

 

[ThirstyBarbarian]

Sweeping the fins back generally does improve the stability by shifting the CP aft.

Short, fat rockets often are more stable than the sims calculate due to “base drag” — the low-pressure that develops at the base of the rocket. However, by adding the tailcone, you are eliminating most of that base drag, so you won’t be able to count on that. So I really do think you need some back-swept fins for stability. And I think back-swept fins will look good on this rocket.

 

[ThirstyBarbarian]

Check out this short, fat rocket with tail cone and back-swept fins:

https://www.rocketryforum.com/threads/estes-big-daddy-2162-gallery.15985/page-5#post-683611

 

[ECayemberg]

Yo Dean,

If you want a set of actual Jart fins, I have an extra set if you want them.

The one caveat: I believe they are for the 5" Jart, so they may be a bit larger than desired; I believe the thickness is .125" (G-10).

 

[David Schwantz]

Here are a couple of ideas to look at. Yes, 2 are Jarts. All fly straight as an arrow, even Dirty Harry!

 

[user 30299]

Here are a couple of ideas to look at. Yes, 2 are Jarts. All fly straight as an arrow, even Dirty Harry!

What did you use to clear coat the red rocket - 2nd from the right (looking at the picture).
It makes that filament-wound fiberglass look pretty good.

 

[David Schwantz]

PPG Omni MC161
They are all cleared with that.
Thank you.

 

[Zeta]

 

[deandome]

Thanks for all the great ideas!! I particularly like ThirstyBarbarian's rounded-tip, little-more-swept-than-jart fins, though Dave's ACTUAL jart fins look great too. I'll PM you, Eric, about your set of fins...might be a great option, and I've already widened my fin slots to fit 1/8" stock (aka .125").