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Large balsa fins -help with strengthening

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sed6

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I'm making some large balsa fins about 12 inches long with a 6 inch span and a basic trapezoidal shape. They are going to go on a 4-foot rocket powered by a D engine. My 6 inch wide balsa stock dictates that the grain runs parallel to the body tube, which of course is not ideal. So I have three ideas and I wonder which is best?

A) 1/8 inch stock, grain parallel to body tube (no balsa waste). Paper for strength.

B) Buy some extra 1/8 inch stock rotate it so the grain is parallel with the leading edge and edge butt several pieces together to get my desired width and cut my fins from that (produces balsa waste). Paper for strength.

C) Laminate two 1/16 inch pieces together at 90 degrees and then don't worry about the grain direction (no balsa waste). Paper for strength.

D) Other

Thanks!
 

Zeus-cat

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I think option C is your best bet, but I have never made fins that big so I am guessing.

What is the expected diameter and weight of this thing? Will a D motor lift it? I love Estes D and E motors, but they have their limits. You may need to use 24mm reloads to get a safe flight.
 

AlfaBrewer

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D) Similar to the 2 x 1/16" is 4 x 1/32" balsa ply.

E) 1/16" balsa with 1/32" ply skins. (like papering, but with thin plywood) You can do this with 1/32" ply or 1/64" ply, just use the right thickness balsa core to get your final desired thickness. One neat thing about the thin plywood, is that it cuts with scissors.
 

dr wogz

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1) Check & select you balsa. Balsa wood comes in different densities, and in turn will have different strengths. Also a little heavier. And yes, butt glue as needed.
1A) have you thought of using 3/16" balsa? or is the thickness important?
2) Laminate with 1/64" or 1/32" ply, with the inner balsa grain running in the right direction.
3) look at Bass wood. slightly stronger..
4) glassing with a light cloth..

I've tried laminating balsa to balsa, and ended up with curly fins..
 

samb

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I like the balsa core/thin ply skin sandwich idea. I would also experiment with cutouts of the core to save weight without compromising strength. Estes Mega Der Red Max is a commercial example of this technique.

MEGA_DER_RED_MAX fin sandwich 1.jpgMEGA_DER_RED_MAX fin sandwich 2.jpg
 

Cabernut

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The Mega Mosquito also has balsa core cut out with thinner balsa skins. Remember though, if you're using wood glue to laminate, keep it pressed until it dries so it hopefully won't warp. Using epoxy would prevent that.

Also a balsa ply with glass cloth core is something I've been wanting to try, you may want to look into that as well.
 

dhbarr

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The Mega Mosquito also has balsa core cut out with thinner balsa skins. Remember though, if you're using wood glue to laminate, keep it pressed until it dries so it hopefully won't warp. Using epoxy would prevent that.

Also a balsa ply with glass cloth core is something I've been wanting to try, you may want to look into that as well.
Thin FG/CF plate skins over a balsa core will be far more rigid & durable.
 

Rex R

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I would suggest 110# cardstock as opposed to standard printer paper
cover the entire fin(including the tabs),
and make them multi-piece with the grain oriented properly (with waste, it is like entropy there will always be waste)
Rex
 

hornet driver

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May I suggest another option. Run the grain parallel to the bt as you suggested. Apply a 1/8 in x 1 inch strip of bass on the leading edge as well as the tip. papering this would add strength.
 

swatkat

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I've done a very similar thing with a 4"x30" tube, and 6" span by 10" wings. I used cardstock (HP printer stuff) attached with CA and TTW fin mount and have been good for 4 launches so far on a 22Oz rocket with 28" parachute. I'd be wary of D-engines though. What is your weight? I've got a 24mm mount, but exclusively use the F44-4 economax or F32-5 SU motors.
 

sed6

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Thanks all! Just 5-6 oz with a D12 motor. Here's a pic.

big dart design.jpg
 
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BABAR

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If you laminate, try thick or medium CA for your laminate glue. You won't have as much of a problem with warping like you will with wood glue. Also the CA soaks into the wood, hard as a rock.

How do you get the nose to fall so the pointy part doesn't hit the ground first and break off? External shock cord mount for nose (maybe 2lb fishing line, would be invisible?)
 

Micromeister

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Another answer is to laminate the core balsa with either 1/64th or 3/64th" 3ply aircraft plywood. I've made some HUGE fins this way with Balsa and Foam Core centers. the Lower stage fins and Transition cone on my 3X UpScale Laser-X are made this way.

167-lp02-sm_Laser-X 3D staged Liftoff_06-10-95.jpg
 

sed6

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If you laminate, try thick or medium CA for your laminate glue. You won't have as much of a problem with warping like you will with wood glue. Also the CA soaks into the wood, hard as a rock.

How do you get the nose to fall so the pointy part doesn't hit the ground first and break off? External shock cord mount for nose (maybe 2lb fishing line, would be invisible?)
Fine idea. As to the recovery, who says I don't want it to stick into the ground? :) It's going to be a 1/2" hardwood dowel, so it won't break. But in truth at 6oz total and only 3oz or so for the front half it might not have enough weight behind it to actually stick in the ground. Either way, the rocket is going to separate behind the lower transition so picture 5' of shock cord separating the two halves, I can control which hits first by the position of the chute. With the chute in the middle the halves will hit together. With the chute closer to the nose the tail will hit first or vice versa.
 
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