Quantcast

Warped fins

The Rocketry Forum

Help Support The Rocketry Forum:

Skyhigh

Well-Known Member
Joined
Jul 15, 2010
Messages
99
Reaction score
0
I am currently building a 4" BSD Thor and have gotten to the stage on mounting the fins but have noticed a warp in the 3/16th plywood fins. Does anyone know of a good technique of fixing these ?
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
put some water on the inside of the curve. try a little gentle heat, like a clothes iron through a towel.

how much of a warp are you seeing?

don't forget, wood does move with changes in temperature and humidity.
 

Skyhigh

Well-Known Member
Joined
Jul 15, 2010
Messages
99
Reaction score
0
Well, the warp is enough to be visible to the eye.... If I put the fin on a flat surface and push on it I can see about 1/16th of a inch movement up and down.
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
sounds like "a lot", as in enough to make the rocket roll. it's not a bad thing but for a 4" rocket you ought to be able to get a clean boost ...


your flat surface probably isn't and your eye can fool you. at least my eyes fool me :) especially when looking at curved, sanded surfaces ...

do you have a straight edge of some kind to check for sure?
 

Skyhigh

Well-Known Member
Joined
Jul 15, 2010
Messages
99
Reaction score
0
I just tried the wet cloth and ironing and it worked like a charm! Thankyou very much for the info. God I love this place !
 

Micromeister

Micro Craftman/ClusterNut
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 19, 2009
Messages
15,074
Reaction score
38
Location
Washington DC
a piece of plate glass is a great way to check flatness and sand airfoils on glider wings. I picked up a 12" x 24" piece of 1/2" plate glass from a local glass company for around 6.00. they even "eased" the edges for me so they aren't as sharp as razors. In the past I've used window glass but it's a little on the delicate side for sanding. but would be OK as a level surface gage.
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
hey, the plate glass is a great tip, I have some in the garage.

God I love this place!!!
 

gerbs4me

Well-Known Member
Joined
Sep 21, 2009
Messages
3,102
Reaction score
3
Location
Iowa
the plate glass is a good idea:)
this place is the greatest
 

Johnnie

Well-Known Member
Joined
Dec 31, 2002
Messages
4,289
Reaction score
0
Micromister,

I'm a little fuzzy on this whole airfoil sanding/plate glass thingy. How does the plate glass ease the job of sanding airfoils, can you elaborate for me please? I have had trouble with airfoils on the past, and have since just left the fins with square edges.
 

swimmer

Well-Known Member
Joined
Feb 20, 2009
Messages
664
Reaction score
0
JP,

Do a search on 'midpwrguy'. He has a way of cutting fin airfoils on his table saw. He also has a photo album showing the steps. He has pictures of several of his finished rockets with airfoiled fins using this technique. His fin to bodytube transitions are flawless.
 

Micromeister

Micro Craftman/ClusterNut
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 19, 2009
Messages
15,074
Reaction score
38
Location
Washington DC
Sure Johnnie:
this tip I picked up from David O'Brian one of our International USA team members, He's really heavy into RC/RG and boost gliders.
this method is mostly used for glider wing airfoils with or without under camber, but can also be used for symmetric airfoiled fins with some care and "practice" ..
My plate glass slab is 12 x 24 inches and 1/2" thick. Place the slab on a table top with something under it like thin corrugated cardboard or the like.. I use a 1/8" polypropylene mat the same size as the glass from tandy leather supply. Funny how all our hobbies lead back to Rocketry:D This mat or cardboard helps keep the glass from moving around while your sanding. Apply a strip of 1" wide double faced masking tape on the forward top surface of the glass even with the edge and longer than the part to be shaped. I usually run my hand and or fingers over the upper surface of the tape to reduce the amount of tac. Remember the thighter the adhesive on the tape hold the wood the more difficult it will be to remove after the part is sanded. Lay the shaped fin or wing slab of wood on the tape making sure the leading edge is even with the edge of the glass. Press it down on the tape to insure it doesn't move while you are sanding. After marking the high point "Center ridge - bout 1/3 of the width" use a sanding block with 120grit or heavier sandpaper and two hands to remove the excess wood to the line, rounding the forward "leading" edge as you go.. Carefully with a putty knife or very thin spatcula pop the wing or fin off the tape, reposition with the tailing edge of the wing even with the edge of the glass, press the wing onto the tape then feather sand the trailing edge of the wing from the high ridge to nearly a knife point at the tailing edge. To keep the wing or fin as even as possible use a long sanding block, I personally use 11" long 1" square blocks or 15" alum. T sanding blocks i'm sure you've seen in the hobby shops. If making a fin, flip the fin over and repeat on the other side.
Once happy with the taper, finish sand the entire wing or fin with 320 and/or 600 grit, pop it off the tape and start the next piece. I usually get 2 wings or fins complete before the d/f tape must be replaced.
What purpose does the glass serve? First; because it is dead flat you are assured the part will not be bowed or warped as you are
attempting to sand the shape. Second; the glass edge resists the sandpaper so it can be used as a tool rest to help insure you are making even passes. That is a point I almost forgot.. make long single direction passes over the entire length of the wing or fin NOT BACK AND FORTH. with as even down pressure as you can. inspect the shape often from both ends.
Please note, you will ruin the first two or three parts you try to airfoil, and/or snap one or two off removing them from the D/F tape.. this is a finess operation. I've been very happy with the results, Hope you will also.
 

astronboy

Well-Known Member
Joined
Sep 11, 2002
Messages
2,437
Reaction score
0
I haven't tried this yet, (and it is not my idea) but I plan on making plywood fins for one of my winter projects in the following manner:

ALL plywood I have found in hobbyshops is warped to one degree or another. So, I will buy sheets that are 1/2 of the thickness that I actually need. I will then glue them together so that the concaved sides are facing each other, and pile a bunch of books on them. When dry, I will have plywood of the thickness that I need, where the warps are actually fighting against one another to keep the wood straight.
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
tell me about warped, I bought some 1/8" aircraft ply today ... ~~~~~ <- ascii graphics wavy line.

in this case I need 1/8" parts and I didn't feel like gluing up 1/16" sheets. but I've had good results gluing two sheets together. just be sure to use lots of even pressure (weight)

when you make up sheets maybe you could use the cheaper "not for aircraft use" ply from Michaels (1/4 the cost!). the cheap stuff seems to be more flexible in one direction so maybe glue the two plies at 90o to each other?

I think I will try that right now with some scrap.
 

jawa

Active Member
Joined
Oct 6, 2003
Messages
33
Reaction score
0
After getting back into rocketry in December, I bought a Fat Boy kit and a Guardian kit from Michael's with their great coupons :D

I read a little on the forum and decided to try and get a little more strength in the fins of the Fatboy by coating them with yellow glue. Holy crap those things warped like crazy! Forgot about the whole moisture content thing...

Anway, once they dried, I coated the other side with yellow glue and covered with a 3x5 index card and piled on the heavy books. I have to say that the index card overlay is about the easiest way to get a smooth finish...so far. I have to paint still, so we'll see how that goes. On a side note, after sanding the edges of the fins, I coated them with thin ca...nice and hard edges!!!

On the Guardian's fins, I coated one side with 5 minute epoxy, let dry (didn't warp), and did the other side with the yellow glue/index card. So hopefully at the end of building both of these I have an idea of which ends up with a better finish. Then I can start on sandman's mini's :)

Anyone else experiment like this? :D

- Jason
 

sandman

Well-Known Member
Joined
Jan 18, 2009
Messages
10,468
Reaction score
5
Listen guys!

I have NEVER...repeat NEVER had a warping problem with my "white glue" coating.

Maybe the trick that no one seams to mention is "GLUE THE FINS ON FIRST!"

After I've glued the fins on and made some nice white glue fillets THEN I apply white glue to both sides of each fin.

I would never even concider doing this before the fins were on the rocket.

The glue doesn't fill the balsa that much but it does strengthen and seal the surface. A light sanding with 220 grit is all you need to get the "nap" sanded off.

Without the glue finish the primer just kinda soaks in and makes the wood fuzzy.

sandman
 

Silverleaf

Well-Known Member
Joined
Feb 22, 2003
Messages
1,124
Reaction score
0
Sandman,

Great tip there !

One question though, you use white glue for the fillets instead of yellow woodwoking glue..any reasons ?

I ask only because I use Titebond II yellow woodworking glue and its excellent. Is it more personal preference ?

Thanks,
 

jawa

Active Member
Joined
Oct 6, 2003
Messages
33
Reaction score
0
Yeah, I guess gluing them on first would solve the warping issue. I would have thought that trying to sand the fins while on the rocket would loosen and weaken the bond between fin and rocket...

- Jason
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
... a little topic drift here but that's OK.

Sandman, how does your white glue grain sealer compare with the "regular" Aero Gloss sanding sealer? mainly I am interested in the weight difference. but if you really get away with just one pass over the surface then that's a huge gain, it seems like sanding sealer takes 3 or 4 coats and a lot of sanding!
 

sandman

Well-Known Member
Joined
Jan 18, 2009
Messages
10,468
Reaction score
5
I haven't used the Aerogloss sealer in a very very long time...I do remember it smells really bad!

I just love Elmer's white glue...I use it for sealer, fillets, gluing fins on, coating nose cones, paper, balsa, cardstock...

I buy it in big bottles.

The yellow glue always gets that disgusting skin on the top (I always forget to close the cap!)

White glue is very forgiving, for me.

Yea, just a personal prefference I guess.

sandman
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,217
Reaction score
0
theoretically yellow glue is stronger than white glue, by quite a bit. although I guess you have found white glue to be "strong enough" for joints.

I am going to try white glue next time. this morning I filled the fins of my Tres (#19!) with Aero Gloss and it looked OK but the primer and paint found those **** pinholes!

oh well, they all look good sitting 30' away out on the pad and that's what counts.


(oh and some unexpected rain this afternoon on the wet laquer gave my upscale tristar the "pox" :p )
 
Top