Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
October 20, 2017, 10:54:52 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: [1]   Go Down
Print
Author Topic: Dual Fins on a Stab Warping  (Read 455 times)
0 Members and 1 Guest are viewing this topic.
OneArm
Bronze Member
***

Kudos: 0
Offline Offline

United States United States

Posts: 99

Topic starter


Ignore
« on: October 12, 2017, 07:15:17 AM »

Hi All,

Finishing up a Hi-Performance Sportster that has dual balsa fins on the stabilizer. Called out as 1/32 sheet. Any tips on keeping these from warping? I tried gluing up two sheets per fin cross grained and they still warped. Thanks
Logged
PB_guy
Gold Member
*****

Kudos: 6
Online Online

Canada Canada

Posts: 381


Just a kid at heart.



Ignore
« Reply #1 on: October 12, 2017, 11:33:10 AM »

You got cross-grained laminates to warp? Cool!
Did you pin the pieces flat as they dried?
Did you cover them with tissue?
Did you dope them, or apply some other finish, and if so, did you pin them flat until dry?
Have you tried steaming to unwarp?
ian
Logged
OneArm
Bronze Member
***

Kudos: 0
Offline Offline

United States United States

Posts: 99

Topic starter


Ignore
« Reply #2 on: October 12, 2017, 12:55:04 PM »

Cross grained 1/32 sheet and glued with wood glue and 10lb weights on top till dry. Even added small cross grain members cyanoed in place. Sanded everything down. Covered with preshrunk Easy Built tissue with glue stick. No dope or tissue shrinking yet done yet. See no reason for the warping
Logged
ZK-AUD
Gold Member
*****

Kudos: 21
Offline Offline

New Zealand New Zealand

Posts: 747



Ignore
« Reply #3 on: October 12, 2017, 12:56:29 PM »

A 'key' section inset at 90 degrees to the grain and sanded back flush helps
Logged
ZK-AUD
Gold Member
*****

Kudos: 21
Offline Offline

New Zealand New Zealand

Posts: 747



Ignore
« Reply #4 on: October 12, 2017, 12:57:43 PM »

Maybe do them again from scratch - you can't be that unlucky twice!
Logged
ZK-AUD
Gold Member
*****

Kudos: 21
Offline Offline

New Zealand New Zealand

Posts: 747



Ignore
« Reply #5 on: October 12, 2017, 01:36:55 PM »

just something else I thought of - tissue orientation.  Most tissues have a grain and shrink more one way than the other so make sure you have both sides oriented the same way to equalise stresses.  Non-shrink dope also a good option
Logged
OneArm
Bronze Member
***

Kudos: 0
Offline Offline

United States United States

Posts: 99

Topic starter


Ignore
« Reply #6 on: October 12, 2017, 01:43:17 PM »

Yep, thought of all those. I'm gonna remake them and see how that goes. Thanks everyone
Logged
danberry
Gold Member
*****

Kudos: 17
Offline Offline

United States United States

Posts: 890



Ignore
« Reply #7 on: October 12, 2017, 08:03:35 PM »

Use C-grain wood and a finish that won't induce a warp.
Logged
faif2d
Platinum Member
******

Kudos: 26
Offline Offline

United States United States

Posts: 1,021


Sun came up I was here to see it = good day



Ignore
« Reply #8 on: October 12, 2017, 10:07:44 PM »

Wood glue uses water as the solvent (to make the solids liquid).  Try using epoxy that is applied and then wipe most all of it off using a credit card or something similar.  I have a jig block to make simplex rib blanks and tried wood glue at first.  After 1 week the glue still had not dried.  I went to a VERY thin layer of epoxy and problem solved.  I think that the layup that you tried under the weight never dried all the way through so when you removed the weight what was left to cure warped the fin.
Logged

I used to like painting with dope but now I can't remember why!    Steve Fauble
mike
Bronze Member
***

Kudos: 4
Offline Offline

United Kingdom United Kingdom

Posts: 68




Ignore
« Reply #9 on: October 13, 2017, 02:59:49 AM »

The water in the wood glue will cause the wood to expand differently across the grain vs. along the grain.  When the wood dries in the 0/90 lay-up, the shrinkage will set up conflicting internal stresses and tend to curve the sheet.  Look up images of anticlastic curvature.  It's a known feature of cross-grained laminates.  In fire/matrix composites it's the heating/cooling during manufacture that causes it to happen in 'unbalanced' laminates.  0/90 is unbalanced, 0/90/0 is balanced.

Why not use 1/16 'C' grain with the grain at 90 deg. to the tail surface.  Lighter and quicker.
Logged
billdennis747
Titanium Member
*******

Kudos: 36
Offline Offline

United Kingdom United Kingdom

Posts: 2,677



Ignore
« Reply #10 on: October 13, 2017, 03:57:12 AM »

Agree that it's the watery wood glue.
I have two oversize pieces, thin cyano on one, spray kicker on the other, slap them together, press down till the smoke and noises stop, cut out.
Logged
Hepcat
Platinum Member
******

Kudos: 206
Offline Offline

United Kingdom United Kingdom

Posts: 1,669



Ignore
« Reply #11 on: October 13, 2017, 08:29:32 AM »

I have my pen name because 72 years ago I built my first really successful own design model and called it ‘Hepcat’. I told my clubmates that its superiority was due to the aerodynamic advantages of twin fins. (In truth the fins were to provide two of the three take off points necessary in those days but I did not broadcast that.)  In those days, before dethermalizers, one built a new model most weeks and build standards were not high.  A damaged fin would probably be replaced with one cut with scissors and stuck on as one cycled to the flying field.
I nearly forgot: the big advantage of twin fins is that your timekeeper nearly always times the correct model and the big disadvantage is that a tailplane with a fin on each end is difficult to transport without damage and warps in a fin or tailplane  are difficult to rectify.
Years later I took up Coupe d’hiver with the passion that only a born again Free Flighter can generate and naturally, with the warped, sunshine memories of ancient aeromodellers the Coupes had to have twin fins. 
Now having established that I have some acquaintance with twin fins I will offer brief advice and a photograph. 
Make the fins detachable. For carrying and storing.  Sandwich the tailplane between two pieces of foam. Hold the fins flat on the outside of the foam with rubber bands.
John
Attached files Thumbnail(s):
Re: Dual Fins on a Stab Warping
Logged
applehoney
Titanium Member
*******

Kudos: 208
Offline Offline

Canada Canada

Posts: 3,026




Ignore
« Reply #12 on: October 13, 2017, 09:55:35 AM »

>Use C-grain wood and a finish that won't induce a warp.

Exactly.   I prefer twin fins on my Mulvihills, Nostalgia rubber (where applicable) and other models in the personal belief that such provide a  cleaner climb pattern than the single fin.

Cut from C-grain, sanded, glued to the stab ...  lightly doped/sanded a couple of times and then colour sprayed for  retrieval visibility in the bean field.   150 sq.in Mulvihill pictured has 1/32" fins, now over ten years old - no warpage..
Attached files Thumbnail(s):
Re: Dual Fins on a Stab Warping
Logged
mkirda
Gold Member
*****

Kudos: 11
Offline Offline

United States United States

Posts: 598

WWW

Ignore
« Reply #13 on: October 13, 2017, 11:40:34 AM »

Another way to do this: Laminating epoxy on tissue. Very light, very warp free.

Wax glass, tissue, epoxy as little as you can get on, vacuum bag with paper towel for a couple minutes, then add balsa. Tissue, epoxy again with paper towel on top. Vacuum.
Then vacuum bag it for 15 hours until epoxy has set. Takes less time then the rest of the suggestions and works really well, plus result is extremely strong.

Regards.
Mike Kirda
Logged

Hepcat
Platinum Member
******

Kudos: 206
Offline Offline

United Kingdom United Kingdom

Posts: 1,669



Ignore
« Reply #14 on: October 13, 2017, 12:27:07 PM »

Response to #12.
I am getting used to this Jim, we must have similar wiring in the brain.  I too think my rubber models climb better with twin fins in clear air rather than one fin right in the slipstream.  It improves the tailplane efficiency as well!
John
Logged
PB_guy
Gold Member
*****

Kudos: 6
Online Online

Canada Canada

Posts: 381


Just a kid at heart.



Ignore
« Reply #15 on: October 13, 2017, 12:28:31 PM »

Mike, do you use a thin liquid epoxy for this, such as is used in coating threads on fly rods?
ian
Logged
Pages: [1]   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!