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Author Topic: My Gyminie Cricket wing has ended up like a prop - help!!  (Read 292 times)
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Newbie_John
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« on: August 13, 2019, 07:27:02 AM »

My next stage in my stubby finger learning curve was to build a Gyminie Cricket from the BMFA kit.
I took on board the advice I've had from on this Forum and face to face about matching wood, not using too much glue, being patient (hard  Cheesy) and pinning everything flat to the board while it dried.
I even pre-shrunk the tissue before sticking it to the wing frame (using thinned-down PVA) and weighed it all down flat on my new magnetic board.
Everything looked good, and I fitted the wing to the motor stick and left it overnight.
In the morning, this was the result!

Now it's easy enough to make another wing, but I don't understand what I'm doing wrong, so I don't want to end up with another propellor Angry Angry Angry

I did try steaming out the warp and weigh the wing down again, but after it's dried the warp comes back worse  Shocked Shocked

Help, please, guys & gals!!
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My Gyminie Cricket wing has ended up like a prop - help!!
My Gyminie Cricket wing has ended up like a prop - help!!
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Jack Plane
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« Reply #1 on: August 13, 2019, 08:41:29 AM »

Did you steam (or over-steam) the tissue after you'd applied it to the wing?
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Newbie_John
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« Reply #2 on: August 13, 2019, 11:13:43 AM »

Jon,
I did what I thought were the right things - I made a frame and pre-shrunk the tissue, then cut out the shape and glued it to the wing. I only tried steaming when I discovered the massive warp when I removed the covered wing from the board.
As far as I can tell I got the tissue grain in line with the span of the wing too.
The only thing I did different to the instructions was to pre-shrink the tissue - the GC instructions actually just say to cover the wing with the tissue as supplied - could that be a clue?
Thanks
John
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Big G
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« Reply #3 on: August 13, 2019, 11:48:47 AM »

Looks like there's quite a lot of glue on the ribs which could possibly have caused the warping. I use Titebond thinned 'quite a lot' with water and apply it to the ribs and spars with a very fine brush. The paper doesn't have to be drum tight, so don't water-shrink it after you've stuck it onto the wing. I've been advised to pre-shrink it on a frame, same as you've done. I screwed up a hell of a lot building my first indoor toys (which were GC's covered with l/w Esaki) - it's quite a learning-curve. I'm still new at the indoor game, but am lucky in that I have one or two experienced local flyers to help me. One good thing is that building new wings, etc., doesn't take long. Good luck!

G
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Indoorflyer
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« Reply #4 on: August 13, 2019, 12:14:09 PM »

Since this is in an Indoor sub forum:  Gluing the tissue onto a frame then spraying/shrinking actually limits the amount of preshrinking the tissue can undergo. A sturdy picture frame will not distort when the tissue shrinks initially.  Once you cut the tissue free, it will continue shrinking with temp and humidity changes. Voila--potato chip! I shrink my tissue unrestrained on a dishrack/drainboard. Then after it has dried (will look like a shriveled up mess) I iron the tissue flat. Apply your adhesive to the wing perimeter and dihedral breaks, hit the tissue with the iron, then immediately attach the tissue to the structure. You don't want a drum tight covering on an indoor model.  I use Elmer's glue stick to attach the tissue (goes on purple then dries clear)  A slight "alligator skin" texture is your "anti warping insurance."


I've attached some pics of a profile scale/nocal Mooney I recently built, for examples...
Attached files Thumbnail(s):
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
« Last Edit: August 13, 2019, 02:02:26 PM by Indoorflyer » Logged

Make the same mistake on both sides; nobody will notice...
Newbie_John
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« Reply #5 on: September 30, 2019, 08:21:11 AM »

At long last I've been able to get back to my building board and take advantage of the advice and encouragement from everyone.

I rebuilt the wing and stab using diluted white glue (is it normal for the dilute glue to be absorbed on some joints so you have to put a second layer on?) and was even more careful with my cutting to minimise stresses.
As you can see from the pics, it came out flat and square  Grin

I shrunk and ironed the tissue then fitted it to the wing and stab using UHU glue stick. Again, not perfect yet, but pretty flat and square still. Grin Grin

I'll hopefully be flight testing it at Berinsfield this weekend, but so far it's looking good! Wink

Thanks for the help, feeling much more confident about this smaller wood now!

Cheers, John
Attached files Thumbnail(s):
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
Re: My Gyminie Cricket wing has ended up like a prop - help!!
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TimWescott
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« Reply #6 on: September 30, 2019, 12:46:47 PM »

I'm late to the party, but here's my two cents, as a long-time outdoor airplane modeler with just a bit of experience with indoor:

The typical ribs & spars construction, covered top and bottom, makes a torsion tube structure with the covering acting as the outside of the tube.  So the covering actually makes the structure torsionally stiff.  With a sufficiently floppy structure, each side (top & bottom) would pull warps, but both together tend to make things better.

When you cover just one surface, then shrink the covering, the covering is just going to want to pull the structure into a pretzel.  That's because the way that the covering can shrink the most (the "least energy condition", if you're a physicist) is to pull everything into a horribly warped condition.  If you've ever worked with sheet metal you've probably noticed that any reasonably large "flat" piece actually wants to be a bit twisted one way or another -- no matter what you do, if you get it flat it wants to go "boing" and twist a bit.  That's why any big flat panels have a bit of contour bent into them, even for ductwork and such.

So for the single-surface, flat wings, you need the covering to be slack (or super-duper wimpy, like microfilm).  You need the covering so slack that the -- exceedingly wimpy -- resistance to twisting of the structure can overcome whatever the covering is trying to do.  Unlike everything else in modeling, wrinkly is good.  I haven't done it, but some people advocate pre-shrinking the covering, then crumpling it into a ball, smoothing it out by hand (no iron), and putting it on.
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