Warped Plywood Fins

[KL2IE]

As the thread title would suggest, I am dealing with 1/8" plywood fins that are noticeably warped when laid down on a flat surface. Never having dealt with this before, I was thinking thinking about soaking the fins in water and then sandwiching them between two flat boards pressed down by a couple of cinder blocks to try to straighten them.
Do any of you have a better/more effective technique than what I'm considering here? Looking for suggestions.

 

[Orien]

See post #46 in this thread:

Build Thread: LOC/Precision 3" Iris 38mm. "The Comeback Kit". First build in 20 years! | Page 2 | The Rocketry Forum

 

[bjphoenix]

See post #46 in this thread

That makes sense- I was watching some woodworking videos and some woods could be bent when wet but would spring back, adding heat would make them hold their shape better.

I had some plywood fins a few years ago that were slightly warped. I tried a little water and clamping them between some straight boards, it helped a little bit. I ended up making a jig to position the fins on the airframe and clamped the fins between two boards on the jig. By holding the fins straight while the initial epoxy cured it took the warp out.

 

[David_Stack]

I recently received some 1/8” plywood fins (3 ply) that too were warped to a degree. Rather than just water I sprayed them with Windex containing ammonia. The latter softens the lignin I the wood fibers, allowing the fins to be ‘tweaked’ as necessary to remove the warp.

I babysat them while the Windex evaporated, checking and making slight adjustments if need be. When dry they remained flat. Days later and they still are…

 

[Sandy H.]

I recently received some 1/8” plywood fins (3 ply) that too were warped to a degree. Rather than just water I sprayed them with Windex containing ammonia. The latter softens the lignin I the wood fibers, allowing the fins to be ‘tweaked’ as necessary to remove the warp.

I babysat them while the Windex evaporated, checking and making slight adjustments if need be. When dry they remained flat. Days later and they still are…

Interesting trick. Is your gut feeling based on your experience that they will remain flat once finished, painted and kept in normal house storage? I have had bad luck in the past with flattening wood when received and eventually it remembering that it was warped. I'm 100% onboard (dang, that was really not intended as a pun) with being able to flatten sheets when I get them and processing them (laser) later if they are likely to stay flat over time.

Sandy.

 

[jqavins]

My gut says cut first, flatten after. In some materials and situations, cutting can allow internal stresses to create warps in the cut piece even if the stock was flat. IF this is one such situation, you could find yourself flattening the stock, cutting the fins, only to have to flatten the fins again.

That's my gut. My head says that the worst that can happen is you have to do the flattening twice, and the best that can happen is you'll discover that my gut doesn't know what it's talking about. Try it. See what happens. Report back.

 

[Grant_Edwards]

I recently received some 1/8” plywood fins (3 ply) that too were warped to a degree. Rather than just water I sprayed them with Windex containing ammonia.

Here's a vidoe where Tim Van Milligan uses an ammonia solution to permanently form flat helicopter blades into an airfoil shape:

https://www.apogeerockets.com/Advanced_Construction_Videos/Rocketry_Video_156

The other way this is done in woodworking is to steam the wood rather than just wet it.

https://www.woodmagazine.com/woodworking-how-to/wood-preparation/steam-bending