Try using a heat gun on the concave side.   

In general, plywood cannot be flattened. You can sometimes minimize the bow, twist or cup by long exposure to heat and moisture, but it's never a predictable thing. You can cut the warped sheet into several smaller pieces and edge-join them to get an almost flat surface. Or you can design the structure to minimize the effect of the warp, compensate for (or even use) the warp.

Bill,

My bad I meant the convex side.  or the other side than what you heated.  The Idea is to dry the convex side out a little so the swelling is less.  Sorry bout that 

Actually, it worked better from the concave side, I think, Tom.  If I lay the ply on a flat surface (outside of the curve down!), weight both ends and apply heat to the middle, it worked better than anything else I have tried.  If I put a sandbag or brick on the outside of the curve to push it down, I have no way to heat the curve.

I think Ralph's idea helps me the most. 

"...Or you can design the structure to minimize the effect of the warp, compensate for (or even use) the warp."

It is probably impractical to straighten a 12"X24" piece of plywood at home.  I can cut out the stab and elevator from the least warped area and have much smaller pieces to straighten.  A rudder that is slightly warped can be helpful if the warp is in the right direction.

Since this is a trainer and the idea was to get it together and ready to fly, I think I am going to stop by Hobby Lobby or Michael's and get some of that really dense balsa in 1/8" or 3/16" and make up a spare tail for this bird.

regards and thanks everyone,
bill marvel

If you'r definitely looking for 1/8 ply, you can cut 2 from 1/16 ply. Try to arrange any warps to counteract each other and laminate together with thin epoxy. Weight it down flat and leave overnight to set.

For durability I would use 1/32" ply (Flat and straight) with 1/32" sheet on either side. I warrant this will be as durable as you could possibly need and it should be lighter as well. I just don't see solid 1/8" ply tail feathers. It is one thing to build durable, another to build like a rock. If it is lite ply then maybe but still...  Overkill?

I would not buy ply as badly warped as you describe. Just doesn't make sense. I would bring it to the attention of the responsible parties and ask him what he would like to use this crud to build and why it was offered for sale if he were not willing to use it himself.

If this were ply that warped after I bought it, I would save it for bulkheads and firewalls  but no more. Certainly for flying surfaces!

There is better ply available. Don't settle for junk! I have never bought ply that was so badly bent. I would not expect anyone else to either.
It really is too much effort for the little result to "fix" warped, bent, or concave ply found in a store. Just pass it by. But do let the owners of this garbage know that it won't sell because of condition.  Ask him if he were building his house if he wanted such poor quality materials making up important structures in his home? I bet not.


off my soap box for now, Robert