I am not used to builing the smaller size models, mainly 80" to 140" spans. Sounds like you have an open airframe or thinly sheeted model. What type of glue are you using? I use CA for the most part and never have a problem with a twisted or bowed airframe. The smaller models can do this more easily than the larger ones. You can get some of this out when covering them. If your building a 1/2a you should have no problem straightening it when you cover it.