I have used green and air dried tulip poplar in a number of frames. The major problem with green poplar, in my experience, is first rapid drying and deep checking, second a strong tendency to wind. So I like to use green poplar as secondary members, i.e., rafters, joists, braces or boards. In my use of air dried timber, I have placed joinery while considering existing checks and either planed the stick or cut corrective joinery to handle the wind. Once dry, poplar seems to be stable.