A point to keep in mind with plank style (ie:no sweep) flying wings is that the CG must never go back beyond the 25% chord point. If it does and if it's by more than a couple of percent the wing will be unstable in the extreme. The only exception to this is the pizza box flat wings that were popular for a while recently. On those the literally square shape added enough vortex flow that it confused the model into flying in a decently OK sort of stable way. But even those were generally only flown at post stall angles due to their tendency to "trip" over their leading edge and tumble if flown "normally" in an unstalled condition when the speed built. A normal CG position range for a plank wing is in the 18 to 22% range. And even that 22% figure needs to be approached carefully.