X-Plane 9 XB-70 Valkyrie 9.31. This latest update to the Valkyrie file restores flight performance that disappeared in the step from 9.21 to 9.31 by tweeking the models airfoils, coeficient of drag AND adding additional engine thrust. The model performs very close to specification at its all up gross weight of approximately 600,000 lbs. Check out the readme file for flying tips. Theres lots more to look at and read in the "reading room" folder as well. By David Austin.