Our pilot had seen Dash 1 try to hover inside the walls above the courtyard and fail. According to two US officials who praised the skill of the pilot, the chopper lost the lift necessary to hover because it entered a “vortex” condition. At least two factors were at play, they said—hotter than expected air temperature and the compound’s eighteen-foot-high walls. In our North Carolina simulations, the walls, although to scale, were essentially fences, allowing air to flow through them. The solid walls in the compound created a bowl effect that affected the aerodynamics just enough to make a
...more
