There is provided an improved weight-shift control flexible wing aircraft and improved systems and methods for recovering or preventing a spiral dive or a stall of a weight-shift control flexible wing aircraft. The aircraft including a frame; a wing assembly defining a flexible sail extending from port to starboard sides of the vehicle, the flexible wing assembly comprising: a wing keel extending from fore to aft ends of the wing assembly; a pair of wings, wherein each side of said pair of wings has a strut coupled to and extending away from the wing keel and defines a leading edge and a trailing edge when the wing assembly is moved though air, wherein the trailing edge of each side of said pair of wings is configured for actuable reversible billowing in at least a portion along a length of the trialing edge.