A nacelle for an aircraft bypass turbojet engine includes downstream, an internal stationary structure surrounding part of the bypass turbojet engine, and an external structure surrounding the internal stationary structure, defining an annular flow path along which an air flow circulates. The external structure includes a mobile flap disposed at the downstream end of the external structure and positioned facing the annular flow path. Each mobile flap can rotate such as to move into a position that increases or reduces the height of the cross-section of the annular flow path in relation to an idle position, in response to the pressure exerted on the mobile flap by the air flow circulating through the facing annular flow path. The mobile flap can return from the cross-section-increasing or -reducing position to another position under the effect of an elastic return means.