Abstract: In an aircraft propulsive power unit a thrust reversing arrangement includes a flow redirecting structure having a flow redirecting downstream wall against which gaseous fluid flow in a first flow direction impinges to turn the flow into a second flow direction to produce reverse thrust and an upstream wall round which the fluid flow turns and which has a boundary surface so formed as to support adherence of a boundary layer of the redirected fluid flow at the boundary surface. The upstream wall is provided with projecting elements to generate vortices which prevent or delay separation of the boundary layer from the boundary surface of the upstream wall. Alternatively, the upstream wall is formed with a plurality of flow apertures which are connected by a duct to a pressure differential region or source to cause gaseous fluid to flow through the boundary surface of the upstream wall to prevent or delay separation of the boundary layer.

Abstract: An aircraft propulsive power unit having an engine (20), a nacelle structure (12) which houses the engine (20) and which has an exterior low drag boundary surface (16) which is subjected to exterior air flow in a boundary layer adjacent the boundary surface (16) and an air flow disruption device (19) activatable to cause at a control location in the boundary surface disruption of exterior air flow in the boundary layer at the control location without reversal or deflection of engine thrust gaseous flow. Where the power unit is a turbofan and the nacelle structure includes a fan duct (22) with inner and outer fan duct walls (25,26) the air flow disruption device (19) is additionally or alternatively activatable to cause at a control location in the outer fan duct wall (26) disruption of air flow in the boundary layer at the control location.