A cavity system that tends to increase the thickness (28) of the shear layer (22), comprising: a cavity (2) and a plurality of rods (4) extending away from the cavity base (3) to a height extending beyond the leading edge (14); the rods (4) being positioned downstream of, and in the proximity of, the leading edge (14). The rods (4) may extend to different heights and/or be positioned longitudinally offset (e.g. in a zig-zag pattern). The rods may be reversibly movable to a configuration in which they are fully enclosed in the cavity (2) when the cavity (2) is closed. Flow alteration elements (34, 38), for example channels (34) passing through the rods (4) and/or protrusions (38) extending from the rod (4) may be provided on the rods (4).

Lifting surface comprising:

• a movable discontinuity (1) located in the surface of the lifting surface, the movable discontinuity (1) being movable between:

o an active position in which the movable discontinuity (1) acts as vortex generator, and

o a passive position in which the movable discontinuity (1) is integrated into the surface of the lifting surface,



• a conduit (2) located in the spanwise direction of the lifting surface and in communication with the movable discontinuity (1), the lifting surface comprising at least two openings (3) in its surface spanwise distant from each other in communication with the conduit (2),

the movable discontinuity (1) and the conduit (2) being configured such that when an airflow goes through the conduit (2), this airflow activates the movable discontinuity (1) to act as a vortex generator of the lifting surface.

A cavity system, comprising: a cavity (2) comprising a cavity opening; and an acoustically reflective structure (18, 20) located at least partially within the cavity (2), the acoustically reflective structure (18, 20) comprising one or more acoustically reflective surfaces (24, 26, 30, 32), each acoustically reflective surface (24, 26, 30, 32) being oblique to a plane of the cavity opening (27). The one or more acoustically reflective surfaces (24, 26, 30, 32) may be arranged to reflect incident acoustic waves out of the cavity opening while avoiding reflection into a region (48) at or proximate to a leading edge (14) of the cavity (2).

A vortex generator (200) includes a depression (202) in an aerodynamic surface (126), and a vortex generator leading edge (300) located in the depression (202). The vortex generator leading edge includes a leading edge upper surface (312). The leading edge upper surface (312) is positioned at or below a tangent line defined at a location along the aerodynamic surface (126) upstream of the depression (202) relative to an oncoming local flow.

A cavity system that tends to increase the thickness (28) of the shear layer (22), comprising: a cavity (2) and a plate (104); the plate (104) comprising a plurality of holes (4) through it, wherein the proportion of the area of the plate (104) taken up by the holes is ≥ 60%, for example ≥ 75% or ≥ 90%; the plate (104) being positioned in the proximity of a leading edge (14) of the cavity (2); the surface of the plate (104) at a perpendicular or oblique angle to the actual or intended flow direction (6). The plate (104) may be positioned at an oblique angle to the flow direction (6). One or more flow alteration elements, for example elongate members (34) positioned across the holes and/or members (38) positioned directly behind or in front of the holes (4).

A fixed wing aircraft has a wing with an aerofoil cross-section defining an upper geometric surface and a lower geometric surface which meet at a geometric leading edge of the wing at the intersection with the aerofoil geometric chord. The wing is configured to define an upper aerodynamic surface and a lower aerodynamic surface when the wing is moved through air. The upper aerodynamic surface and the lower aerodynamic surface meet at an aerodynamic leading edge of the wing at the intersection with an attachment line dividing the air that passes over the upper aerodynamic surface from the air that passes over the lower aerodynamic surface. The lower geometric surface adjacent the geometric leading edge has a roughness strip with a step height of at least 50 microns over the lower geometric surface. The roughness strip is located on the lower aerodynamic surface of the wing when the aircraft is flown at a load factor -the ratio of the lift of the aircraft to its weight - of 1g, and the roughness strip is located on the upper aerodynamic surface when the aircraft is flown at a load factor of 1.2g or more.