Propulsor

1. A boundary layer propulsor comprising:

a rotor having an axis of rotation;

a plurality of first aerofoil blades extending radially from the rotor and arranged in a circumferential array around the axis of rotation, each first aerofoil blade has, in a radially outward sequence, a radially proximal portion, a middle portion, and a radially distal portion,

wherein the radially proximal portion has a first cambered cross-section, the middle portion has a second uncambered cross-section, and the radially distal portion has a third cambered cross-section, the first cambered cross-section being cambered in an opposite sense to the third cambered cross-section, wherein the boundary layer propulsor is not actively powered by an engine or other equivalent source of motive power; and

a plurality of second aerofoil blades extending radially from the rotor and arranged in a circumferential array around the axis of rotation,

wherein each second aerofoil blade has a fourth cambered cross-section, the fourth cambered cross-section being cambered in the same sense as the first cambered cross-section,

and wherein a radial length of each second aerofoil blade is equal to a radial length of each radially proximal portion of the first aerofoil blades.

2. The boundary layer propulsor as claimed in claim 1 , wherein the first cambered cross-section is a negative camber, and the third cambered cross-section is a positive camber.

3. The boundary layer propulsor as claimed in claim 1 , wherein each first aerofoil blade has a first surface and an opposite second surface, each of the first surface and second surface being provided with a smoothed transition between the radially proximal portion and the middle portion, and between the middle portion and the radially distal portion.

4. The boundary layer propulsor as claimed in claim 1 , wherein the plurality of first aerofoil blades comprises between two and six first aerofoil blades.

5. The boundary layer propulsor as claimed in claim 1 , wherein the plurality of first aerofoil blades is equi-spaced around the axis of rotation.

6. The boundary layer propulsor as claimed in claim 1 , wherein the plurality of second aerofoil blades is equi-spaced radially between the first aerofoil blades.

7. The boundary layer propulsor as claimed in claim 1 , wherein each of the first aerofoil blades has a first chord length, and the first chord length decreases from the radially innermost end of the first aerofoil blade to the radially outermost end of the first aerofoil blade.

8. A boundary layer propulsion system comprising:

a machine body having an upstream end and an opposite downstream end; and

a boundary layer propulsor comprising:

a rotor having an axis of rotation; and

a plurality of first aerofoil blades extending radially from the rotor and arranged in a circumferential array around the axis of rotation, each first aerofoil blade has, in a radially outward sequence, a radially proximal portion, a middle portion, and a radially distal portion,

wherein the radially proximal portion has a first cambered cross-section, the middle portion having a second uncambered cross-section, and the radially distal portion has a third cambered cross-section, the first cambered cross-section being cambered in an opposite sense to the third cambered cross-section,

wherein the boundary layer propulsor is positioned at the downstream end of the machine body such that a fluid flow passing over the machine body, from the upstream end to the downstream end, flows over the plurality of first aerofoil blades, the fluid flow passing over the radially distal portions of the first aerofoil blades causes a rotation of the boundary layer propulsor relative to the machine body, and thereby causes the radially proximal portions to entrain a boundary layer flow passing over a surface of the machine body to thereby accelerate the boundary layer flow, and

wherein the boundary layer propulsor is not actively powered by an engine or other equivalent source of motive power; and

a plurality of second aerofoil blades extending radially from the rotor and arranged in a circumferential array around the axis of rotation,

wherein each second aerofoil blade has a fourth cambered cross-section, the fourth cambered cross-section being cambered in the same sense as the first cambered cross-section,

and wherein a radial length of each second aerofoil blade is equal to a radial length of each radially proximal portion of the first aerofoil blades.

9. The boundary layer propulsion system as claimed in claim 8 ,

wherein the rotation of the boundary layer propulsor relative to the machine body causes the second aerofoil blades to entrain a boundary layer flow passing over the surface of the machine body to thereby accelerate the boundary layer flow.

10. The boundary layer propulsion system of claim 8 included in an aircraft.