Aircraft structure comprising a skin panel

1. An aircraft structure comprising;

(i) a skin panel, having;

an outer surface forming an external face of the structure, and

an inner surface internal to the structure;

(ii) a plurality of stringers extending along a length of the inner surface of the skin panel, wherein each stringer is a discrete element of the structure; and

(iii) an inner panel spaced apart from the inner surface of the skin panel and being internal to the structure, so as to form an internal cavity adjacent to the skin panel, wherein the stringers are located within the cavity.

2. An aircraft structure as claimed in claim 1 , wherein the stringers have a height so as to extend across the entire height of the cavity from the inner surface of the skin panel to the inner panel.

3. An aircraft structure as claimed in claim 1 , wherein the stringers are in the form of hollow struts, so as to provide a plurality of elongate hollow stringer cells extending along the cavity.

4. An aircraft structure as claimed in claim 3 , wherein a plurality of the stringer cells have a cross-section substantially in the shape of a hollow rectangle and wherein the stringer cells are arranged side-by-side in the cavity such that a side wall of each stringer cell abuts a side wall of an adjacent stringer cell so that there are substantially no gaps in between the cells.

5. An aircraft structure as claimed in claim 1 , wherein the inner panel is bonded to the inner surface of the skin panel at an outer edge portion of the inner panel.

6. An aircraft structure as claimed in claim 5 , wherein the inner panel comprises:

a central section spaced apart from the inner surface of the skin panel;

the outer edge portion bonded to the inner surface of the skin panel; and

an angled intermediate portion extending towards the inner surface of the skin panel from the central section to the outer edge portion.

7. An aircraft structure as claimed in claim 1 , wherein the stringers comprise a layered composite material.

8. An aircraft structure as claimed in claim 7 , wherein each layer of the composite material has fibres orientated in a fibre direction and wherein the angles of the fibre direction in each of the composite layers are chosen such that, where two stringers are bonded to one another, the composite layers across the two stringers are symmetrical with respect to fibre direction angles either side of a notional line.

9. An aircraft structure as claimed in claim 7 , wherein at least one of the skin panel and the inner panel also comprises a layered composite material, each layer of the composite material having fibres orientated in a fibre direction and wherein the angles of the fibre direction in each of the composite layers are chosen such that, where the stringers are bonded to said at least one of the skin panel and the inner panel, the composite layers across the bonded stringers and said at least one of the skin panel and the inner panel are symmetrical with respect to fibre direction angles either side of a notional line.

10. An aircraft structure as claimed in claim 1 , wherein aircraft structure further comprises;

(i) a second skin panel providing an opposite external face of the structure, and

(ii) at least one rib connected across a depth of the structure in between the two skin panels.

11. An aircraft structure as claimed in claim 10 , wherein the structure further comprises a second inner panel spaced apart from the inner surface of the second skin panel and being internal to the structure, so as to form a second internal cavity adjacent to the second skin panel and wherein a second plurality of stringers extending along a length of the inner surface of the second skin panel are located within the second cavity.

12. An aircraft structure as claimed in claim 11 , wherein the rib is connected between the two inner panels.

13. An aircraft structure as claimed in claim 1 , wherein the structure is at least a part of a wing box, horizontal tail plane or vertical tail plane.

14. An aircraft structure as claimed in claim 13 , wherein at least some of the stringer cells are arranged to transport fluids from one part of an aircraft to another.

15. An aircraft comprising the aircraft structure of claim 1 .