External fan duct casing in a turbomachine

1. A twin-flow turbo-jet engine having a longitudinal axis, comprising a substantially cylindrical casing mounted downstream of an intermediate housing and externally defining an annular flow space for a secondary flow around the turbo-jet engine, wherein the cylindrical casing is formed from a lattice framework and from removable cowling panels fixed to the framework, said framework comprising at least one upstream annular flange for fixing to the intermediate housing, a downstream annular flange for connecting to an exhaust housing and rigid beams connecting the two flanges to one another.

2. The turbo-jet engine as claimed in claim 1 , wherein the lattice framework comprises a first plurality of beams parallel to the longitudinal axis and regularly distributed about the longitudinal axis.

3. The turbo-jet engine as claimed in claim 2 , wherein the lattice framework comprises a second plurality of rigid beams inclined relative to the longitudinal axis, each of the rigid beams of said second plurality having an upstream end fixed to the upstream annular flange and a downstream end fixed to the downstream annular flange.

4. The turbo-jet engine as claimed in claim 1 , wherein the lattice framework comprises an intermediate reinforcing ring arranged coaxially between the upstream and downstream annular flanges and connected by beams to the upstream and downstream annular flanges.

5. The turbo jet engine as claimed in claim 3 , wherein the inclined beams are arranged in a zigzag pattern between the annular flanges.

6. The turbo jet engine as claimed in claim 3 , wherein each inclined beam forms the same angle α with each of the two adjacent beams.

7. The turbo-jet engine as claimed in claim 1 , wherein the cowling panels comprise a coating providing acoustic protection.

8. The turbo-jet engine as claimed in claim 1 , wherein the lattice framework is produced from metal and is in one piece or formed from elements which are mechanically welded or bolted together.

9. The turbo-jet engine as claimed in claim 1 , wherein the lattice framework is formed from elements made of composite material which are screwed together, riveted or bonded.

10. The turbo-jet engine as claimed in claim 1 , wherein modules for equipment are fixed to the annular flanges of the lattice framework.

11. The turbo-jet engine as claimed in claim 1 , further comprising a fan housing upstream of said intermediate housing, and a fan duct downstream of said intermediate housing, wherein said fan duct comprises an internal casing and said cylindrical casing being externally and coaxially located relative to said internal casing, said internal casing and cylindrical casing defining said annular flow space for said secondary flow.

12. The turbo-jet engine as claimed in claim 3 , where the rigid beams from said first and second pluralities of rigid beams form a plurality of triangular shaped zones within said framework.

13. The turbo-jet engine as claimed in claim 3 , wherein the lattice framework comprises an intermediate reinforcing ring arranged coaxially between the upstream and downstream annular flanges and connected by beams to the upstream and downstream annular flanges.

14. The turbo jet engine as claimed in claim 13 , wherein the inclined beams are arranged in a zigzag pattern between the upstream and downstream annular flanges.