Containment casing

1. A gas turbine engine, comprising:

a tubular containment casing comprising (i) a series of axially-extending wall portions each having a radially outer surface and a radially inner surface, (ii) a series of axially-extending external edges between the outer surfaces of adjoining ones of the wall portions, and (iii) a series of axially-extending internal joins between the inner surfaces of adjoining ones of the wall portions; and

a rotary fan blade assembly,

wherein a radially outer perimeter and a radially inner perimeter of a radial cross-sectional profile of a portion of the containment casing surrounding the fan blade assembly are polygonal.

2. The gas turbine engine according to claim 1 , wherein at least one of the radially outer perimeter and the radially inner perimeter is a square, pentagon, hexagon, heptagon, octagon, nonagon, decagon, hendecagon or dodecagon.

3. The gas turbine engine according to claim 1 , wherein the number of fan blades in the rotary fan blade assembly is not divisible by the number of axially-extending wall portions.

4. The gas turbine engine according to claim 1 , wherein at least one of the radially outer and inner surfaces of each axially-extending wall portion is substantially planar.

5. The gas turbine engine according to claim 1 , wherein:

the tubular containment casing comprises opposing axial end portions each having a circular cross-sectional profile; and

the axially-extending wall portions extend between the axial end portions of the containment casing.

6. The gas turbine engine according to claim 1 , wherein the axially-extending wall portions are formed of a fibre-reinforced organic matrix composite.

7. The gas turbine engine according to claim 1 , further comprising a radially outer layer of ballistic wrapping.

8. The gas turbine engine according to claim 1 , further comprising a radially inner liner for defining an annular fan blade path.