Flexible duct having no appreciable variation in length under the effect of an internal pressure

A reinforced flexible duct substantially invariable in axial length due to effects of changes of internal pressure in the flexible duct, the flexible duct comprising an internal pressure resisting reinforcement having at least one layer formed from rings, or wires, cables or extruded sections which would be at a high angle with respect to an axis of the duct and a traction resisting reinforcement comprising at least one pair of crossed layers or wires, cables or extruded sections wound at an angle of at most equal to 55.degree. with respect to the axis of the duct, wherein a ratio ##EQU4## of the stiffness of traction resisting reinforcement to the stiffness of the internal pressure resisting reinforcement is between 2.5 and 8 and the winding angle .alpha. of said pair of crossed layers or wires, cables or extruded sections constituting the traction resisting reinforcement has a value of between 20.degree. and 50.degree. with respect to the axis of the flexible duct, the stiffness K of each reinforcement being defined by a product of:

• K=E.multidot.e,
• where:
• E is the modulus of elasticity of the material forming the reinforcement considered, and
• e is the total useful thickness of the reinforcement,
• said ratio ##EQU5## of the stiffness and the angle .alpha. satisfy the relationship:
• ##EQU6##