towing cable 3 to body 2 is not restricted to that shown in the drawings the drag producing structure of the aerodynamic device 11 incorporates two kinds of blades 4 and 5 blades 4 are alternately mounted with blades 5 around the circumference of a slidable sleeve 6 such that blades 4 lie outermost the blades 4 and 5 are pivotably attached at one end portion to the slidable sleeve by hinge means 7 sleeve 6 is mounted coaxially to the main tubular body 8 of the device and can slide axially relative to body 8 sleeve 6 is constrained to move between limits imposed by lugs 9 attached to the main tubular body 8 of the device and which lie in slots 10 in the sleeve 6 this arrangement shown in fig2 a also prevents rotation of the sleeve 6 about body 8 the sleeve 6 rests against resilient means in the form of a helical spring 11 the other end of which rests against collar 12 ring 13 is securely mounted onto the tubular body 8 the volume occupied by this configuration is a minimum for this particular towed body 2 and aerodynamic device 1 and so this configuration is suitable for storage of the body and device when in storage the towed body 2 and device 1 are kept inside a storage container not shown until the pilot of the towing aircraft releases them from the container by e g pressing a button in the cockpit the towed body 2.