In recent times, there has been a rapid increase in airborne sporting and leisure activities such as parachuting, ballooning, hang-gliding and microlite flying, so much so that the presence of these craft in the air is becoming a hazard to the safety of low flying aircraft such as light aircraft and military aircraft on low flying exercises. This is especially the case because of the small size of these craft and the fact that they are generally invisible as far as radar detector systems are concerned.

An object of the present invention is to make airborne craft and appliances of the aforesaid kind more readily detectable.

This is achieved according to the invention by arranging that the flexible material of the wing

or like structure is radar reflective at least in part, and/or providing an additional panel or panels of flexible radar reflective material so that they are tensioned in use and form a suitable radar reflective structure either alone or with the wing.

In one embodiment, it may be sufficient simply to make the wing or like structure radar reflective. However, the shape of the wing may not give it a suitable radar reflective characteristic; for example, reflections may be only of low intensity or may be directionally sensitive so that it can only be detected when viewed in a particular orientation. Preferably, therefore, the craft has attached to it flexible radar reflective panels which are tensioned in use to form a suitable radar reflector, the wing or part of it possibly also being radar reflective to form part of the reflector with the panels. For example, the panels may define an array of corner reflectors such as are well known for their optimum radar reflective characteristics. These panels may be tensioned between opposed portions of the wing or enclosed within an envelope that forms the wing.

The invention is illustrated in the accompanying drawings in which:

Figure 1 is a sketch of a hang-glider adapted according to a first embodiment of the invention,

Figure 2 is an underview of the hang-glider of Figure 1,

Figure 3 is a longitudinal section through a hang- glider wing according to a second embodiment of the invention,

Figure 4 is a longitudinal section through a hang- glider wing according to a third embodiment of the invention,

Figure 5 is a longitudinal section through a hang- glider wing according to a fourth embodiment of the invention, and

Figure 6 is a lateral section through the hang- glider wing of Figures 3 and 4.

Figure 1 shows a hang-glider having flexible radar

reflective panels 1 tensioned between the king¬ post 2 and the central longitudinal frame member 3 so as to extend longitudinally of the glider and serve as a reflector for radar beams incident from the sides of the craft. Further flexible radar reflective panels 4 are tensioned between the lateral frame member 5 and the bracing wires 6 that support the pilot's grab rail 7 beneath the glider. These panels 4 extend laterally of the glider so as to reflect radar beams incident from the front or rear of the glider. The wing 8 of the glider may also be made of a radar reflective material in which case it forms angled corners with the panels to give improved radar reflective characteristics in the manner of a dihedral reflector.

In the second embodiment illustrated in Figures 3 and 6, the hang-glider is such as to have a frame member 9 along the leading edge of the wing 8 which takes the form of an inflatable tube. This type of inflatable tubular frame construction has been adopted in some designs of hang-glider and gives the opportunity of including a radar reflective construction within these inflatable

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tubes. As shown in Figure 3, the reflector consists of four longitudinally extending flexible radar reflective panels 10 to 13 connected together along their inner edges at 14 and connected to the tube 9 at their outer edges so that they form a cross with right angles between the panels when the tube 9 is inflated and the panels tensioned. Additional flexible radar reflective panels 15 are provided laterally at intervals along the length of the tube 9 (see Figure 6) and are connected to the wall of the tube and the longitudinal panels 11 to 13 so as to form trihedral reflectors with the other panels where they all meet at right angles in a corner. Thus a plurality of trihedral radar reflectors are formed along the length of the tube 9 at the leading edge of the wing 8, and a plurality of dihedral radar reflectors are formed between these trihedral reflectors in the angles between the longitudinal panels 11 to 13.

In the third embodiment of Figure 4, the whole wing 8 of the hang-glider is formed as a double- walled inflatable structure and a radar reflector is provided within it in the form of a plurality

of interconnected flexible radar reflecting panels 16 that are connected to the walls of the wing at their outer edges so as to be tensioned thereby and form a plurality of trihedral and dihedral reflectors in a similar manner to that shown in Figures 3 and 6.

In the fourth embodiment of Figure 5, the whole wing 8 is similarly formed as an inflatable double-walled structure as in Figure 4, however, instead of providing a plurality-of interconnected panels 16 throughout the wing, one or more radar reflectors 17 are located locally within the wing and are such as to have a radar reflecting characteristic over an azimuth of 360°. Each reflector comprises a plurality of interconnected flexible radar reflecting panels connected to the walls of the wing so as to be tensioned thereby. Figure 5 illustrates a reflector comprising a series of trihedral reflectors arranged around a central upright axis in the manner described in our corresponding U.K. Patent Application No. 8903534.

It will be appreciated that the inflatable tube 9

and wing 8 in the embodiments of Figures 3 to 6 are composed of non-radar reflecting materials so as not to screen or effect the reflecting characteristics of the panels within them.

In yet other embodiments of the invention in which the frame of the hang-glider is composed of rigid tubular members, provided these are composed of non-radar reflecting material, they can house within them a radar reflecting structure which may be rigid and could be similar in form to that illustrated in Figures 3 and 6.

It will be appreciated that the illustrated alternative arrangements for providing radar reflectors in hang-gliders, can be readily adapted for use within the envelopes of balloons or within the canopies of para-gliders.