GB2254489A

GB2254489A - Radome nose cap

Info

Publication number: GB2254489A
Application number: GB8520559A
Authority: GB
Inventors: Neil Williams; Ronald William Lyon; Norman Robert Gilbert; Alexander Mckay Munro
Original assignee: ERA Patents Ltd; UK Secretary of State for Defence
Current assignee: ERA Patents Ltd; UK Secretary of State for Defence
Priority date: 1984-08-21
Filing date: 1985-08-16
Publication date: 1992-10-07
Anticipated expiration: 2005-08-16
Also published as: GB2254489B

Abstract

A cap 3 is provided for protecting the nose of an aircraft or missile radome against rain erosion, the cap being made of a low loss plastics material, and having associated with it, an array of metallic elements, preferably in the form of a network 4 of thin metal conductors, designed to compensate for the insertion phase change of incident RF radiation caused by its passage through the thickness of the cap. The array 4 of metallic elements is preferably embedded within the cap 3, but may be located on its surface, or even spaced therefrom in the outer skin of the radome itself. <IMAGE>

Description

Radome Nose Cap This invention relates to the problem of rain erosion of aircraft and missile radomes made of plastics.

A radome is a member which forms the end of an aircraft or missile nose cone and which is made of a dielectric material so that it forms a protective window through which radar or other radio frequency (RF) sending and/or receiving equipment housed in the nose cone can operate. Ideally, the radome should be substantially invisible to the RF beam at the operating frequencies and orientations, and for this purpose the insertion loss and phase change of the radome should be as small as possible.

For aircraft and missiles which travel at subsonic and low supersonic speeds, it is usual to make the radome from a suitable plastics material, and a preferred construction comprises a layer of plastics foam sandwiched between two layers of glass reinforced plastics. A major problem with such plastics radomes, however, is that of rain erosion, i.e. the gradual degradation and destruction of the radome resulting from the impact of hydrometeors on its outer surface when the aircraft or missile flies through rain or hail. The effect of the impact is greatest where the angle of incidence is near normal, and it is common to taper the nose of the radome to a point to alleviate this problem. However, a rounded nose provides better electrical performance.

Low loss plastics materials are available which are suitable for use in the manufacture of complete radomes and which are capable of providing an acceptable level of resistance to rain erosion, but to achieve this it is necessary to use an appreciable thickness of the material, whereupon the resulting radome structure becomes too heavy. Currently therefore, the problem of rain erosion is dealt with by coating the outer surface of the radome with a layer of impact absorbing rubbery material. However, such materials are generally lossy and it is therefore necessary to minimise the thickness of the protective coating. Furthermore, the coating is difficult to apply evenly, and variations in the thickness can result in insertion phase change variations and a degradation of the electrical characteristics of the radome.

It is also known to provide the radome with a pointed metallic tip to avoid rain erosion in this region, but this produces scattering of the incident RF energy.

According to the present invention, we provide a cap for fitting over the nose of an aircraft or missile radome to protect it against rain erosion, the cap being made of a low loss plastics material and having a thickness sufficient to render acceptable protection against rain erosion, and the cap having associated with it an array of metallic elements which, in use, compensates for the insertion phase change of the incident RF radiation caused by its passage through the thickness of the cap.

The array of metallic elements is designed to compensate for the insertion phase change of the RF radiation passing through the cap by inductively matching the impedance of the cap at the frequency of operation, and thus renders the cap substantially invisible to the radiation at the operating frequency.

The array may comprise a grid of thin metal conductors, such as wires, and preferably the array is a bi-directional array which is preferably substantially orthogonal at least across the central region of the cap, thus rendering the cap insensitive to the polarisation of the RF radiation.

The array is preferably embedded in the cap, and for optimum electrical performance the array should be positioned mid-way between the inner and outer surfaces of the cap. However, for practical reasons, it may be preferred to locate the array closer towards, or even on, one or other of the cap surfaces, preferably the inner surface. Furthermore, it may be electrically acceptable in some situations even to locate the array outside the dielectric material of the cap, for example in the outer skin of the radome itself.

As will be appreciated, the present invention makes use of a sufficient thickness (e.g. about 2 millimetres) of a suitable known low loss plastics material to provide a radome with acceptable rain erosion protection where it matters most, i.e., in the nose region where the angles at which hydrometeors impact are close to the normal to the randome surface and where erosion is expected to be severe. It is considered that rain erosion of the rest of the radome surface will be negligible since hydrometeors will make glancing impact only. The weight of the protective nose cap is not likely to be a problem since the size of the cap required to protect the nose region, which is only a relatively small area of the radome, will be correspondingly small, and the problem of insertion phase change is overcome by the provision of the array of metallic elements.

The solution to the problem of rain erosion of plastics radomes proposed by the present invention has the advantages that a standard optimum construction may be used for the radome, and that the degradation in electrical performance of the nose capped radome is less than that of a corresponding radome treated for protection against rain erosion in a conventional manner. Indeed, it may even be possible in some cases to design the array so that, in addition to compensating for the insertion phase change caused by the cap thickness, it improves the electrical performance of the radome itself. Also, since the cap is relatively small, it is relatively easy to manufacture accurately to the required contours, and within reasonable limits the thickness of the cap may be varied to match rain erosion and aerodynamic criteria.Furthermore, the cap will permit operation of the RF equipment at millimetre wavelengths, in contrast to the currently used protective coatings which become electrically thick and unacceptably absorptive at such wavelengths.

An example of a protective cap in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of a conventional aircraft or missile radome having the protective nose cap fitted to it; and Figure 2 is a diagrammatic perspective view of the cap, partly cut away to show its internal construction.

The radome 1 shown in Figure 1 is of conventional plastics construction, such as the sandwich construction described earlier, and has a rounded nose, indicated by the dotted line 2, covered by a close fitting protective cap 3 which is held in place by suitable mechanical means (not shown), or by a thin layer of adhesive. As shown, the cap 3 covers only the nose region where the effects of rain erosion would be most severe.

The cap 3 is made of a low loss dielectric plastics material, such as polyether-etherketone, chosen chiefly for its rain erosion resistant qualities, and the thickness of the cap is dependent upon the plastics material from which it is made and the degree of rain erosion protection which the cap is to provide.

As shown in Figure 2, the cap 3 has embedded within it a metal wire lattice network 4 which extends over the whole area of the cap and which effectively divides the plastics material of the cap into inner and outer dielectric layers 5 and 6 respectively. In this example the layers 5 and 6 are of equal thickness and the dimensions of the network 4 are such as to match inductively the impedance of the cap at a particular operating frequency. In practice, however, the network 4 should render the cap 3 operative acceptably over a frequency bandwidth of about 20%.

Claims

1. A cap for fitting over the nose of an aircraft or missile radome to protect it against rain erosion, the cap being made of a low loss plastics material and having a thickness sufficient to render acceptable protection against rain erosion, and the cap having associated with it an array of metallic elements which, in use, compensates for the insertion phase change of the incident RF radiation caused by its passage through the thickness of the cap.

2. A cap according to claim 1, in which the array comprises a grid of thin metal conductors.

3. A cap according to claim 1 or claim 2, in which the array is a bi-directional array which is sub stantially orthogonal at least across the central region of the cap.

4. A cap according to any one of the preceding claims, in which the array is embedded within the cap.

5. A cap according to claim 4, in which the array is embedded mid-way between the inner and outer surfaces of the cap.

6. A cap according to any one of claims 1 to 4, in which the array is located on or near the inner surface of the cap.

7. A cap according to any one of the preceding claims, in which the plastics material of the cap is polyether-etherketone.

8. A cap according to claim 1, substantially as ,described with reference to the accompanying drawings.

9. An aircraft or missile radome having the end of its nose covered by a cap according to any one of the preceding claims.

10. An aircraft or missile radome having the end of its nose covered by a cap to protect the nose against rain erosion, the cap being made of a low loss plastics material and having a thickness sufficient to render acceptable protection against rain erosion, and the nose capped radome including an array of metallic elements, which, in use, compensates for the insertion phase change of the incident RF radiation caused by its passage through the thickness of the cap.