RU2369530C1 - Device for reducing effective scattering area of aircraft engine channel - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: device is a metallic shield with regular openings, placed in front of engine turbine blades, forming a double-wall with the surface of the channel and held by spacers. Inside the shield and coaxially with it, there is a pair of identical single-thread wire cylindrical spirals with different winding directions. The median values of effective scattering area are reduced (on probability level of 0.5) in the sector of angles of location 0±60° relative the normal to the opening of the channel from 2 dB to 16 dB in the wavelength range from centimetres to decimetres.

EFFECT: reduced effective scattering area of the channel of aircraft engine.

24 dwg

Description

The invention relates to the field of radar technology and can be used to reduce the effective scattering area (EPR) of the cavity of the channel of the aircraft engine: an air intake or nozzle.

As the results of experimental studies of the EPR of aircraft show (Radar signature of airplanes (based on open foreign press). TsAGI review, No. 665. 1986, p.20.), In the centimeter and decimeter wavelength ranges in the front and rear hemispheres the main contribution to their EPR are reflected from the channels of the air intakes and nozzles of the propulsion systems.

Currently, to increase the survival rate of combat aircraft, technologies are being introduced that ensure their low radar visibility. First of all, designers seek to reduce reflections from propulsion systems (Aviation Week & Space Technology, 03.19.2001, v. 154, No. 12, p.90-93.), Developing new methods and devices for reducing the EPR of air intakes and nozzles (Bad A. P., Shabonov DS Radar reflectors and their application. "Foreign Radio Electronics", No. 8. 1992. S. 85.). The closest technical solution to the proposed device is a device that implements a method of reducing the EPR of the cavity of the channel of the engine of the aircraft, based on diffuse energy dissipation without its absorption (Method of reducing the effective area of the channel scattering. US Patent No. 4148032. 1979, NKI 343-18A (prototype )). The application of this method is justified in cases where the specific ESR of the cavity (i.e., the ratio of the ESR to the area of the inlet) exceeds 2. In practice, this value in some cases can reach 20. To achieve the effect of reducing the specific ESR of the cavity, special wall designs are used that , for example, can be double metal with a variety of holes. A significant advantage of the method is the high strength and heat resistance of the walls, which is especially important for using the device in a nozzle. The design of the device in which diffuse scattering is achieved is shown in FIG. The device for reducing the effective dispersion area of the cavity of the channel 1 of the engine 2 of the aircraft is a metal screen 4 with regular holes 5 located in front of the blades 3 of the engine turbine, forming a double wall with the surface 6 of the channel cavity and held by spacers 7. In the absence of the screen 4, the radio wave enters the channel cavity 1, one or more times is reflected from the surface 6 of the cavity of the channel and the blades 3 of the engine turbine and leaves in the direction of the source of the radio wave. The presence of a metal screen 4 with regular openings 5 causes the radio wave to experience diffuse re-reflections in different directions, thereby weakening the radio wave extending in the opposite direction. Options for constructing screens are shown in figure 2.

However, as shown by the results of experimental studies (Test Act ...), this device has a number of the following disadvantages:

a screen 4 with regular holes 5 provides diffuse scattering in a narrow range of radio wavelengths, where the dimensions of the holes (inhomogeneities) are comparable with the wavelength;

with normal incidence of the radio wave towards the opening of the cavity, when the back reflection from the cavity of the engine channel 1 is maximum, the “positive” effect of the screen 4 with regular openings is less pronounced (less re-reflections).

These shortcomings significantly reduce the potential capabilities of the device, and also limit its use for a wide range of radio wavelengths and location angles.

The objective of the present invention is to reduce the EPR of the cavity of the channel of the engine of the aircraft by increasing the diffuse scattering inside it for a wide range of radio wavelengths and location angles.

The problem is solved due to the fact that at least one pair of identical single-starting cylindrical spirals 8 with different winding direction (Fig. 3) located from each other and from the engine turbine blades at a distance k and held by spacers is placed inside the screen 4 coaxially with it, while the parameters of the spirals are selected from the relations:

k≈0.85λ _cp , S≥0.5λ _cp , n = 6-8, d≈ (0.06-0.08) λ _cp ,

where k is the distance between the spirals;

S is the distance between the centers of adjacent turns of the spiral (step of the spiral);

λ _cf - the average wavelength of the range of radio emissions;

n is the number of turns of the spiral;

d is the diameter of the wire of the spiral.

The use of single-shot cylindrical wire spirals (PCS) is as follows. It is known that the PCB is the basis for the design of a spiral antenna belonging to the class of traveling wave antennas (G. Eisenberg and other VHF antennas. T.2. - M .: Communication. 1977. P.239). The advantage of such antennas is their broadband and the ability to work with both circular and linear polarization of the field. In this case, the turns of the PCBs do not have to have a round shape. Their shape can be any, for example, in the form of squares or polygons (Antennas and microwave devices. Calculation and design of antenna arrays and their radiating elements. Edited by DI Voskresensky. - M .: Sov. Radio. 1972. P.257) .

The PCB as a passive radar reflector, like any other receiving antenna, retains its inherent qualities to a greater extent. In this case, the radio wave falls on the PCB, as a coordinated load: it is partially absorbed, exciting waves of type T in the spiral wire, partially scattered, repeatedly reflecting from the walls of the metal screen in different directions. The shape of the backward reflection diagram (DOO) of the PCB is determined by the type of wave induced by the currents in the wire of the spiral, which in turn depends on the relationship between the length of the coil (spiral pitch) and the radio wavelength. The most favorable condition for ensuring the conditions of diffuse scattering and attenuation of the incident radio wave is the formation of a spatial DOE in the form of a volume cone (Fig. 4) with an axis coinciding with the axis of the PCB (λ = λ _cf , where λ _cf is the average wavelength of the radio emission range). This condition will be met for a spiral coil length greater than 1.5λ _sr , when in the center-and-center center, in addition to the main type of wave T, waves T ₁ , T ₂ , etc. appear.

It can be approximately assumed that the amplitude of the traveling wave along the spiral wire is constant. Then the directivity pattern F (θ) of the DSP as an antenna can be represented by the product of the directivity pattern of a single turn F ₁ (θ) by the directional pattern of the array of n omnidirectional emitters F _n (θ):

F (θ) = F ₁ (θ) F _n (θ),

where θ is the angle relative to the axis of the spiral;

n is the number of turns of the spiral.

The radiation pattern of a single turn is approximately described by the expression:

F (θ) = cos (θ).

The lattice multiplier is equal to:

where in relation to the average wavelength λ _{sr of the} working range of the PCB we have:

- сдвиг фазы между токами соседних витков ПЦС;Where

- phase shift between the currents of adjacent turns of the PCB;

- замедление фазовой скорости

- slowdown of phase velocity

V ₁ in a spiral;

s = 3 · 10 ⁸ m / s.

Considering that c / V ₁ = 1.22, for calculating the DEC of the PCB we obtain the following approximate expression (Antennas and microwave devices. Calculation and design of antenna arrays and their radiating elements. Edited by D.I. Voskresensky. - M .: Sov. Radio. 1972. S. 248), in which it is necessary to substitute the number of turns n, the pitch of the spiral S and the length of the coil L of the spiral (L> 1.5λ _sr ):

The width of the directional pattern of the half-power half-wave center, expressed in degrees for this case, is determined from the expression:

It was experimentally established that the optimal geometric parameters of a cylindrical spiral antenna fed from the base side are the following (Glagolevsky V.G., Shishov Yu.A. Antennas of radar stations. Military Publishing House. - M .: 1977. S.99):

k≈0.85λ _cp , S≥0.5λ _cp , n = 6-8, d≈ (0.06-0.08) λ _cp ,

where k is the distance between the spirals with different winding directions;

S is the distance between the centers of adjacent turns of the spiral (step of the spiral);

λ _cp is the average wavelength of the range of radio emissions;

n is the number of turns of the spiral;

d is the diameter of the wire of the spiral.

For DSS, the width of the DOO will vary from 50 to 90 °. A relatively wide directivity pattern (DOE) and a relatively high level of side lobes, of the order of 18 dB, provide the PCB with additional scattering and re-reflection of the incident field in a closed volume of the channel cavity.

However, it is known (Antennas and microwave devices. Calculation and design of antenna arrays and their radiating elements. Edited by DI Voskresensky. - M.: Sov. Radio. 1972. P.253) that in complex antenna systems as elements of the lattice with small interconnections also use spirals. So, the decoupling coefficient between spirals having the same winding direction, with a distance between turns of S≥0.5λ _cp, exceeds 15 dB. Spirals with the opposite direction of winding are decoupled among themselves by 40 dB. A spiral antenna having two oppositely directed windings creates two counterpropagating waves with circular polarization. In this case, a linearly polarized wave is formed in the far zone, the direction of polarization of which changes due to a phase shift between the currents in both windings due to their spatial separation by a distance equal to k≈0.85λ _cp . The ability of a spiral to receive and radiate a field with only one direction of rotation of polarization determined the order of sequential placement of two identical spirals with different directions of winding. In this case, the fact that the field radiated by the spiral, when reflected from a conductive surface, such as, for example, a turbine blade of an engine, takes into account the opposite rotation of the polarization and cannot be taken back by the same spiral, was taken into account.

In the case when λ <λ _{cf, the} PCB is a source of additional diffuse scattering (Fig. 5) due to the spiral-shaped volume diffraction grating of equal turns with a wire diameter d≈ (0.06-0.08) λ _cf and period (step) S≥0.5 λ _cf.

To estimate the maximum values of the EPR during reflection (re-reflection) by turns of the radio wave, we apply the method of physical optics. It is obvious that the shape of the PCB turns will depend on the shape of the cross section of the walls of the metal screen. By analogy with the turns, individual PCBs can be considered as independent identical reflectors arranged in a linear equidistant lattice with a period (step) S (Kovalev S.V., Nesterov S.M., Skorodumov I.A. // RE. 1995. T. 40. No. 9. P.1346). Then, the DOO of such a lattice, provided that λ <0.5S, in the plane coinciding with the electric field vector when the angle θ changes due to interference of reflections from individual n turns, will be a multilobe structure, which, in turn, increases the diffuse effect scattering inside the cavity of the channel with a metal screen. In this case, the maximums of the petals in the DOE, where the field reflected from the turns will be in-phase, are formed in the angular directions θ, based on the expression:

where p = 1, 2, 3, ...

The maximum value of the EPR of the interference lobes σ _l will be determined from the expression:

σ _l = σ _i · n ² ,

where σ _l is the EPR value of the i-th turn;

n is the number of turns of the spiral.

Obviously, when a radio wave is incident at an angle θ ≠ 0 ° to the plane of an individual turn, the value of its EPR (σ) will be determined from the expression (Crispin, Muffett. Estimation of the radar cross section of simple bodies. TIIER, 1965, v. 53, No. 8. S.960-975):

where d is the diameter of the wire of the spiral;

D is the diameter of the coil of the PCC (figure 3).

For λ> λ _{cf, the} metal screen with pairs of periodic DSPs is an antenna cylindrical continuous decelerating structure with a period (step) S≤0.1λ, which forms a radiation field in the direction of the turbine blades of the engine of one or more traveling waves (Fig.6). With the inevitable deceleration of such waves, the amount of loss (attenuation) in the spiral structure is inversely proportional to the wavelength to the degree 3/2 (Antennas and microwave devices. Calculation and design of antenna arrays and their radiating elements. Edited by D. I. Voskresensky. - M .: Sov.Rad. 1972.P.214). At the same time, in a cylindrical continuous retarding structure, each individual PCB from 6-8 turns becomes an independent reflector. Pairs of identical PCBs with different winding directions are elements of a periodic diffraction grating, but with a larger wavelength than wire coils. Separate DSPs, commensurate with the wavelength of radio emission, are isotropic (omnidirectional) reflectors and, by analogy with the previous statements, enhance diffuse scattering in the internal volume of a metal screen already due to interference of reflections from each individual DSP.

The above-described principle of operation of the PCB does not take into account the complexity of the processes and, in particular, the fact that in reality there is a slight reflection of energy from the base of the spiral. In addition, the wave along the PCB propagates both directly along the wire and through the spatial connection between the turns, which creates a more complex picture of the scattered field. These considerations relate to PCS as a reflector and are not strict enough, although they give an idea of the physics of the ongoing process, which, as will be shown below, is in good agreement with the results of experimental studies.

The device operates as follows.

The plane front of the electromagnetic wave at some angle falls on the opening of the cavity of the channel of the engine of the aircraft. The wave passes into the cavity of the channel 1 of the engine 2, is reflected one or more times from the walls of the metal screen 4, and is partially absorbed. The presence inside the screen 4 of at least one pair of identical one-way cylindrical wire spirals 8 with different winding directions, aligned with the incident direction when the radio waves are incident on the DSP, forms a spatial DOO in the form of a volume cone, reinforcing rereflections from the walls of the metal screen 4. Formed in two adjacent DSPs with different In the direction of winding, two counterpropagating waves with circular polarization form a linearly polarized wave in the far zone, which cannot be received by the radar. In the short-wave part of the wavelength range, absorption is predominant due to the enhancement of diffuse scattering and interference of waves on the PCS coils. In the long-wavelength region, the attenuation of a field emitted by one or several traveling waves in a spiral structure along a metal screen and propagating towards the blades of an engine turbine with subsequent repeated reflection from them. As a result, weakening of the reflected radio wave extending in the direction of the radar.

The proposed device was tested in an open measuring range ("Reference radar measuring complex of the open type (ERIC)." Weapons and technologies of Russia. Encyclopedia. XXI century. Air defense and missile defense. Volume IX. - M .: "Weapons and technologies". 2004, p. 385), as evidenced by the "Test Report ...".

The studied sample of the cavity of the channel of the aircraft engine was a hollow metal cylinder 100 cm long and 30 cm in diameter, open on one side. The prototype device included a metal screen with regular round holes with a diameter of 0.75 cm and a period of ~ 1.5 cm, forming a double wall with the surface of the cylinder cavity and held by spacers at a distance of ≈1.3 cm. The proposed device was different in that the metal screen is aligned with it on distance ≈1.6 cm from the surface of the metal screen on the spacers placed three pairs of identical wire cylindrical spirals with different winding directions, located from each other and from the "turbine blades engine "at a distance of k = 2.7 cm, while the parameters of the spiral were selected based on the above relationships and provided that λ _cf = 3.2 cm: the distance between the centers of adjacent turns of the spiral S = 1.6 cm; the number of turns of the spiral n = 7, the diameter of the wire of the spiral d≈2 mm

The essence of the proposed technical solution is illustrated in Fig.7 ... 10.

7 shows the geometry and dimensions of the test sample of the cavity of the channel of the aircraft engine (a hollow metal cylinder open on one side).

On Fig presents a diagram of the measurement of the EPR of the engine sample with the proposed device to reduce the EPR and the device prototype.

Figure 9 shows the EPR diagrams of a sample of the cavity of the channel of the engine of the aircraft with the proposed device to reduce the EPR (a) and the prototype device (b) in the sector of location angles (θ) 0 ± 60 ° relative to the normal to the opening of the cavity for wavelengths: 3.2 cm - e, 4.6 cm - f, 10.1 cm - g, 17 cm - h, 36 cm - m.

Figure 10 shows the integral laws of the EPR distribution (P (σ)) of a sample of the engine channel cavity with the proposed EPR reduction device (a) and the prototype device (b) in the sector of location angles 0 ± 60 ° relative to the normal to the opening of the channel cavity, obtained for wavelengths (e, f, g, h, m).

An analysis of the results shown in Figs. 9 and 10 allows us to conclude that (Test report ...) that the proposed device for reducing the effective scattering area of the cavity of the aircraft engine channel in comparison with the prototype device can reduce the median EPR values (probability level 0.5 ) in the sector of location angles 0 ± 60 ° relative to the normal to the opening of the channel cavity from 2 dB to 16 dB in the wavelength range from centimeters to decimeters (from 3.2 cm to 36 cm). The implementation of the claimed device is not difficult. Obviously, the invention is not limited to the foregoing example of its implementation. Based on its scheme, other options for its implementation may be provided, without going beyond the scope of the subject invention. For example, it is obvious that the use of two-way or similar spirals will improve the masking properties without a significant deterioration in the operational parameters of the device itself.

Claims (1)

A device for reducing the effective area of the scattering of the cavity of the channel of the engine of the aircraft, which is a metal screen located in front of the turbine blades of the engine with regular holes, forming a double wall with the surface of the channel of the channel and held by spacers, characterized in that at least one pair of identical one-way wire cylindrical spirals with different winding directions located from each other and from the turbine blades the engine at a distance k and held by spacers, while the parameters of the spirals are selected from the relations:
k≈0.85λ _cp , S≥0.5λ _cp , n = 6-8, d≈ (0.06-0.08) λ _cp ,
where k is the distance between the spirals;
S is the distance between the centers of adjacent turns of the spiral (step of the spiral);
λ _cf - the average wavelength of the range of radio emissions;
n is the number of turns of the spiral;
d is the diameter of the wire of the spiral.

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