Abstract:
A propulsion gas exhaust assembly in an aircraft propelled by hot gases produced along an axis of the aircraft by a gas generator is disclosed. The propulsion gas exhaust assembly includes a transition element emerging in two duct elements each communicating with an ejection half-nozzle, wherein each of the two duct elements forms an elbow downstream of the transition element.

Description:
The invention relates to the field of aircraft propulsion by ejection of gas flows, and relates to the exhaust assembly downstream of the gas generator up to the nozzle. 
   BACKGROUND OF THE INVENTION 
   For aircraft propelled by turbojets, with or without pilot, drones, for military applications, one objective is stealth. 
   Stealth is defined in particular in relation to two parameters: the radar cross section (RCS) and the infrared signature (IRS). The RCS is the surface area likely to appear on a radar, taking account of the geometry of the aircraft. The IRS is the heat signature that the aircraft leaves, particularly at its ejection nozzles. 
   DESCRIPTION OF THE PRIOR ART 
   To reduce this heat signature, or even delete it, it is a known practice to mask the infrared radiation emitted by the hot gases exiting the gas generator. For example U.S. Pat. No. 3,693,880 is known that describes a device of this type. It comprises a screen in the shape of a body of revolution placed in the gas exhaust channel while being at a distance from the walls of the latter and held by radial arms. It has an aerodynamic shape guiding the gases in the best way possible along its ovoid profile and its surface is cooled by the colder air carried by the radial arms. The cylindrical duct becomes annular between the screen and the wall of the duct. The flow again becomes cylindrical downstream of the screen to be exhausted through a nozzle. The diameter of the duct becomes larger at this location and follows a profile parallel to that of the screen to conserve a sufficient opening. The maximum diameter of the screen is sufficient to mask the turbine from view from the rear. In addition the downstream portion of the screen has a double wall traversed by the cooling air in order to prevent any heating thereof. 
   The present applicant has furthermore developed a double nozzle, called a bifid nozzle, in which the gas flow exiting from the gas generation means is divided into two flows and guided to two nozzles. The two flows of equal size are ejected in parallel in the axis of the thrust. The advantage of this arrangement is to allow the guidance of the aircraft, particularly in yaw by a control of the two flows either by orientation of their thrust vector or by the delivery rates. Being separated from one another, they are also offset from the axis of the generator. In this manner, in the case where the generator means comprises a turbine, the latter is capable of being invisible from the rear through the opening of the nozzles. The infrared signature is therefore small. 
   SUMMARY OF THE INVENTION 
   The subject of the present invention is an exhaust device whose infrared signature is further reduced relative to the former solution. 
   The invention succeeds in achieving this objective with a propulsion gas exhaust assembly, in an aircraft propelled by hot gases produced along the axis of the latter by a gas generator, comprising a transition element emerging in two duct elements each communicating with an ejection half-nozzle, wherein each of the two duct elements forms an elbow downstream of the transition element in such a manner that the elements inside the duct that lie upstream of the elbow are not visible from the rear. 
   In the present application, the term “elbow” is understood in the following manner. Each duct element, tubular in shape, comprises a first portion guiding the gas flow in a radial direction away from the axis and a second portion downstream of the first, guiding the gas flow in a radial direction toward the axis. Downstream of the elbow, the flow is returned to the axis. 
   Advantageously, the two elbows are symmetrical with respect to each other and more particularly lie in the same plane. The invention applies especially to an aircraft propelled by at least one gas generator consisting of a turbojet. This may be of the single-flow or multiple-flow type. 
   Surprisingly, it has been found that by bending the two gas flows in this manner it was possible to effectively mask not only the disk of the turbine but also the hottest portions downstream of the latter. Furthermore, this concept also makes it possible to produce a transition element minimizing the aerodynamic pressure drops while favoring the mixture of the gases before ejection. 
   Thus, the exhaust assembly comprises a transition element with an upstream cylindrical portion of especially circular cross section communicating toward the downstream with the two ducts. Preferably, the cross section of the transition element changes toward the downstream until forming two adjacent elliptical cross sections. Advantageously, these two ellipses may have a vertical or horizontal major axis. 
   The invention also relates to the guidance capability supplied by this type of exhaust. The solution described in the patent application filed by the present applicant under number FR 0551857 has been applied. Specifically, in the case of a nozzle as intended to be fitted to a military drone, an objective of IRS and RCS stealth is linked with the need for vectored thrust. This leads to designing flat two-dimensional nozzles, which can have an elongation of the order of 5 for IRS and RCS stealth and with a pointed external shape for RCS stealth. 
   A further object of the present invention is the production of an aircraft controlling device, particularly for controlling an aircraft in yaw, that is effective and is associated with a control of the engine delivery rate. 
   This device must be applicable to single-engined or twin-engined aircraft and in particular to drones. 
   This device must be capable of continuously providing a low amplitude vectorization without causing a gas generator performance penalty. 
   It must be capable of providing a major vectored thrust for the needs of controlling the aircraft. 
   These objectives can be met with the exhaust assembly of the invention, which is shaped so as to divide a main propulsion gas flow into a first and a second flow for an ejection into a first and a second half-nozzle and comprising at least one of the following two controlling means: a means of distributing the main flow into each of the two half-nozzles and a means of orienting the thrust vector produced by each of the two half-nozzles. 
   “Half-nozzle” means, in the present application, a gas ejection nozzle that receives a portion of the main flow downstream of the turbine. This term is not associated with a particular shape. Use is made of the two-flow solution to separately control the two half-thrust vectors in modulus and in orientation. 
   Advantageously, at least one of said two means, preferably both, use fluid injection and still preferably the exhaust assembly comprises said two means. This solution has the advantage of being simple and of operating with a reduced number of fluid injection devices, ensuring a high reliability and low cost. However, the means of controlling the two flows may be mechanical. 
   In the first place, said half-nozzles are placed for a yaw orientation of the thrust vector. In this way the absence of a vertical stabilizer is alleviated. 
   According to one variant, said half-nozzles are placed for a pitch or roll control or else the nozzle comprises two pairs of half-nozzles, for example one for the yaw orientation, the other for the pitch orientation. Other arrangements or combinations are possible. 
   According to another feature, the means for controlling the distribution of the flows comprises means for fluid injection at the throat of each of the half-nozzles. More precisely, the gas generator being a turbojet, the fluid injection means are supplied by the air that can be tapped from the compressor of the generator. This solution is particularly advantageous because it allows a balanced operation in all the flight phases. Particularly, a nozzle operation procedure is planned according to which the air is continuously tapped from the generator compressor. 
   According to a variant embodiment, the main flow is generated by two gas generators. In this case, the exhaust assembly preferably comprises only one means of orienting the thrust vector produced by each of the two half-nozzles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is now described in greater detail with reference to the appended drawings in which: 
       FIG. 1  shows in a top view an example of an aircraft to which the invention is applied; 
       FIG. 2  shows in a top view a nozzle of the bifid type as described in patent application FR 05 51 857; 
       FIG. 3  shows, in a top view, an exhaust assembly according to the invention; 
       FIGS. 4 to 6  show the assembly of  FIG. 3  seen respectively from the side, from behind and in rear three-quarter perspective; 
       FIG. 7  shows the shape of the cross section of the exhaust assembly on sections VIIa-VIIa, VIIb-VIIb, VIIc-VIIc and VIId-VIId respectively; 
       FIG. 8  is a schematic representation of the arrangement of the control means of the invention in a half-nozzle; 
       FIG. 9  illustrates the operation of the control means placed at the throat; 
       FIG. 10  illustrates the operation of the control means placed in the divergence of the half-nozzles. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The aircraft  1  represented in the figure is a nonlimiting example. It has a nose  2 , two wings  3  and  4  and is propelled by one or two turbojets that are not visible. It is shaped so as to have the smallest possible RCS and IRS. Its rear portion in particular has no vertical stabilizer and terminates in a point  5  with an appropriate apex angle, for example 40°, to reject the radar waves to infinity. The exhaust assembly  10  participates in this requirement by being bifid. It distributes the main flow exiting the channel  12  at the entrance into two flows in two symmetrical channels  12 A and  12 B that terminate in two half-nozzles  14  and  16  of rectangular section. The channels  12 ,  12 A and  12 B have a shape suitable for ensuring the separation of the flow into two flows but also the transition from a circular or substantially circular section cylindrical shape to a rectangular section shape. Where appropriate, the channels include an additional elbow for masking the turbine. As may be seen in the figures, this masking is already at least partly provided by the separation between the half-nozzles  14  and  16 . 
   According to the present application, the shape of the exhaust assembly is enhanced so as to ensure the masking of the turbine block irrespective of the position of a rear observer. As may be seen in  FIG. 2 , one portion of the bifid nozzle, because of its beveled ejection plane, is visible when viewed from the side. This also reduces its signature. 
   With reference to  FIGS. 3 to 7 , the geometry of the exhaust assembly  20  according to the invention can be seen. 
   This assembly comprises a transition element  21  with a cylindrical upstream portion  21 A, of circular or other cross section. The transition element emerges in a first duct element  22  and a second duct element  23 , these two being parallel. The two duct elements each terminate in a respective half-nozzle  24  and  26 . 
   The upstream portion  21 A communicates directly with the exit of the gas generator (not shown), such as a turbine of a gas turbine engine. As may be seen in  FIG. 7 , its cross section is preferably circular. However, it may deviate therefrom. The shape changes from the entrance  21 A.  FIG. 7  shows two shapes  21 B and  21 C corresponding to intermediate planes of section between the entrance and the separation into two duct elements  22  and  23 . 
   The shape of the transition element progressively changes toward the downstream so as to adopt the contour  21 B consisting of two ellipses, which partially overlap, as may be seen in the plane of section VIIb-VIIb. The two ellipses, which are identical, here have a vertical major axis. On going toward the downstream, they progressively move apart until adopting the outline at  21 C of two ellipses, as may be seen in the plane of section VIIc-VIIc. The separation takes place in the plane of section VIId-VIId. 
   On moving axially downstream, the two ducts each form an elbow. They progressively move apart and deflect the gas flows radially toward the outside until reaching a maximum separation at  22 M and  23 M where the flows become axial. Downstream, they converge on each other, deflecting the flows radially toward the axis until reaching  22 N and  23 N where they are returned to the axis. At this point, the separation between the two ducts is still sufficient to correspond substantially to that of the diameter of the entrance plane  21 A. Each duct terminates in a half-nozzle,  24  and  26  respectively, which diverges downstream of the throat that lies in the plane  22 N- 23 N. Here they have a rectangular cross section, but other shapes are possible. The shape of the cross sections of the ducts  22  and  23  progressively changes until they have the shape of the half-nozzles. The areas are determined according to the requirements of the fluid dynamics. 
   Preferably, the assembly has at least one of the following dimensional relationships:
         L elbow /L channel  is between 0.5 and 0.7;   L int /L ext ≧½;   L int /L channel  close to ⅓;   L ext /L channel ≧½;   L separation /L channel ≦0.3,
 
where:
   L elbow  is the length measured axially from the entrance plane  21 A to the point where the elbow is at its maximum lateral deviation from the axis;   L channel  is the length measured axially from the entrance plane  21 A to the throat of the half-nozzles;   L int  is the width of the elbow measured transversely from the engine axis to the internal wall of the duct, at the point where the duct element is at its maximum departure;   L ext  is the width of the elbow measured from the axis to the external wall of the duct, at the point where the duct element is at its maximum departure; and   L separation  is the length measured along the engine axis from the entrance plane  21 A to the plane of section VIId-VIId.       

   As illustrated by the straight lines D 1  and D 2 , such a geometry allows effective masking of the hot zones of the engine and in particular the zones of the transition elements through which the gas flow passes. These straight lines constitute the limits of visibility of these zones. 
   The means of yaw guidance of the aircraft will now be described with reference to  FIGS. 8 to 10 . In this example, each of the half-nozzles consists of a rectangular throat,  24 C and  26 C respectively, with a high horizontal elongation, width/height ratio, as seen in  FIG. 8 . The elongation of the nozzles may be 2.5. Downstream of the throat, the divergence is formed by two vertical walls. It is short on the external side  24 D E  and  26 D E . The vertical walls on the internal side  24 D I  and  26 D I  are longer. This gives a beveled shape of the downstream edge of the nozzles,  24  and  26 . The top and bottom walls are either parallel with one another or divergent. 
   The assembly is preferably optimized to provide, in the cases with no injection and no vectorization, a minimum transverse thrust of each half-nozzle. Specifically, the latter results in a loss of axial thrust that must be reduced to a minimum. The overall lateral thrust remains zero because of the symmetry of the system. 
   According to a feature of the invention, to provide the guidance of the aircraft  1  without a tail unit, control means are provided by which action is taken on the two flows. These control means may be mechanical or fluidic. 
   The convergent-divergent nozzle, for example  24 , comprises the throat  24 C and downstream the two divergent walls  24 D I  and  24 D E . Here the nozzle comprises a fluid injector  28  placed on a wall at the throat and a fluid injector  29  situated on the wall  24 D I  of the divergence. The injector is preferably situated close to the end of the divergence. 
   In a symmetrical manner, the half-nozzle  26  is fitted with a fluid injector  28  at the throat  26 C and a fluid injector  29  on the wall of divergence  26 D I . 
   The injectors  28  and  29  are advantageously supplied with air tapped from the turbojet compressor that supplies the main flow, as appropriate. 
   Operation is as follows.  FIG. 9  shows by arrows  28 / 24  and  28 / 26  the air injections via the injectors  28 . The yaw moment is created by controlling the distribution of the delivery rate in each of the two half-nozzles  24  and  26  by means of fluid injections at the two throats. The value of the delivery rate is illustrated by the length of the arrow, and here one arrow is longer than the other. According to this example, the half-nozzle  24  receives a strong injected delivery rate  28 / 24 , and consequently sustains a major restriction of the effective section at the throat. Conversely, the half-nozzle  26  receives little or no delivery rate at the throat. The result of this is the creation of an axial thrust differential. The thrust F 1  on the half-nozzle  26  is greater than the thrust F 2  on the half-nozzle  24 . The result of this is a yaw moment. 
   It is observed however that a sudden obstruction of the nozzle would instantaneously create an increase in pressure in the channel and a risk of pumping the compressor. According to a preferred operating mode, a nominal permanent injection is created. This is done at equal delivery rate tapped off in such a way that the generator does not undergo a sudden variation during the mission while regulating the nozzle at total equal effective section at the throat. The thermodynamic cycle of the engine is directly optimized under this constraint of constant tapping. In this manner, the system of regulating the tapped air operates continuously and does not undergo any transitional startup phase. 
   Therefore this operating mode in accordance with the invention provides, with a low impact on the performance of the engine, a vectored thrust that makes it possible to compensate for the absence of cell tail unit, particularly for cruising or slow transitional speeds. 
   The operation of the injection device situated in the divergence of the nozzles  24  and  26  is now described with reference to  FIG. 10 . 
   The injectors  29 , in this embodiment, are preferably placed at the end of the long wall of divergence. By injecting a fluid into the nozzle  24 , the direction of which is represented by the arrow  29 / 24 , a deviation of the thrust vector produced by the nozzle and shown by the arrow F′ 2  is induced. The thrust F′ 1  provided by the half-nozzle  26  remains axial since nothing disrupts its direction. This results in the creation of a yaw moment relative to the center of gravity of the aircraft. This operating mode provides a substantial vectored thrust in order to control the aircraft, to the detriment however of the performance of the generator. This deterioration is however controlled. 
   One embodiment of the invention has been described. However, many variants are possible without departing from the context of the invention. For example, a channel has been shown supplied by a single gas generator. In the case of a twin-engined aircraft, the two half-flows of exhaust are generated by two distinct engines whose regulation is synchronized. Preferably, only the injectors in the divergence are used. 
   Variants of the arrangement and operation of the control means comprise the presence of a single control means. It is possible to operate it at the same time as the other means or separately. 
   According to an embodiment not shown, the nozzles may be of the fluid type with ejector, that is to say a secondary flow emerging in or downstream of the main channel. 
   The control means according to the invention may be combined partly with mechanical means of orienting the flows.