Abstract:
This trailing edge is of the type with moving chevrons ( 9   a,    9   b,    9   c ). It comprises actuating means able to cause these chevrons ( 9   a,    9   b,    9   c ) to move from a passive position in which they are directed substantially in the direction of the airflow leaving the said engine, into an active position in which at least part of each chevron is inclined with respect to this direction. This trailing edge is notable in that the said active position is derived from the said passive position by pivoting, possibly combined with a translational movement, of at least part of each chevron about an axis (Aa, Ab, Ac) substantially parallel to the said direction.

Description:
TECHNICAL FIELD 
     This invention relates to a trailing edge for an aircraft engine, of the type with moving chevrons. This invention relates also to a nacelle incorporating such a trailing edge. 
     BACKGROUND 
     From the prior art a noise abatement device is known for an aircraft engine, of the type with moving chevrons: these chevrons can be placed on the trailing edge of the exhaust nozzle of the hot gases of a turbojet, and/or on the trailing edge of the outer cowl of the turbojet. 
     These chevrons are moving between a passive cruse position, wherein they extend in the extension of the walls onto which they are mounted, and thus practically do not interfere with the involved airflow and an active position used during takeoff or landing, in which these chevrons are inclined with respect to the walls on which they are mounted, causing blending of hot air leaving the exhaust nozzle of gases with the cold air originating from the secondary flow, and/or the blending of this cold air with the external air. 
     With this blending, it is possible to limit the effect of mutual tearing of the different airflows, and thereby reduce the sound level emitted downstream of the turbojet. 
     A drawback of these moving chevrons of the prior art is that considerable energy needs to be provided to cause them to move from their passive position to their active position, which requires the use of relatively high actuating means. 
     BRIEF SUMMARY 
     Therefore, in particular this invention provides a trailing edge of the type with moving chevrons which does not have these drawbacks. 
     More particularly, the invention provides a trailing edge such as a trailing edge for hot gases exhaust or an aircraft nacelle thrust inverter incorporating a noise abatement device for an aircraft engine of the type with moving chevrons, the trailing edge comprising actuating means causing these chevrons to move from a passive position in which they are directed substantially in the direction of airflow leaving the said engine, to an active position wherein at least a part of each chevron is inclined with respect to this direction, this trailing edge being notable in that the said active position is derived from the said passive position by pivoting, possibly combined with a translation movement of at least a part of each chevron about an axis substantially parallel to the said direction. 
     Due to these characteristics, actuating of the chevrons in view of obtaining a noise abatement is therefore made by rotating by at least a part of each chevron about a direction parallel to the airflow leaving from the turbojet: during this movement, the chevrons have only a weak air intake leaving from the turbojet (and flowing outside the latter), so as that a relatively small amount of energy is sufficient to cause these chevrons to move from their passive position into their active position. 
     Thus, actuating means relatively light are sufficient to implement this change of position. 
     According to other optional characteristics of the trailing edge according to the invention:
         the said active position is derived from the said passive position by a set of pivoting of each chevron about the said axis;   the said active position is derived from the said passive position by the torsional deformation of a part of each chevron about the said axis;   each chevron comprises a fixed part and a swiveling mounted moving position, by appropriately hinged means on this fixed part;   each chevron has an originally twisted shape;   the said actuating means are able to cause the vibration of at least part of each chevron about its active position: this actuating vibration, which can be achieved with little energy given the direction of the chevrons according to the direction of the airflows, causes increasing the blending of these flows, and thus achieve even further noise abatement;   the said actuating means comprise elastic returns of each chevron towards the passive position thereof: these means of elastic return cause to be freed from any means of active actuating for returning the chevrons in their active position, which constitutes a safety vis-à-vis the risks of failure of such means of actuating;   at least one part of each chevron is mounted on a rod equipped with a helical groove cooperating with a fixed pawl: this arrangement causes to achieve in a simple way the rotational movement of the chevrons between their active position and their passive position: in fact, it is sufficient to provide a linear actuator able of pushing the rod of each chevron to cause rotation;   the said trailing edge comprises chevrons of different sizes: this causes adapting at best the trailing edge to any given nacelle, and so to optimize noise abatement;   active positions of some of the said chevrons are obtained by pivoting in the opposite direction with respect to the active positions of other chevrons: this pivoting possibility in the opposite direction provides an increased acoustic effectiveness;   the said chevrons are mounted projecting;   the said chevrons are mounted in inlay;   the said chevrons are distributed only on a part of the circumference of the said trailing edge.       

     This invention also relates to a nacelle for an aircraft engine, notable in that it comprises at least one trailing edge complying with the above. 
     According to other optional characteristics of this nacelle:
         the said chevrons are arranged on the trailing edge of the exhaust nozzle of gases of this nacelle;   the said chevrons are arranged on the trailing edge of the thrust inverter of this nacelle.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of this invention will become evident in the light of the following description and by the examination of the hereby attached figures, wherein: 
         FIG. 1  is a perspective view of a rear part of the nacelle for a turbojet aircraft intended to be equipped with a trailing edge according to the invention, 
         FIG. 2  is a perspective view of three chevrons according to the invention in passive position, 
         FIG. 3  is a view similar to that of  FIG. 2 , the three chevrons being shown in active position 
         FIG. 4  shows in passive position two chevrons made in accordance with an embodiment of the invention 
         FIG. 5  shows in perspective a chevron made by yet another embodiment of the invention, and 
         FIG. 6  shows a side view of a chevron according to the invention and its actuating mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , it can be observed that the trailing edge according to the invention can be arranged for example on the rear edge  1  of a thrust inverter  3  of a nacelle aircraft. 
     This rear edge  1  is located at the boundary between the outside airflow flowing along the nacelle and the cold airflow generated by the turbojet fan, and flowing in a stream of cold air  5 . 
     The chevrons forming the trailing edge according to the invention can be arranged across the entire circumference of the trailing edge  1 , or only a part of this circumference, as symbolized by arrow  7  in  FIG. 1 . 
     Referring to  FIGS. 2 and 3 , it can be observed that the trailing edge according to the invention therefore comprises a plurality of chevrons  9   a ,  9   b ,  9   c , that is to say plates of substantially triangular shape, the acute tips of these triangles being directed towards the rear of the nacelle. 
     According to a characteristic of the invention, chevrons  9   a ,  9   b ,  9   c  are mounted on the trailing edge  1  being able to rotate about respective axes Aa, Ab and Ac substantially parallel to the airflow inside the nacelle  3 . 
     In other words, these axes Aa, Ab and Ac are each substantially parallel to axis A of the nacelle  3 , shown in  FIG. 1 . 
     In its normal cruising position, chevrons  9   a ,  9   b ,  9   c  are arranged as shown in  FIG. 2 , that is to say they are arranged in the extension of the trailing edge  1  of the nacelle, so as to have a minimum intake of airflow flowing through the cold air stream  5  as well as to the outside of this nacelle. 
     In situations of takeoff or landing, where it is important to have an abatement of the noise emissions from the nacelle, each chevron  9   a ,  9   b ,  9   c  is pivoted about its respective axis Aa, Ab and Ac, as shown in  FIG. 3 , so as to prevent the airflow flowing on the one hand inside the cold air stream  5  and on the other hand outside of the nacelle. 
     As shown in  FIG. 3 , the two chevrons,  9   a ,  9   b  can pivot in a first direction, and the third chevron  9   c  can pivot in a second direction opposite the first. 
     It may also be provided, as shown in  FIG. 4 , that all the chevrons do not have the same size, depending on the acoustic optimization needs. 
     It may also be provided, as shown in  FIG. 5 , that the chevrons, instead of projecting downstream of the rear edge  1  of the nacelle, are in fact each arranged in a well  11  of complementary shape, formed within the trailing edge  1 . 
     It can be observed with more precision, by referring to  FIG. 6 , a possible embodiment of actuating means for the chevrons  9 , causing them to move from their passive position to their active position, and vice versa. 
     As it is shown in this  FIG. 6 , there can be a linear, electric, electromagnetic or hydraulic actuator, able to exert a thrust effort according to the direction Aa of the chevron  9   a , as indicated by arrow  13 . 
     Chevron  9   a  is mounted on a rod  15  having a helical groove  17  able of cooperating with a fixed pawl  19 . 
     Thus, under the effect of the thrust  13 , the helical groove  17  scrolls on the pawl  19 , causing rotation of the chevron  9   a  along with its movement rearward of the nacelle. 
     Resilient means such as a spring  21  are arranged so that when the thrust force  13  ceases, the chevron  9   a  returns to its passive position, after having pivoted in the opposite direction. 
     It will be observed that it can be advantageously provided that the actuating means causing to exert the thrust  13  are endowed with a vibration capacity, thus causing the chevrons to vibrate about their active position. 
     The operation method and the advantages of the trailing edge which have been hereby described, result directly from the foregoing. 
     In the phase of landing or takeoff, that is to say when it is important to have a significant abatement of the noise emitted by the turbojet of an aircraft, the chevrons are caused to move from their passive position, as shown in  FIG. 2 , to their active position, as shown in  FIG. 3 , wherein these chevrons interfere with the airflow flowing on both sides of the wall on the trailing edge  1  onto which they are mounted. 
     This interference with these airflows can promote blending of these two airflows, and thus limit the effects of mutual tear of these airflows, sources of very strong acoustic waves. 
     Both in their passive position and in their active position, the chevrons extend according to the direction of airflows, so that only a little energy is needed to cause them to move from one position to the other of these positions: low-dimensioned actuator means can therefore be used, such as electrical actuators, available on the market. 
     The weight problem inherent to the use of these actuators may thereby be reduced. 
     The presence of elastic return means  21  causes the return of the chevrons to their passive position even when the actuator means fail, which constitutes a safety feature. 
     Of course, this invention is not limited to the embodiment described and shown above, provided as a simple non limiting example: the number of chevrons, their circumferential distribution, their direction of rotation, their respective sizes, as well as their installation location (on the rear edge of a thrust inverter or on the rear edge of an exhaust nozzle of hot gases) can indeed vary considerably while staying within the framework of this invention. 
     This is also how an embodiment could be considered wherein each chevron would comprise a solidly mounted part onto the trailing edge  1 , and a moving part mounted in rotation, by appropriate hinge means, onto this fixed part. 
     This is still how it could be considered that each chevron is formed in a deformable material, the movement from the passive position to the active position being therefore effected by torsional deformation of the chevron by appropriate actuating means. 
     This is still how it could be considered that each chevron has an originally slightly twisted shape, so as to increase the penetration of the chevron in the related airflow when driven into rotation. 
     This is still how the teachings of this invention could be extended to other trailing edges other than those of a turbojet nacelle, such as those of wings or the tail group of an aircraft.