Abstract:
Disclosed is a method for producing a modified aircraft bypass turbojet engine having reduced sound output, in which the method is based on modifying an initial configuration of a rear portion of the turbojet engine to produce a modified rear portion of the turbojet engine. The modified rear portion includes a modified outer convex rear part shaped with a modified concave inner rear part that delimits an intermediate space beyond an initial cold stream outlet orifice. The intermediate space has a thickness at least equal to the thickness (E) of a sound deadening coating, which is placed in the intermediate space.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for reducing the sound output at the back of an aircraft bypass turbojet engine and to a turbojet engine improved by implementing this method. 
     BACKGROUND OF THE INVENTION 
     It is know that bypass turbojet engines comprise a nacelle defining an air inlet at the front and axially containing a cold stream fan, a central hot stream generator and a fan duct of annular section provided, at the rear, with a jet pipe nozzle for said cold stream, and that, in at least some of these turbojet engines:
         said cold stream jet pipe nozzle is formed by an outer fan cowl and by an inner fan cowl of which the initial rear parts are respectively convex and concave and converge toward one another until they meet to form an initial outlet orifice for the cold stream;   a sound deadening coating of annular section that has to have a preset optimum thickness in order effectively to deaden the noise generated by said fan and carried along in said cold stream, said coating being borne internally by said inner fan cowl at the location where the distance between said converging parts of said inner and outer fan cowls is at least equal to said optimal thickness of the sound deadening coating;   said hot stream generator is enclosed in an axial engine cowl that has at least approximately the shape of a divergent front conical surface and of a convergent rear conical surface opposing one another on a common base which lies forward of said initial cold stream outlet orifice, the initial jet pipe nozzle throat and the initial cold stream outlet section being delimited between the initial rear part of the inner fan cowl and the rear conical surface of said engine cowl, said rear conical surface comprising, in its rear part, at least one opening which is positioned on the outside with respect to said cold stream initial outlet orifice and which is intended to discharge to the outside a stream of ventilating air bled from said cold stream and introduced into said engine cowl to regulate the temperature of said hot stream generator; and   said fan duct is delimited between said inner fan duct and said engine cowl.       

     In a turbojet engine such as this, the rear part of the cold stream jet pipe nozzle may have noise-deadening characteristics that are not optimal because throughout that part of it in which the distance between the converging rear parts of said inner and outer fan cowls is smaller than said optimal thickness of said noise deadening coating, there is no space to house said coating. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to remedy this disadvantage by allowing a greater area of sound deadening coating to be housed between said convergent rear parts of the inner and outer fan cowls. 
     To this end, according to the invention, starting out from a turbo jet engine initial status, which turbo jet engine comprises inner and outer fan cowl rear parts, a cold stream outlet orifice, a jet pipe nozzle throat and a cold stream outlet section all arranged in the initial way described hereinabove, the method is notable:
         in that, without making any modifications to said axial engine cowl:
           said concave initial rear part of the inner fan cowl is modified:
               by progressively diverting it away from the axis of said turbojet engine and lengthening it rearward beyond said initial cold stream outlet orifice,   then by extending it rearward in the form of a convex rear end part the rear edge of which defines a modified cold stream outlet orifice, the latter orifice being positioned near said opening through which the ventilation air is discharged, but forwards thereof, and   by shaping said convex rear end part in such a way that it, with said rear conical surface of the axial engine cowl, delimits:
                   a modified jet pipe nozzle throat the area of which is equal to that of said initial jet pipe nozzle throat, and   a modified cold stream outlet section the area of which is equal to that of said initial cold stream outlet section; and   
                   
               said convex initial rear part of said outer fan cowl is modified:
               by progressively diverting it away from the axis of said turbojet engine and lengthening it rearward to beyond said initial cold stream outlet orifice,   then by extending it rearward in the form of a concave rear end part the rear edge of which meets said rear edge of said convex rear end part in order jointly to form said modified cold stream outlet orifice, and   by shaping said modified convex rear part in such a way that it, with the modified concave rear part of the inner fan cowl, delimits an intermediate space of which the thickness is, just beyond said initial cold stream outlet orifice, at least equal to said optimum thickness for said sound deadening coating, and   
               
           in that said sound deadening coating is placed in all of said intermediate space.       

     Thus, by virtue of such a transverse expansion and such a lengthening of the cold stream jet pipe nozzle it is possible for the axial length (parallel to the axis of said turbojet engine) of the sound deadening coating that can be installed at the periphery of the fan duct to be increased considerably rearward. This then results in excellent reduction in the noise output by the fan at the back of the turbojet engine. 
     In addition, implementing the method according to the present invention yields the advantageous results that the increase in axial length obtained for the noise deadening coating is greater than the ensuing increase in axial length (distance between the initial and modified cold stream outlet orifices) of the cold stream jet pipe nozzle. Experience has shown that this increase in axial length of the sound deadening coating may be up to 25% greater than the increase in axial length of the cold stream jet pipe nozzle. 
     It must be pointed out that the modification, according to the present invention, to the convex initial rear part of the outer fan cowl leads to the formation of a zone of inflection where it meets the concave rear end part. The variation in curvature that occurs in this zone of inflection needs not to cause an inversion of the pressure gradient, as this would have the effect of causing boundary layer separation in the rear part of the outer fan cowl. To avoid such a disadvantage, steps are taken to ensure that the shape parameter Hi of the zone of inflection remains lower than 1.6. 
     Of course, the present invention additionally relates to a turbojet engine that is improved in accordance with the abovementioned method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the attached drawing will make it easy to understand how the invention may be embodied. In these figures, identical references denote elements that are similar. 
         FIG. 1  is a schematic axial section of a bypass turbojet engine. 
         FIG. 2  is a schematic and partial transversely expanded half-section of the rear part of the initial cold stream jet pipe nozzle of the turbojet engine of  FIG. 1 , the modified rear part being depicted in dotted line. 
         FIG. 3  is a schematic and partial transversely expanded half-section of the rear part of the modified cold stream jet pipe nozzle, said  FIG. 3  being comparable with  FIG. 2  and the initial rear part being depicted therein in dotted line. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The bypass turbojet engine  1  of longitudinal axis L-L depicted in  FIG. 1  comprises a nacelle  2  delimiting an air inlet  3  at the front and axially containing a fan  4  generating the cold stream symbolized by arrows  5 , a central generator  6  generating the hot stream symbolized by arrows  7  and an annular-section fan duct  8  provided with a jet pipe nozzle  9  for said cold stream  5 . 
     As also shown, and on a larger scale, in  FIG. 2 , the cold stream jet pipe nozzle  9  is formed by an outer fan cowl  10  and by an inner fan cowl  11  of which the rear parts  10 R and  11 R, which are respectively convex and concave, converge toward one another toward the rear to form the cold stream  5  outlet orifice  12 . 
     A noise deadening coating  14 , of annular cross section, for example of the known cellular type, is borne internally by the inner fan cowl  11 . In order effectively to deaden toward the rear the noise generated by the fan  4  and carried along in the cold stream  5 , the sound deadening coating  14  has to have an optimum thickness equal to E. As a result, the sound deadening coating  14  cannot be fitted into the annular rear tip  15  of the nacelle  2 , adjacent to the outlet orifice  12  and beginning in the rearward direction at the transverse plane  15 P, in which tip the distance between the convergent parts of the cowls  10  and  11  is less than the thickness E. 
     The hot stream generator  6  is enclosed in an axial engine cowl  16  that has at least approximately the shape of a divergent front conical surface  16 A and of a convergent rear conical surface  16 R which are opposed to one another on a common base  17  which lies forward of the cold stream  5  outlet orifice  12 . 
     The rear part  11 R of the inner fan cowl  11  and the rear conical surface  16 R of the engine cowl  16  between them delimit the cold stream  9  jet pipe nozzle throat  18  and the outlet section  19  for said cold stream  5 , said throat  18  and said outlet section  19  each being formed by a slightly conical annular surface coaxial with the axis L-L of the turbojet engine  1 . 
     The rear conical surface  16 R comprises, to the rear of and on the outside of the cold stream outlet orifice  12 , at least one opening  20  (for example in the form of an annular slot) intended to discharge to the outside a ventilation air stream symbolized by the arrows  21  and bled (in a known way that has not been depicted) from the cold stream  5  and introduced into the engine cowl  16  (again in a known way that has not been depicted) in order to regulate the temperature of said hot stream generator  6 . 
     The fan duct  8  is thus delimited between said inner fan cowl  11  (or the sound deadening coating  14 ) and said engine cowl  16 . According to the present invention, in order to be able to increase the length of the sound deadening coating  14 , of optimum thickness E, rearward parallel to the axis L-L of the turbojet engine  1  and thus reduce the noise at the rear of said turbojet engine without thereby detracting from engine performance:
         no modification is made to the engine cowl  16  of the hot stream generator  6  but, as illustrated by  FIG. 2 :
           said concave initial rear part  11 R of the inner fan cowl  11  is modified:
               by progressively diverting it away from the axis L-L of said turbojet engine and lengthening it rearward beyond said initial cold stream outlet orifice  12  (see dotted line  11 RM),   then by extending it rearward in the form of a convex rear end part  22  the rear edge of which defines a modified cold stream outlet orifice  12 M, the latter orifice  12 M being positioned near said opening  20  through which the ventilation air is discharged, but forwards thereof, and   by shaping said convex rear end part  22  in such a way that it, with said rear conical surface  16 R of the axial engine cowl  16 , delimits:
                   a modified jet pipe nozzle throat  18 M the area of which is equal to that of said initial jet pipe nozzle throat  18 , and   a modified cold stream outlet section  19 M the area of which is equal to that of said initial cold stream outlet section  19 ; and in addition   
                   
               said convex initial rear part  10 R of said outer fan cowl  10  is modified:
               by progressively diverting it away from the axis L-L of said turbojet engine and lengthening it rearward to beyond said initial cold stream outlet orifice  12  (see dotted line  10 RM),   then by extending it rearward in the form of a concave rear end part  23  the rear edge of which meets said rear edge of said convex rear end part  22  in order jointly to form said modified cold stream outlet orifice  12 M, and   said modified convex rear part  10 RM is shaped in such a way that it, with the modified concave rear part  11 RM of the inner fan cowl  11 , delimits an intermediate space  24  of which the thickness is, just beyond said initial cold stream outlet orifice  12 , at least equal to said optimum thickness E for said sound deadening coating  14 , and   
               
           said sound deadening coating  14  is placed in all of said intermediate space  24 , as far as the plane  24 P beyond which, rearward, the thickness of said space becomes smaller than the optimum thickness E for the coating  14  (see also  FIG. 3 ).       

     Thus, the sound deadening coating  14  can extend as far as the transverse plane  24 P positioned to the rear of the initial cold stream outlet orifice  12 . 
     In  FIG. 3 , in which the rear part  9 RM of the jet pipe nozzle  9 , modified as indicated hereinabove, has been depicted in solid line, with the outline of the initial jet pipe nozzle  9  indicated in dotted line, it may be seen that the lengthening ΔL of the axial length of the coating  14  thus obtained exceeds the lengthening Δl of the jet pipe nozzle in the rearward direction. 
     It will be noted that, where the modified convex part  10 RM and the concave rear end part  23  meet, a profile of inflection  25  is formed on the outer fan cowl. This profile of inflection  25  is additionally shaped in such a way as to cause no boundary layer separation. To do this, the shape parameter Hi of the profile of inflection  25  is chosen to be equal to 1.6 at most.