Abstract:
An aircraft comprising an aeroengine and a noise reflective surface, the aeroengine capable of generating a hot exhaust jet and noise characterized in that the reflective surface is profiled to reflect noise from the aeroengine into the hot exhaust jet thereby attenuating reflected noise.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a configuration of a surface of an aircraft, adjacent an aeroengine, for noise attenuation of the aeroengine. 
   BACKGROUND OF THE INVENTION 
   Noise generated by aeroengines is a significant proportion of the overall noise generated by an aircraft. As more stringent environmental noise pollution legislation comes into effect it becomes more important to reduce noise by previously uneconomic airframe configurations. 
   One source of noise pollution from the aircraft is noise generated by the engine, which is then reflected from an aerofoil surface of the aircraft such as a fuselage, tailplane or wing. The engine noise may be either from the engine itself or from the exhaust jet. 
   In the paper, “Model Tests Demonstrating Under-wing Installation effects on Engine Exhaust Noise”, AIAA-80-1048, Way, D. J. &amp; Turner, B. A., 1980, it is recited that reflection of the jet noise from the wing under-surface is evident, but is less than predicted. The reduced reflected noise is believed to arise principally from attenuation by the exhaust jet as the reflected noise passes through the jet exhaust. 
   Furthermore, in “Wing Effect on Jet Noise Propagations” AIAA-80-1047, 1980, Wang, M. E. acknowledges that the engine noise is reflected by the underside of the wing and part of which is then refracted and attenuated by the jet exhaust. Wang proposes a number of measures to reduce noise enhancement due to wing effects, firstly the jet engine should be positioned so that the major source distribution of the jet noise would be downstream of the trailing edge of the wing and secondly surface treatment of the underside of the wing. However, positioning the engine toward the trailing edge of the wing to avoid reflected noise would cause the engine to receive undesirable airflow from the underside of the wing and suffer an interference drag penalty. Furthermore, an engine mounted rearward of the wing trailing edge would need to be sufficiently far back that in the event of a rotor burst, fragment trajectories would not pass through the wing. Surface treatment of an aerofoil surface would cause a negative impact on the aerodynamic performance of the aircraft. 
   SUMMARY OF THE INVENTION 
   Therefore it is an object of the present invention to provide an aircraft comprising an aeroengine and a noise reflective surface, the aeroengine capable of generating a hot exhaust jet and noise characterized in that the reflective surface is profiled to reflect noise from the aeroengine into the hot exhaust jet thereby attenuating reflected noise. 
   Preferably, the reflective surface comprises a substantially arcuate profile with respect to a plane normal to the aeroengine centre line. 
   Preferably, the arcuate profile is generated by a radius from the engine centre-line. Alternatively, the arcuate profile is generated by a radius from below the engine centre-line. 
   Alternatively, the reflective surface comprises a substantially arcuate profile that extends between the leading edge of the wing to the trailing edge of the wing. 
   Alternatively, the noise is generated by the engine. Alternatively, the noise is generated by the mixing of the exhaust jets issuing from the engine and the ambient air. 
   Preferably, the reflective surface is that of a wing, alternatively the reflective surface is that of a fuselage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully described by way of example with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic section of part of a ducted fan gas turbine engine and an aircraft wing in accordance with the present invention; 
       FIG. 2  is a rear view of a wing mounted gas turbine aeroengine. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , and intermediate pressure turbine  18 , a low-pressure turbine  19  and a core nozzle  20 . A core duct  22  is partly defined radially inwardly by a core plug  23  and radially outwardly by the core nozzle  20 . A nacelle  21  or other aircraft architecture generally surrounds the engine  10  and defines the intake  12 . 
   The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  24  to a bypass exhaust nozzle  34  to provide propulsive thrust in the form of a generally annular bypass exhaust jet  31 . The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
   The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust in the form of a generally circular core engine exhaust jet  30 . The high, intermediate and low-pressure turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts  25 ,  26 ,  27 . 
   The fan  13  is surrounded by a structural member in the form of a fan casing  28 , which is supported by an annular array of outlet guide vanes  29 . A pylon  32  extends from an aircraft wing  33  and attaches the engine  10  to the wing  33  in conventional fashion. 
   Where an aero-engine  10  is installed under an aircraft wing  33 , or below a tail-plane assembly, the aerofoil underside surface  35  provides a substantial reflective surface  35 , which increases the noise level below the aircraft. However, the engine&#39;s core exhaust jet  30  partially disperses and attenuates the reflected signal, reducing the reflected noise increase apparent below the aircraft. The dispersion of reflected noise is caused by refraction of the sound passing through the core jet  30 , which has a higher speed than the bypass and ambient streams and is substantially circular in cross section. The attenuation is attributed to unsteadiness in the heated flow impeding the acoustic propagation of sound for noise having a wavelength that is small relative to the diameter of the jet  30 . 
   The present invention comprises profiling the underside of the wing  33 , or other reflective aircraft aerofoil surface  35 , to reflect a greater proportion of the reflected noise into the core jet  30 .  FIG. 2  best shows a suitable profile  36  for focussing reflected noise into the core exhaust jet  30 . The profile  36  is a defined by the arc of a radius R from the engine centre line  11 . Thus any noise generated by the engine or exhaust jet will radiate from the centre-line and be reflected back substantially along the same radial path when looking at  FIG. 2 . 
   Profile  36  is an ideal profile considering only focussing reflective noise into the centre of the core exhaust jet  30 . However, when considering also the aerodynamic performance of the wing  33  a shallower profile  37  is preferable. The shallower profile  37  is still capable of reflecting noise into part of the core exhaust nozzle jet  30  and thus remains beneficial to reducing reflected noise from the underside of the wing  33 . 
   A benefit of the under wing profile  36 ,  37  is that there is an increase gully depth, that is the distance between the engine and the wing. This increased gully depth is advantageous in that there is either less aerodynamic interference drag for a given engine size or a larger engine with an increase fan diameter may be used with no aerodynamic interference drag reduction. 
   The profiling should be such that the incident sound rays emanating from the engine  10  and jet mixing  38 ,  39  noise sources are reflected back towards the hot jet. The resulting concave profiling  36 ,  37  has an axial extent D along the wing chord such that the engine  10  and jet mixing  38 ,  39  sources are reflected back to the hot jet for as much of the wing chord as practicable. In  FIG. 2 , the extent D of the profiled surface  36 ,  37  is substantially from the leading edge  41  of the wing  33  to the trailing edge  42 . The profile extends from the trailing edge  41  as some noise is radiated towards this region and is then reflected towards and into the core exhaust stream  30  as the aircraft and engine travel forward out of the way of the reflected noise. 
   It is not necessary for the profile  36 ,  37  shown in  FIG. 2  to be part circular, merely that the profile is capable of reflecting noise into the core or hot exhaust jet  30 . Thus other arcuate profiles, such as an ellipse or a combination of dihedral and anhedral, are substitutable without departing from the scope of the present invention. 
   As shown in  FIG. 1  the profiled under wing extends rearward to a wing flap  40  of the wing  33 . However, the profile  36 ,  37  extends substantially between the leading edge  41  and the trailing edge  42  of the wing. The profile  36 ,  37  is generally arcuate, in respect of the section shown in  FIG. 1 , and is capable of directing the reflected noise into the core exhaust jet  30 . 
   Considering the air flow in this region, the effects of aircraft motion, the engine and jet noise source distribution as well as other mechanisms affecting the acoustic propagation, the aircraft geometry—for instance, wing dihedral, and the observer position relative to the individual engines on the aircraft, the optimized design may vary from this simplified case. For instance, the radius of the concave surface R may vary both along the axial extent, D, and across the width-wise extent, L. L may also vary along the axial extent D as the radial extent of the core exhaust jet  30  increases. The axial extent 
   For engines mounted under an aircraft tailplane, the underside of the tailplane surface should be profiled over an appropriate extent L to enable reflected noise reduction on the ground over a significant region. The radius R can vary azimuthally and axially along the length of the wing to optimize the hot jet dispersion and attenuation of the acoustic reflection in balance with the aerodynamic and structural considerations. It may be possible to extract aerodynamic advantage from this invention as a result of enabling the engine to tailplane underside spacing to be increased and to vary less with azimuthal angle, than with conventional designs. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.