Abstract:
The nacelle drag reduction device comprises a substantially circular and axis symmetrical external airfoil concentric with a aft section of the nacelle and located outside a propulsive jet zone defined behind the engine when operating, the airfoil being positioned at a location providing a maximum streamline angle with reference to the main axis of the engine and a highest streamline curvature.

Description:
TECHNICAL FIELD 
   The invention relates to a nacelle drag reduction device, and in particular to a device and a method for enhancing nacelle external flow conditions on a confluent flow turbofan gas turbine engine. 
   BACKGROUND 
   A turbofan gas turbine engine is generally located in an aircraft enclosure which is referred to as the nacelle. The nacelle provides a smooth contour around the gas turbine engine. In a confluent flow turbofan gas turbine engine, the nacelle covers entirely or almost entirely the engine. 
   When operating, the propulsive jet behind a turbofan gas turbine engine forces the nacelle external flow stream lines to bend inwards near the rear thereof. The stream line curvature is associated with a pressure gradient pointing away from the nacelle, such that the aft section of the nacelle is subjected to a low pressure which increases nacelle drag when the aircraft is moving. The jet induced nacelle drag is somewhat directly proportional to the jet core flow rate and the mixing intensity at the jet boundary behind the engine. Nacelle drag can also increase due to noise reduction measures such as chevrons or lobed nozzles, which increase the entrainment rate of the ambient fluid around the nacelle. 
   Overall, it was desirable to provide a way to mitigate the nacelle drag resulting from the ambient fluid entrainment by the propulsive jet. 
   SUMMARY 
   In one aspect, the present concept provides a nacelle drag reduction device for a confluent flow nacelle of a turbofan gas turbine engine having a main axis, the device comprising a substantially circular and axis symmetrical external airfoil concentric with a aft section of the nacelle and located outside a propulsive jet zone defined behind the engine when operating, the airfoil being positioned at a location providing a maximum streamline angle with reference to the main axis of the engine and a highest streamline curvature. 
   In another aspect, the present concept provides a method of enhancing external flow conditions around a nacelle of an aircraft-mounted confluent flow gas turbine engine, the method comprising: operating the engine and creating an ambient fluid entrainment behind the engine; and deflecting ambient air entrained around the nacelle to reduce jet induced nacelle drag, the ambient air being deflected upstream the propulsive jet zone. 
   Further details of these and other aspects of the nacelle drag reducing device and method will be apparent from the detailed description and figures included below. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     For a better understanding and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying figures, in which: 
       FIG. 1  schematically shows an example of a prior art generic gas turbine engine to illustrate an example of a general environment around which the nacelle drag reduction device can be used; 
       FIG. 2  is a schematic cross-sectional view of an example of a nacelle drag reduction device; and 
       FIG. 3  is a schematic cross-sectional view showing the nacelle of  FIG. 2  without the device. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a confluent flow turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. The engine  10  is located inside a nacelle  20 . In a confluent flow turbofan engine, such as engine  10 , the nacelle  20  at least covers a significant portion of the aft section of the engine  10 . In some designs, it may cover the entire aft section. 
   Referring now to  FIG. 2 , there is shown a schematic example of the upper half of an aft section of the nacelle  20 , in a longitudinal cross section, on which is provided a nacelle drag reduction device  22 . The nacelle drag reduction device  22  comprises a hypercritical airfoil  24  which is substantially circular and axis symmetrical with reference to the central axis  26  of the aft section of the nacelle  20 . The airfoil  24  is concentric with the aft section of the nacelle  20  and is located outside or adjacent to the propulsive jet zone  30  defined behind the engine to avoid excessive flutter. The propulsive jet zone  30  is delimited by a boundary, hereafter called the jet boundary  32 . 
   Because of the relatively high momentum of the gases at the outlet of the engine, the ambient air surrounding the engine is entrained and thereby accelerated near the outer surface of the nacelle  20 . 
     FIG. 3  shows an example of the upper half of a nacelle  20 ′ without the nacelle drag reduction device. As can be seen, the nacelle rear external flow stream lines bend inwards. The aft section  20   a ′ of the nacelle  20 ′ is then subjected to a low pressure which increases nacelle drag. The jet induced nacelle drag is somewhat directly proportional to the jet core flow rate and the mixing intensity at the jet boundary  32 ′ behind the engine. 
   Referring back to  FIG. 2 , the airfoil  24  deflects ambient air entrained around the nacelle  20  to reduce the jet induced nacelle drag. The airfoil  24  is supported around the nacelle  20  by mean of narrow supports  40  located at various locations around the circumference of the nacelle  20 . The airfoil  24  is configured and disposed to improve the nacelle external flow conditions. The exact shape, angle of attack and configuration of the airfoil  24  will vary in accordance with the specific operational parameters of the gas turbine engine. 
   The optimal position of the airfoil relative to the nacelle, under any conditions, is at the location where the flow streamlines have the highest deflection angle relative to the engine axis and the highest curvature. 
   In use, operating the engine creates a propulsive jet zone behind it. The airfoil  24  is provided to deflect ambient air entrained around the nacelle  20 . The airfoil  24  is designed to increase the pressure on the aft section of the nacelle  20 , by reducing or even changing the direction of the stream line curvature at distances at least comparable to the airfoil chord. The airfoil lift and drag give a forward pointing component while the radial component is cancelled due to symmetry. 
   The operation of the nacelle drag reduction device  20  can be optimized using one or more air circulation control devices normally encountered on aircraft wings such as trailing edge flaps, leading edge slots, blown flaps, Coanda effect leading and trailing edge jets, plasma actuators, jet actuators and shape control actuators, all of which are generically illustrated in  FIG. 2  with the block diagram  42 . These devices  42  may work intermittently, continuously or in a periodic manner in function of the flight speed and engine thrust settings. If desired, the circulation control devices  42  can be used asymmetrically for the purpose of thrust vectoring. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that other changes may also be made to the embodiments described without departing from the scope of the invention disclosed as defined by the appended claims. For instance, the present invention is not limited to a nacelle drag reduction device including an airfoil as illustrated in  FIG. 2 . Other shapes can be used as well. Similarly, the turbofan gas turbine engine may be different from the one shown in  FIG. 1 . If desired, the airfoil may be retractable at high transonic speeds. The airfoil may be provided in different circumferential sections separated by spaces. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.