Abstract:
A system for cooling a transmission of a hover-capable aircraft, the system having: a stator; a heat exchanger connectable thermally to the transmission; a fan for creating a current of a first heat-carrying fluid from the heat exchanger to the fan itself, to remove heat from the heat exchanger; a rotary member, which rotates about an axis to rotate an impeller of the fan about the axis; and a bearing supporting the rotary member for rotation about the axis; the system also having cooling means for cooling the bearing, and in turn having conducting means for conducting a current of a second heat-carrying fluid along a path from an outside environment, external to the bearing, to and to cool the bearing itself.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to European Patent Application. No. 12425158.8, filed Sep. 28, 2012, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a system for cooling a hover-capable aircraft transmission. 
     The present invention also relates to a method of cooling a hover-capable aircraft, e.g. helicopter, transmission. 
     BACKGROUND OF THE INVENTION 
     As is known, helicopters are normally equipped with a number of transmissions for transmitting power from one or more turbines to the main and/or tail rotor, and/or to various accessory devices, e.g. for powering operation of on-board instruments. 
     Lubricating fluid, typically oil, is circulated in known manner inside the transmission to both lubricate and cool the moving parts. 
     For effective lubrication and cooling, the lubricating fluid circulating inside the transmissions must be cooled. 
     So, helicopters are equipped with cooling systems substantially comprising:
         a heat exchanger for exchanging heat between the transmission oil and the air circulating inside the cooling system; and   a fan for creating airflow from the heat exchanger to the fan itself.       

     More specifically, the airflow draws heat from the heat exchanger, and hence the transmission, and flows over the fan at a temperature of about 125° C. 
     The cooling systems also comprise:
         a casing;   a shaft connected to a drive member to rotate the fan; and   one or more bearings supporting the shaft with respect to the casing.       

     The hot airflow from the heat exchanger to the fan heats the area around the bearings, thus reducing their working life and dependability. 
     A need is therefore felt within the industry for a system of cooling helicopter transmissions without impairing the working life and dependability of the fan shaft bearings. 
     Known helicopter transmission cooling systems are described in GB 591,982 and KR-A-20100109717. 
     EP-A-2409919 discloses a system for cooling a transmission of an aircraft, comprising a stator, a fan for creating a current of a heat-carrying fluid towards the heat-exchanger, a shaft rotating about an axis to rotate an impeller of the fan, and a bearing supporting the shaft in rotation about the axis and with respect to the stator. 
     Due to the fact that the fan creates a current of the heat-carrying fluid directed towards the heat-exchanger, the bearing of EP-A-2409919 does not run a substantial risk of overheating. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a helicopter transmission cooling system designed to meet the above demand in a straightforward, low-cost manner. 
     According to the present invention, there is provided a system for cooling a transmission of a hover-capable aircraft, according to claim  1 . 
     The present invention also relates to a method of cooling a transmission of a hover-capable aircraft, according to claim  10 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  shows a view in perspective of a helicopter comprising a cooling system in accordance with the present invention; 
         FIG. 2  shows an axial section of the  FIG. 1  cooling system; 
         FIG. 3  shows the  FIG. 1  cooling system with parts removed for clarity. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  1  in  FIG. 1  indicates a helicopter comprising two turbines  2 , a main rotor  4 , and a tail rotor  5 . 
     Helicopter  1  also comprises a number of secondary transmissions  6  for transmitting power from one of turbines  2  to respective known accessory devices (not shown), e.g. for powering respective on-board equipment. 
     One of transmissions  6  is of the type described in Patent Application 05425470.1. 
     Helicopter  1  also comprises a cooling system  7  for cooling the lubricating fluid circulating inside transmission  6 . 
     System  7  ( FIGS. 2 and 3 ) substantially comprises:
         a stator  3 ;   a heat exchanger, e.g. a radiator,  8  connected thermally and adjacent to transmission  6 ;   a fan  9 ; and   a shaft  10  rotating about an axis A.       

     Fan  9  is located downstream from heat exchanger  8 , and substantially comprises an impeller  20   a  rotated by shaft  10  about axis A; and a diffuser  20   b  fixed with respect to axis A and located downstream from impeller  20   a.    
     Fan  9  circulates a current P (indicated by the pale arrow in  FIG. 2 ) of a heat-carrying fluid—in the example shown, the hot air inside stator  3 —from heat exchanger  8  to diffuser  20   b  to draw heat from, and cool the lubricating oil of, transmission  6 . 
     In the flow direction of current P, stator  3  substantially comprises:
         a casing  11  fixed by a flange  35  to the fixed part of transmission  6 ;   a tubular body  23  fixed to casing  11  and forming part of diffuser  20   b ; and   an outlet pipe  13  (only shown schematically in  FIGS. 2 and 3 ) connected to tubular body  23  and located on the opposite side of fan  9  to heat exchanger  8 .       

     More specifically, casing  11  comprises:
         a curved portion  33 , which defines an inlet  12  facing heat exchanger  8 , and houses fan  9 ; and   an axial portion  34  projecting from portion  33  and connected by flange  35  to the fixed part of transmission  6 .       

     More specifically, portion  33  is interposed between heat exchanger  8  and tubular body  23  of diffuser  20   b.    
     In the example shown, casing  11  is curved and/or made of aluminium. 
     Shaft  10  is housed partly in portion  33  and partly in portion  34 . 
     Impeller  20   a  substantially comprises:
         a predominantly radial wall  14  fixed to shaft  10 ;   a wall  15  sloping with respect to axis A and extending from both sides of wall  14 ; and   a number of blades  19 , which are spaced angularly about axis A, project radially from wall  15 , on the opposite side to wall  14 , and are separated by an annular gap from a wall of portion  33 , adjacent to diffuser  20   b  and opposite heat exchanger  8 .       

     Blades  19  interact with the hot air inside casing  11  to create a low pressure at inlet  12  and, hence, current P. 
     Inside, impeller  20   a  defines a cavity  29  housing bearing  16 . 
     Being swept by current P, impeller  20   a  heats up during operation of system  7 , and transmits heat to bearing  16 , which thus operates at a temperature of roughly 125° C. 
     In the direction of current P from heat exchanger  8  to diffuser  20   b , impeller  20   a  also comprises:
         an annular inlet section  37  upstream from blades  19  and extending radially between the portion of wall  15  upstream from blades  19 , and the inner wall of portion  33  of casing  11 ; and   an annular outlet section  38  downstream from blades  19  and extending radially between the portion of wall  15  downstream from wall  14 , and the inner wall of portion  33  of casing  11 .       

     In the example shown, the reaction of impeller  20   a  is such that outlet section  38  is at a lower pressure than the outside environment  50 . 
     Fan  9  distributes pressure radially inside cavity  29 . 
     More specifically, when fan  9  is running, the radially innermost area, close to shaft  10 , of cavity  29  is at a lower pressure than inlet section  37  of fan  9 . 
     Diffuser  20   b  substantially comprises:
         an ogival body  21  tapering radially from fan  9 , in the direction away from heat exchanger  8 ;   a number of ribs  22  projecting towards axis A from ogival body  21 ;   tubular body  23  surrounding ogival body  21 ; and   a number of blades  26  spaced angularly about axis A and extending between tubular body  23  and ogival body  21 .       

     Tubular body  23  and ogival body  21  define an inlet section  24  and an outlet section  25  for current P. 
     Inlet section  24  and outlet section  25  are annular with respect to axis A. 
     Inlet section  24  of diffuser  20   b  is defined radially between an axial end  27  of tubular body  23  facing fan  9 , and an end  28  of ogival body  21  facing fan  9 . 
     Outlet section  25  of diffuser  20   b  is defined radially between an end  31 , opposite end  27 , of tubular body  23 , and a corresponding portion  32  of ogival body  21 . 
     In the example shown, tubular body  23  is truncated-cone-shaped, of axis A, and tapers from end  27  to end  31 . 
     Inside, ogival body  21  defines a cavity  30  tapering radially from end  28  in the direction away from impeller  20   a.    
     Cavity  30  is open on the impeller  20   a  side and communicates fluidically with cavity  29 , and is closed on the opposite side to impeller  20   a.    
     Bearing  16  is housed in a portion of cavity  29  radially inwards of wall  15  of impeller  20   a.    
     Bearing  16  is interposed radially between a bushing  17  fitted to shaft  10  and to fan  9 , and a flange  18  fixed to ribs  22  of diffuser  20   b.    
     Bushing  17  is located radially inwards of flange  18  with respect to axis A. 
     More specifically, bearing  16  is located radially inwards of end  28  of ogival body  21 , and is interposed axially between wall  14  of impeller  20   a  and ribs  22 , so the action of fan  9  and diffuser  20   b  places bearing  16  at a lower pressure than the outside environment  50 . 
     System  7  advantageously comprises cooling means for cooling bearing  16 , and which comprise conducting means  40  for conducting a current Q (shown by the bold arrows in  FIG. 2 ) of a second heat-carrying fluid—in particular, ambient-temperature air—along a path from outside environment  50  to bearing  16 , so as to cool bearing  16 . 
     It is important to note that conducting means  40  conduct current Q using only the pressure gradient between outside environment  50  and cavity  30 , i.e. with no need for any powered devices, such as pumps, in addition to impeller  20   a.    
     More specifically, conducting means  40  comprise a cavity  41  formed in shaft  10  and coaxial with axis A; cavity  30  defined by ogival body  21 ; and cavity  29  defined by impeller  20   a.    
     Cavity  41  communicates fluidically with the outside environment through a number of holes  42  in the lateral surface  43  of shaft  10 , and communicates fluidically with cavity  30  defined by diffuser  20   b.    
     More specifically, shaft  10  comprises:
         an open axial end  44  located on the diffuser  20   b  side to fluidically connect cavity  41  to cavity  30 ; and   a closed axial end  47  opposite end  44  and outside stator  3 .       

     Holes  42  are formed through a portion of surface  43  opposite end  44  and housed inside portion  34  of casing  11 . 
     More specifically, holes  42  are equally spaced angularly, and connect outside environment  50  fluidically to cavity  41 . 
     More specifically, holes  42  are located, radially with respect to axis A, over a slot  36  formed through portion  34  of casing  11 , and which connects outside environment  50  fluidically to the inside of portion  34 . 
     For any position of shaft  10  about axis A, current Q therefore flows from outside environment  50  through slot  36  into portion  34 , and from the inside of portion  34  through holes  42  into cavity  41 . 
     Impeller  20   a  and diffuser  20   b  define between them an annular passage  45  for current Q. 
     Passage  45  communicates fluidically with outlet pipe  13  to expel current Q together with current P along outlet pipe  13 . 
     More specifically, passage  45  extends between an annular end  46  of wall  15 , and end  28  of ogival body  21 . 
     More specifically, end  46  defines cavity  30  radially outwards. 
     Passage  45  also fluidically connects cavity  30  and inlet section  24  of diffuser  20   b.    
     The Applicant has observed that, when fan  9  is run, passage  45  is at a lower static pressure than cavities  29  and  30 . 
     When transmission  6  is running, the lubricating oil inside overheats. 
     By virtue of heat exchanger  8 , the air in stator  3  reaches a temperature of about 125° C. 
     Rotation of shaft  10  about axis A rotates impeller  20   a.    
     Rotation of impeller  20   a  creates hot-air current P, which flows from heat exchanger  8  to outlet pipe  13 , and draws heat from heat exchanger  8  and, therefore, from transmission  6 . 
     More specifically, current P flows through inlet section  37  of impeller  20   a , interacts with blades  19 , and flows away from impeller  20   a  along outlet section  38 . 
     Impeller  20   a  is thus immersed in hot-air current P. 
     The reaction of fan  9  lowers the pressure inside inlet section  37  with respect to that of outside environment  50 . 
     The Applicant has observed a pressure difference of roughly a few KPa between outside environment  50  and the area of cavity  29  housing bearing  16 . 
     Fan  9  generates ambient-temperature air current Q. 
     More specifically, the pressure difference draws current Q through slot  36  in portion  34 , and through holes  42  in shaft  10  into cavity  41  inside shaft  10 . 
     From there, the pressure difference draws current Q through end  44  into cavity  30  of diffuser  20   b.    
     The difference in pressure produced by impeller  20   a  first directs current Q (along the path shown in  FIG. 2 ) onto the radially outermost areas of cavity  30  in diffuser  20   b , i.e. in a spinning direction with respect to axis A. 
     Current Q then flows into, and produces a vortex field inside, cavity  29 , and so reaches and cools bearing  16 . 
     Passage  45  being the low static pressure point of cavities  29  and  30 , current Q then flows through passage  45  to inlet section  24  of diffuser  20   b , where it mixes with current P. 
     Finally, currents P and Q interact with fixed blades  26  of diffuser  20   b , and flow through outlet section  25  of diffuser  20   b  into outlet pipe  13 . 
     The advantages of system  7  and the method according to the present invention will be clear from the above description. 
     In particular, current Q cools bearing  16 , thus improving its dependability by preventing it from operating at high temperature, e.g. of about 120° C. 
     Current Q being generated by the static pressure difference between outside environment  50  and cavities  29  and  30 , bearing  16  is cooled using the static pressure difference produced by fan  9 , i.e. with no need for any further drive devices. 
     Current Q flows into cavity  41  in shaft  10 , cavity  30  in diffuser  20   b , and cavity  29  in impeller  20   a , which means it is produced with no need for any additional component parts, over and above the normally existing parts of known cooling systems. 
     Clearly, changes may be made to system  7  and the method as described and illustrated herein without, however, departing from the protective scope of the accompanying Claims.