Patent Publication Number: US-8985517-B2

Title: Passive cooling of transmission using mast mounted heat pipes

Description:
BACKGROUND 
     1. Field of the Invention 
     The present application relates generally to the removal of heat from a helicopter gearbox. 
     2. Description of Related Art 
     Aircraft drivetrains include various components that produce and transfer power. For example, engines and gearboxes are common components. Such components generate heat and require lubrication. Excessive levels of heat can cause premature failure and create safety risks. Proper lubrication serves to reduce heat production and assist in heat removal from within moving components. 
     Typically, aircraft use a variety of primary lubrication systems to provide wear protection and control heat transfer within components. Under normal operating conditions, primary lubrication systems provide proper lubrication and heat removal. However, in cases of emergency, primary lubrication systems can fail resulting in excessive wear and failure of components, such as a gearbox or transmission. 
     Aircraft are generally required to maintain manageable flight operations for selected durations of time if the primary lubrication system fails (low-pressure). One method used to satisfy the requirements of manageable flight during an emergency is to increase the amount of lubricant reserves and increase the weight of the lubricant. Another method is to use a secondary lubrication system to operate when the primary lubrication system fails. Although not commonly used, secondary systems typically provide only sufficient lubricant to lubricate moving parts but fail to adequately remove heat. Both methods increase the overall weight of the aircraft and fail to remove adequate amounts of heat. An improved method of controlling heat transfer from an aircraft is required. 
     Heat pipes are a device commonly used to transfer heat. Heat pipes are a transfer mechanism that can transport large quantities of heat with a very small difference in temperature between hot and cold interfaces. However, heat pipes are typically used in static environments and experience design limitations from difficulties in moving a working fluid between a condenser end and an evaporator end. External forces, such as gravitational and centrifugal forces, can hinder performance of the heat pipe. 
     Although great strides have been made in managing heat transfer in aircraft, considerable shortcomings remain. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the application are set forth in the appended claims. However, the application itself, as well as a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a helicopter using a cooling system in the drivetrain according to the preferred embodiment of the present application; 
         FIG. 2  is a tilt rotor aircraft using the cooling system of  FIG. 1 ; 
         FIG. 3  is a perspective view of a transmission used in the drivetrain of  FIG. 1 ; 
         FIG. 4  is an exploded view of selected components of the transmission of  FIG. 3 , including a planetary carrier assembly for use in the cooling system of  FIG. 1 ; 
         FIG. 5  is an exploded view of a drive linkage used to engage the transmission, the drive linkage forming part of the cooling system of  FIG. 1 ; 
         FIG. 6  is a section view of the transmission of  FIG. 3  along the lines VI-VI, the transmission having the planetary carrier assembly and a mandrel assembly used in the cooling system of  FIG. 1 ; 
         FIG. 7  is an isometric view of the mandrel assembly used in the cooling system of  FIG. 1 ; 
         FIG. 8  is a partial section view of the mandrel assembly of  FIG. 7  coupled within an input quill; 
         FIG. 9  is a schematic view of a rotating heat pipe used in the cooling system of  FIG. 1 ; 
         FIG. 10  is a schematic view of an alternative heat pipe with a wick used in the cooling system of  FIG. 1 ; 
         FIG. 11  is an isometric view of the planetary carrier assembly used in the cooling system of  FIG. 1 ; 
         FIGS. 12 and 13  is an isometric view and a partial section view of a supplemental heat exchanger used in the cooling system of  FIG. 1 ; 
         FIGS. 14 and 15  are an isometric view and a partial section view of an alternative embodiment of the supplemental heat exchanger of  FIGS. 12 and 13 ; 
         FIG. 16  is a schematic of a conventional loop heat pipe system; and 
         FIG. 17  is the cooling system according to an alternative embodiment of the present application. 
     
    
    
     While the system and method of the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the application to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the process of the present application as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the preferred embodiment are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     In the specification, reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the present application, the devices, members, apparatuses, etc. described herein may be positioned in any desired orientation. Thus, the use of terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components, respectively, as the device described herein may be oriented in any desired direction. 
     Referring to  FIG. 1  in the drawings, a helicopter  11  according to the present application is illustrated. Helicopter  11  has a fuselage  13  and a main rotor assembly  15 , including main rotor blades  17  and a main rotor mast  18 . Helicopter  11  has a tail rotor assembly  19 , including tail rotor blades  21  and a tail rotor mast  20 . Main rotor blades  17  generally rotate about a vertical axis  16  of main rotor mast  18 . Tail rotor blades  21  generally rotate about a lateral axis  22  of tail rotor mast  20 . Helicopter  11  also includes a cooling system according to the present application for removing heat within the drivetrain of helicopter  11 . The cooling system utilizes heat pipes configured to remove heat from a first medium in communication with the drivetrain to a second medium in communication with the drivetrain. Transmission  23  includes the cooling system of the present application. Transmission  23 , also known as a gearbox, is mechanically coupled between engine  25  and main rotor assembly  15 . It is understood that the cooling system may be used in other locations within helicopter  11 . Furthermore, the cooling system may be used in other applications outside of aircraft. 
     The cooling system of the present application may also be utilized on other types of rotor wing aircraft. Referring now to  FIG. 2  in the drawings, a tilt rotor aircraft  111  according to the present application is illustrated. As is conventional with tilt rotor aircraft, rotor assemblies  113   a  and  113   b  are carried by wings  115   a  and  115   b , and are disposed at end portions  116   a  and  116   b  of wings  115   a  and  115   b , respectively. Tilt rotor assemblies  113   a  and  113   b  include nacelles  120   a  and  120   b , which carry the engines and transmissions of tilt rotor aircraft  111 , as well as, rotor hubs  119   a  and  119   b  on forward ends  121   a  and  121   b  of tilt rotor assemblies  113   a  and  113   b , respectively. 
     Tilt rotor assemblies  113   a  and  113   b  move or rotate relative to wing members  115   a  and  115   b  between a helicopter mode in which tilt rotor assemblies  113   a  and  113   b  are tilted upward, such that tilt rotor aircraft  111  flies like a conventional helicopter; and an airplane mode in which tilt rotor assemblies  113   a  and  113   b  are tilted forward, such that tilt rotor aircraft  111  flies like a conventional propeller driven aircraft. In  FIG. 2 , tilt rotor aircraft  111  is shown in the helicopter mode. As shown in  FIG. 2 , wings  115   a  and  115   b  are coupled to a fuselage  114 . Tilt rotor aircraft  111  also includes a cooling system according to the present application for managing the heat transfer within mechanical components of the aircraft drive train. 
     It is understood that helicopter  11  and tiltrotor aircraft  111  are both aircraft. The term aircraft is not so narrow as to be limited by these types of aircraft. For purposes of this application, aircraft may include any machine supported for flight in the air, by buoyancy, or by the dynamic action of air on its surfaces. Examples of possible aircraft include powered airplanes, gliders, tiltrotors, and rotorcraft such as helicopters and compound helicopters. Therefore the term “aircraft” will relate to rotorcraft  11 , tiltrotor aircraft  111 , and all other forms of aircraft. 
     Referring now also to  FIGS. 3-5  in the drawings, an isometric view of transmission  23 , used within the cooling system of the present application, is illustrated in  FIG. 3 .  FIG. 3  is a representative embodiment of a transmission that may be configured to use the cooling system of the present application. It is understood that other styles and sized transmissions may also be used.  FIG. 4  shows an exploded view of selected components within transmission  23 . Transmission  23  includes an upper housing  26   a  and a lower housing  26   b  that couple together to form a single unitary housing configured to encompass internal components. Internal components may include at least a planetary carrier assembly  27  having planetary pinions  28 , a planetary ring gear  29 , a planetary sun gear  30 , and an input shaft  31 . Although described with specific internal components, it is understood that transmission  23  may function with and utilize any number of selected components other than those depicted and described. For simplicity, other internal components within transmission  23  were not depicted. It is understood that different embodiments of transmission  23  may utilize either a plurality of planetary carrier assemblies  27 , or may refrain from using any planetary carrier assemblies  27  depending on the configuration. 
     Lower housing  26   b , planetary ring  29 , planetary carrier assembly  27  and upper housing  26   a  are each configured to accept main rotor mast  18  along vertical axis  16 . It is understood that vertical axis  16  is not limited in orientation to that of being vertical. Other orientations are possible depending on the type of aircraft and power train configuration. Input shaft  31  is a portion of drive linkage interconnecting engine  25  and transmission  23 . 
     Transmission  23  is configured to receive mechanical energy from engine  25  via a rotating member called an input adapter  37 , as seen in  FIG. 5 . Input adapter  37  is inserted onto input shaft  31  to form an input drive quill  35 . Input drive quill  35  is operably coupled at one end to engine  25  and at the other end within transmission  23 . As engine  25  rotates input adapter  37 , and therefore input drive quill  35 , input shaft  31  rotates within transmission  23 . Transmission  23  is configured to utilize this rotational energy to rotate main rotor shaft  18 . Other members of the drive linkage interconnecting engine  25  and transmission  23  is a driveshaft adapter  39 . A mandrel assembly  33  is inserted within a hollow cavity of input shaft  31 . Mandrel assembly  33  is included within input drive quill  35  and is configured to transfer heat energy generated within transmission  23 . 
     Referring now also to  FIG. 6  in the drawings, a section view of transmission  23  is illustrated. Heat is generated by the gears and bearings within transmission  23  and is conducted through the steel gears and shafts. The gears and bearings surrounding input shaft  31 , namely a nose roller bearing  43 , a spiral bevel gear  45  used to engage the teeth of input shaft  31 , and a triplex bearing  47 , produce a significant amount of the heat within a transmission. Furthermore, the areas surrounding planetary carrier assembly  27  also account for a significant amount of the heat generated within a transmission. Lubrication systems are used to decrease friction and to remove heat from transmission  23 . However, lubrication systems are susceptible to failure, resulting in a transmission operating without lubrication and without the ability to remove heat generated. Internal components can fail if not cooled or lubricated sufficiently by the lubrication system. 
     The cooling system of the present application includes a passive heat removal system that is configured to provide continual heat removal from transmission  23  for a period of time during a “run dry” or emergency condition. A “run dry” condition can exist when the primary pressurized lubrication supply has been terminated (low-pressure) through a system malfunction, battle damage, or the like. During the run dry condition, the cooling system of the present application utilizes one or more heat pipes to provide continual heat removal from components within transmission  23  without active command from a pilot or aircraft control system. The cooling system of the present application is a passive system. 
     Aircraft regulatory agencies, such as the Federal Aviation Administration (FAA) may require that transmission  23  be operable for a requisite period of time after the primary lubrication system has failed. Such a requirement in aircraft transmissions may be referred to as a “run dry” capability requirement. Therefore, aircraft are required to maintain manageable flight operations for selected durations of time if the primary lubrication system fails (low-pressure). The primary lubrication system refers to the lubrication system or systems within the transmission of the aircraft. The cooling system of the present application is configured to maintain manageable flight operations of the aircraft for a selected duration in accordance with the “run dry” capability requirement. For example, the cooling system of the present application is configured to prevent failure of transmission  23  due to heat build-up for a specified time (possibly thirty minutes) after failure of the primary lubrication system. It is understood the time period may be lengthened or shortened. Manageable flight operations refer to a degree of control a pilot has over the flight controls and drivetrain of an aircraft to sufficiently and safely land the aircraft. Transmission  23 , when equipped with the cooling system of the present application, is configured to operate during a loss of lubrication event for the duration of the time period prior to failure of transmission  23 . 
     Transmission  23  uses hollow shafts and gears to transfer power. The shafts and gears typically have one or more hollow sections or bores to reduce weight. Transmission  23 , with the cooling system of the present application, is configured to place uniquely designed, light weight heat pipes into hollow shafts and gears for the purpose of improving heat removal afforded by primary lubrication systems. The cooling system may act in combination with the primary lubrication system or independently, such as in times when the lubrication system fails. To accomplish this, the cooling system includes mandrel assembly  33  and/or planetary carrier assembly  27 , and/or supplemental heat exchangers (see  FIGS. 12-14 ). 
     Referring now also to  FIGS. 7 and 8  in the drawings, mandrel assembly  33  is illustrated. Mandrel assembly  33  includes at least a rotating member or body  50 , one or more channels  54  formed within rotating member  50 , and at least one heat pipe  70  (see  FIG. 9 ) secured within channel  54 . Mandrel assembly  33  is configured to transfer heat energy between two mediums by efficiently utilizing available space within body  50  to house one or more heat pipes. The heat pipes are uniquely oriented with respect to body  50  to permit centrifugal loads to assist in the movement of the working fluid from a condenser end to an evaporator end of the heat pipe. 
       FIG. 7  is an isometric view of mandrel assembly  33 . Mandrel assembly  33  has a first end  51  and a second end  52 . First end  51  engages input shaft  31  and is inserted within transmission  23  during operation. Body  50  maintains a general cylindrical shape of varied diameters, defining a central axis  53 . Body  50  is configured to have one or more channels  54  extending between first end  51  and second end  52 . In the preferred embodiment, channels  54  are symmetrically aligned about central axis  53 . Channels  54  are sequentially spaced about central axis  53  in a radial pattern and are configured to accept one or more heat pipes each. Maintenance of a symmetric pattern assists in maintaining the balance of mandrel assembly  33  during rotation. Although depicted having a total of six channels in  FIG. 7 , it is understood that mandrel assembly  33  may have one or more channels  54 . Where a single channel  54  is used, channel  54  is located coaxial to central axis  53 . 
     As seen in particular with  FIG. 8 , channels  54  are hollow passageways formed within the interior of body  50 . Each channel  54  defines a channel axis  55 . Channel axis  55  and central axis  53  are orientated in a non-coaxial manner, such that channel axis  55  and central axis  53  form an angle α at an intersection  56  adjacent second end  52 . In the preferred embodiment, the portion of channel  54  adjacent second end  52  is closer to central axis  53  than the portion of channel  54  adjacent first end  51 . Other embodiments may switch the misaligned orientation such that intersection  56  of channel axis  55  and central axis  53  is adjacent first end  51 . Although depicted as utilizing linear cylindrical channels  54 , it is understood that other embodiments may permit channels of varied shapes and sizes. In such alternative embodiments, the distance between channel axis  55  and central axis  53  would still diverge along the length of body  50  from one end ( 51 ,  52 ) to the other end ( 51 ,  52 ). 
     The misaligned orientation of channel  54  is important due to how heat pipes function. A typical heat pipe consists of a sealed hollow pipe made of a thermally conductive material, e.g., a thermally conductive metal such as copper or aluminum. The heat pipe contains a relatively small quantity of a “working fluid” or coolant (such as water, ethanol or mercury) with the remainder of the heat pipe being filled with a vapor phase of the working fluid, all other gases being substantially excluded. Heat is transferred from an evaporator end of a heat pipe to an opposing condenser end of the heat pipe by a rapid transition of heat vaporized working fluid from the evaporator end to the condenser end. 
     During operation, heating the evaporator end of the heat pipe will cause the working fluid inside the heat pipe at the evaporator end to turn to vapor, thereby causing a pressure differential in the heat pipe. This pressure difference drives a rapid mass transfer of the heated vaporized working fluid from the evaporator end to the condenser end of the heat pipe where the vapor condenses, thereby releasing its latent heat and heating the condenser end of the heat pipe. The condensed working fluid then flows back to the evaporator end of the heat pipe. 
     In order for a heat pipe to function, the condensed working fluid must be able to travel from the condenser end to the evaporator end of the heat pipe. Uses of heat pipes can be limited as a result of difficulties in moving the condensed working fluid. External forces, such as gravitational forces, can impede or assist in the movement of the condensed fluid. In the case of heat pipes that are vertically-oriented with the evaporator end down, the fluid movement is assisted by the force of gravity, known as a thermosyphon. For this reason, heat pipes can be the longest when vertically oriented with the evaporator end of the heat pipe below the condenser end. The length of a heat pipe will be most limited when the heat pipe is vertically oriented with the evaporator end of the heat pipe above the condenser end. In this orientation, gravity attracts the condensed fluid to the condenser end of the heat pipe rather than the evaporator end. When horizontal, the maximum heat pipe length will be somewhere between the maximum heat pipe lengths in the two vertical orientations. Wicks are used to return the working fluid to the evaporator end by capillary action as a help in overcoming external forces. 
     Centrifugal forces are also external forces that can impede or assist in the movement of the condensed fluid. As opposed to static environments where typically only gravitational forces act against the working fluid, the heat pipes in mandrel assembly  33  are configured to overcome gravitational and centrifugal forces while body  50  rotates about central axis  53 . The diverging misaligned orientation of channels  54  with respect to central axis  53  allow for external forces to be overcome. 
     Referring now also to  FIGS. 9 and 10  in the drawings, a schematic of a rotating heat pipe  70  is illustrated. Heat pipes are highly efficient heat removal devices that can conduct heat much more efficiently than steel. Rotating heat pipe  70  assists in the removal of heat generated by bearings and gears during normal flight operation. The heat removed by the heat pipes  70  during normal aircraft operation reduces the required heat removal capacity of the primary lubrication system. Therefore the oil reserve in the primary lubrication system and the weight associated with that oil can be reduced. In addition, heat pipe  70  removes heat from gears and bearings during a loss of lubrication event (i.e., zero oil pressure) allowing the aircraft transmission to function until the aircraft has landed. 
     Heat generated by the gears and bearings is conducted through the steel gears and shafts to heat pipes, such as rotating heat pipe  70 , located in the bore or hollow of the gear or shaft. The heat pipe moves heat from the gear or shaft to the transmission case or directly to the ambient air around the transmission. 
     Heat pipe  70  is configured to rest within channels  54  of mandrel assembly  33 . Heat pipe  70  has a condenser region  73  and an evaporator region  75 . In operation, heat energy is received in evaporator region  75  which turns the working fluid into a vapor that travels in the direction of arrow B to condenser region  73 . Heat energy is rejected in condenser region  73  turning the working fluid back into a liquid state. The working fluid travels along internal walls  71  back toward evaporator region  75  as noted by arrow A. The internal walls  71  of heat pipe  70  are set at an angle β with respect to pipe axis  77 . As heat pipe  70  rotates about central axis  53  of mandrel assembly  33 , working fluid is forced against internal walls  71 . Angle β and α, independently, are sufficiently sized to permit working fluid to translate along internal walls  71  as a result of centrifugal forces. Heat pipe  70  works whether located in a non-coaxial orientation with central axis  53  or in a coaxial orientation with central axis  53 . 
     The non-coaxial orientation of channels  54  with that of central axis  53  permit other types of heat pipes to operate successfully within mandrel assembly  33 , see  FIG. 10 . For example, heat pipe  80  may be used in place of rotating heat pipe  70 . Heat pipe  80  is similar in form and function to that of heat pipe  70 , except that heat pipe  80  has internal walls  81  that are parallel to heat pipe axis  87 . Heat pipe  80  has a condenser region  83  and an evaporator region  85 . Heat energy is received in evaporator region  85  which turns the working fluid into a vapor that travels in the direction of arrow B to condenser region  83 . Heat energy is rejected in condenser region  83  turning the working fluid back into a liquid state. The working fluid travels along internal walls  81  back toward evaporator region  85  as noted by arrows A. Furthermore, heat pipe  80  also uses an optional wick  89  to aid in the translation of working fluid from condenser region  83  to evaporator region  85 . 
     Heat energy is released from heat pipe  70 , 80  in condenser region  73 . Mandrel assembly  33  is configured to use convective airflow to accelerate the transfer of heat energy and create a heat sink for mandrel assembly  33 . One such method is to rely on induced convection from the spinning of body  50 . However, airflow over condenser region  73  may be limited. A more preferred method is to increase or generate more directed airflow over condenser regions  73 . In generating this convective airflow, a condenser  38  may be used as seen in  FIG. 5 . In an alternative embodiment, mandrel assembly  33  may include condenser  38 . Condenser  38  is one of the drive linkages seen in  FIG. 5  and is located between driveshaft adapter  39  and input adapter  37 . Condenser  38  has a central aperture configured to accept body  50 . Central axis  53  and central aperture are coaxial. Condenser  38  is configured to generate a heat sink for the heat energy by generating a continual flow of air over condenser region  73 . Condenser  38  may use one or more fan blades having a plurality of shapes and designs to produce the flow of air. Examples may include a turbo compressor fan or a radial fin stack. 
     Referring now also to  FIG. 11 , planetary carrier assembly  27  is illustrated. As stated previously, heat generated by the gears and bearings is conducted through the steel gears and shafts to heat pipes located in the bore or hollow of the gears or shafts. Heat pipes remove heat from the gear or shaft to the transmission case or directly to the ambient air around transmission  23 . Heat pipes  70 , 80  have been described previously with respect to mandrel assembly  33 , a rotating shaft. The cooling system of the present application also includes planetary carrier assembly  27  wherein heat pipes are located within the hollows or bores of planetary pinion gears  28 . 
     Planetary carrier assembly  27  generates a large portion of the heat energy associated with transmission  23 . Planetary carrier assembly includes one or more planetary pinions  28 , a housing  95 , a heat exchanger  94 , and a heat pipe  70 , 80 . Planetary carrier assembly  27  is configured to improve heat transfer from internal bearings and improve surface area for convective heat transfer. Planetary pinions  28  are radially aligned in a common plane around a central aperture  91  formed in housing  95 . Each planetary pinion  28  has a pinion bore  92  having a pinion axis  93  (see  FIG. 6 ). Planetary pinions  28  are configured to spin about pinion axis  93 . Heat pipes  70 , 80  are integrated into planetary pinion  28  in a coaxial relationship with pinion axis  93 . Evaporator region  75 , 85  are located within pinion bore  92  and condenser region  73 , 83  are located within heat exchangers  94 . Use of heat exchanger  94  increases the surface area for convective heat transfer. 
     Convective heat exchangers  94  may be located in any number of positions within transmission  23 , however, the present application identifies two locations that work well. Heat exchangers  94  are coupled to housing  95  above or below planetary pinions  28  adjacent a convection area. A convection area is an open relatively unobstructed volume of space within transmission  23 . Upper convection space  96  and lower convection space  97  are convection areas in transmission  23  (see  FIG. 6 ). As depicted in  FIG. 11 , heat exchangers  94  are radially positioned about central aperture  91 , although other positions or orientations within convection spaces  96 , 97  are possible. Heat exchangers  94  increase the surface area of the condenser region  73 , 83  to more efficiently release heat energy. 
     It is understood that either type of heat pipe  70 ,  80  may be used with planetary carrier assembly  27 . Furthermore, other embodiments of the cooling system may include one or more heat pipes  70 , 80  embedded within upper housing  26   a  and/or lower housing  26   b . In one embodiment, the evaporator region  75  of the heat pipes  70 , 80  are within housing  26   a , 26   b  walls adjacent upper and/or lower convection space  96 , 97 ; while the condenser region  73  is in communication with a heat sink external to the housing of transmission  23 . The heat sink could be a supplemental heat exchanger or the ambient air. Such heat pipes are configured to help remove heat from upper and/or lower convective space  96 , 97 . Heat pipes used in the walls of upper and/or lower housing  26   a , 26   b  may include heat pipes  70 , 80  in any other locations. 
     Referring now also to  FIGS. 12 and 13  in the drawings, a dipstick heat exchanger  100  is illustrated. The cooling system of the present application may also further include supplemental heat exchangers. It is understood that supplemental heat exchangers may work independently of or in combination with mandrel assembly  33  and planetary carrier assembly  27 . Dipstick heat exchanger  100  is a type of supplemental heat exchanger that may be used with transmission  23 . Supplemental heat exchangers are a component built to efficiently transfer heat energy, usually incorporating a finned body for increased surface area. Dipstick heat exchanger  100  includes one or more heat pipes  70 , 80 , a cylindrical finned shaft  101 , and a supplemental fin structure  102 . Shaft  101  defines a central axis  105  extending the length of shaft  101 . Shaft  101  includes a pedestal  107  coupled to a face of shaft  101  in a relatively concentric alignment with axis  105 . Pedestal  107  separates cylindrical shaft  101  from supplemental fin structure  102 . Structure  102  is formed in two halves, although it is understood that structure  102  may be a unitary part in certain embodiments. Structure  102  uses a plate  109  extending along a plane parallel to and adjacent with central axis  105 . Fins  108  protrude from plate  109  outward to provide as much surface area as possible to release heat energy. Heat pipes  70 , 80  are arrayed in a radial pattern within the length of shaft  101  and dispersed within plate  109 . The fins of shaft  101  operate to transfer heat energy between heat pipes and to increase the ability of dipstick heat exchanger  100  to absorb heat energy within transmission  23 . The two halves of structure  102  are configured to couple together surrounding heat pipes  70 , 80 . 
     In operation, dipstick heat exchanger  100  is configured to engage portions of transmission  23 , such that shaft  101  protrudes through walls of transmission housing  26   a , 26   b  and into a heated lubricant, such as oil, or into a heated air space, such as a convection area. Pedestal  107  preferrably contacts housing  26   a , 26   b  and may, in some embodiments, provide bolt holes to permit dipstick heat exchanger  100  to be secured to transmission  23  via fasteners. The evaporator region  75  of heat pipes  70 , 80  are located within transmission  23  and the condenser region  73  is located within structure  102 . Heat energy is transported from within transmission  23  to an external heat sink through structure  102  via the working fluid in heat pipes  70 , 80 . It is understood that dipstick heat exchanger  100  is removable from transmission  23 . Furthermore, other methods of coupling dipstick heat exchanger  100  include the use of external threads around shaft  101 , adhesive, and welding to name a few. 
     Referring now also to  FIGS. 14 and 15  in the drawings, a T-shaped heat sink  131  is illustrated. Heat sink  131  is similar to dipstick heat exchanger  100  in that both are supplemental heat exchangers. Heat sink  131  includes a finned body  133 , a pedestal  135 , and one or more heat pipes  70 , 80 . T-shaped heat sink  131  is different from dipstick heat exchanger  100  in that heat pipes  70 , 80  fail to penetrate transmission  23 . Finned body  133  is configured to accept the condenser region  73 , 83  and evaporator region  75  extends toward and is dispersed within pedestal  135 . Pedestal  135  bends relatively orthogonally so as to create a generally flat base plane  137 . Heat sink  131  is configured to couple externally to a portion of housing  26   a , 26   b  by using fasteners to secure base plane  137  to an external surface. It is understood that base plane  137  may be configured to take a plurality of shapes depending on the mating surface. Furthermore, other methods of securing heat sink  131  to the mating surface may be employed, namely, adhesive for example. 
     It is understood that any devices, components, items, or parts described above with any and all of the associated embodiments may be used with respect to transmission  23  and other portions of the drivetrain of the aircraft. Furthermore, the cooling system of the present application may interchange and be composed of any and all of the devices, components, items, or parts described previously. Additionally, the cooling system of the present application may be used to cool engines and transmissions in static ground environments; such as pumps, test stands, and industrial equipment resting on the ground; or in automotive applications with respect to automotive drivetrains. 
     Referring next to  FIG. 16  in the drawings, a schematic of a conventional cooling system  1601  is shown. System  1601  is a closed loop heat pipe (LHP) circuit, wherein heat from an external source is extracted and subsequently cooled through a series of one or more heat sinks having working fluid channeled therein. In particular, a LHP circuit is a two-phase heat transfer device that uses capillary action to remove heat energy from a source and passively move the heat energy to a condenser or radiator. LHP circuits are similar to heat pipes but have the advantage of being able to provide reliable operation over long distances and the ability to operate against gravity. It will be appreciated that LHP can transfer a large heat load over a long distance with a small temperature difference. 
     LHP system  1601  includes a heat pipe  1603  configured to receive and dissipate heat energy. A working fluid, preferably water, is channeled within the inner cavity (not shown) between an evaporator  1605  and a condenser  1607 . As depicted, input heat “Qin” is extracted from an external source (not shown) and thereafter dissipates during travel to and within condenser  1607 . Also, the figure depicts the phase change from a liquid to a vapor state and then back to a liquid state during the cooling process. 
       FIG. 17  is a cross-sectional view of a cooling system  1701  according to an alternative embodiment of the present application. System  1701  can be substantially similar in form and function to one or more of the cooling systems discussed herein. Thus, the features discussed herein are interchangeable between the different types of cooling systems unless otherwise stated. For example, system  1701  could incorporate one or more of the heat pipes discussed above in the mast of the rotor assembly. 
     One unique feature believed characteristic of cooling system  1701  is the use of LHP technology for cooling an aircraft transmission. In particular, system  1701  overcomes a long-felt need to reduce engine components for cooling the transmission by incorporating a passive LHP that is cooled via ambient air through one or more heat sinks operably associated with the rotor assembly. These features provide the following advantages: no moving parts; less maintenance than conventional oil coolers; no leaks potential for loss of lube and general aircraft cleanliness; low cost, mass-produced technology; easily implemented on pre-existing aircraft, in particular, on aircraft with de-icing systems; and can easily be monitored via thermocouple monitoring. Further advantages include: high heat flux capability; capability to transport energy over long distances; routing or the liquid and vapor lines; ability to generate over a range of g-force environments; vapor and liquid flow separate, therefore no entrainment; and no wick is required within the transport lines. The LHP system also reduces parts currently being utilized to cool the aircraft transmission, including: lube lines, oil cooler, blower housing, blower shaft, in addition to bearings, hanger, hardware, and other operably associated devices. These and other features of system  1701  are discussed below and illustrated in the accompanying drawings. 
     In the preferred embodiment, system  1701  removes heat from transmission  23  via the aircraft rotor assembly  1703 ; however, it is also contemplated removing heat energy from the transmission via one or more alternative devices associated with the aircraft incorporating LHP technology and/or another similar suitable technology adapted to transfer heat. Cooling system  1701  comprises a evaporator section 1705 disposed within the transmission  23  and in communication with the engine oil (not shown) or other operably associated working fluid or means for transferring heat. 
     Evaporator  1705  receives heat input “Qin” from the engine oil preferably via a plurality of fins  1707  or similar means configured to increase the exposed surface area. A heat pipe  1709  is in thermal communication with evaporator  1705  and is configured to both dissipate heat energy and to carry a working fluid disposed therein to rotor assembly  1703 . It should be understood that rotor assembly  1703  includes a hollow rotor mast  1711 ; thus, in the preferred embodiment, pipe  1709  extends within an inner cavity  1712  of mast  1711  from the evaporator section to the condenser section. 
     In the contemplated embodiment, rotor assembly  1703  is considered the condenser section of the LHP circuit, wherein the rotor assembly provides means to dissipate heat energy via the rotor mast  1711  and/or through a plurality of heat sinks  1713   a ,  1713   b  carried by one or more rotors  1715   a ,  1715   b , as depicted “Qout.” Heat sinks  1713   a ,  1713   b  are in thermal communication with heat pipe  1709 . 
     Alternative embodiments could utilize different components of rotor assembly  1703  as means for dissipating heat energy, for example, one or more heat sinks  1717  carried on the rotor assembly hat  1719  or other devices operably associated with rotor assembly  1703 . 
     In the exemplary embodiment, the rotor blades act as effective means for dissipating heat energy through the process of exposing the heat sinks to ambient air and/or rotorwash. In the preferred embodiment, heat sinks  1713   a ,  1713   b  are position on the exposed surfaces  1721   a ,  1721   b  of respective rotors  1715   a ,  1715   b ; however, it will be appreciated that the heat sinks can alternatively be carried within or disposed therein the rotor structure. Although shown positioned at the root of the rotor blade, it is also contemplated providing a heat sink that extends the entire length of the rotor blade. It is also contemplated selectively positioning the heat sinks on other surfaces of the aircraft in alternative embodiments. 
     It should be understood that system  1701  operates in the following steps: the working fluid travels around the LHP circuit from the evaporator to the condenser and back to the evaporator; at the evaporator, the fluid changes from liquid to vapor phase and acquires the heat of vaporization; and the vapor moves along the loop without transferring significant amount of heat (adiabatic section) until the vapor reaches the condenser; the condenser is cooler than the vapor resulting in condensation of the fluid back to the liquid state; the heat of vaporization is released to the condenser; and, the liquid working fluid returns to the evaporator by gravity and may be assisted in return by centrifugal force caused by the rotating mast if the LPH is configured to take advantage of the centrifugal forces. 
     The current application has many advantages over the prior art including the following: (1) not requiring a separate oil system that is non-functional until an emergency lubrication event occurs; (2) assist in the removal of heat during normal flight operation, so as to reduce the required heat removal capacity of the primary lubrication system; (3) increased efficiency at transferring heat out of a transmission; (4) decrease the weight of lubricant and decrease the lubricant reserves required; and (5) passively dissipating heat energy from the transmission through the rotor assembly.