Patent Publication Number: US-2015075010-A1

Title: Aerodynamic Trimmer Head For Use In Flexible Line Rotary Trimmers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/717,908, filed Mar. 4, 2010 entitled “Aerodynamic Trimmer Head For Use In Flexible Line Rotary Trimmers”. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved trimmer head for use in flexible line rotary trimming devices used to trim grass, weeds and other vegetation. More particularly, the invention is directed to an improvement in the aerodynamics of the trimmer head so that with the same drive input, the head will rotate faster than a conventional trimmer head, enhancing the cutting efficiency of the rapidly rotating cutting line or rotate at the same speed as a conventional head with less drive input, saving energy. The improved aerodynamics of the head in the present invention also has been found to reduce vibration and noise during use. 
     Trimmer heads used in flexible line rotary trimmers generally carry one or two lengths of flexible nylon cutting line typically wrapped about an interior spool with the ends of the line or lines projecting outwardly through opposed apertures in the side wall of the trimmer head. The head is threadably mounted on the end of an elongated shaft and rotated at a high velocity by a gas or electric motor so that the ends of the cutting line project radially from the head and sever weeds or other vegetation. When cutting line projecting from the head breaks off or becomes overly worn, it must be severed and fresh line extended from the spool through the line outlet eyelets in the side of the housing. Bump-feed type heads such as those disclosed in U.S. Pat. Nos. 4,458,419, 4,959,904 and 6,901,667, include a line feed-out mechanism which responds to the operator intentionally bumping the rotating head against the ground during use to feed out a measured length of fresh cutting line which is typically cut to the desired length by a knife blade projecting from a shield attached to the trimmer above the cutting head and spaced a predetermined distance from the perimeter of the trimmer head housing. Manual heads such as that disclosed in U.S. Pat. No. 7,275,324, do not include any such line feed-out mechanism. The spool must be manually rotated relative to the housing to pay out additional cutting line. Automatic heads such as that disclosed in U.S. Pat. No. 5,063,673, include a mechanism that detects a loss of mass in the cutting line projecting from the head during use and, without the need for any action by the operator, pay out additional lengths of line from the spool through the eyelets. Fixed line heads such as that disclosed in U.S. Pat. No. 6,928,741, utilize one or more relatively short lengths of cutting line, typically under 12 inches in length, that project from the trimmer head housing and are held proximate their inner ends by a line gripping mechanism disposed within the housing. The improvement of the present invention is adaptable for use with each of these types of head to improve the efficiency thereof and reduce noise and vibration. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention is directed to an improved trimmer head for use in flexible line rotary trimmers that is more efficient and generates less noise and vibration than conventional trimmer heads. The trimmer head of the present invention comprises a low profile housing, preferably of a generally elliptical configuration. A spool for carrying one or more lengths of cutting line can be mounted within the housing and preferably configured such that the lower spool surface is generally curvilinear and merges smoothly into the contour of the lower portion of the trimmer head. The outer surfaces of the upper and lower portions of the head define aerodynamic features in the form of surface irregularities therein configured to enhance the aerodynamics of the trimmer head and reduce the drag on the head and line during use. 
     In a preferred embodiment of the present invention, the outer surface of the upper portion of the housing is provided with a plurality of coordinated arcuate troughs angularly disposed therein so as to draw air downwardly, during use, about the central axis of rotation of the trimmer head and onto the upper surface of the head where the air is moved outwardly and downwardly therefrom by the troughs at a forward inclination in the direction of rotation of the head and into the plane of the rotating cutting line. The downward air flow will pass about the gear box in a gasoline powered rotary trimmer, cooling the gears, or about the motor and batteries in the case of an electric powered trimmer, cooling and prolonging the life thereof. Directing air into the plane of the rotating line in the direction of rotation reduces the relative velocity between the air and the line, reducing the drag on the line. The aerodynamic features also preferably extend about the periphery of the housing below the level of the line outlet apertures in the side of the head to cooperate with the arcuate troughs and direct the air flow generated by the troughs into a substantially horizontal flow from the head so as to maintain the forwardly directed air flow in and about the plane of the extended cutting line, further decreasing the drag on the line and maintaining the extended rotating line in a substantially horizontal disposition to provide an even cutting of the vegetation and reduce the vibration of the line and the noise generated thereby. 
     The outer surface of the lower portion of the housing preferably is provided with a plurality of patterned dimples therein to break up the air boundary adjacent the lower housing surface reducing the areas of laminar flow adjacent to the head and thereby reducing the drag on the lower housing surface and the overall drag on the head. The volume of air movement effected by the dimples is quite small so as to reduce the drag on the lower portion of the rotating trimmer head without disturbing the vegetation below the head sufficiently to impair uniform cutting thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first embodiment of the trimmer head of the present invention as seen from above. 
         FIG. 2  is a perspective view of the embodiment of the trimmer head illustrated in  FIG. 1  as seen from below. 
         FIG. 3  is an exploded perspective view as seen from above of the various elements comprising the embodiment of the trimmer head of the present invention illustrated in  FIG. 1 . 
         FIG. 4  is an exploded perspective view of the various elements comprising the first embodiment of the trimmer head similar to  FIG. 3  but as seen from below. 
         FIG. 5  is a sectional view of the embodiment of the trimmer head shown in  FIGS. 1-4 , illustrating the spool and trimmer head housing before the cutting line is loaded. 
         FIGS. 6A and 6B  are sectional views of the trimmer head as shown in  FIG. 5  illustrating the loading of the cutting line. 
         FIG. 6C  is the sectional view of the trimmer head as shown in  FIG. 5  with the cutting line wound thereon. 
         FIG. 6D  is a top plan view of the trimmer head shown in  FIGS. 1-6C  illustrating the zone of outwardly and forwardly moving air generated by the head during rotation thereof. 
         FIG. 6E  is a side view of the trimmer head shown in  FIGS. 1-6C  illustrating the downward air flow about the trimmer head drive mechanism on a gasoline powered rotary trimmer and the elevation and thickness of the outwardly and forwardly moving zone of air generated by the head during rotation thereof. 
         FIG. 6F  is a side view of the trimmer head shown in  FIGS. 1-6C  illustrating the downward air flow about the trimmer head drive mechanism on an electric powered rotary trimmer and the elevation and thickness of the outwardly and forwardly moving zone of air generated by the head during rotation thereof. 
         FIG. 7A  is a bottom plan view of the interior of the upper portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 7B  is a perspective view of the interior of the upper portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 8A  is a top plan view of the interior of the lower portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 8B  is a perspective view of the interior of the lower portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 9  is a cross-sectional view of the lower portion of the housing of the trimmer head taken along the line  9 - 9  in  FIG. 8A . 
         FIG. 10A  is a bottom plan view of the trimmer head shown in  FIGS. 1-6C  in the drive mode showing the relative positioning of the lower cam followers on the spool with respect to the lower cams on the housing wherein the lower cam followers and cams are shown in dotted lines. 
         FIG. 10B  is a bottom plan view of the trimmer head shown in  FIGS. 1-6C  in the line winding mode showing the relative positioning of the lower cam followers on the spool with respect to the lower cams on the housing wherein the lower cam followers and cams are shown in dotted lines. 
         FIG. 11A  is a perspective view of the spool of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 11B  is a cross-sectional view of the spool of the trimmer head shown in  FIGS. 1-6C  and is taken along the line  11 B- 11 B in  FIG. 10 . 
         FIG. 12A  is a cross-sectional exploded view of the spool of the trimmer head shown in  FIGS. 1-6C  as viewed along the line  12 A- 12 A in  FIG. 10  but prior to the insertion of the channel-forming inserts to illustrate an economical method of forming the spool. 
         FIG. 12B  is a cross-sectional view of the spool of the trimmer head shown in  FIGS. 1-6C  and is taken along the line  12 A- 12 A in  FIG. 10  illustrating the spool with the channel-forming inserts installed. 
         FIG. 12C  is a cross-sectional view of the spool of the trimmer head shown in  FIGS. 1-6C  and is taken along the line  12 A- 12 A in  FIG. 10  illustrating the spool with the channel-forming inserts installed and with the cutting line extending through the line receptor channel in the spool. 
         FIG. 12D  is a cross-sectional view of a spool for use in the embodiment of the trimmer head shown in  FIGS. 1-6C  and is of single-piece construction. 
         FIG. 13  is a bottom plan view of the trimmer head shown in  FIGS. 1-6C  showing the relative positioning of the upper cam follower with respect to the lower cam follower and with the upper cam follower and line openings being shown in dotted lines. 
         FIG. 14A  is a side view of the spool of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 14B  is a front view of the spool of the trimmer head shown in  FIGS. 1-6C  as viewed from the left side of  FIG. 14A . 
         FIG. 14C  is a rear view of the spool of the trimmer head shown in  FIGS. 1-6C  as viewed from the right side of  FIG. 14A . 
         FIG. 15A  is a perspective view of the trimmer head shown in  FIGS. 1-6C  as seen from below and with the cutting line loaded thereon. 
         FIG. 15B  is a perspective view of the trimmer head shown in  FIGS. 1-6C  as seen from below and illustrating the cutting line as it begins to be withdrawn from the lower open portion of the line receptor channel. 
         FIG. 15C  is a perspective view of the trimmer head shown in  FIGS. 1-6C  as seen from below and illustrating the cutting line as it further withdrawn from the lower open portion of the line receptor channel. 
         FIG. 16  is a perspective, sectional view of an alternate embodiment of a spool for use in the present invention. 
         FIG. 17A  is a top plan view of the upper portion of the housing of the trimmer head shown in  FIGS. 1-6C  and illustrating various dimensional parameters of the aerodynamic troughs formed therein. 
         FIG. 17B  is a side view of the upper portion of the housing illustrating various dimensional parameters of the aerodynamic troughs formed therein. 
         FIG. 17C  is a partial sectional view of the upper portion of the housing of the trimmer head shown in  FIGS. 1-6C  showing the configuration of the aerodynamic troughs formed therein. 
         FIG. 18A  is a bottom plan view of the exterior of the lower portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 18B  is a side view of the exterior of the lower portion of the housing of the trimmer head shown in  FIGS. 1-6C . 
         FIG. 19  is a top plan view of an alternate embodiment of the aerodynamic features in the outer surfaces of the housing of the present invention. 
         FIG. 20A  is a side view of another alternate embodiment of the aerodynamic features in a trimmer head of the present invention. 
         FIG. 20B  is a side view of yet another alternate embodiment of the aerodynamic features in a trimmer head of the present invention. 
         FIG. 21  is a partial side view of still another alternate embodiment of the aerodynamic features in a trimmer head of the present invention. 
         FIG. 22  is a sectional view of one of the finger/thumb receiving recesses in the lower end of the spool in the trimmer head shown in  FIGS. 1-6C . 
         FIG. 23A  is a perspective view as seen from above of the application of the present invention to a fixed line head. 
         FIG. 23B  is a perspective view as seen from below of the application of the present invention to the trimmer head shown in  FIG. 23A . 
         FIG. 23C  is a bottom plan view of the upper housing portion of the trimmer head shown in  FIGS. 22A and 22B . 
         FIG. 24A  is a top plan view of still another alternate embodiment of the aerodynamic features in a trimmer head of the present invention. 
         FIG. 24B  is a side view of the trimmer head shown in  FIG. 24A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in detail to the drawings, a first embodiment of a trimmer head  10  embodying the teachings of the present invention is illustrated in  FIGS. 1-18B . Trimmer head  10  is a bump-feed type and is designed to be mounted on the extended end of a rotatable drive shaft  12  on a gasoline or electric powered rotary trimmer (not shown). Trimmer head  10  comprises a housing  14 , spool  16 , coil spring  18  and drive bolt  20 . The housing comprises an upper portion  14   a  and a lower portion  14   b  that are releasably secured together about the spool  16 . The drive and line feeding mechanism employed in trimmer head housing  10  is similar to that disclosed in detail in U.S. Pat. No. 4,959,904 and the rapid line loading and line replacement mechanism employed in trimmer head  10  is disclosed in U.S. Pat. No. 7,797,839. However, the aerodynamic profile of the head  10  and the cooperative aerodynamic elements defined by the exterior surfaces of the head are novel. While primarily described below in connection with a bump-feed type head having the above noted line drive, feeding, loading and replacement features, the aerodynamic features of the present invention can be employed in trimmer heads having a wide variety of drive configurations, line feeding mechanisms and line loading and replacement systems. 
     In the illustrated embodiment of trimmer head  10 , the housing  14  is of a slightly flattened, generally ellipsoidal configuration so as to define a relatively low profile aerodynamic shape. By way of example, a trimmer head embodying the present invention and having the same configuration as trimmer head  10  defines a maximum height H of 2.750 inches and a diameter D of 5.600 inches. The upper portion  14   a  of the housing is perhaps best illustrated in  FIGS. 7A ,  7 B,  17 A and  17 B and includes an outer surface  22  defining a generally flat upper annular area  22   a  extending about and projecting radially from a central opening  23  disposed about the central axis of rotation of the trimmer head. Area  22   a  can include printed indicia thereon and merges at its outer perimeter into an annular curvilinear surface  24  extending outwardly and downwardly and terminating at its lower end in a more gradually curved skirt portion  25  extending slightly past vertical as seen, for example, in  FIG. 17B . 
     The interior of the upper portion  14   a  of the housing defines a centrally disposed depending tubular extension  26  axially aligned with the central axis of rotation of the head. Tubular extension  26  circumscribes an axial channel  26 ′ extending downwardly from the central opening  23  in the housing surface and defining a cylindrical upper channel portion  26   a  and a hexagonal lower portion  26   b . Channel  26 ′ is configured to receive the drive bolt  20  in a mating configuration with the upper cylindrical portion  20   a  of the bolt being disposed in the upper cylindrical portion  26   a  of the channel and the hexagonal lower portion  20   b  of the bolt being disposed in the lower hexagonal portion  26   b  of the channel. The lower portion of the drive bolt and the tubular extension in the housing could also be square or otherwise configured to provide the desired interference fit between the drive bolt and housing. 
     The drive bolt  20  preferably is secured within the tubular extension by utilizing the natural shrinkage of extension  26  during the cooling of the freshly molded head  10  about the bolt. The upper and lower portions of the trimmer head housing  14  are preferably molded of a plastic material with a polyamide (nylon 6), 15% glass reinforced material being most preferred. Objects molded of such materials will shrink as the freshly molded material cools. An annular groove  20   c  can be provided in the drive bolt to enhance the interference fit between the bolt and housing upon the shrinkage of the freshly molded plastic housing material about the bolt and thereby to enhance the securement of the bolt to the housing. Other means for securing the drive bolt to the housing also could be employed. The drive bolt  20  may define an internally threaded cylindrical bore  20   d  extending axially therethrough for threaded engagement with the drive shaft of the trimmer. 
     The upper portion of housing  14  also defines a pair of opposed slots  30  in the housing skirt  25 . The slots are open at their lower ends and are adapted to receive a pair of opposed metal outlet eyelets  32  in a press fitment. Perimeter portions  33  of the slots are radiused so that the outer ends of the eyelets fit substantially flush with the outer surface of the housing. A plurality of equiangularly disposed and outwardly projecting radial tabs  36  also are provided at the lower end of the skirt portion  25  of the housing for the securement of the lower housing portion  14   b  to the upper portion  14   a , as will be described. 
     While the present invention is described herein in connection with trimmer heads in which the cutting line exits the head through opposed openings, the aerodynamic features of the present invention are equally applicable for use on trimmer heads having a single line outlet opening. Such heads are in widespread use on low end rotary trimmers wherein adequate torque is generally lacking and improved aerodynamics in the heads would be very beneficial. 
     The upper housing portion  14   a  additionally includes an interior depending vertical wall  38  outwardly spaced from tubular extension  26 . As shown in  FIGS. 7A and 7B , the wall  38  is configured to define four pair of inwardly angled surfaces  40 , each pair forming a projection  44  that projects radially inwardly toward the central axis of rotation of the head at the center of tubular extension  26 . Each pair of angularly disposed surfaces  40  are offset by 135° and extend parallel to the axis of rotation of the head. The projections  44  formed by surfaces  40  define cam surfaces wherein the leading edge of each of the projections  44  defines an upper cam  44   a  (assuming counterclockwise rotation of the head as seen from above the head) and the trailing surfaces on each projection define an upper slide surface  44   b . The surface of each of the cams is again parallel to the axis of rotation of the head. 
     The lower housing portion  14   b  of the trimmer head  10  is perhaps best illustrated in  FIGS. 8A ,  8 B,  18 A and  18 B and defines an outer arcuate inclined surface  45  circumscribing an enlarged central opening  46  and merging proximate its upper end at  45 ′ into a slightly curved and more steeply inclined upper end portion  47 . The uppermost surface of end portion  47  merges along a radiused surface  47 ′ into an upper generally horizontal annular surface  47 ″. The interior of the lower housing portion  14   b  includes an upstanding annular interior wall  48  that circumscribes central opening  46 , an annular interior surface  43  that slopes upwardly from wall  48  to vertical surface  49  and an annular horizontal upper housing support surface  51  that extends radially outwardly from the upper end of all surface  49  to a steeply inclined wall surface  53 . Surfaces  51  and  53  are adapted to abut the lower end surfaces of the upper housing portion  14   a  upon the upper and lower housing portions being secured together. 
     A plurality of equiangularly disposed slots  50  (four being shown) are disposed in the lower portion of the housing for receiving a corresponding number of locking tabs  36  on the upper housing portion  14   a . Slots  50  extend vertically through upper end portions of inclined surface  45  exteriorly adjacent the interior housing support surface  51  and are provided with narrow offset access portions  50 ′ as seen in  FIGS. 8A and 8B . To attach the upper housing portion  14   a  to the lower housing portion  14   b , a locking tab  54  defined by a resilient cantilevered portion  24 ′ of the upper housing portion  14   a  must first be aligned with a small protrusion  56  provided on the lower housing portion  14   b . The user then inserts the four locking tabs  36  of the upper housing  14   a  into the four slots  50  in the lower housing portion  14   b . The tab  54  is then pressed radially inwardly and the housing portions  14   a  and  14   b  are pressed together, mating together the upper housing portion  14   a  and the lower housing portion  14   b  such that the lower annular end surface  25 ′ of the upper housing portion  14   a  is disposed on the annular support surface  51  in the lower housing portion  14   b . The user then rotates the upper housing portion  14   a  counterclockwise with regard to the lower housing portion  14   b , causing the locking tabs  36  to translate into the offset portions  50 ′ of the slots  50 . When the locking tabs  36  have translated completely into the offset portions  50 ′, the tab  54  that had been pressed inwardly and then became biased outwardly against interior wall surface  53  upon the initiation of the rotation of the upper housing portion, now is aligned with one of the slots  50 , allowing the cantilevered portion  24 ′ of the tab to return to its original, unstressed extended position and project outwardly into the aligned slot, whereupon the upper housing portion  14   a  is securely attached to the lower housing portion  14   b.    
     The lower portion  14   b  of the housing  14  also defines four equiangularly disposed, upstanding projections  58  on the interior annular surface  43  (see, e.g.  FIG. 8B ). Projections  58  each define a radially extending, upwardly inclined foot portion  58 ′ at the trailing end thereof that merges into an inclined surface  58 ″ which merges into an outwardly projecting portion  58 ′″ at the leading end of the projection  58  (assuming counterclockwise rotation). The outwardly projecting portions  58 ′″ of the projections  58  define vertical, angularly disposed walls  58   a  that function as lower cams as will be later described. The foot portions  58 ′ and inclined surfaces  58 ″ on the projections define lower slide surfaces and cooperate with lower cam follower surfaces formed on spool  16 , as will be later described. 
     The spool  16  in trimmer head  10  defines an upper flange  60 , a middle flange  61  and a lower flange  62  carried by and projecting radially from a cylindrical body portion  64  so as to define two annular storage areas  66 ′ and  66 ″ between flanges  60  and  61 , and between flanges  61  and  62 , respectively, for carrying coils of flexible nylon cutting line  17  wrapped about body portion  64  such that upon assembly, the end portions  17 ′ of the cutting line will extend outwardly through the opposed eyelets  32  (see  FIGS. 6C-6E ). An annular chamber  68  is provided in the depending tubular extension  26  in the interior of the upper housing  14   a  about the lower portion of the extension. Chamber  68  is open at its lower end and defines an upper spring abutment surface  70 . A cylindrical chamber  69  having an open upper end is disposed about the central axis of rotation in the upper end of the spool and defines a lower spring abutment surface  71 . When head  10  is assembled, the tubular extension  26  on the upper housing projects into chamber  69  in the spool and the coil spring  18  extends between and bears against the spring abutment surfaces  70  and  71  as seen in  FIGS. 5 and 6 . A lower portion  72  of spool  16  projects outwardly from body portion  64  and defines at its lowermost end a ground abutment surface  73 . 
     As seen, for example, in  FIG. 2  and as will be discussed later herein in more detail, the lowermost surface  73  of the spool  16  is generally curvilinear and is shaped so as to merge smoothly into the contour of the lower portion of the head as defined by arcuate surface  45  on the lower portion  14   b  of the housing so as to provide the trimmer head  10  with a relatively low profile and the lower portion of the head with a relatively smooth aerodynamic shape. Also, spool surface  73  is provided with a plurality of depressions or recesses  77  adapted to conveniently receive the fingers and thumb of the user for rotating the spool with respect to the trimmer head housing to load the cutting line onto the spool without interrupting the operative connection between the spool and the housing as also will be later described. 
     Spool  16  preferably defines a line receptor channel  74  extending therethrough from a first open end  74   a  to a second opposed open end  74   b  and having an open accessible portion  74   c  therebetween. In trimmer head  10 , the opposed openings  74   a  and  74   b  of the line receptor channel  74  are located in the middle flange  61  and when the spool  16  is secured within housing  14 , the channel openings are radially aligned with the eyelets  32  in the upper housing  14   a  (see, e.g.,  FIGS. 5 and 6 ), enabling the cutting line  17  to be inserted into the line receptor channel through one of the eyelets  32  without having to remove the spool from the housing. The line receptor channel  74  extends radially inwardly from the opposed channel openings  74   a  and  74   b  through flange  61 , turns downwardly at  75   a  and  75   b  in relatively wide radius curves and extends downwardly through opposed interior portions of the lower spool body portion  72 , outwardly through laterally spaced openings  74 ′ and  74 ″ in a lower recessed area  76  in the lowermost surface  73  of the spool and transversely across a recessed area  76  in the spool surface  73 , interiorly of the gripping recesses  77  formed therein. The portion of channel  74  extending across the lower recessed area  76  of the spool is the open or exposed portion  74   c  of the channel providing access to the portion of cutting line  17  extending therethrough. Channel portion  74   c  preferably is defined by a concave surface or is of an inverted U-shaped configuration to assist in guiding the cutting line through the lower recessed portion  76  of the spool. Recessed area  76  extends transverse to channel portion  74   c  to facilitate gripping of the cutting line extending thereacross and protect the exposed line in channel portion  74   c  during use. 
     The upper flange  60  on spool  16  defines an upper cam follower  80  on its upper surface and the lower spool flange  62  defines a lower cam follower  82  on its lower surface as seen in  FIGS. 3 ,  4 , and  11 A. While other configurations could be employed, both cam followers are preferably of a square configuration, defining four perpendicular surfaces  80 ′ and  82 ′ respectively, and are offset by 45° with respect to the central axis of rotation of the trimmer head. The cam abutment surfaces  80   a  and  82   a  defined by the trailing surfaces of the upper and lower cam followers are again parallel to the axis of rotation of the head. The leading surfaces of the lower cam follower which define sliding surfaces  82   b  may be inclined upwardly proximate the corners thereon so as to provide smoother ratcheting if needed. Also, the leading surfaces of the upper cam follower may also be inclined downwardly to facilitate line feeding. Such a downward inclination of the trailing surfaces is particularly preferable on the smaller heads where the length of each of the cam follower surfaces is shorter which otherwise might make the relative rotation of the spool and housing during line winding more difficult. It may also prove desirable to incline the trailing surfaces of the upper and lower cams. In the preferred configuration, the sliding surfaces  82   b  on the lower cam follower are defined by inclined ramps as seen in  FIGS. 10A ,  10 B and  13  to provide a smoother ratcheting of the spool during the winding of the cutting line thereon due to the more gradual incline than that which would be provided by radiused surfaces. 
     During use in the drive mode, the lower cams  58   a  on the lower housing portion  14   b  are aligned with and abut the lower cam abutment surfaces  82   a  on the trailing surfaces of the lower cam follower  82  (see  FIG. 10A ). The configuration of projections  58  provides a relatively large contact area for cams  58   a . Accordingly, as the housing is rotated in a counterclockwise direction by the trimmer drive, the spool is rotated with the housing. In this drive position, the upper cams  44   a  are upwardly spaced from upper cam follower  80  so that the driving force is generated solely by the lower cams. When the lowermost surface  73  of the spool is pressed or bumped against the ground, the spool  16  is forced upwardly within the housing, disengaging the lower cam abutment surfaces  82   a  on the lower cam follower from the lower cams  58   a  and bringing the upper cam abutment surfaces  80   a  on the upper cam follower  80  into alignment and immediate abutment with the upper cams  44   a  on the upper portion  14   a  of the housing  14  whereupon the driving force is effected solely by the upper cams. When the lower spool surface  73  is lifted from the ground, the coil spring  18  forces the spool downwardly, disengaging the upper cam follower from the upper cams and re-engaging the lower cam follower with the lower cams. With the first embodiment of the invention, with each bump on the ground effects a relative rotation of the spool and housing of 90° results, regardless of the duration of the bump. During this relative rotation, centrifugal force causes a predetermined amount of fresh cutting line to be paid out through the opposed eyelets  32  in the trimmer head housing. 
     To load the cutting line  17  about spool  16 , the opposed open ends  74   a  and  74   b  of the receptor channel  74  are aligned with the eyelets  32  such that a single length of cutting line  17  can be inserted through one of the outlet eyelets  32  of the upper housing portion  14   a  and into and through the adjacent aligned open end  74   a  or  74   b  of the receptor channel  74 . Printed indicia, such as the arrows  92  illustrated in  FIG. 15A , can be provided on the outer surface of the lower housing portion  14   b  and in the recessed area  76  in the lower end of the spool to facilitate proper alignment of the line receptor channel with the eyelets  32  in the housing skirt. As the cutting line is pushed into the line receptor channel  74  through, for example, open end  74   b , the end portion of the line passes about the curvilinear channel portion  75   b  and downwardly through the channel and out the opening  74 ″ in the recessed area  76  in the lower end of the spool as illustrated in  FIG. 6A . The end portion of the line projecting through opening  74 ″ can then be readily grasped and pulled downwardly, causing more of the cutting line to be drawn inwardly through the eyelet and the adjacent portion of the line receptor channel  74 . The downwardly extending end portion of the line is then redirected inserted back up through lower opening  74 ′ and pushed upwardly through the remainder of the channel  74  and out through the other channel end  74   a  and the aligned eyelet. As the cutting line is pushed upwardly through the lower channel opening  74 ′, it is drawn along the exposed transverse portion  74   c  of the channel and is directed by the surrounding channel wall outwardly through the channel  74  and to and through the adjacent eyelet to the position illustrated in  FIG. 6B . 
     The cutting line  17  continues to be pulled through the trimmer head  10  until the midway point on the length of cutting line  17  to be loaded onto the head  10  is disposed in the open or exposed portion  74   c  of the line receptor channel. In this position, approximately equal lengths of line project through each of the opposed eyelets  32 . Alternatively, the two end portions of the length of cutting line to be loaded on the spool  16  could each be inserted through one of the separate channel openings  74 ′ and  74 ″ in the bottom of the spool and pushed upwardly through their respective channel portions and out through the opposed ends of the channel  74  and aligned eyelets. To wind the line onto the spool  16 , it is only necessary to rotate the spool forwardly with respect to the housing, i.e., in the direction of rotation of the trimmer head  10  during use in the drive mode. 
     To assist in the winding of the cutting line  17  on to the spool  16  such that the portions of the line projecting from each of the two eyelets is drawn back into the separate spool storage areas  66 ′ and  66 ″ and wrapped about the spool as illustrated in  FIG. 6C , the opposed ends  74   a  and  74   b  of the line receptor channel are configured by the middle spool flange  61  to direct the opposed portions of the line into the different spool areas. This can be accomplished by removing portions of the channel wall defined by flange  61  adjacent to the two channel ends such that lateral openings  74   a ′ and  74   b ′ are formed in the side wall, communicating channel end  74   a  with area  66 ′ and channel end  74   b  with area  66 ″ as shown in  FIGS. 14A-14C . By inserting one&#39;s thumb and fingers into recesses  77  in the lowermost surface  73  of the spool and rotating the spool in a clockwise direction using recesses  77 , while holding the housing stationary with the head in an inverted position (i.e., with the lower spool end  73  facing upwardly), the portion of the trimmer line  17  projecting from the open end  74   b  of the line receptor is directed into the upper spool storage area  66 ′ and the portion of the trimmer line projecting from channel end  74   a  is directed into the lower line storage area  66 ″ (see  FIG. 14C ). Other guide surface configurations could also be employed to direct the opposing line portions into the different spool chambers or storage areas  66 ′ and  66 ″. 
     By recessing the portion  76  of the lower end of the spool through the exposed portion  74   c  of the line receptor channel extends, the exposed line is protected when the rapidly rotating head is bumped against the ground to pay out fresh lengths of line through the eyelets  32 . If desired, additional protection can be afforded by, for example, a protective bridge, a removable cap or other protective element (not shown) that would allow the user to access the trimmer line extending thereover in channel portion  74   c . Any such protective element should be configured so as to provide a continuous curvilinear surface consistent with the contour defined by the lower spool surface  73  and the adjacent outer lower housing surface  45 . Such a protective element may be particularly desirable for use on abrasive terrain such as gravel. 
     When the spool  16  is gripped and rotated relative to the housing as described above, the lower slide surfaces  82   b  on the lower cam follower  82  will contact the upwardly inclined foot portions  58 ′ and surfaces  58 ″ on the lower projections  58  in the lower portion of the housing and ride upwardly over the inclined surfaces (see  FIG. 10B ), causing the spool to translate upwardly with respect to the housing and compressing the coil spring  18 . As the manual rotation of the spool continues, the corner portions of the lower cam follower  82  can ride over and clear the leading ends of projections  58  in the lower portion of the housing, whereupon the spring causes the spool to snap downwardly such that the lower projections and lower cam abutment surfaces are again in planar alignment. Thus, this reciprocating movement of the rotating spool, which, although not necessary, is preferably employed in the present invention to provide for an even distribution of the cutting line in the two storage areas  66 ′ and  66 ″ about the upper body of the spool. As a result, the lengths of line tend to roll over themselves and fill the two spool areas without becoming entangled on the spool. Because the upper slide surfaces on the upper projections  44  and the leading (slide) surfaces  80   b  on the upper cam follower  80  are in abutment, it may prove desirable to incline the trailing surfaces  44   b  of the upper projections  44  and/or the leading (slide) surfaces  80   b  of the upper cam follower  80  to provide smoother rotation of the spool with respect to the housing during the loading of the line. 
     The above description of the trimmer head  10  and its components is based on using the head on a conventional rotary trimmer in which the gear box (not shown) typically imparts a counterclockwise rotation to the drive shaft  12  and thus to the trimmer head as viewed from above. If the head were used on a trimmer without a gear box or with one that imparted a clockwise rotation to the head, the leading and trailing surfaces on the cams and cam follower abutment members would simply be reversed. Accordingly, the orientation of the cam features and slide surfaces could be reversed to accommodate a rotary trimmer in which the gear box imparts a clockwise rotation to the drive shaft  12 . 
     The bump-feed mechanism provided by the cams and cam abutment surfaces need not be limited to square cam followers. The same is true of trimmer head  10 . The upper and lower cam followers formed by upper and lower portions of the spool, for example, could be three or five sided. Three and five sided cam followers would cooperate with an equal number of cam abutment members in the upper and lower housings as shown in the referenced drawings. The function, cooperation and operation of such cams and cam followers would be otherwise essentially unchanged from that described above. 
     In the embodiment of the spool employed in the trimmer head shown in  FIGS. 1-6C  and  11 A- 12 C, the line receptor channel  74  is formed by affixing a pair of channel forming or line guide inserts  90  in the interior of the spool. Spool  16  is preferably formed by an injection molding process and by using inserts  90  to form the line receptor channel  74 , the cost of manufacturing the spool is substantially reduced. Alternately, the same channel configuration could be obtained without the need for inserts  90  in a single-piece construction, albeit at a substantially higher cost. Such a spool  116  is illustrated in  FIG. 12D . Other than its method of manufacture, spool  116  is substantially identical to spool  16 . In the spool  16  (see, e.g.  FIGS. 4 and 12A ) a pair of generally rectangular, radially spaced, axially extending, slots  92  are provided in the interior of the spool. The slots  92  are configured to receive inserts  90  such that the end surfaces of the slots cooperate with inner end surfaces on the inserts to define the portions of the line receptor channel  74  disposed with the body of the spool. In the embodiment shown in the drawings, each insert  90  defines an upper concave end surface  91 ′ that merges along a curvilinear end surface  91 ″ into a substantially vertical concave end surface  91 ′″. Upon affixing inserts  90  in slots  92 , the concave end surfaces  91 ′- 91 ′″ of the inserts and the adjacent interior surfaces of the spool thus define surrounding wall for the interior portions of the line receptor channel  74 . The inserts  90  and the spool  16 , like housing  14 , are preferably both formed of a polyamide (nylon 6), 15% glass reinforced material, although other materials could be used. Currently, the inserts  90  are molded separately from and prior to the spools to allow for a natural shrinkage of the inserts as the material cools. The inserts are then inserted into the freshly molded spool in a relatively tight fitment such that the subsequent shrinkage of the spool as it cools, locks the inserts in place. It has been found that roughening the adjacent contract surfaces of the inserts  90  and the walls about slots  92  helps prevent any slippage of the inserts during fabrication. Providing small crushable ribs  90 ′ on the adjacent contact surfaces have been found to inhibit any slippage of the inserts (see  FIG. 12A ). Also, a small lateral rib (not shown) can be provided on the extended end surfaces of the inserts that are received in mating recesses formed in the outer walls of the slots into which the line guides are inserted. A depending foot portion (also not shown) can be formed at the lower outer end of each insert such that upon insertion of the line guide insert into the freshly molded spool, the spool will cool and shrink not only against the insert, but about the foot, enhancing the securement of the insert within the spool. Adhesives and other attachment means also could be used and, as noted above and illustrated in  FIG. 12D , the spool could alternatively be molded of a single-piece construction. 
     In the event that the cutting line were to break during use proximate one of the eyelets  32 , the trimmer line inwardly adjacent the break retracts into the head so that additional fresh line cannot be payed out by simply bumping the head on the ground. With the present invention, the user can grasp the portion of the cutting line extending across the open or exposed portion  74   c  of the line receptor channel  74  and pull the line downwardly as illustrated in  FIGS. 15B and 15C . The downward pulling on the line in a direction parallel, if not coincident to the axis of rotation of the spool, will effect an unraveling of the line off the spool, allowing all of the line to be pulled downwardly from the spool through the laterally-spaced openings  74 ′ and  74 ″ in the bottom of the spool. Thus, the old line can be removed from the trimmer head without having to split the head (remove the spool from the housing) or otherwise interrupt the operative connection between the spool and the housing. The removed line or a new length of fresh line can then be re-loaded onto the head using the line receptor channel as earlier described, again without having to split the head. 
     The spools illustrated in  FIGS. 1-15C  are dual area spools in which the portion of the line extending from one eyelet is wrapped about one area of the spool or spool chamber (e.g.  66 ′) and the portion of the cutting line exiting another eyelet is wrapped about another of the spool (e.g.  66 ″). It is to be understood that the present invention could also be employed with a single chamber spool such as that illustrated in  FIG. 16 . 
     To enhance significantly the aerodynamics of the low profile shape of trimmer head  10  during use, aerodynamic features, are provided in the outer head surfaces. By configuring the aerodynamic features so as to move air outwardly from the rapidly rotating head in and about the plane of the extended cutting line and in a forward inclination in the direction of head rotation (see  FIG. 6D ), the relative velocity between the air and the rotating length(s) or portions  17 ′ of cutting line  17  projecting from the head is reduced as compared to the length(s) of rotating line moving through still air, significantly reducing the aerodynamic drag on the projecting line and thereby significantly reducing the drag on the rotating head. As a result, the trimmer head can run at a higher velocity with the same input drive or the same velocity with a lower input drive, saving energy. This increase in efficiency is particularly significant for lower end rotary trimmers having less torque. Also, the aerodynamic features that move the air outwardly from the head to significantly reduce the drag on the line can draw that air downwardly about the axis of rotation from above the head (see  FIGS. 6E and 6F ) such that the air can first flow about the trimmer head drive mechanism  11  on the rotary trimmer, which is positioned proximate the upper end of the trimmer head, providing a beneficial cooling effect on the drive mechanism (i.e., about the gear box  11 ′ of a gasoline powered rotary trimmer (see  FIG. 6E ) or about the motor and batteries (collectively identified at  11 ″ in  FIG. 6F ) of an electric trimmer). To facilitate such cooling, the conventional gear box  11 ′ on a gasoline powered rotary trimmer could be provided with exterior cooling fins  11   a  and the cover  11   c  surrounding the batteries and motor  11 ′ on an electric trimmer should be provided with a plurality of louvers  11   c  or other appropriate openings therein to allow the cooling air flow to pass therethrough. Further, by maintaining the air flow moved outwardly and forwardly from the head in a generally horizontal disposition about the plane of the extended cutting line such that the length(s) of rotating cutting line projecting from the housing are continuously traveling within the forwardly directed air flow, not only is the drag on the rotating line significantly reduced, but also the line is maintained in a substantially planar disposition to effect an even cutting of the vegetation. Vibration in the line and noise also are reduced. 
     In the above disclosed embodiment of trimmer head  10 , the curvilinear outer head surface  24  defined by the upper portion  14   a  of the trimmer head housing  14  is provided with a plurality of uniformly spaced and angularly disposed arcuate troughs  100  formed into a turbine-like pattern about surface  24  (see, e.g.,  FIG. 17A ). Troughs  100  are preferably configured and arranged to draw air downwardly during use from above the central portion of the head onto the upper surface  24  of the rapidly rotating head, providing the above described cooling effect on the trimmer drive mechanism. The troughs  100  then drive the air outwardly and downwardly from the head at a forward inclination in the direction of rotation of the head (see  FIG. 6D ). As troughs  100  direct air outwardly in a forward and slightly downward inclination, the trimmer head  10  preferably positions the generally horizontal annular upper end surface  47 ″ of the lower housing portion  14   b  outwardly of the opposed slots  30  in the upper housing portion  14   a  such that surface  47 ″ is disposed adjacent to and slightly below (e.g. about 0.125 in.) the bottom of the line outlet apertures in eyelets  32  so that the air abuts and is directed by surface  47 ″ outwardly in a relatively flat disposition about the plane of the extended length(s) of cutting line during rotation of the head (see  FIG. 6E ). Thus, the troughs  100  will create a relatively flat zone of air  400 , approximately 0.5 inches thick, that projects at a forward inclination from the trimmer head (see  FIG. 6D ), the lower boundary  402  thereof being substantially defined by the positioning of surface  47 ″ on the upper end portion  47  of the lower housing  14   b . As a result, during use, the entire length(s), or at least substantially the entire length(s)  17 ′, of the rapidly rotating cutting line projecting from the housing  14  are effectively maintained within the forwardly moving zone  400  of air created by troughs  100  which further enhances the effectiveness of the trough generated air flow in reducing the drag on the line. It also has been found that the air flow generated by troughs  100  reduces the aerodynamic drag on the upper portion of the housing, further contributing to the reduction of the drag on the head during use. In addition, by directing the air flow about at least substantially the entire length(s) of the extended line as above described, the projecting line remains substantially planar during use to provide an even cutting of the vegetation and, as noted above, line vibration is reduced as is the noise generated by both the line and the rotating head. 
     While the annular upper end surface  47 ″ of the lower housing portion  14   b  is illustrated and described as being generally horizontal and interrupted solely by locking tab slots  50  and troughs  104 , it is to be understood that alternate surface configurations could be employed. For example, surface  47 ″ could be smooth, without any troughs or other aerodynamic elements formed therein. The surface could be angled upwardly or downwardly to vary the elevation of the outwardly and forwardly moving zone of air  400  relative to the trimmer head to better accommodate different head configurations. Other modifications in the surface  47 ″ might also be employed. While the surface could be eliminated as an aerodynamic element, it is preferably employed as above described and for the reasons stated. 
     In a presently preferred configuration, troughs  100  are generally V-shaped in cross-section, inclined forwardly with respect to the horizontal, and extend downwardly and forwardly (with respect to the rotational direction of the head—counterclockwise as shown in the drawings) along curvilinear paths from an upper inner portion of surface  24  to a lower outer portion thereof. By way of example, a forward trough inclination of about 54° has been employed (see  FIGS. 17A-C ). Such a configuration has been found to move the air off surface  24  at a forward inclination within the range of about 30 to 40 degrees in the direction of rotation as illustrated in  FIG. 6D . As will be later described, the curvature defined by troughs  100  can be a segment of a constant radius circle. Also, because sharp edges are preferably avoided on the surfaces of trimmer head  10  for aerodynamic reasons, such edges, including the edges and bottoms of the troughs, are typically radiused as shown, for example, in  FIGS. 1 and 2 . Accordingly, the term “generally V-shaped troughs”, as used herein, includes recesses that are curvilinear or generally U-shaped in cross-section. 
     Supplementing troughs  100  are a plurality of dimples  102 , preferably of a circular configuration, that are formed in the lower annular surface  45  of the housing, circumscribing the ground abutment surface  73  of spool  16 . The dimples  102  in surface  45  are preferably arranged in a plurality of staggered, radially spaced, concentric rings, as seen in  FIG. 18A , to minimize the flat surface areas between the dimples. So positioned, the dimples  102  break up the air boundary adjacent the lower housing surface on which they are formed, reducing the areas of laminar flow adjacent the head and thereby further reducing the drag on the head. Dimples  102  will move less air than the troughs  100  in the upper housing portion, but are preferably utilized on the lower portion of the housing so as not to overly disturb the vegetation disposed below the head and thereby adversely impact the uniform cutting of the vegetation with the cutting line  17 . 
     To further enhanced the stability of the extended cutting line during use, aerodynamic elements  104 , preferably in the form of relatively small, angularly disposed troughs or channels, can be provided in the upper end portion of lower portion  14   b  of the housing. Troughs  104  are configured so as to offset the line lifting effect of the upper troughs  100 . Troughs  104  extend outwardly at a rearward inclination θ of about 28 to 29 degrees (see  FIG. 8A ) about the annular surface  47 ″ at the upper end of the lower housing portion  14   b  and through the adjacent radiused surface  47 ′, as seen in  FIGS. 1 and 3 . So positioned and configured, troughs  104  direct a relatively stable flow air outwardly from the housing and slightly downwardly, under the extended cutting line  17 . 
     It has been found that by so directing the air flow about the lower portion of the rotating housing and the extended cutting line utilizing the dimples  102  and troughs  104 , in combination with the forwardly inclined outward flow directed by the larger troughs  100  on the upper head surface  24  and by the upper end portion  47  of the lower portion of the housing, the drag on the head and particularly on the extended cutting line is substantially reduced and the air pressure above and below the extended cutting line appears relatively balanced as the extended line remains substantially planar during use to provide an even cutting of the vegetation. Testing has shown that trimmer head  10  using 0.095 in. diameter twisted nylon cutting line requires 24.5% less power (217 watts vs. 275) to drive trimmer head  10  at about 5,000 rpm than an identically shaped head and equipped with the identical 0.095 in. cutting line but without troughs  100  and  104  and dimples  102 . At approximately 7,000 rpm, the power savings was 24.5% (492 watts vs.  652 ). 
     By way of example, in a trimmer head  10  having a maximum height of 2.750 inches as measured along its central axis Y (axis of rotation) and a diameter of 5.600 inches, the troughs  100  formed in the arcuate upper surface  24  of the head are configured and oriented so as to each define a radius of about 1.50 inches as measured from a point X as seen in  FIG. 17A , located on surface  24  1.438 inches below a horizontal line extending through the axis of rotation of the head and along a first vertical line located 0.573 inches to the right of a second vertical line extending through the axis of rotation of the head. Thirty-six such troughs are formed in surface  24  separated by an arc 10° as measured from the axis of rotation. The troughs  100  are generally V-shaped in cross-section and each of the troughs is angularly inclined toward the trailing edge of the trough at an angle of about 54° as illustrated in  FIGS. 17B and 17C . The troughs range from about 0.090-0.150 inches across and from 0.040-0.065 inches in depth. The spacing between the troughs ranges from about 0.150-0.175 inches. The dimple pattern on lower surface  45 , as noted above, is evenly and uniformly distributed and each dimple defines a diameter within the range of 0.090-0.125 inches and a depth of 0.009-0.015 inches. The spacing between the dimples should be at a minimum and the molded dimples should have no undercut areas. The troughs  104  are fewer in number than troughs  100  in the upper surface of the housing due to the presence of slots  50 . In the illustrated example, there are 27 troughs  104  spaced about annular surfaces  47  and  47 ′. The troughs  104  are angled about 10° apart at a rearward angle of inclination of about 28-29 degrees. The troughs are spaced approximately 0.300 inches apart and each trough defines a width of about 0.100 inch and a depth of about 0.025 inch. The angular orientation of the troughs  104  is illustrated in  FIG. 8A . 
     The above dimensions and configurations of the trimmer head  10 , troughs  100  and  104  and dimples  102  are by way of example only. Other trough and dimple sizes and shapes and combinations thereof could also be employed. It has been found that while the above described angular disposition of troughs  100  and  104  facilitate the molding of the upper housing portion of the head, it may be preferable to configure the troughs  100  on the upper surface of the trimmer head housing such that they do not trace a constant radius but form a tighter curvature in the lower end portions  100 ′ thereof as shown in  FIG. 19 . Additionally, dimples and/or troughs could be provided in the lower skirt portion  25  and/or upper end portion  47  of the upper and lower housing portions as shown, for example, in  FIGS. 20A and 20B . Again, other configurations and dimensional changes in the aerodynamic features could be employed. 
     Aerodynamic features in the form of protruding elements, if appropriately configured spaced and sized, such as raised ridges  100 A and bumps  102 A, could be employed in the present invention in lieu of or in addition to immersed cavities such as troughs and dimples to reduce the frictional drag on the head and line (see  FIG. 21 ). As with the use of troughs and dimples, a variety of shapes and sizes of ridges, bumps and/or other protruding elements should be suitable for use in the outer surfaces of the trimmer head to effect the desired air movement and the resulting reduction of drag on the head and line. Indentations in the form of troughs and dimples are preferred over protruding elements from a cost standpoint as the formation of cavities requires the use of less material, whereas the addition of air directing protrusions requires the use of more material. 
     In certain instances wherein the cooling of the trimmer head drive mechanism may be of primary concern, the aerodynamic elements on at least the upper head surface could be configured to enhance the downward flow of cooling air about the trimmer drive mechanism while providing little or no reduction in aerodynamic drag on the trimmer head during use. Such an application could include large electric rotary trimmers in which the operating lives of the batteries and electric motors are of primary concern and could be meaningfully extended by adequate cooling. More typically, however, such elements would continue to provide both a reduction in the aerodynamic drag on the head and a cooling air flow for the drive mechanism. 
     As seen, for example, in  FIGS. 2 and 22 , and as noted above, the lowermost surface  73  of the spool  16  in trimmer head  10  has a plurality of finger and thumb receiving recesses  77  formed therein for rotating the spool  16  with respect to the housing  14  to effect the winding of the cutting line  17  onto the spool in the manner described above. To minimize the turbulence created by the rapidly rotating head in the area within and adjacent to those gripping recesses  77 , the lower or bottom surface  95  of each recess is inclined upwardly into the spool from the trailing edge  95   a  of the recess to the leading edge  95   b  thereof so as to define a substantially vertically surface  95   c  at the leading edge of each recess and a lower bottom surface  95  that slopes downwardly from the bottom of surface  95  to the lowermost surface  73  of the spool, as shown in  FIG. 22 . This configuration both provides a pushing surface  95   c  to enable the user to grip and rotate the spool within the housing in the rotational direction of the head while holding the housing stationary to effect the winding of the line  17  onto the spool and prevents the trailing sides of the gripping recesses from functioning like an air dam and trapping air within the recesses, creating turbulence and slowing the rotational speed of the head. This configuration also allows the user only to wind the spool in one direction obviating the need for directional indicia on the exposed bottom surface  73  of the spool where room is limited. 
     While the aerodynamic features of the present invention have been disclosed in connection with a trimmer head  10  of the bump-feed type having a particular line feeding mechanism and line winding mechanism, a variety of such mechanisms could be employed with the aerodynamic features of the present invention. Also, as noted earlier herein, the present invention is not limited to heads of the bump-feed type but could be employed with manual heads, automatic heads and fixed line heads. 
       FIGS. 23A-23C , for example, illustrate the use of the present invention in a fixed line head  210 . As seen therein, the aerodynamic elements in the upper and lower housing portions  14   a  and  14   b  of the previously discussed trimmer head  10  can be employed in the upper and lower housing portions  214   a  and  214   b  of the housing  214  of the fixed line head  210 . Again, the upper and lower housing portions  214   a  and  214   b  are configured and mated together so as to provide head  210  with a generally elliptical, aerodynamic low profile configuration. Because a fixed line head does not include an interior spool, the profile of head  210  may be slightly flatter than head  10 . 
     As with trimmer head  10 , the upper housing portion  214   a  of head  210  may be provided with troughs  200  on the upper surface  224  thereof that can be configured like troughs  100  in the upper portion  14   a  of head  10 . Similarly, the lower housing portion  214   b  can be provided with a plurality of dimples  202  in the lower housing surface  245 , that can be of the same configuration and positioned in the same general patterned array as dimples  102  in the lower housing portion  14   b  of head  10 , except that as seen in  FIG. 22B , dimples  202  can cover a larger surface area than dimples  102  as the available surface area is larger. The access opening  264  in the underside of the housing portion  214   b  on the fixed line head is smaller than the central opening  46  for the spool  16  in the lower end of housing portion  14   b  of head  10 . The access opening is provided in fixed line head  210  to enable the user to grasp the inwardly extended end portions of the lengths of cutting line and to pull the line inwardly through the eyelets  232  and out of the housing through opening  264  as will be briefly described. To provide the lowermost portion of the lower housing portion with a continuous curvilinear configuration, a removable cap  265  is provided to cover aperture  264  to prevent the creation of turbulence proximate the lower end of the head. The outer surface of cap  265  also can be provided with dimples  202 , as shown in  FIG. 23B . 
     The lower housing portion  214   b  also preferably includes a plurality of arcuate troughs  204  positioned in the upper end portion of the lower portion  214   b  of housing  210 . As with trough  104  in head  10 , troughs  204  are configured so as to project in the opposite direction of the troughs  200  in the upper housing portion (i.e., forwardly as opposed to rearwardly) and extend through the adjacent radiused surface  247 ′ and generally horizontal surface  247 ″ to direct a relatively stable flow of air outwardly from the housing and downwardly under the extended cutting line, as do troughs  104  in head  10 . As with head  10 , various changes can be made in the size, configuration and positioning of the aerodynamic elements in head  210 . 
     While not part of the present invention, the mechanism for loading, gripping and replacing the cutting line  217  in head  210  is described in detail in U.S. Pat. No. 6,928,741 and illustrated in  FIG. 23C . As seen therein, short lengths of cutting line  217  project through opposed eyelets  232  and into aligned radial channels  244  within the head  210 . The lengths of line are held in place by a pair of spring-biased toothed cams  216  that press the inner end portions of the line against channel walls  244 ′. To remove the line when it becomes damaged or worn, the line is simply pulled inwardly and downwardly through the central opening  264  in the lower housing portion  214   b  of the head. Opening  264  is axially aligned with central area  48  in the upper housing portion, providing access to the end portions of the lengths of cutting line. Due to the angular configuration of the locking cams  216  and teeth  250  formed thereon, the cams allow for the sliding movement of the line inwardly through channels  244  to load the lengths of line on the head, hold the line in place during use and allow the line to be pulled inwardly for removal and replacement. 
     While the present invention has been illustrated and described in connection with a bump-feed head  10  and a fixed line head  210 , those particular heads were illustrated and described by way of example only. As noted earlier, the present invention is not limited to a particular type or types of trimmer head or heads having a particular drive, line feeding, loading or replacement mechanism. Also, as noted earlier, various changes and modifications also may be made to the aerodynamic features of the present invention, including, but not limited to, the shape of the head and the shape, size and location of the aerodynamic elements. For example, very shallow troughs in the form of scoring, as opposed to the deeper cavities or depressions formed, for example, by troughs  100 , channels or dimples  102 , could be employed in the outer surfaces of the head to reduce the drag on the head during use. While the scoring may not move sufficient air to effect a reduction of the drag on the cutting line during use, it could reduce somewhat the drag on the housing, increasing the speed of rotation and reducing vibration. An example of such a trimmer head  310  is illustrated in  FIG. 24 . 
     As in the prior embodiments, trimmer head  310  preferably defines an aerodynamic generally ellipsoidal profile, although, again, other head configurations could be employed. As seen in  FIGS. 24A and 24B , the upper portion  314   a  of the trimmer head housing  314  includes aerodynamic elements in the form of score lines  300  tracing a turbine-like pattern. The score lines  300  extend in a closely spaced uniform disposition about upper housing surface  324  such that they trace curvilinear paths extending downwardly and rearwardly from an upper inner portion of surface  324  to a lower outer portion thereof, similar to troughs  100  of trimmer head  10 , except that the score lines  300  preferably form a tighter curvature in the lower portions thereof, similar to the end portions  100 ′ of the troughs shown in  FIG. 19 . The lower housing portion  314   b  can also be provided with score lines  301  in the lower housing surface  345 . The score lines  301  formed in the annular surface of the lower portion  314   b  of the housing preferably form a mirror image of score lines  300 , extending upwardly and rearwardly with respect to the rotational direction of the head as seen in  FIG. 24B . As with the troughs and dimples of the prior embodiment, other score line patterns and configurations could be employed to provide the desired aerodynamic effect. While the size of the score lines also could be varied, by way of example, such lines can be about 0.020 in. wide and 0.010 in. deep with a radial spacing of about 0.200 in. at their midpoints. The edges of the score lines  300  and  301  preferably are radiused at about 0.005 in. to avoid the adverse aerodynamic effects created by sharp edges in the outer surfaces of the head. As seen in  FIG. 24 , the score lines preferably are slightly tighter at their upper end portions than their lower end portions and the opposite is true with respect to the score lines in the lower portion of the housing which are more closely spaced at their lower ends. 
     As seen from the above description, various changes and modifications can be made in the present invention. Insofar as the above described changes and modifications and/or any other changes or modifications are within the purview of any of the appended claims, they are to be considered as part of the present invention.