Low speed rotary mower
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 494,167, filed Aug. 2, 1974. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to lawn mowing equipment and more particularly concerns an improved rotary lawn mower of the type in which a cutting blade is mounted for rotation about a vertical axis within a housing which, together with the ground surface, substantially encloses the blade. 
     Rotary lawn mowers function by impact cutting of blades of grass, that is, by driving the cutting edge to impact the blades of grass at such a substantial velocity that the blades are cut cleanly by the cutting edge before individual grass blades may bend out of the cutting plane. This distinguishes the mowing concept from that of a reel type mower, where blades of grass are cut by means of a pair of cutting edges which operate against one another. 
     It has been known for some time that impact cutting of grass of various varieties and in varying moisture and toughness conditions may be cleanly made at cutting edge or tip speeds as low as 5,000 per minute (FPM). Despite this knowledge, many prior art mowers have been constructed to operate with their cutting edges moving at a speed of as much as 20,000 FPM. The primary reason for the high blade velocities is that it is desirable to have a lawn mower which will impart sufficient momentum to the cut particles so that they may be collected in a porous bag or other receptacle connected to a discharge opening in the mower housing. 
     Unfortunately, driving the cutting blade at such high speeds results in substantial noise generation by the engine used to drive the blade and from interaction of the driven blade and mower housing. Driving the blade of a rotary mower at high tip speeds in the range of 20,000 FPM also means that when the blade contacts foreign objects such as stones, wires, or other harder-than-grass debris on the lawn, it is possible to impart sufficient momentum to the debris to cause them to become hazardous projectiles. Since the kinetic energy available for transfer to the foreign objects increases with the square of velocity, it is highly desirable to operate at a lower velocity. 
     Despite the shortcomings of rotary mowers having blades which are driven at relatively high speeds, mowers available to the consuming public have typically operated at such high blade speeds because of the significant difficulty of constructing a low speed mower which would cleanly cut and bag its cut grass particles, without leaving significant amounts of cut particles on the mown surface or so bruising the grass tips that the lawn takes on a brownish cast within a short time after mowing has been completed. With increased emphasis on safety considerations and sources of noise pollution, there has been corresponding concentration on the problem of constructing a quieter mower with a blade operating at a lower speed without sacrificing other performance features felt to be convenient and desirable by the user, but the problem has not been satisfactorily solved. 
     The present invention provides a solution to these problems. It is a mower in which a substantially lower blade tip speed is used, yet clean cutting bagging is achieved, even in embodiments using rear discharge openings, which as is well known to those of skill in the art, is considerably more difficult than bagging using a side discharge opening. This is true because cutting of grass occurs in the front two qradrants of blade rotation. Therefore grass must be transported a significantly greater distance for rear than for side discharge. The improved rotary mower of the present invention provides momentum to cut grass particles by means of upward, inward mechanical throwing of cut particles by its trailing sail portions in combination with an axial flow pump effect providing a controlled upward flow of air from the cutting region. Use of an upwardly, inwardly raked sail driven at a relatively low tip speed in contrast to previous machines results in a combined mechanical throwing and fluid pumping action which transports cut grass particles into the expansion channel of the housing. 
     Although the mechanism of transport has not been analytically defined, it appears to be an entirely different concept of cut particle transport than the very common transport method in which grass particles are cut and thrown essentially tangentially outward to the side wall of the housing where the air turbulence generated by the blade and sail portions thereof transport grass angularly around the side wall to a discharge opening therein. 
     Another problem at the lower speeds is now more fully appreciated as a result of the present invention. One of the limitations of effective cutting has been discovered to be the problem of keeping the cutting edges &#34;clean&#34; or free of large numbers of cut grass particles which dull the effective cutting edge and result in bruising of grass contacted by the &#34;loaded&#34; cutting edge. As a result of this discovery, certain modifications have been made in specific embodiments of the invention that allow the cutting edges to clean themselves more effectively and thus promote impact cutting at the lower speeds. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a cutting blade is mounted for rotation about a substantially vertical axis. The cutting blade has a pair of cutting tips at opposite ends. Each tip includes a leading cutting edge, a body, and a following trailing sail portion. The sail portions of the cutting blade have a substantial upward and inward rake angle. The cutting blade is mounted within a mower housing having an expansion or channel portion with a top wall having a significant rise through its extent about the housing. The housing has a substantially continuous exterior side wall surrounding the blade at the height thereof and extending above the sail portion. The mower discharge opening is located vertically above a portion of this side wall. Means for driving the cutting blade rotatably at a tip speed below 14,000 feet per minute, such as an internal combustion engine, are provided. Means such as wheels mounted on the housing enable movement of the housing and enclosed blade over the lawn surface at a controlled height above it. A grass-catching bag may also be attached to the discharge for collection of grass clippings therein. 
     It has been found that, at these lower speeds it is highly desirable to maintain certain sail parameters within established ranges. For example, the trailing sail portion preferably has substantial upward rake angle between substantially fifteen degrees (15°) and substantially fifty degrees (50°) near the upper end of the tip speed range, say at approximately 10,500 feet per minute and above, down to a range of substantially twenty-five degrees (25°) to substantially fifty degrees (50°) at approximately 8500 feet per minute tip speed. For the purpose of these ranges, the upward right angle is defined as that angle between the front surface of the sail and a plane perpendicular to the cutting plane defined by rotation of the cutting edges about the rotational axis of the blade. This provides the proper upward mechanical throwing action. 
     In addition, it is important to achieve a mechanical throwing action which is inward rather than tangential to blade rotation to help prevent precipitation of clippings at the housing edges. To promote this preferred range of substantial inward rake angles, that is, the angles between the sail portions and the blade center line measured in the plane of rotation, has been established. Based upon available data, this preferred range is substantially from ten degrees (10°) to sixty degrees (60°). 
     In certain embodiments of the present invention, particularly those embodiments in which cutting blade tip speeds below 10,500 feet per minute are desired, it has been discovered that performance may be improved by introducing cutting edge sweepback or by selecting a lesser blade diameter than is typically used for mowers at the upper end of the range. Both of these parameter selections are believed to be effective to increase the available centrifugal force to clear cut grass particles from the cutting edge at a lower cutting edge speeds being used. Blade sweepback accomplishes this because of an increased component of outward force on cut grass particles when they impact a sweptback edge. Decreased blade diameter for a given tip speed results in increased angular velocity, which provides additional cutting edge cleaning. It has also been found desirable to increase sail height or sail frontal area to improve evacuation at the lower cutting blade tip speeds under some conditions. 
     As previously mentioned, it is particularly difficult to achieve a rear discharge bagging type mower. Therefore, it is an important achievement of the present invention that it is possible to construct a rear discharge bagging mower having a blade which rotates at a relatively low cutting edge speed. In certain rear discharge opening embodiments of the present invention, the mower cutting blade may be so mounted with respect to the housing that, in the rear two quadrants of blade rotation, the blade tip clearance between the housing side wall and the blade tip is at a maximum of three-sixteenths (3/16) inch. This tends to prevent clippings which may not exit the housing in the initial contact and transport from the blade from being dropped to the lawn surface at the rear of the mower and enables them to be carried through another revolution for subsequent discharge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon a reading of the following detailed description and upon reference to the drawings showing specific embodiments of the invention. Referring particularly to the drawings, wherein the use of like reference numerals throughout the several views denotes like elements: 
     FIG. 1 is a perspective view of a rear discharge rotary mower constructed in accordance with the present invention, the handle portion thereof being partially cut away; 
     FIG. 2 is a sectional view vertically through the center of the housing of the mower of FIG. 1, with a blade mounted therein to show the position of the blade with respect to the housing and rear discharge opening; 
     FIG. 3 is a perspective view of a cutting tip of a blade constructed according to an embodiment of the present invention, particularly illustrating an important angle defining a sail parameter; 
     FIG. 4 is a view of a portion of the cutting tip of FIG. 3 taken perpendicular to the front surface of the sail portion to particularly illustrate a second important angle; 
     FIGS. 5, 6 and 7 are plan, elevational and side views respectively of cutting tips of the blade of FIGS. 2, 3 and 4 with vector diagrams thereon for the purpose of illustrating and explaining the mechanism of operation of the trailing sail portions of a mower constructed according to the present invention; 
     FIG. 8 is a plan view showing the housing of the mower of the present invention in dotted line form and portions of a cutting tip in successive positions about the housing for the purpose of discussion of particle trajectories followed by the clipped grass impacted and thrown by the cutting blade; 
     FIG. 9 is a generated view taken from FIG. 8 showing, inter alia, the rise of the expansion portion of the mower housing which enables discharge of cut grass particles through the combined pumping and mechanical throwing effects; and 
     FIG. 10 is a plan view of a blade cutting tip similar to the view shown in FIG. 5, particularly showing sweepback of the cutting edge utilized in certain embodiments of the invention to aid in clearing of cut grass particles from the cutting edge. 
    
    
     While the invention will now be described in connection with specific embodiments, it should be clear that the invention is not limited in scope to those embodiments. On the contrary, all alternatives, modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims are covered. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a rear bagging rotary lawn mower generally designated 10 is shown in perspective, a portion of an operator handle therefore being cut away since it is not essential to the invention. Rear bagging rotary lawn mower 10 has a mower housing 12 to which other mower elements may be attached. Mower housing 12 substantially surrounds and encloses a cutting blade (not shown in FIG. 1) which is driven about a vertical axis by an internal combustion engine 14. 
     Housing 12 has mounted thereon front and rear wheels 15 which, by their height with respect to the housing, control the height and inclination of the cutting blade, as is typical in modern rotary lawn mowers. Front and rear wheels 15 may be fastened so that the axis of rotation of the blade, although perpendicular to certain housing surfaces, will not be quite perpendicular to the ground surface. Normally, the wheel mounting will be such that housing 12 and the cutting plane will be slightly closer to the ground surface in the front of the mower than in the rear. The purpose of this is twofold. First, the amount of power needed to drive the blade is decreased since contact with the grass occurs only in the front two quadrants of the circle swept out by the cutting blade. Second, because the blade is higher in the rear two quadrants it should not contact nor bruise the cut grass tips, which would cause undesirable and unsightly browning of the grass top edges. 
     Housing 12 has a symmetrical channel or expansion 16 of essentially uniform width but with a top wall thereof of gradually increasing height. The top wall of expansion or channel 16 in the particular embodiment shown in FIG. 1 is essentially a circular helix form which terminates in a discharge chute or opening 18, the outer surfaces of which are shown in FIG. 1. While it is not essential that the expansion 16 be of circular helix form, it is significant that there is a substantial rise of the expansion portion between the cutting region and the discharge opening. 
     Discharge chute 18 has mounted therein a pivotable arm 20 to which is attached a discharge closure door (not shown) located interior of chute 18. This discharge closure may be used to close the discharge for use of the mower in a mulching mode or as a safety feature in the event the mower is started with its grass-catching receptacle removed. Attached to the discharge opening is a clipping receptacle 22 which may be a grass-catching bag of a woven porous fabric. Also shown in FIG. 1 is an operator handle 23 with a cross brace 24 attached thereto against which the back of grass-catching bag 22 may rest, particularly in embodiments which use a bag with a reinforced top frame. 
     FIG. 2 shows a vertical section taken through housing 12 along a plane parallel to the intended direction of travel through the center of the mower. An unsectioned cutting blade is placed in the sectionally shown housing to illustrate the relative blade and housing positions in the embodiment being described. Mower housing 12 includes an exterior side wall 26, substantially vertical in extent and, in the embodiment shown, circularly cylindrical in form, which surrounds and provides a horizontal enclosure for the cutting blade. 
     In the housing of FIG. 2, a discharge insert 27 provides continuity to the exterior side wall at the level of the cutting blade so that the lower section of side wall 26, in the particular embodiment shown, is substantially a complete circular cylinder. It is preferable that the housing be substantially enclosed in the cutting region, but it should be apparent that it is not necessary that this be accomplished with an insert. It may be possible to mold housing 12 so that the insert structure is an integral part thereof or achieve required minimum clearances between blade and housing in yet another way. 
     Continuing with description of the features of the specific embodiment of housing 12 shown, housing 12 has, connected to exterior side wall 26 by means of a rounded but substantially right angle portion, a channel or expansion to wall 28. Expansion top wall 28 is configured with a substantial vertical rise which occurs in its three hundred sixty degrees (360°) of angular extent above the housing, so that the vertical height of expansion top wall 28 proximate the discharge opening above the portion of wall 28 where the expansion commences is considerably higher than is typically encountered. 
     For example, expansion top wall 28, in an embodiment recently constructed, had an angular rise of approximately twenty degrees (20°) and a vertical rise of somewhat over six (6) inches in its excursion about the mower housing. 
     On the opposite or inner side of the expansion or channel portion which defines an upward circular helix from the substantially cylindrical cutting region of the housing is an inner expansion side wall 29 which generally follows the expansion top wall and increases in its height to provide continuity between expansion top wall 28 and a substantially planar housing inner top wall 30 located centrally of the mower. Housing inner top wall 30 is generally a circular plate in form with a motor mounting wall 32 rearwardly offset from the center thereof. A vertical axis is represented by a vertical center line extending downward from motor mounting wall 32. This represents a rotational axis of a mower cutting blade 34 mounted to the drive shaft of an engine (not shown for clarity of representation). 
     Referring now particularly to cutting blade 34, it will be noted that cutting blade 34 is mounted offset with respect to the center of the chamber defined by side wall 26, insert 27 and the elements 28, 29, 30 and 32 which define the top wall of the housing. It is not essential to the present invention that such offset mounting occur. However, it has been found preferable that, in the rear two quadrants of blade rotation, there be a sufficiently small clearance between the blade and the housing to create a substantial pumping effect and to prevent grass cuttings and other debris from collecting on the rear wall and sliding downward therefrom to cause unsightly accumulation on the lawn. It has been found with the cutting edge speeds used, that is cutting edge speeds below 14,000 FPM, that it is preferable to maintain a tip to side wall clearance less than three-sixteenths (3/16) inch in the rear two quadrants of blade rotation. 
     Cutting blade 34 includes a central portion 35 having upwardly extending flanges 36 for engagement with a blade retainer element or other drive means. Angularly discontinuous from central portion 35 are a pair of cutting tips 38 at opposite ends of blade 34. Cutting tips 38 are mounted lower than central portion 35 to reduce the power required to drive the mower. If the entire blade were substantially planar, so that the cutting edge was at the same level as the remainder of the blade, much more contact between blade and grass would instantaneously occur. Such contact would create additional drag without providing additional performance. 
     As previously mentioned, expansion top wall 28 extends around the housing 12, preferably in a circular helix form, to a discharge opening. In the sectional view of FIG. 2, a portion of the discharge opening is visible at the interior of the housing. A discharge opening bottom edge 39, which is the top of the vertical wall of discharge insert 27, defines the bottom of the discharge area. An important aspect of the present invention is that the discharge opening is completely away from, in this case above, the cutting region. 
     Additional features of the housing 12 shown in FIG. 2 include front and rear wheel mounting flanges 41 and 42 respectively and a recessed portion for a discharge closure or door 44 which in the figure appears as a discontinuous extension of expansion top wall 28. 
     In viewing the discharge opening of housing 12, it should be particularly noted that the vertical dimension of the opening exceeds the width dimension. It is important to the present invention that there be substantial area available for the discharge opening. As an example, in one particular embodiment of the present invention which has been constructed, the height of the opening was approximately eight (8) inches and the width thereof was approximately four and seventy-five hundredths (4.75) inches. It is also important to effective collection that the clippings be lifted and carried upward and away from the cutting region rather than forced therefrom by a tremendous expenditure of power with its attendant high blade speed requirement. 
     Moving now to discussion of FIG. 3, there is shown an enlarged perspective view of one of cutting tips 38 of the rotary mower blade. Each cutting tip 38 has a body portion 46 with a beveled leading or cutting edge 47 which is, of course, the lowest point on the blade, with body 46 being slightly higher so that only the cutting edges define a cutting plane and bruising of the grass does not result because there is no additional contact between the blade body and the grass tips. Located rearwardly of beveled cutting edge 47 is a trailing sail portion 48, in the embodiment shown generally trapezoidal in form with a planar front surface. Trailing sail portion 48 may be constructed of a portion of blade material formed upwardly with respect to body 46. Trailing sail 48 has a sail exterior edge 51 and upper edge 49. The front surface of sail 48 in the embodiment shown is substantially planar and has a substantial inward rake with respect to a line parallel to a radial line from the axis of rotation of the blade. 
     Although the exact theory of operation of the present invention to provide low speed cutting and bagging discharge of cut grass particles is not rigorously defined, it is known that the inward rake angle identified as angle A in FIG. 3 is an important parameter to be controlled in construction of a mower which operates in accord with the present invention. The inward rake angle A is defined as the angle between the blade center line measured in the plane of rotation and a line defined by intersection of the front surface of sail 48 with the cutting plane or the body portion of the tip 46. Experimentation with inward rake angles of varied values has shown that a substantial inward rake angle is necessary to provide the inward momentum to cut grass particles necessary to prevent the particles from being tangentially thrown to adhere to the exterior side wall of the housing. 
     At this point, the range of preferred tip speeds for the blades on mowers of the present invention will be briefly discussed. This range of preferred velocities is from the lower limit of impact cutting to a maximum of 14,000 feet per minute. The lower limit of impact cutting is not precisely defined. Because the mechanism of impact cutting is basically cutting a cantilevered grass filament by an impact blow delivered faster than the grass can flex with a sufficiently sharp edge to sever the tip, the lower limit in terms of cutting edge speed is dependent upon the condition of the grass being cut, the type of grass being cut, the sharpness of the blade presented to the grass filaments, and other factors. At one time, the lower limit of impact cutting was thought to be 5,000 feet per minute, it being believed that it was not possible to generate sufficient momentum and fluid flow to clear the grass from the cutting edge at tip speeds below this level. It has since been discovered that, by selection of a lesser blade diameter and/or introducing sweepback of the blade cutting edge, it is possible to operate at somewhat below this previously assumed minimum, at least under optimum mower conditions with particular grass varieties. 
     At the upper limit, above 14,000 feet per minute, the substantial upward and inward rake of the sail, designed to provide predominantly mechanical suspension and transport rather than pneumatic transport, tend to over-pressurize the housing and cause mower &#34;blowout&#34;, with entrained air (and clippings) being driven out around the lower edges of the housing rather than channeled into the bag. In addition, such high speeds are likely to result in undue turbulence within the housing, and to interfere with orderly cutting and evacuation of the housing. 
     Unlike conventional high blade speed mowers which rely almost exclusively on pneumatic suspension and transport of cut grass particles, the present invention is believed to suspend the cut grass particles and provide initial transport of the particles by combined mechanical throwing and pneumatic pumping effects using the sail surfaces. This makes sail parameters extremely important. 
     For example, it has been determined that the inward rake angle A shown in FIG. 3 should preferably be somewhere in the range of greater than substantially ten degrees (10°) to substantially sixty degrees (60°). Below ten degrees (10°) of inward rake, the discharge stream has too great a radial component, causing grass buildup on the housing side wall. The sixty degree (60°) upper limit for the inward rake angle occurs because at angular velocities within the working range, an angle greater than sixty degrees (60°) results in undue turbulence, creates high chamber pressures, and due to over-agitation of the lawn surface, results in poor cuts. As would be expected, it also consumes large amounts of power. 
     Experimentation with and analysis of the inward rake angle parameter have produced a more accurate definition of the available range of inward rake angles. In addition, a hypothesis to explain the significance of the inward rake angle has been developed. According to that hypothesis, the inward rake angle of the sail is important because it aids in neutralizing the outboard vortex which tends to develop around the outboard edge of the sail and to allow cut grass particles to escape and be deposited along the grass surface, rather than discharged. At the higher speed which have been traditional with rotary mowers, the outboard vortex effect is not so significant because of the greater pneumatic pumping effects generated by the sails themselves. However, at the lower speeds at which the present invention operates, the deleterious affects of the outboard vortex are very significant. Therefore, it is important to the invention that the cutting blades have sails with inward rake angles within the range specified above. 
     It is presently believed that inward rake simultaneously works against discharge flow by tending to cause the cut grass particles to circulate with the blade rather than exiting the discharge opening. At lower speeds, less centrifugal force is imparted to the cut grass particles which may make a lesser inward rake angle preferable to lessen recirculation of such particles. 
     Since the front surface of the trailing sail is used in a combined mechanical throwing and pneumatic pumping action, it is also important that the sail have a substantial vertical height above the body portion, and sufficient front surface area to provide an effective impact and pumping surface at the speeds being used. It is preferable that the sails have a vertical height above the body of the cutting tips of at least one-half inch. As an example, in a specific blade recently constructed, the sail portions were approximately two and five tenths (2.5) inches long and sixty-two hundredths (0.62) inches in vertical height above the body. 
     Further recent developments and testing have shown that at tip speeds toward the lower end of the specified range, it may be desirable to have additional sail height, that is, distance between the cutting tip body and the sail top edge measured along the sail front surface, in order to aid in effective evacuation of cut grass particles from the mower housing and to maintain a cohesive discharge stream. As an example, in one test a machine with a twenty-one (21) inch diameter blade driven at a tip speed of 8,500 FPM, a cutting edge sweepback angle, that is, the angle between its cutting edge and the blade center line, of twenty degrees (20°), and inward and upward rake angles of fifteen degrees (15°) and thirty degrees (30°) respectively, the sail height was increased to achieve a vertical height of 1.38 inches. The result was improved evacuation of the housing at the 8,500 FPM tip speed with acceptable bagging efficiency and quality of cut over the lawns being tested. In view of these results, it is believed that it may be helpful and desirable at lower tip speeds to increase sail area by means of increases in the sail height. 
     FIG. 4 is another view of a portion of cutting tip 38 taken perpendicular to the front surface of trailing sail portion 48 to aid in definition of a preferred range of angles of upward rake of trailing sail 48. In that figure, cutting edge 47, sail exterior edge 51 and trailing sail portion 48 are shown. An angle B shown in FIG. 4 is defined as the angle between the front surface of trailing sail portion 48 and a line perpendicular to the body of tip 46. For purposes of this specification, this will be referred to as the upward rake angle of the trailing sail. 
     A substantial upward rake of the sail, that is, a relatively low upward rake angle, has been found important to achieve proper discharge flow and promote bagging. It has been discovered that the range of acceptable upward rake angles is dependent upon the tip speed of the cutting blades. A preferred range for the upward rake angle B has been established empirically by test of different types of trailing sails. It has been found preferable at tip speeds of approximately 10,500 FPM and above, to select the upward rake angle to be in the range of substantially fifteen (degrees (15°) to fifty degrees (50°). Experimental results indicate that angles less than fifteen degrees (15°) may result in too low a component of upward momentum transmitted to the grass particles. Therefore they may not be readily suspended and transported upward from the cutting region to the expansion or channel portion of the housing. This complicates discharge into a bagging receptacle. 
     To better understand the invention, one should realize that at least some of the grass particles apparently impact and rebound from the trailing sail portion. If the particles&#39; angle of incidence is not sufficiently large (a small upward rake angle), the particles will tend to rebound and remain within the cutting region. Furthermore, with greater upward rake, that is, less than fifteen degrees (15°) upward rake angle, the flow behind the blade appears to be much more turbulent and result in a lower upward amount of pneumatic transport. 
     On the other hand, if the angle is too great, say greater than fifty degrees (50°), the sail will not have sufficient authority and a substantial number of clippings will tend to slide over the trailing sail portion without receiving substantial upward momentum or mechanical throw therefrom. Additionally, there will be a lesser amount of wake or turbulence behind the sail which may decrease the component of horizontal velocity necessary to achieve effective pneumatic transport. 
     Thus, in both cases, angles outside the range of recommended upward rake angle B may result in a less effective discharge stream. 
     As mentioned previously, selection of a preferred upward rake angle is dependent upon the tip speed of the particular mower being used. At the lower ranges of speed, say, in the vicinity of 8,500 FPM or less, rake angles are preferably in the narrower range of from substantially twenty-five degrees (25°) to fifty degrees (50°). At this lower speed, it appears to be important to have a more laid back sail which results in a more coherent discharge stream than sails with rake angles less than twenty-five degrees (25°). 
     FIGS. 5, 6 and 7 are vector diagrams showing a cutting tip 38 in plan, front and side views respectively for purposes of discussion of components of velocity imparted to cut grass particles and debris by the trailing sail portion of a blade constructed according to the present invention. In these figures, the velocity-representing vectors are identified with lower case letters with certain important angles between these velocity vectors identified with upper case letters. 
     While the vectors shown represent relationships for a specific embodiment of the present invention, they are helpful in understanding the concepts underlying the present invention. In the specific embodiment on which the diagrams are based, a nineteen (19) inch blade was used and driven at approximately 2,100 RPM so that the tip velocities were in the vicinity of 10,500 FPM, approximately a nominal tip speed for the mower of the present invention. The trailing sail portion in FIGS. 5, 6 and 7 has an inward rake angle of approximately forty degrees (40°) and an upward rake angle of approximately thirty degrees (30°). 
     The vectors radiate outwardly from a representative point on the trailing sail portion of the cutting tip. A vector extends perpendicular to the cutting edge of the blade and represents the velocity of the blade with respect to the mower at that radius. Since the blade is rotating about an axis, the instantaneous velocity of the cutting edge is directed perpendicularly to a radial line from the blade axis of rotation. Erect blades of grass are impacted by the cutting edge of the blade and severed thereby. As a result of this impact, they gain some amount of forward velocity. However, they gain less than the blade velocity and thus move back along the body of the cutting tip until they contact or are carried by air flow over the front surface of the trailing sail portion. In either case, the sail action imparts substantial additional forward momentum. The cut grass particles and debris will not acquire the same tangential component of velocity as the blade itself. Therefore, with respect to the blade they will have a component of rearward velocity, respresented in FIGS. 5 and 7 by vector b. 
     Due to the substantial inward rake of the trailing sail&#39;s front surface, clippings which impact or are pneumatically pumped by the front surface will also attain a substantial inward component of velocity represented in FIGS. 5 and 6 as vector c and directed perpendicular to vectors b and a. This inward velocity is a direct result of the inward rake angle of the trailing sail portion. The resultant rearward and upward velocity of the grass, the sum of vectors b and c, is identified as vector d in FIG. 5 and has an angle X with respect to vector b or a tangential line. 
     For the specific embodiment described and specified above, the angle X which describes the movement of grass particles upward and inward along the sail has been measured at thirty degrees (30°) to thirty-five degrees (35°) experimentally. Thus, there is a very substantial inward component of velocity imparted to the grass by combined mechanical throwing and pneumatic pumping action of the inwardly raked sail. 
     Vectors b, c and d all represent velocities with respect to the blade. The grass particles will, of course, actually be moving in the same direction as the blade with respect to the mower or ground surface. The velocity of the grass with respect to the mower itself is represented by vector d&#39;, the sum of vector d and vector a, the velocity of the blade with respect to the mower. The angle between vectors a and d&#39; or the angle of inward throw of grass particles with respect to the housing is identified as angle Y in FIG. 5. For the embodiment with the specific parameters previously described, this inward throw angle will be approximately twenty degrees (20°). 
     Due to the upward rake angle of the sail there will be a substantial upward component of grass velocity which brings the grass clippings into the expansion channel where they may be easily transported outward to the discharge opening. Vector e in FIG. 6 represents this upward component of velocity. It is substantially equal to the rearward component of velocity b in magnitude but directed perpendicular thereto. Therefore, the resultant upward and inward velocity, vector f, which is the sum of vectors e and c, is at an angle Z to the perpendicular and angle Z is substantially equal in magnitude to angle X. 
     FIG. 7 shows a side view of cutting tip 38 so that the resultant upward and rearward velocity with respect to the blade, labeled vector g, may be combined with the velocity of the blade with respect to the mower to give vector g&#39;. This illustrates the substantial upward throw with respect to the housing throwing clippings into the expansion or channel portion of the area where it may be aided by the pneumatic transport effect of the housing and rotating blade combination. 
     FIGS. 8 and 9 are views which show successive positions of a blade cutting tip for purposes of discussion of suspension and transport phenomena. FIG. 9 is a generated view taken from FIG. 8 in which the expansion top wall of the housing is effectively unwound. In FIG. 8, successive positions of the cutting tip are identified with reference numerals 60 through 65 respectively. Taking a circle defined by the rotation of the cutting blade about its driven axis, a number of trajectories of grass particles are shown. For example, the trajectory of the particles from cutting tip 60 is initially substantially inward from a tangential line. However, the particles eventually gather toward the outer edge of the top expansion wall and are carried by pneumatic transport generated by an axial flow pumping effect created by the blade and continuous housing to the discharge opening. In providing this pumping action insert 27 is particularly important, since it provides side continuity therefore enabling a substantially greater axial pumping effect. It should be understood that for grass cut in successive positions 61, 62, 63, 64 and 65 there is less need for pneumatic transport since cutting occurs closer to the discharge opening and less angular redirection of cut particles is required. Also, the substantial rise of the expansion top wall shown particularly in FIG. 9, aids in enabling grass cut on the same half of the housing as the discharge opening to rise more sharply and be transported into the bag with a high trajectory. 
     It is believed that it is the combination of this inward upward mechanical throwing action of the sail, which suspends and transports cut grass particles, with pneumatic flow which aids transport that enables the mower of the present invention to achieve rear discharge and bagging despite a relatively low cutting blade tip speed. 
     As previously alluded to, there are a number of important criteria which must be met to achieve an acceptable rotary mower configuration. First, there must be sufficient cutting edge sharpness and velocity to sever grass filaments at the speed at which the cutting edges are driven. Secondly, since cutting edges of a rotary mower tend to load up with severed filaments, particularly &#34;hairpins&#34;, that is, filaments which break off below the point of cutting edge impact and wrap around the cutting edge, there must be sufficient centrifugal force or other forces to clean or clear severed grass filaments from the cutting edges. 
     For a rotary mower having a blade diameter typical of machines on the market, for example, a twenty-one (21) inch diameter, as tip speed of the blade is decreased, a point is reached at which blade cleaning becomes difficult to achieve. For a twenty-one (21) inch machine with no sweepback to its cutting edges, that is one with cutting edges parallel to the blade center line measured in the cutting plane, it is estimated that this will occur in the vicinity of 8,500 FPM. 
     There are a number of factors which are believed to combine in their effects at lower speeds to cause this limitation. The dynamic pressure generated by blade rotation decreases with the square of blade speed. The centrifugal force which aids in cleaning cut grass particles from the mower blade also decreases with the square of blade speed. In addition, when the tip speed decreases, assuming the forward speed at which the mower is moved over the lawn remains constant, the grass loading per cutting edge must necessarily also increase. 
     It has been discovered that by introducing sweepback to the cutting edges, it is possible to improve cleaning of the cutting edges for a given tip speed and blade diameter. Blade sweepback is illustrated in a plan view of a cutting tip of a blade in FIG. 10. FIG. 10 shows a cutting blade tip generally designated 70 in plan view. Tip 70 is at the end of a rotary cutting blade having a blade center line 72 measured in the cutting plane. Tip 70 has a cutting edge 74 and a trailing sail portion 76. Cutting edge 74 has substantial sweepback. Sweepback, as referred to herein, is the angle between center line 72 and the cutting edge of tip body 70. In FIG. 10 the sweepback angle is designated angle S. The blade shown in plan view in FIG. 5 is a blade with zero sweepback. The effect of sweepback is to lessen the normal component of impact force when a cutting edge contacts grass filaments and to increase the radial or outward component of force transmitted to the filaments. This gives the filaments an outward momentum which aids in cleaning of the cutting edges. 
     It has also been discovered that there are limitations to the amount of cutting edge sweepback which may be employed to aid in cleaning of the blade. As one increases the cutting edge sweepback angle, a point is reached at which the cutting edge will no longer sever filaments cleanly, but will instead tend to push filaments sideways out of the cutting plane. At this point, while the blade may clean itself properly it will not satisfactorily cut the lawn. It is believed that blade sweepback angles substantially greater than twenty-five degrees (25°) may be above this practical limit. 
     It may be helpful in understanding the advantages of sweepback as applied to the present invention to consider a specific example. For a mower with a twenty-one (21) inch blade diameter and a zero sweepback angle (as shown in FIG. 5), it is believed that a tip speed of approximately 8,500 FPM is necessary to generate sufficient centrifugal force for the purpose of satisfactorily cleaning the cutting edges of severed grass filaments under certain lawn conditions with certain grass varieties. If a fifteen degree (15°) sweepback angle is introduced, experimental results indicate that a tip speed as low as 6,000 FPM may be sufficient to achieve satisfactory cleaning of the blade under the same mowing conditions. 
     Another way of achieving improved cleaning at a given tip speed is to select a lesser blade diameter. At a lesser blade diameter, maintaining tip speed constant necessitates increased angular velocity and therefore, results in increased available centrifugal force for cleaning of the cutting edges. Considering a mower with a fifteen degree (15°) sweepback angle and a twenty-one (21) inch blade diameter, results indicate that adequate cleaning of the cutting edges may cease below approximately 6,000 FPM. If the blade diameter is decreased to twelve (12) inches, results indicate that satisfactory cleaning of the cutting edges may be maintained at a tip speed as low as approximately 4,600 FPM, although other problems including loss of discharge efficiency may occur in some grass conditions at those levels. 
     The tip speeds and other important parameters which define the present invention have been found to be interrelated. Thus, as mentioned above, adjustments in the sail inward and upward rake angles may be desirable to achieve somewhat improved performance depending upon the blade tip speed which has been selected. It also follows from the discussion above that the selection of a given blade diameter or cutting edge sweepback angle may limit available tip speeds. At the larger blade diameters which are common with riding type lawn mowers, it may be necessary to introduce substantial sweepback to the cutting edges or accept a higher tip speed in order to achieve desired blade cleaning and quality of cut. 
     It should be stressed that it is not claimed that mere use of a trailing sail alone, use of a symmetrical expansion type housing, or the use of a sail with an inward rake or upward rake are in themselves the invention. What has been accomplished here is the development of a mower with a combined mechanical throwing and pneumatic transport action in which a combination of certain important features regarding the configuration of the cutting blade in combination with features of the housing which allows construction of rotary mowers which operate in a low speed range and are capable of functioning in a bagging mode. 
     From the foregoing, it should be apparent that a low speed rotary mower overcoming the shortcomings and disadvantages of prior art rotary mowers has been provided. The invention has been described in the specification in conjunction with specific embodiments thereof. However, it is evident that a number of alternatives and modifications will be apparent to persons skilled in the art after a reading of the foregoing description. Accordingly, it is intended to embrace all such alternatives modifications and variations falling within the spirit and broad scope of the appended claims.