Model airplane engine starter and drive train
BACKGROUND OF PRIOR ART 
     The closest prior art is believed to be U.S. Pat. No. 4,440,122, issued to Roger D. Theobald, on Apr. 3, 1984, and entitled &#34;Model Airplane Engine Starter&#34;. The contents thereof is incorporated herein by reference thereto. Particular reference is made to the prior art discussion contained therein, in Column 1 thereof and to the structure disclosed and claimed therein. 
     The device shown in the above Theobald patent constitutes a definite contribution to the art in that it positively precludes running of the engine in reverse, and in that the starter spring automatically engages the propeller and becomes energized when the latter is rotated in a contra-normal direction. Extended usage thereof has shown, however, that the connector member which connects the drive shaft to the propeller tends, upon occasion, to become loose and to slip, with the result that although the engine continues to operate properly, the propeller is no longer adequately driven because the connector member slips relative to the drive plate carried by the drive shaft. When the engine is shut off or runs out of fuel, the rapidly rotating propeller tends to continue to rotate. Since this tendency is directed in a direction opposite to that in which the propeller-securing nut tightens, the nut is urged by the propeller to turn toward a loosened position, causing the propeller to be held in a less-secured relation against the drive plate. As a consequence, power is lost initially and finally, the propeller may not be driven at all. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a positive guarantee against the problem described hereinabove in that there is no way that the propeller can slip relative to its driving member as a result of slight loosening of the nut which secures the propeller to the drive shaft. This is accomplished by ensuring positively against any slippage between the propeller and its associated driving member. Toward this end, we have embedded a metal hub within the center of the otherwise plastic connector which provides the advantages referred to hereinabove in the above Theobald patent. This metal hub is press-fit upon an end of the drive shaft which is splined to preclude slippage between the shaft and hub. The hub is provided with a hexagonal circumferential surface to preclude relative rotation between the connector and its hub. The outer end of the hub terminates abreast of the outer end of the rest of the connector and is knurled to positively drive the propeller which it engages and against which it is secured by the threaded propeller-securing pin which is threaded into the end of the drive shaft. As added insurance, the connector carries an outwardly extending fixed and rigid drive element in position to engage the propeller and to drive the same, if and when it is so engaged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of one preferred embodiment of the MODEL AIRPLANE ENGINE STARTER AND DRIVE TRAIN is hereafter described with specific reference being made to the drawings in which: 
     FIG. 1 is a left side elevational view of a self-starting model internal-combustion engine embodying my invention; 
     FIG. 2 is an exploded perspective view of the propeller and self-starter assembly elements, with the more remote parts of the propeller and engine shaft journal broken away; 
     FIG. 3 is an outer end elevational view of the connector member; 
     FIG. 4 is an inner end elevational view of the connector member; 
     FIG. 5 is a perspective view of the metal hub of the connector member; and 
     FIG. 6 is a vertical sectional view of a modified connector member without the propeller driving element thereon, viewed from a position such as along line 6--6 of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of our invention, as shown in FIGS. 1-6, inclusive, may include a conventional model internal-combustion engine 10 which includes a cylinder 11, a drive shaft 12, the outer end 13 of which is splined, as shown in FIG. 2. The shaft 12 is carried in its tubular journal 14. The engine also includes a propeller 15 and a machine screw 16 which secures the propeller to the drive shaft and is threaded into the internally threaded counter-bore in the outer end of the shaft 12. 
     As shown, the self-starter includes a helically wound wire spring 17 made of wire as described in U.S. Pat. No. 2,751,898. This spring has its inner end formed into a loop or anchor element 18 which extends around the cylinder 11 to anchor the spring thereto. The other end of the spring 17 is formed into a hook member 19 which extends radially outwardly in a radial plane from the outer end convolution. The anchor element 18 extends axially outwardly from the opposite end of the spring in a plane tangential to the convolutions of the spring. 
     As shown in FIGS. 1 and 2, the spring 17 is mounted in coaxial surrounding relation to the drive shaft 12 and is tubular journal 14. One of its ends bears against the end of the crankcase of the engine 10 and at its opposite end against a cylindrically-shaped plastic connector member 20. 
     The plastic connector member 20 has a metal hub element 21 embedded therein which is fixedly mounted at its axis on the splined end 13 of shaft 12 for driven rotation therewith. As best shown in FIG. 5, the hub element 21 has a central bore 22. The outer end portion 23 of the hub element is cylindrical in configuration and its outer end surface is knurled as at 24. The diameter of this end portion 23 is equal to the diameter of the hub 25 of the propeller 15 and bears thereagainst to rotatively drive the same when the self-starter is properly assembled. 
     The inner end portion 26 of the hub element is also cylindrical in configuration and is of reduced diameter as compared to the outer end 23 and bears against the outer end of the journal 14 when assembled. The intermediate portion 27 of the hub element 21 is hexagonal in configuration and keys the hub element to the inner-more plastic portion of the connector member 20, since it is embedded therein. 
     Since the hub element 21 is embedded in the inner-more plastic portion of the connector element 20, the inner end of the plastic portion has a hexagonally-shaped recess therein in which the correspondingly shaped intermediate hub portion 27 lies. Peripheral walls 28 and 29 extend circumferentially and axially inwardly from the areas of the connector surrounding the intermediate portion 27 of the metal hub, defining ramps which terminate abruptly at spring-engaging elements or tangs 30 and 31. It will be seen by reference to FIGS. 1 and 2 that peripheral walls 28 and 29 each slope axially outwardly away from the propeller until they reach the next spring-engaging element 30 or 31, at which point the next inward cut is made to form the next spring-engaging element. 
     The diametrical dimensions of the recess of the connector member 20 defined by the peripheral walls 28 and 29 are slightly larger than the external diameter of the spring 17 so that the end of the latter which carries the hook 19 extends into that recess. As a consequence, the hook 19 rides upon the ramps 28 and 29 at all times as the propeller and connector member rotate while free running, and snaps into the recess adjacent the spring-engaging element each time that point passes the hook, and thereafter is gradually cammed axially away from the propeller until the next spring-engaging element 30 or 31 passes. At the opposite end of the connector member 20, a drive element 32 extends axially outwardly from the periphery thereof. In a second form of the invention, as shown in FIG. 6, this drive element 32 may not be included. 
     As stated hereinbefore, the machine screw 16 secures the propeller 15 to the engine, passing through the axial opening of the propeller, the central bore 22 of the hub element 21, and extending into the threaded counter-bore of the shaft 12. When tightened, the machine screw 16 forces the hub of propeller 15 against the knurled end surface 23 of the hub 21 and holds the two against relative rotation. Since the latter is keyed to the shaft 12 by the splined end thereof, it is impossible for relative rotation to occur therebetween. 
     The drive element 32 is positioned so as to engage the propeller blades and thereby drive the propeller when so engaged. Since it extends axially beyond the propeller blade, there is no way slippage can occur between the drive shaft 12 and the propeller 15. 
     Although there is a slight drag created by the hook member 19 riding along the ramps 28, 29, the drag and the attendant wear upon the ramps is not appreciable because the latter are formed of plastic which has self-lubricating qualities. Thus, it does not materially affect the operation or performance of the engine 10 and does not cause undue wear upon either the portion of the spring which rides upon the ramp or upon the ramp walls themselves. The plastic material from which the connector member 20 is made is sold commercially under the trademark ZYTEL Nylon by Dupont de Nemours E I &amp; Co., of Wilmington, Del. 
     The self-starter assembly described herein is operated to effect starting in the same manner as described in U.S. Pat. No. 4,440,122, and the spring member rides upon the ramps when the engine is running in the same manner as described therein. 
     From the above, it can be seen that we have provided a self-starter for a model internal-combustion engine which is so constructed and arranged as to make it impossible for slippage of the propeller, and consequently failure of the system to properly function, to occur. We have retained the previously known advantages of the plastic connector, but have eliminated the undesirable aspects thereof, as hereinbefore described. The structure described hereinabove positively precludes propeller slippage and provides a reliable, consistent and inexpensive drive train for the propeller. 
     In considering this invention, it should be remembered that the present disclosure is illustrative only and the scope of the invention should be determined by the appended claims.