Patent Publication Number: US-10780989-B2

Title: Aircraft propeller drive system

Description:
CROSS-REFERENCE 
     The present application is a division of U.S. patent application Ser. No. 14/870,971, filed Sep. 30, 2015, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The present technology relates to aircraft propeller drive systems. 
     BACKGROUND 
     Many airplanes are powered by one or more propellers driven by one or more intermittent combustion internal combustion engines, such as two-stroke or four-stroke internal combustion engines. In many such airplanes, during operation, the engine turns its corresponding propeller at a constant speed, which is commonly referred to as a constant-speed propeller. In order to address the changes in power requirements, the pitch of the propeller blades of the propeller is changed instead of changing the speed of the engine. 
     Each engine is connected to its corresponding propeller via a propeller drive system. The propeller drive system typically includes a plurality of gears arranged such that the propeller turns at a slower speed than a crankshaft of the engine. 
     Due to its mass and dimensions, the propeller has a high moment of inertia. Therefore, when the propeller turns during operation of the airplane, it tends to do so at a constant speed. On the other hand, the speed of rotation of the crankshaft of the engine varies. The speed of rotation of the crankshaft increases during power strokes of the engine and decreases during compression strokes of the engine. As a result, the portion of the propeller drive system that is connected to the propeller rotates at a constant speed, but the portion of the propeller drive system that is connected to the crankshaft of the engine varies in speed. This causes stress and wear of the gears in the propeller drive system and also causes noise and vibration. 
     Also, should resonance of the propeller drive system occur, the vibrations increase and the problems associated with these vibrations are exacerbated. In order to avoid resonance during most operating conditions, some propeller drive systems are designed with a stiffness that results in a resonance frequency that occurs at a speed of rotation that is below the idle speed of the engine. Therefore, during the normal operation range of the engine, which is at idle speed and higher, resonance of the propeller drive system should not occur. However, during engine start-up, the engine goes from rest to the idle speed of the engine, and as such at some point will turn at the speed that causes resonance of the propeller drive system. As such, during engine start-up, resonance of the propeller drive system occurs, which causes strong vibrations and may even cause the crankshaft to rotate backwards momentarily. As a result, the engine control unit that is responsible for the control of the fuel injection and ignition, among other things, may receive erroneous signals regarding the speed of rotation and position of the crankshaft, which could prevent successful engine start-up. 
     There is therefore a need for a propeller drive system that can dampen the torque peaks associated with the changes in speed of the crankshaft during operation of the engine and/or can avoid or reduce the impact of resonance of the propeller drive system during start-up of the engine. 
     SUMMARY 
     It is an object of the present technology to ameliorate at least some of the inconveniences present in the prior art. 
     According to one aspect of the present technology, there is provided an aircraft propeller drive system for an aircraft. The aircraft has a propeller driven by an intermittent combustion internal combustion engine via the propeller drive system. The propeller drive system has a gear adapted to be operatively connected to and driven by the engine. The gear has a first plurality of teeth. A spacing of adjacent teeth of the first plurality of teeth along a pitch circle of the gear has an arc length. A torsion bar has a first end and a second end opposite the first end. The first end is connected to and driven by the gear about a torsion axis. The second end is rotatable relative to the first end about the torsion axis by a torsion angle. An output shaft is fixedly connected to and driven by the second end of the torsion bar. The output shaft is adapted for being connected to the propeller. A clutch has a driving member and a driven member. The driven member is rotationally fixed to the output shaft. The driving member has a second plurality of teeth. Teeth of the second plurality of teeth have a circular thickness. The second plurality of teeth selectively engages the first plurality of teeth. The circular thickness is less than the arc length of the spacing of adjacent teeth of the first plurality of teeth such that when one tooth of the second plurality of teeth is received and centered between two adjacent teeth of the first plurality of teeth the one tooth of the second plurality of teeth is angularly spaced from each of the two adjacent teeth of the first plurality of teeth by a clearance angle of at least 0.75 degrees. The second plurality of teeth engages the first plurality of teeth when a variation in the torsion angle from a mean torsion angle is greater than the clearance angle. Torque is transferred between the gear and the output shaft via the clutch when the second plurality of teeth engages the first plurality of teeth. 
     In some implementations of the present technology, the output shaft is at least partially hollow. The torsion bar is disposed at least partially inside the output shaft. The output shaft and the torsion bar are coaxial. 
     In some implementations of the present technology, the gear, the torsion bar, the output shaft and the clutch are coaxial. 
     In some implementations of the present technology, the output shaft is disposed inside the driven member of the clutch. The first plurality of teeth is a plurality of internal teeth. The second plurality of teeth is a plurality of external teeth. The driving member of the clutch is disposed at least partially inside the gear. 
     In some implementations of the present technology, the torsion bar passes through the gear and is coaxial with the gear. 
     In some implementations of the present technology, the clutch is a first clutch. The aircraft propeller drive system also has a second clutch having a driving member and a driven member. The driven member of the second clutch is rotationally fixed to the first end of the torsion bar. The driving member of the second clutch is formed by a portion of the gear. The gear drives the torsion bar via the second clutch. 
     In some implementations of the present technology, the first and second clutches are slip friction clutches. The second clutch has a higher slip torque than the first clutch. 
     In some implementations of the present technology, the gear is a first gear having a third plurality of teeth. The aircraft propeller drive system also has a second gear adapted to be operatively connected to and driven by the engine. The second gear has a fourth plurality of teeth engaging the third plurality of teeth. The first gear is driven by the engine via the second gear. 
     According to another aspect of the present technology, there is provided an aircraft having a fuselage, wings connected to the fuselage, an intermittent combustion internal combustion engine connected to the fuselage or one of the wings, a propeller drive system connected to the engine, and a propeller connected to the propeller drive system and driven by the engine via the propeller drive system. The propeller drive system has a gear operatively connected to and driven by the engine. The gear has a first plurality of teeth. A spacing of adjacent teeth of the first plurality of teeth along a pitch circle of the gear has an arc length. A torsion bar has a first end and a second end opposite the first end. The first end is connected to and driven by the gear about a torsion axis. The second end is rotatable relative to the first end about the torsion axis by a torsion angle. An output shaft is fixedly connected to and driven by the second end of the torsion bar. The output shaft is connected to the propeller. A clutch has a driving member and a driven member. The driven member is rotationally fixed to the output shaft. The driving member has a second plurality of teeth. Teeth of the second plurality of teeth have a circular thickness. The second plurality of teeth selectively engages the first plurality of teeth. The circular thickness is less than the arc length of the spacing of adjacent teeth of the first plurality of teeth such that when one tooth of the second plurality of teeth is received and centered between two adjacent teeth of the first plurality of teeth the one tooth of the second plurality of teeth is angularly spaced from each of the two adjacent teeth of the first plurality of teeth by a clearance angle of at least 0.75 degrees. The second plurality of teeth engages the first plurality of teeth when a variation in the torsion angle from a mean torsion angle is greater than the clearance angle. Torque is transferred between the gear and the output shaft via the clutch when the second plurality of teeth engages the first plurality of teeth. 
     In some implementations of the present technology, the output shaft is at least partially hollow. The torsion bar is disposed at least partially inside the output shaft. The output shaft and the torsion bar are coaxial. 
     In some implementations of the present technology, the gear, the torsion bar, the output shaft and the clutch are coaxial. 
     In some implementations of the present technology, the output shaft is disposed inside the driven member of the clutch. The first plurality of teeth is a plurality of internal teeth. The second plurality of teeth is a plurality of external teeth. The driving member of the clutch is disposed at least partially inside the gear. 
     In some implementations of the present technology, the torsion bar passes through the gear and is coaxial with the gear. 
     In some implementations of the present technology, the clutch is a first clutch. The propeller drive system also has a second clutch having a driving member and a driven member. The driven member of the second clutch is rotationally fixed to the first end of the torsion bar. The driving member of the second clutch is formed by a portion of the gear. The gear drives the torsion bar via the second clutch. 
     In some implementations of the present technology, the first and second clutches are slip friction clutches. The second clutch has a higher slip torque than the first clutch. 
     In some implementations of the present technology, the gear is a first gear having a third plurality of teeth. The propeller drive system also has a second gear adapted to be operatively connected to and driven by the engine. The second gear has a fourth plurality of teeth engaging the third plurality of teeth. The first gear is driven by the engine via the second gear. 
     According to another aspect of the present technology, there is provided a method for transmitting power from an intermittent combustion internal combustion engine to a propeller of an aircraft. The propeller is connected to an output shaft. The method comprises: driving the output shaft with the engine via a torsion bar, the torsion bar having a first end operatively connected to the engine and a second end connected to the output shaft, the second end being rotatable relative to the first end about a torsion axis by a torsion angle; and transferring torque between the output shaft and the engine via a clutch only when a variation in the torsion angle from a mean torsion angle is greater than or equal to a predetermined torsion angle. 
     In some implementations of the present technology, driving the output shaft with the engine via the torsion bar comprises driving the first end of the torsion bar via a gear driven by the engine; and transferring torque between the output shaft and the engine via the clutch comprises transferring torque between the output shaft and the gear via the clutch. 
     In some implementations of the present technology, the clutch is a first clutch. Driving the first end of the torsion bar via the gear comprises driving the first end of the torsion bar with the gear via a second clutch. 
     In some implementations of the present technology, the gear has a first plurality of teeth. A spacing of adjacent teeth of the first plurality of teeth along a pitch circle of the gear has an arc length. The clutch has a driving member having a second plurality of teeth. Teeth of the second plurality of teeth have a circular thickness. The second plurality of teeth selectively engages the first plurality of teeth. The circular thickness is less than the arc length of the spacing of adjacent teeth of the first plurality of teeth such that when one tooth of the second plurality of teeth is received and centered between two adjacent teeth of the first plurality of teeth the one tooth of the second plurality of teeth is angularly spaced from each of the two adjacent teeth of the first plurality of teeth by a clearance angle of at least 0.75 degrees. The predetermined torsion angle is the clearance angle. 
     For purposes of the present application, the term “intermittent combustion internal combustion engine” refers to an engine which operates as a result of the periodic combustion of air and fuel, such as in a two-stroke, four-stroke or Wankel rotary engine. Also for purposes of the present application, terms related to spatial orientation such as forward, rearward, left and right are as they would be understood by a pilot of an aircraft sitting in the aircraft in a normal piloting position with the aircraft being at 0 degree of pitch and 0 degree of roll. 
     Implementations of the present technology each have at least one of the above-mentioned object and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present technology that have resulted from attempting to attain the above-mentioned object may not satisfy this object and/or may satisfy other objects not specifically recited herein. 
     Additional and/or alternative features, aspects and advantages of implementations of the present technology will become apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present technology, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
         FIG. 1  is a left side view of an airplane; 
         FIG. 2  is a schematic right side elevation view of an engine, propeller drive system and propeller of the airplane of  FIG. 1 ; 
         FIG. 3  is a perspective view taken from a front, right side of the engine of  FIG. 1 ; 
         FIG. 4  is a perspective view taken from a rear, right side of the propeller drive system of  FIG. 2 ; 
         FIG. 5  is a perspective view taken from a rear, right side of a longitudinal cross-section of the propeller drive system of  FIG. 4 ; 
         FIG. 6  is a right side elevation view of the propeller drive system of  FIG. 4 ; 
         FIG. 7  is a cross-sectional view of the propeller drive system of  FIG. 4  taken through line  7 - 7  of  FIG. 8 ; 
         FIG. 8  is a cross-sectional view of the propeller drive system of  FIG. 4  taken through line  8 - 8  of  FIG. 7 ; 
         FIG. 9  is a close-up view of an upper left portion of  FIG. 8  with a housing of the propeller drive system removed for clarity and in a first position of the torsion bar of the propeller drive system; 
         FIG. 10  is a close-up view of an upper left portion of  FIG. 8  with a housing of the propeller drive system removed for clarity and in a second position of the torsion bar; and 
         FIG. 11  is a close-up view of an upper left portion of  FIG. 8  with a housing of the propeller drive system removed for clarity and in a third position of the torsion bar. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology will be described with respect to an airplane having a single variable pitch propeller powered by an intermittent combustion internal combustion engine. It is contemplated that at least some aspects of the present technology could be applied to an airplane having a fixed pitch propeller and/or having multiple propellers and/or in a different type of aircraft. 
     As can be seen in  FIG. 1 , an aircraft, specifically an airplane  10 , has a fuselage  12  defining a cockpit  14  for accommodating a pilot. It is contemplated that the fuselage  12  and the cockpit  14  could be designed to also accommodate one or more passengers. A vertical stabilizer  16  is connected to the rear end of the fuselage  12 . A pair of horizontal stabilizers  18  (only the left one being shown) is connected to the vertical stabilizer  16 . A pair of wings  20  (only the left one being shown) is connected to the bottom of the fuselage  12 . It is contemplated that the wings  20  could be connected to the top or the sides of the fuselage  12 . It is also contemplated that airplane  10  could have more than two wings  20  and/or that the wings  20  could be integrally formed with the fuselage  12 . Three landing gears  22  are provided on the bottom of the fuselage  12 . In the present implementation, the landing gears  22  are provided with wheels  24  and are not retractable. It is contemplated that other types of landing gears  22  could be used. For example, the wheels  24  could be replaced with skis, skids or floats and/or the landing gears  22  could be retractable. A variable pitch propeller  26  is provided at the front of the fuselage  12 . It is contemplated that the airplane  10  could have two or more propellers  26  and/or that the propeller(s)  26  could be provided on the wings  20  or elsewhere on the airplane  10 . It is also contemplated that the propeller  26  could be a fixed pitch propeller  26 . The engine  28  (schematically shown in  FIG. 1 ) powering the the propeller  26  is located in and connected to the front of the fuselage  12 . The airplane  10  has many other components, but these will not be described herein. 
     As can be seen in  FIG. 2 , the propeller  26  is driven by the engine  28  via a propeller drive system  100 . The engine  28  has a crankshaft  30  (schematically shown in  FIG. 2 ), that drives the propeller drive system  100 . The propeller drive system  100  has an output shaft  102  defining a propeller mounting flange  104 . As can be seen in  FIG. 1 , the flange  104  is located outside the fuselage  12 . The propeller mounting flange  104  defines a plurality of holes  106  (see  FIG. 4 ). The propeller  26  has a central hub  32  from which the propeller blades  34  extend. The central hub  32  defines holes (not shown) corresponding to at least some of the holes  106  in the propeller mounting flange  104 . Fasteners  36  are inserted through the holes in the central hub  32  of the propeller  26  and through the hole  106  of the flange  104  to fasten the propeller  26  to the flange  104 . 
     The engine  28  is best seen in  FIG. 3 . The engine  28  is an intermittent combustion internal combustion engine. In the present implementation, the engine is a fuel injected, four-stroke boxer engine  28 , such as the Rotax 915 iS™ for example. Other types of intermittent combustion internal combustion engines are contemplate, such as, but not limited to, V-type engines, two-stroke engine, and Wankel rotary engines. The engine  28  has a central crankcase  38  and four horizontally extending cylinders  40  (two on each side). The crankshaft  30  is rotationally supported in and extends from the crankcase  38 . The reciprocating motion of the pistons (not shown) inside the cylinders  40  rotate the crankshaft  30 . The engine  28  is also provided with a turbocharger  42  and an intercooler  44 , but it is contemplated that these components could be omitted. The engine  28  has many other components, but these will not be described herein. 
     Turning now to  FIGS. 4 to 11 , the propeller drive system  100  will be described herein. The propeller drive system  100  has a housing  108  inside which most of the components described below are housed. The housing  108  has an upper generally frustoconical portion  110  and a lower generally frustoconical portion  112  that is integrally formed with the upper portion  110 . As can be seen, the lower portion  112  is smaller than the upper portion  110 . As best seen in  FIG. 6 , the output shaft  102  extends from a front of the upper portion  110  of the housing  108 . The rear end of the housing  108  defines a flange  114 . The flange  114  defines a plurality of apertures  116 . Fasteners  118  (only some of which are labeled for clarity) are inserted through the apertures  116  to fasten the housing, and therefore the propeller drive system  100 , to the front of the engine  28  as shown in  FIG. 3 . 
     The propeller drive system  100  has an input gear  120  located in the lower portion  112  of the housing  108 . The input gear  120  has internal splines  122  and a plurality of external teeth  124 . The crankshaft  30  extends through the input gear  120 . The crankshaft  30  has external splines that engage the internal splines  122  of the input gear  120  such that the crankshaft  30  drives the input gear  120 . The end of the crankshaft  30  is received in a recess  126  formed in the lower portion  112  of the housing  108 . A bearing  128  is disposed around the end of the crankshaft  30  to rotationally support the end of the crankshaft in the recess  126 . 
     The plurality of external teeth  124  of the input gear  120  engages a plurality of external teeth  130  of a gear  132  such that the input gear  120  drives the gear  132 . As can be seen, the gear  132  has a larger diameter than the input gear  120 . The gear  132  defines an outer sleeve  134  and an inner sleeve  136 . A body or web  137  of the gear extends radially between the sleeves  134 ,  136 . A front end of the outer sleeve  134  defines a radially outwardly extending flange  138 . A plurality of internal teeth  140  (see  FIG. 8 ) are also defined at the front end of the outer sleeve  134 . A rear end of the inner sleeve  136  defines a plurality of external teeth  142 . The plurality of external teeth  142  engage the teeth of a gear (not shown) connected to a governor (not shown). 
     The propeller drive system  100  also has a slip friction clutch  144 . In the present implementation, the slip friction clutch  144  is a multiple-disk friction clutch, but it is contemplated that other types of friction clutches could be used. The friction clutch  144  has a driving member formed by the outer sleeve  134  of the gear  132 , a driven member  146 , driving friction disks  148 , driven friction disks  150 , a front annular plate  152 , and a rear annular plate  154 . The driving and driven friction disks  148 ,  150  are disposed in an alternating arrangement between the front and rear annular plates  152 ,  154 . The driving friction disks  148  are rotationally fixed to the outer sleeve  134 . The driven friction disks  150  are rotationally fixed to the driven member  146 . A spring  156  is disposed radially between the outer and inner sleeves  134 ,  136  of the gear  132  and axially between the web  137  of the gear  132  and the rear annular plate  154 . The spring  156  applies pressure onto the clutch  144  to compress the driving and driven friction disks  148 ,  150 . When the torque applied to the clutch  144  is less than a slip torque of the clutch  144 , the outer sleeve  134  of the gear  132  and the driven member  146  rotate together. The clutch  144  and spring  156  are designed and selected such that the clutch  144  has a slip torque that is sufficiently high so that during normal operation of the airplane  10 , the clutch  144  does not slip and the outer sleeve  134  and the driven member  146  rotate together. Should the torque applied to the clutch  144  exceed the slip torque of the clutch  144 , the driving and driven friction disks  148 ,  150  slip relative to each other and the outer sleeve  134  and the driven member  146  no longer rotate together. For example, should the propeller  34  hit the ground during a difficult landing, the torque applied to the clutch  144  exceeds the slip torque of the clutch  144 , causing the clutch  144  to slip, thereby preventing the force of the impact to be transferred to the engine  28 , thus helping to prevent damage to the engine  28 . In one implementation, the slip torque of the clutch  144  is 600 Nm, but other slip torques are contemplated. 
     The driven member  146  of the clutch  144  is integrally formed with a flange  158  of a hollow shaft  160 . The inner sleeve  136  of the gear  132  is disposed around the shaft  160 , but is not fixed to the shaft  160 . From the flange  158 , the shaft  160  extends rearward and out of the rear of the housing  108 . The rear end of the shaft  160  is rotationally supported by a bearing  162 . The bearing  162  is supported inside a recess (not shown) formed in the front of the engine  28 . The rear end of the shaft  160  has internal splines  164 . The internal splines  164  engage external splines  166  of a rear end of a torsion bar  168 . As can be seen, the torsion bar  168  is hollow. From its rear end, the torsion bar  168  extends forward and out the front of the housing  108 . The front end of the torsion bar  168  has external splines  170 . The external splines  170  engage internal splines  172  defined in the front end of the output shaft  102 . As can be seen, the output shaft  102  is hollow. The output shaft  102  extends rearward around the torsion bar  168  and into the shaft  160 . The shaft  160  is disposed around the output shaft  102  but is not fixed to the output shaft  102 . A fastener  174  fastens a washer  176  to the front end of the torsion bar  168 . An O-ring  178  is held between the washer  176  and the output shaft  102 . A clip  180  is inserted in the output shaft  102  in front of the washer  176 . As a result, the output shaft  102  and the torsion bar  168  are axially fixed to each other. The output shaft  102  is rotationally supported by a ball bearing  182  located in the front portion of the upper portion  110  of the housing  108 . The bearing  182  has an outer race  184  and an inner race  186 . The outer race  184  is held between a lock ring  188  and a step  190  formed in the upper portion  110  of the housing  108 . The inner race  186  is held between a screw nut  192  threaded onto the output shaft  102  and a flange  194  formed by the output shaft  102 . As a result of the arrangement, the bearing  182  and its associated components ( 188 ,  190 ,  192 ,  194 ) limit the axial displacement of the output shaft  102 . A bearing seal  196  is disposed in front the bearing  182  and is held radially between the step  190  and the flange  194 . 
     The propeller drive system  100  also has a slip friction clutch  200 . In the present implementation, the slip friction clutch  200  is a multiple-disk friction clutch, but it is contemplated that other types of friction clutches could be used. The friction clutch  200  has a driving member  202 , a driven member  204 , driving friction disks  206 , driven friction disks  208 , a front annular plate  210 , and a rear annular plate  212 . The driven member  204  is rotationally fixed to the output shaft  102 . The driving and driven friction disks  208 ,  210  are disposed in an alternating arrangement between the front and rear annular plates  210 ,  212 . The driving friction disks  206  are rotationally fixed to the driving member  202 . The driven friction disks  208  are rotationally fixed to the driven member  204 . The rear annular plate  212  is disposed adjacent to the front annular plate  152 . A spring  214  is disposed radially between the driving and driven members  202 ,  204 . The spring  214  is disposed axially between the front annular plate  210  and a ring  216  abutting an inner side of the driving member  202 . The ring  216  is prevented from moving axially forward by a clip  218  received in a notch in the inner side of the front portion of the driving member  202 . The spring  214  applies pressure onto the clutch  200  to compress the driving and driven friction disks  202 ,  204 . When the torque applied to the clutch  200  is less than a slip torque of the clutch  200 , the driving member  202  and the driven member  204  rotate together. The clutch  200  and spring  214  are designed and selected such that the clutch  200  has a slip torque that is less than the slip torque of the clutch  144 . Should the torque applied to the clutch  200 , as will be described below, exceed the slip torque of the clutch  200 , the driving and driven friction disks  206 ,  208  slip relative to each other and the driving member  202  and the driven member  204  no longer rotate together. In one implementation, the slip torque of the clutch  200  is 250 Nm, but other slip torques are contemplated. 
     The driven member  204  has an inner sleeve  220  and an outer sleeve  222 . The inner sleeve  220  is fixed to the output shaft  102 . A ring  223  is disposed radially between a flared front end of the inner sleeve  220  and the output shaft  102  to limit axial displacement of the clutch  200 . The outer sleeve  222  is disposed rearward of the inner sleeve  220 . The outer sleeve  222  is radially spaced from the output shaft. As can be seen in  FIGS. 5 and 7 , a forward portion of the hollow shaft  160  is disposed radially between the outer  222  sleeve of the driven member  204  and the output shaft  102 . The driven friction disks  210  are rotationally fixed to the outer sleeve  222  of the driven member  204 . 
     The rear portion of the driving member  202  is disposed inside the front portion of outer sleeve  134  of the gear  132 . The rear portion of the driving member  202  has a plurality of external teeth  224  which selectively engage the plurality of internal teeth  140  of the gear  132  as will be discussed in greater detail below. 
     As best seen in  FIGS. 7 and 8 , the output shaft  102 , the gear  132 , the clutch  144 , the torsion bar  168 , and the clutch  200  are coaxial and rotated about a rotation axis  226  (see  FIGS. 6 to 9 ). As will be discussed below, the torsion bar  168  twists when the engine  28  drives the propeller  26 . When the torsion bar  168  twists, the front end of the torsion bar  168  rotates relative to the rear end of the torsion bar  168  about a torsion axis  228  (see  FIGS. 6 to 9 ), which corresponds to the rotation axis  226 . 
     Turning now to  FIG. 9 , the internal teeth  140  of the gear  132  and the external teeth  224  of the driving member  202  of the clutch  200  will be described in more detail. The gear  132  and the driving member  202 , which defines a gear with the teeth  224 , have a common pitch circle  250 . Each spacing between adjacent internal teeth  140  of the gear  132  has an arc length S measured along the pitch circle  250 . Each tooth  224  of the driving member  202  of the clutch  200  has a circular thickness t measured along the pitch circle  250 . The circular thickness is the length of the arc between the two sides of the tooth  224  along the pitch circle  250 . As can be seen, the circular thickness t is smaller than the arc length S. As a result, when the teeth  224  are centered between the teeth  140  they are angularly spaced from the teeth  140 . Each tooth  224  is angularly spaced from each of its two adjacent teeth  140  by a clearance angle C. The clearance angle C is measure about the rotation axis  226  and is the angle between the intersection of the pitch circle  250  with the face of the tooth  224  and the intersection of the pitch circle  250  with the adjacent face of the tooth  140 . In the implementation, the clearance angle C is 2.5 degrees. In an alternative implementation, the clearance angle C is 1.5 degrees. In other alternative implementations, the clearance angle C is at least 0.75 degrees. The clearance angle C is greater than the backlash angle resulting from manufacturing tolerances that would be present in a constantly engaging pair of gears having the same diameters as the gear  132  and the driving member  202 . 
     When the propeller  26  turns during operation of the airplane  10 , it tends to do so at a constant speed, but the speed of rotation of the crankshaft  30  of the engine  28  varies. The speed of rotation of the crankshaft  30  increases during power strokes of the engine  28  and decreases during compression strokes of the engine  28 . As a result, the output shaft  102  rotates at a constant speed, but the input gear  120  varies in speed. The torsion bar  168  and the clutch  200  contribute to diminish the stress and wear of the propeller drive system  100  as explained below. 
     When the engine  28  is operating at full power, the crankshaft  30  drives the input gear  120 , the input gear  120  drives the gear  132 , the gear  132  drive the torsion bar  168  via the clutch  144  which does not slip, the torsion bar  168  drives the output shaft  102 , and the output shaft  102  drives the propeller  26 . Under such operating conditions, the front end of the torsion bar  168  rotates (i.e. twists) relative to the rear end of the torsion bar  168  by a varying torsion angle, due to the torque variations of the engine  28 . The average angle of this varying torsion angle is referred to herein as the mean torsion angle. In the present implementation, the mean torsion angle is 5 degrees. When the torsion bar  168  is twisted by the mean torsion angle, the teeth  224  of the driving member  202  are centered between the teeth  140  of the gear  132  as shown in  FIGS. 8 and 9 . The variation in torsion angle due to torque variations is less than the clearance angle C. In the present implementation, the variation in torsion angle is 1.5 degrees in each direction from the mean torsion torsion angle. Since the teeth  224  only move by 1.5 degrees in each direction from their position shown in  FIG. 9  and the clearance angle is 2.5 degrees, the teeth  224  never engage the teeth  140 . As such no torque is transferred between the gear  132  and the output shaft  102  via the clutch  200  and the clutch  200  does not slip. During such a steady state operation, the torsion bar  168  absorbs the vibrational energy. 
     During engine start-up, a resonance situation can occur as previously explained. During such a situation, should the variation in torsion angle from the mean torsion angle exceed the clearance angle C, the teeth  224  of the driving member  202  make contact with and engage the teeth  140  of the gear  132  as shown in  FIGS. 10 and 11 . When the excess variation in torsion angle results in the front end of the torsion bar  168  rotating clockwise relative to the rear end of the torsion bar  168  (as viewed from a front of the airplane  10 ), the teeth  140  and  224  engage as shown in  FIG. 10 . When the excess variation in torsion angle results in the front end of the torsion bar  168  rotating counter-clockwise relative to the rear end of the torsion bar  168  (as viewed from a front of the airplane  10 ), the teeth  140  and  224  engage as shown in  FIG. 11 . When the teeth  224  of the driving member  202  engage the teeth  140  of the gear  132 , torque is transferred between the output shaft  102  and the gear  132  via the clutch  200 . Should the torque transferred between the driving member  202  and the gear due to the engagement of the teeth  224 ,  140  exceed the slip torque of the clutch  200 , the driving member  202  of the clutch  200  rotates relative to the driven member  204  of the clutch  200 . When this occurs, the clutch  200  is said to be slipping. When the clutch  200  slips, the driving and driven friction disks  206 ,  208  move relative to each other, thereby generating a significant amount of friction. This friction acts to dampen the variations in torsion angle of the torsion bar  168  during such operating conditions. Another situation where the teeth  224  of the driving member  202  can engage the teeth  140  of the gear  132  is during a load change of the engine  28 , even when a resonance situation does not occur. When the load of the engine  28  changes, the mean torsion angle also changes, which can initially cause contact between the teeth  140  and the teeth  224 . After a few engine cycles, the gear  132  and the driving member  202  realign such that at the mean torsion angle the teeth  224  are centered between the teeth  140  as shown in  FIG. 9 . 
     Modifications and improvements to the above-described implementations of the present technology may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present technology is therefore intended to be limited solely by the scope of the appended claims.