Aircraft propeller drive system

1. An aircraft propeller drive system for an aircraft, the aircraft having a propeller driven by an intermittent combustion internal combustion engine via the propeller drive system, the propeller drive system comprising:

a gear adapted to be operatively connected to and driven by the engine, the gear having a first plurality of teeth, a spacing of adjacent teeth of the first plurality of teeth along a pitch circle of the gear having an arc length;

a torsion bar having a first end and a second end opposite the first end, the first end being connected to and driven by the gear about a torsion axis, the second end being rotatable relative to the first end about the torsion axis by a torsion angle; and

an output shaft fixedly connected to and driven by the second end of the torsion bar, the output shaft being adapted for being connected to the propeller; and

a clutch having a driving member and a driven member, the driven member being rotationally fixed to the output shaft, the driving member having a second plurality of teeth, teeth of the second plurality of teeth having a circular thickness, the second plurality of teeth selectively engaging the first plurality of teeth,

the circular thickness being 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 engaging the first plurality of teeth when a variation in the torsion angle from a mean torsion angle is greater than the clearance angle, and

torque being transferred between the gear and the output shaft via the clutch when the second plurality of teeth engages the first plurality of teeth.

2. The aircraft propeller drive system of claim 1 , wherein:

the output shaft is at least partially hollow;

the torsion bar is disposed at least partially inside the output shaft; and

the output shaft and the torsion bar are coaxial.

3. The aircraft propeller drive system of claim 2 , wherein the gear, the torsion bar, the output shaft and the clutch are coaxial.

4. The aircraft propeller drive system of claim 3 , wherein:

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; and

the driving member of the clutch is disposed at least partially inside the gear.

5. The aircraft propeller drive system of claim 1 , wherein the torsion bar passes through the gear and is coaxial with the gear.

6. The aircraft propeller drive system of claim 1 , wherein the clutch is a first clutch;

the aircraft propeller drive system further comprising a second clutch having a driving member and a driven member, the driven member of the second clutch being rotationally fixed to the first end of the torsion bar, the driving member of the second clutch being formed by a portion of the gear; and

wherein the gear drives the torsion bar via the second clutch.

7. The aircraft propeller drive system of claim 6 , wherein:

the first and second clutches are slip friction clutches; and

the second clutch has a higher slip torque than the first clutch.

8. The aircraft propeller drive system of claim 1 , wherein the gear is a first gear having a third plurality of teeth;

the aircraft propeller drive system further comprising a second gear adapted to be operatively connected to and driven by the engine, the second gear having a fourth plurality of teeth engaging the third plurality of teeth, the first gear being driven by the engine via the second gear.

9. An aircraft comprising:

a fuselage;

wings connected to the fuselage;

an intermittent combustion internal combustion engine connected to one of:

the fuselage, and

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 comprising:

a gear operatively connected to and driven by the engine, the gear having a first plurality of teeth, a spacing of adjacent teeth of the first plurality of teeth along a pitch circle of the gear having an arc length;

a torsion bar having a first end and a second end opposite the first end, the first end being connected to and driven by the gear about a torsion axis, the second end being rotatable relative to the first end about the torsion axis by a torsion angle; and an output shaft fixedly connected to and driven by the second end of the torsion bar, the output shaft being connected to the propeller; and

a clutch having a driving member and a driven member, the driven member being rotationally fixed to the output shaft, the driving member having a second plurality of teeth, teeth of the second plurality of teeth having a circular thickness, the second plurality of teeth selectively engaging the first plurality of teeth,

the circular thickness being 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 engaging the first plurality of teeth when a variation in the torsion angle from a mean torsion angle is greater than the clearance angle, and

torque being transferred between the gear and the output shaft via the clutch when the second plurality of teeth engages the first plurality of teeth.

10. The aircraft of claim 9 , wherein:

the output shaft is at least partially hollow;

the torsion bar is disposed at least partially inside the output shaft; and

the output shaft and the torsion bar are coaxial.

11. The aircraft of claim 10 , wherein the gear, the torsion bar, the output shaft and the clutch are coaxial.

12. The aircraft of claim 11 , wherein:

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; and

the driving member of the clutch is disposed at least partially inside the gear.

13. The aircraft of claim 9 , wherein the torsion bar passes through the gear and is coaxial with the gear.

14. The aircraft of claim 9 , wherein the clutch is a first clutch;

the propeller drive system further comprising a second clutch having a driving member and a driven member, the driven member of the second clutch being rotationally fixed to the first end of the torsion bar, the driving member of the second clutch being formed by a portion of the gear; and

wherein the gear drives the torsion bar via the second clutch.

15. The aircraft of claim 14 , wherein:

the first and second clutches are slip friction clutches; and

the second clutch has a higher slip torque than the first clutch.

16. The aircraft of claim 9 , wherein the gear is a first gear having a third plurality of teeth;

the propeller drive system further comprising a second gear adapted to be operatively connected to and driven by the engine, the second gear having a fourth plurality of teeth engaging the third plurality of teeth, the first gear being driven by the engine via the second gear.