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Aviation History
1929
1929 - 0460.PDF
of the starter through a'solenoid switch mounted integrally with the electric motor. Pushing the switch button closes the electric circuit and allows the motor to accelerate the starter flywheel to operating speed, the time required usually being about five seconds. As soon as this speed is reached a pull on the switch button opens the electric circuit to the motor, and further pulling on the switch button acts on the operating lever of the starter through the medium of a cable or rod fastened to the end of the switch button rod, so as to engage the rotating starter drive jaw with the engine crankshaft. As soon as the engine fires, the starter jaw is automatically disengaged from the engine crankshaft by means of a quick thread. The operator then releases the switch button, and a return spring brings it back to its normal neutral position. In addition to the electric attachment a hand-cranking shaft is provided so that the electric inertia starter can also be used as a hand inertia starter. As an automatic device is provided for connecting the electric motor armature to the B.T.H. hand inertia starter (horizontal) fitted to Wright " Whirlwind " engine. flywheel only when the electric motor is running, the starter is relieved at all times of any losses due to brush friction, and any excessive drag on the starter when it is operated by hand is avoided. Twelve volts is the standard for these starters, and, for aeroplane engines, the starters have the following weights :— B.T.H. Starters Weight Combined Flywheel Hand Electric and Axis Pattern Hand Pattern Horizontal 21 lb. 31 lb. Horizontal 231b, 35 1b. Vertical 30 J lb. 41* lb. The standard arrangement is with the axis of the flywheel running in the horizontal plane, but for the larger engines (up to 2,500 cub. in.) the starters are also made with the axis of the flywheel in the vertical plane, whereby the overall length is decreased, but the weight increased as stated in the table. Starters for marine service weigh 2 lb. more in all instances. Besides the inertia starters described, a starter which has given satisfactory performance in many commercial aero- planes is that known as the " Aviation Hand Turning Gear." This starter has been designed for cranking an engine directly by hand, and, although the weight has been cut down as much as possible, it can be depended on for reliability in operation. It consists of a worm reduction gear operating an automatic meshing and demeshing mechanism through an adjustable torque-overload release, which protects both operator and starter in the event of engine backfire, as described in con- nection with the hand and electric inertia starters. As a Engine Capacity Series Up to 1,300 cub. ins. VI Up to 2,500 cub. ins. ^ MARCH 7, 1929 protection to the operator against " kicking," a ratchet is provided on the hand crankshaft. The automatic meshing and demeshing mechanism is such that rotation of the hand crank by the operator causes the starter drive shaft to travel longitudinally lor a fixed distance, thus effecting its engagement with the internal spline at the end of the engine crankshaft, or a separate B.T.H. combined hand and electric inertia starter (vertical model] fitted to " Liberty " engine. extension of it. As soon as the starter drive shaft pinion is fully engaged, it rotates as a unit with the drive shaft revolv- ing the engine. Positive engagement is assured and the automatic disengagement occurs the instant the engine fires. Friction is reduced to a minimum by the careful design of all details, the use of ball-thrust bearings, etc., and ease of B.T.H. Aviation hand-turning gear mounted on " Liberty " engine. It is operated with an external cranking unit. starting depends entirely on temperature conditions, and stiffness and size of the engine. For engines equipped with magneto ignition, it is recom- mended that the aviation hand turning gear be used with an integrally-mounted booster magneto driven by speed- increasing-gear from the hand crankshaft. By this means engine starting is greatly facilitated, as, at low engine speeds, a hot spark is available for distribution to the cylinders through the regular engine magnetos. 182
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