impulse coupling

impulse starter

impulse starterclick for a larger image
Initially the body of the starter is rotated while the rotating magnet remains stationary. This results in winding of the spring in the coupling. At the appropriate time the fly weights are released and the spring unwinds. The rapid movement of the magnet results in a better spark. As soon as the engine starts running, the flyweights are held in release position by centrifugal force and the magnet fires in the normal advance position. However, during engine start up the retard spark is produced when the magneto rotation is held back by the impulse starter of inertia coupling.
A spring-loaded clutch device between the engine drive and the magneto spindle that helps increase momentarily the rate of rotation of the magnet, which increases the voltage produced. An impulse starter is required while starting the engine. Also known as impulse coupling.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
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It also uses standard ignition wiring harnesses, but there's no impulse coupling because the system controls the ignition timing by manifold pressure and RPM.
During a 500-hour magneto inspection, the impulse coupling was discovered to have cracks in its flyweight base plate.
Impulse coupling found cracked at advance stop notches of base plate.
I had the idea it was about a 6 hp with an external magneto and an impulse coupling and weighed about 150 pounds.
Many Continental engines are equipped with a "shower of sparks" system to multiply the number of sparks from each plug per activation of a magneto's impulse coupling. The system has been around since the Ford Model T because it works.
Magneto impulse couplings disengage at an RPM as low as 180 and Sky-Tec's starters will turn up at about 120-140 RPM--so there's not a lot of room for improvement.
The stiff/frozen bearing was causing the impulse coupling to be ineffective.
Developers (including SureFly) recognized this and have pursued STCs that require the retention of the right-hand direct-drive mag (or typically the one without the impulse coupling), while replacing the left mag with the electronic unit.
The starter, or in the case of an airplane with no electrical system, the human arm, is required to swing the propeller and crankshaft, engaging an impulse coupling typically built into the left magneto.
While doing so will prevent the impulse coupling from winding up and snapping, Hughes cautioned doing so could damage vanes in the engine-driven vacuum pump, especially if it's a dry pump.