• Re: Poor Starting 912iS

    by » 8 months ago

    Jim Flock Wrote: 

    The Sprag Clutch has been mentioned a few times.  Could someone explain to the group what it is and how it works on the 912iS?

    First, let us understand that there are two clutches in many engines.

    The first is the Propeller Overload Clutch located up front in the Propeller Gearbox.

    It is also known as the Slipper Clutch.

    This is an optional component so not every engine has one.

    Its sole purpose is to allow the engine to come to a brisk, but not an instantaneous and damaging stop, in the event of a propeller strike.

    You might lose the propeller and the gearbox, but the engine crankshaft and everything behind the gearbox should survive the prop strike.

    This clutch will only slip if the torque applied to it is in the order of 5 times what a normal running engine can deliver to the prop.

    So for the 999 out of 1000 engines that never have a prop strike, this clutch never moves even 1 degree.


    The second clutch, the Sprague Clutch, is located near the starting motor in the rear of the engine.

    The common automotive stater that most people are familiar with consists of a toothed heavy flywheel gear firmly attached to the crankshaft that is momentarily engaged by a Bendix Gear mounted on the starter motor shaft.

    An Aircraft engine has no need for a flywheel as the inertia of the propelled serves this purpose.

    The Rotax Starting mechanism consists of a large but lightweight starter gear loosely attached to the crankshaft by the Sprague clutch.

    The starter gear is driven by the starter motor through a small pinion gear that remains in constant engagement with the larger starter gear.

    The Sprague clutch can be thought of as a One-Way Bearing.

    When the starter motor is energized its pinion gear rotates the starter gear which in turn rotates the Sprague clutch.

    The One-Way feature in the clutch engages tightly to the crankshaft and spins the motor until it starts.

    Once the engine starts and the crankshaft starts to outrun the starter mechanism, the Sprague clutch releases its grip on the crankshaft.

    The crankshaft is now free to rotate as the Starter motor, Starter Gear, and Sprague Clutch come to a complete stop as the engine continues running.

    If you have ever used a Pipe Wrench you are familiar with how it will grip tightly in one direction and slip easily in the other. The Sprague Clutch is just a circular pipe wrench.

    - - -

    You can feel the Sprague clutch in operation by rotating the propeller.

    Take notice of how the engine sounds as you rotate the propeller in the normal Counter Clockwise direction.

    Then rotate the propeller backward in the Clockwise direction slowly for no more than a quarter of a revolution and notice what sounds like something being dragged across a sheet of sandpaper.

    This is the sound of the electric starter motor being rotated in reverse.

    Also, notice how the Sprague clutch engages with even the slightest of reverse rotation.  There is No Slippage!

    This will not harm the starter motor, but just don't make a habit of turning the prop backward for a dozen rotations.

    - - -

    In early model engines without the Soft-Start Ignition modules, A piston kickback would cause the engine to kick vigorously in reverse while the starter motor was still engaged, resulting in huge stresses on the Sprague Clutch eventually causing it to slip and requiring the clutch to need replacement. 

    The Soft Start Ignition Modules have virtually eliminated this problem.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

    Thank you said by: RotaxOwner Admin, Jon Tensfeldt

  • Re: Poor Starting 912iS

    by » 3 months ago

    Following this thread as the 912iS in my Kitfox has started to act up during start.  Same symptoms as the others.  In my case ambient outside temperature could be causing it.  I travel with the Kitfox around the US.  www.blackpearladventures.com  Due to Covid we were in Cottonwood, AZ for an extended stay.  I use premium Mogas with only occasional AVGAS.  The plane ran fine in Cottonwood and then in Santa Fe.  As I worked east the heat cranked up.  In San Marcos, TX the problem started to show up.  600' elevation and temps in the 90 deg F every day.  Flew in Memphis with high temps and it showed.  Back to NE Texas and yesterday when starting (90 deg and hot sun) it was really nasty.  290 hrs. on the engine.  

    Will try Bill's starting procedure and see if it helps.  Will post back.  

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