Debunking the "Too Fast" Myth

 

Ever hear one of those mythological "Hangar Tales" just one too many times and wish you could set the record straight?  Us too!

 

This myth generally goes something like this:  Lightweight starters spin your engine so fast that the impulse couplers kick out and the engine won't start*.

 

Wow, we wish we could be so cool.  Spinning an engine faster than impulse coupler kick-out RPM would indeed be a very impressive feat.  After all, most all Lycoming mag impulse couplers are designed to kick out at around 400 RPM.  

 

So what are impulse couplers?  They are both a) a retard mechanism and b) a hot spark generator.

 

The retard mechanism is necessary to avoid a kickback.  Advance timing (generally 24 degrees before TDC) is optimal for a running/performing engine.  It gives the air, fuel and pressure time to get to know each other and ignition is easy and optimized for performance.  But inducing spark before a piston reaches TDC while the starter is trying to crank an unlit engine is fertile ground for a kickback.  So impulse couplers retard the spark to TDC or after to avoid kickbacks.  Once the engine lights and the RPM's exceed a certain level (400 RPM in most cases), the impulse coupler is deactivated and the mag timing returns to advanced timing.  So if the mag/engine manufacture determine that the impulse coupler can kick out at as little as 400 RPM, then the mag/engine manufacturer must firmly believe the engine can run with advance timing at 400 RPM.  Therefore, if the starter can crank the engine past 400 RPM, the mag/engine builder have faith the engine will run (start) and so it would hold that there'd be no need for impulse couplers if a starter could crank an engine above the impulse coupler kick-out RPM.  

 

At low RPM's, a hot spark is a very helpful aid to getting an engine started.  With timing delayed to TDC (or after), cylinder air/fuel mixture and pressures are sub optimal.  Impulse couplers give us a hotter spark to help that sub optimal mixture light.  But spin an engine faster than the impulse couplers and what do you get?  Advanced timing with arguably optimal ignition mixture.  In other words, a very easy starting engine and no need for the "hot spark."  How can it be true?  Well once again, if the mag/engine manufacturer both firmly believe the engine can run with advance timing at 400 RPM, they must have faith that the engine will run (start) and so it would hold that there'd be no need for a hot spark from the impulse couplers if a starter could crank an engine above the impulse coupler kick-out RPM.  

 

So impulse couplers are indeed helpful gizmos and that's why it's important to keep them properly maintained and functioning properly.  But if they can be outrun, using the logic above, they wouldn't be needed at all. 

 

But just for the sake of argument, let's take a look at how fast can a lightweight, high-torque starter can spin a Lycoming engine.  Let's use the most aggressive configuration: a Hi-Speed, 24V, 149-24HT starter running on a low compression 4-cylinder (320 or 360?) in an aircraft connected to a ground power cart jacked up to the maximum 28V.  In this configuration, it is theoretically possible that the 149-24HT could spin the engine at close to 360RPM.  Whew!  That's fast!  But none the less still more than 10% below the impulse coupler kick-out speed of 400RPM.

 

All other Sky-Tec starters wouldn't spin as fast as this scenario.  Most aren't even close.   That's the fastest one we can come up with.  And it's still below impulse coupler spring kick-out RPM.

 

Now, let's say for the sake of argument, a starter COULD spin an engine above impulse coupler kick-out.  Heck, let's say Les and Gene run amuck and develop a starter that cranks at 800RPM.  We'll call it "Smokey."  If Smokey were capable of cranking an engine at an RPM the mag/engine builder determined can handle advanced timing, then the engine would start and run at 800RPM without any impulse couplers all together.

 

So, that's why this myth is BUNK.  A) Sky-Tec starters CAN'T outrun impulse coupler kick-out RPM and B) Even if they could, the engine would START without impulse couplers.

 

*Now, all that said, we also have heard ACTUAL tales of a situation where, while cranking (holding the key to "start") an engine would not start, but upon releasing the key, the engine would IMMEDIATELY start.  Naturally, the attending mechanic is often tempted to further perpetuate the "starter outran the impulse couplers" myth.  Upon further inspection, however, what most have found is that the ignition key switch was simply mis-wired.  The R-L-Both-Start key switch should be wired such that the P-lead on any non-impulse coupled mag on the engine is GROUNDED when the key is in the "start" position.  This prevents the non-impulse coupled mag's advance timing from instigating a kickback during starter cranking.  However, if the switch also happens to be grounding the impulse coupled mag also, both P-leads will be grounded.  Therefore, when the key is released to the "run" position from the "start" position, the mag P-leads are again powered, the mag can fire and sometimes there is enough momentum in the prop/flywheel to carry the cylinder(s) through compression and allow the engine to start.  So check the mag switch wiring if you experience this phenomenon.  It's usually the culprit.  Another contributing factor can be old/tired mags or a mag with a failed impulse coupler spring.  Check those impulse coupler spring AD's.  There's a reason they were written.

 

A quick note regarding shower of sparks ignition systems (contributed by Scott Norsworthy Pro Aero Engines)

 

The shower of sparks system (or buzz box, starting vibrator, SlickStart etc.) supplies interrupted DC voltage to the main and retard contact assemblies of the starting mag (usually the left) through the starting vibrator. If the aircraft voltage drops below a certain level while starting the engine the staring vibrator may stop working. Again, when the key or switch is released and the starter is no longer using most of the voltage, the engine starts and the customer is wondering what the expletive is going on.

 

Using Sky-Tec's Troubleshooting Guide to determine the cause of the voltage drop is the best way most have found to solve this issue. If the voltage to the starting vibrator is within allowable limits then usually the starting vibrator is the culprit and should be replaced. Many of these units are the original ones supplied with the airframe and are have been overlooked at engine overhaul or replacement.  (thank you, Scott!)

 

I'm going to talk to Les and Gene about possibly developing Smokey.  It sounds like a fun product.  And this might just keep them out of my hair for a while.

 

 

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