Debunking the "Too
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
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.
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
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.