SkyTec Starter sticking?

wrote:
skytec stuff snipped because 'tin' doesn't auto-wrap columns

Hi Cory. I have a sky-tec starter sitting in the back of my Cherokee 180
that I bought to replace the Prestolite unit. After buying it, I did some
more detailed reading and found quite a few notes about operation like yours.
As skytec says, the PM motor spins down and generates quite a few volts until
it stops. Because the motor and solenoid are both fed from the starter
contactor the solenoid is the path to ground and it stays pulled in far too
long. The skytec-like design works on cars because the solenoid that kicks
out the pinion gear also energizes the motor but is powered from a different
circuit, so when the solenoid drops out the motor's residual voltage goes
somewhere else. (I suspect that you knew all that.)

The notes that I turned up were from a Grumman Tiger owner that installed
the sky-tec and then had the same "sticky" pinion. The Tiger owner eventually
went back to the Prestolite starter because he was afraid of ruining the ring
gear.

Perhaps sky-tec could have minimized this little glitch with a couple
diodes, and I think it could be completely eliminated with a second starter
contactor to drive the solenoid only. My sky-tec has a heavy external wire
between the solenoid and the motor, so they could be separately driven
easily (in an electrical sense: YMMV paperwork-wise).

The conclusion that I came to was to fix the bendix on my Prestolite starter.
So, now I have a spare sky-tec for when the Prestolite breaks.
--
Aaron Coolidge (N9376J)

Interesting. I was thinking a free-wheeling diode might help the situation,
but in order do it, you'd need a true isolator-type diode arrangement. That would
require a diode in the starter motor path... not too great of an idea (WAY too much
current to be reasonable/reliable). The separate contactor for the solenoid would
probably work fine, but the FAA goons are already circling on that one. It's not a
PMA'd replacement if you need to install additional stuff on the airframe. Then it's
an STC and would require more certification and paperwork, etc.

The more I think about it, the more I realize that it was probably a necessary
compromise. Even if you put the contactor *ON* the starter for just the solenoid,
there's no way for it to "know" that the voltage on the starter motor lug is from the
battery vs. the back EMF in the motor. I suppose you could measure the motor current
with a shunt (haha!) or winding, but again... it's more complicated and failure-prone.
The system they have is safe and simple, if a bit inelegant. I doubt the starter gear
could damage the ring gear much since there's no load on it at the time. All it's
doing is free-wheeling the sprag clutch. I suppose it would be good to keep the RPMs
as low as possible until it stops (but then again I do that anyway while the oil
pressure comes up).

Interesting... I think it might even be worth it for the 10 lb weight savings, though.

-Cory


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************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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