Arwel,
In my Troubleshooting guide I mention the starter issues.
It is my opinion all Rotax engines should upgrade to the high torque starte
r.
I=92ve tried them both. The Rotax high torque is what I installed ($1100US
D), mainly because my plane was kept stock as much as possible. For the Ex
perimenter I recommend the Sky-Tec as it costs half (<$500 USD) as much and
is quite torquey.
Both spin the prop with authority, easily surpasses the ignition excitation
speed. With plugs in and ignition off, I see 500 RPM for a 914/912 and 40
0-450 for a 912S/iS.
Even with a slightly low battery, starts are crisp. This lengthens the lif
e of the sprag clutch. But these clutches do wear and somewhere within 100
0 hours even on a well maintained 912S engine with good starting, it may fa
il.
In my Rotax troubleshooting update in March, I went into the soft start/sta
rting ignition problems of the Rotax. As many have learned, once the start
er can=92t push past the compression peak at ignition, the engine will kick
back which will quickly wear the sprag clutch. What I didn=92t explain co
mpletely was when the sprag clutch begins to weaken and slightly slip, this
will exacerbate the kickback issues. Adding a high torque starter on a de
fective clutch will not be the total answer to the kickback. Because the R
otax sprag clutch is a trapped un-round roller type one way clutch, if the
spring that holds the un-round roller in place is loaded excessively, the s
pring stretches which causes the clutch to slip so it no longer is an effec
tive one way clutch. The good news is there is no damage to the starter or
the crank shaft when this happens unless the engine continues to have =93k
ick backs=94 with the defective clutch, then things break and fowl the engi
ne with metal parts. Which means it goes from a somewhat expensive repair t
o a very expensive repair. Youtube has videos on the different types of the
se one way clutches.
I recommend that if you have an XS firewall forward (with the ring mount) t
he high torque starter will fit. (Note the Rotax high torque may come with
the tabs still attached which must be cut/ground off. Our dealers in the
States supply them ready to install without the tabs.) I found the Sky-Tek
to be well built and spun up quickly on some light sport aircraft we have
around here. I think it is good insurance for preserving the sprag clutch.
The Sky-Tek is cheaper also.
For those with the Classic FWF, I=92m afraid neither of the high torque sta
rter options will fit the original Classic mount. It is too long by only a
few millimeters, but it is too long. Service your starter for good operat
ion, keep the battery up, and on very cold starts, use engine preheat.
Best Regards,
Bud Yerly
www.customflightcreations.com
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s 10
From: arwel pritchard<mailto:arwel.pritchard@powercom.ltd.uk>
Sent: Tuesday, December 1, 2020 8:51 AM
Subject: Europa-List: Rotax 912ULS starter motor
Good morning
Has anybody replaced the starter motor on a 912ULS with a Sky-Tec model. Ap
parently, it cranks the engine faster than the standard Rotax model.
I have been quoted =A3660 for this unit, seems quite expensive, but if it p
reserves the sprag clutch (which I had to replace last year at much more co
st) its got to be a good thing?
Just wondering if it=92s worth it.
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