I've done a bit of detective work in the on-line Illustrated Parts
Manual for the Rotax 912/914, and have determined that someone is
blowing smoke,at least for the 914. One would have to replace the 29
tooth drive / 22 tooth spline gear pair with something more like a 31/22
to get the necessary 8% speedup. There isn't one listed. Curiously
enough, there are listed two sets of prop drive gears in 2.43:1 (912S,
914) and 2.27:1 (912) ratios. 8%! If you're flying behind a 912, you
can get more oomph from the vacpad alternator because of the faster prop
speed. But it's the 914 that needs the most help.
Indeed, a simple gear change would do the trick - but it's the main
drive gears!
Feeling like the Amazing Randi -
Shaun Simpkins
A207
----- Original Message -----
From: Shaun Simpkins
Subject: Speeding up the Rotax 914 Vacuum Pump PTO
Hi, me again.
I've been exploring going all-electric and was looking at the B&C
Specialties alternators that run off the vacuum pump PTO on the Rotax
914. An extra 5-10 Amps would be quite nice to have, as well as the
redundancy, given that only about 10-12A is left over for everything in
the plane after the engine pumps and computer take their portion.
The little 8-amp alternator would be great, except for the fact that
the Rotax PTO runs at only 54% engine speed, which at typical cruise
RPMs is below the knee of the alternator's output curve - only 3.2 to
4.3 A are available. Their 20-A alternator could also be used, and
would generate about 10A, but it's twice as heavy and quite tall.
Bill at B&C told me that someone at Sun'n'Fun claimed to have made a
simple change to the PTO gearing in the engine block to get the PTO RPMs
up to the 3500 range that these alternators prefer. So - anybody know
anything about this? Is this possible without tearing the engine block
apart, and, more importantly, voiding the warranty? Anyone install the
20A alternator?
Yep, you guessed it, the prop will be electric, too. Motorglider.
Thanks,
Shaun Simpkins
A207
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