Shaun, I'm surprised to learn the B & C will put out only about 4A on
the Rotax, when they market it for vacuum pad installation on
Lycomings. Your 54% calc is correct (for the 914 at 75%, pump speed
of 2983 RPM; 912 is 3194). However, the Lycoming vacuum pump pad has
1.3:1 gearing, thus turning 3250 at an assumed 2500 for 75% power. Is
B & C saying a 9% reduction in alternator RPM will halve its output?
How so?
Regards,
Fred F., A063
> Shaun Simpkins wrote:
>
> 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?
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