Not exactly. I was making my remarks with reference to economy cruise
settings of around 4800-5000 RPM. Check out www.bandcspecialty.com. By their
own documentation, and confirmed by Bill, the SD-8, SD-20, and 914 internal
generator perform so:
SD-8/SD-20 Rotax 914 Generator
PTO RPM Output Engine RPM Output
2000 2.3 2000 7
2500 4.7 2500 11
3000 6.8 12 3000 14
3500 8.4 20 3500 16
4000 9.4 26 4000 19
4500 10.2 31 4500 19.5
5000 10.7 33 5000 20
5500 20.5
6000 21
Note the clear knee in the output curves. The SD-8 and 20 are clearly
designed for the Lycoming, and, perhaps conveniently, the 912, but on the
914 the SD-8 is down 40% or so at economy cruise, and down 15% at max RPM.
The SD-20 is no better than 50% of rating! Note also that at most cruise
power settings the Rotax internal generator is producing full output!
The 914 PTO needs to have about a 0.7:1 ratio, not 0.54:1.
Shaun
----- Original Message -----
From: "Fred Fillinger" <fillinger@ameritech.net>
Subject: Re: Speeding up the Rotax 914 Vacuum Pump PTO
> 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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