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Date: Tue, 22 Feb 2005 13:00:45 -0500
From: Ernest Christley <echristl@cisco.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Back-up alternator thoughts (was: Amps required to
 run engine    & amp- hours available)
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Bill Dube wrote:

>       I would think that it would be pretty simple to put together a 
> small permanent magnet back-up alternator to go behind the accessory 
> pulley. If enough folks are interested, I think I could come up with 
> something.
>
>     If you think about it, the very basic back-up alternator can be 
> pretty simplistic. It just has to be powerful enough to get you to the 
> nearest airport. Thus, it really only has to put out full power at 
> cruise RPM, and it doesn't have to put out much more than a trickle at 
> idle. This could make it quite compact and would likely reduce the 
> cost as well.

If you're using an EWP and can replace the alternator, then there's no 
need for the pulley at all.  I've not been able to find any reason that 
you can't produce 40A from a very small and light permanent magnet 
setup.  B&C sells a 35A PM unit for the Lyc accessory pad.

Consider that alternators have to be built beefy because it is being 
driven by a belt which adds a hefty side load.  If you drove it from the 
center, you could use a comparatively thin sheet metal housing vs the 
cast monstrosity that is usually seen.  All you're trying to do is the 
center driven case is to keep part from flying away due to centrifugal 
forces.

The eccentric shaft is spinning at 6000 RPM.  Generator outputs 
increases with RPM, and the B&C 35A generators I've seen have been rated 
for less that 3000RPM.  Using the metering oil pump seemed like a good 
option to me, because it would allow the use of a off the shelf part 
(the RPM is divided by two, as I understand it).  I looked into it, but 
I would have to much to learn to design a generator from scratch. It 
doesn't seem like it would be that hard for someone with a little 
background.

Magnets on the shaft, coils with pickup mounted to the housing.  No 
parts touching. No wear. (Do the magnets in generators wear out.)

So far, I believe an electric engine with 1 EWP and 1 fuel pump can be 
driven with just under 20A.  If a backup PM generator only produces 15A 
and the system requires 20A, then a 20AH battery will last beyond the 
point of usable bladder.   I used to drive a truck with an underated 
alternator.  Drive at night and the headlights would slowly get dim to 
the point where they were almost useless by the time the sun was coming 
up.  It would take two days of running to get the batteries back up 8*)