Message text written by "Shaun Simpkins"
>I'm worried that a 914 with an Airmaster prop will
be rather nose-heavy, and thought that a RG battery of the minimum required
for cranking up front with a very
tiny battery in the back for backup might work and possibly save cabling
weight. Redundancy would also be desired
for a motorglider restart. Since I haven't got to this point, either, I
thought I'd throw out some of my findings for comment as well.
I started by asking Bob Ruckolls (Aeroelectric connection). He said:
1. Rotax 912/914 series crank with low power requirements - 0.6KW ( about
50A ) for a few seconds. 8AH RG
batteries will crank a Rotax.
2. 8AH is about all that's needed if you size your battery to allow
comfortable termination of flight (1HR after failure)
with essential electrical loads. And if you load-test your battery
regularly to avoid suprises<
I have a 912S with an NSI VP prop. Although the engine might be slightly
lighter than the 914 the heavier NSI prop is more significant in terms of C
of G. Even with this combination and a heavy panel it is possible to get
the trim towards the aft end of the envelope with a 17AHr battery in the
aft position.
Although 8AHr may well be adequate for engine starting all batteries
progressively lose capacity with age. A 16AHr battery will therefore last a
lot longer in service for a small weight penalty. Having 2 batteries for
redundancy is only of value for engine starting if each can be selected
individually, otherwise if one battery has a cell go low it will pull the
cranking voltage down with it. The RG batteries certainly give better
cranking amps. Mine actually turns the engine faster than a standard full
sized car battery.
To protect your expensive onboard battery it would be helpful if the first
start of the day (particularly in cold weather) was from an external
battery. It is much easier and safer to have an external power connection
aft of the wing and a rear mounted battery makes this simpler.
As the 914 needs electrical power to keep the engine running (unlike the
912 and 912S) I think that a backup power source for the electric fuel
pumps is a wise precaution. If this is not done I would consider a voltage
crowbar unit essential to prevent a single failure becoming a total
electrical failure. Provided this is fitted it is reasonable to treat the
alternator and the battery as two seperate power sources. If this is the
case it is important that the two fuel pumps do not share any fuses or
c/b's (ie the main busbar fuse) as should it fail neither pump will run.
Regards
Nigel Charles
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