Date: Wed, 3 May 2000 09:53:25 -0400
From: James Frantz <LFrantz@compuserve.com>
Subject: Alternator sizing
Sender: James Frantz <LFrantz@compuserve.com>
To: Marve Kay <Lancair.list@olsusa.com>
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>On the IV the hydraulic pump draws over 500 watts. <

This is also true of the 320/360, however, the hydraulic pump is an
intermittant load.  The way to size a charging system for aircraft is to
add up all of your continuous loads and insure that they do not exceed 80%
of your charging capacity. See AC43.13-1A section 2 - 426 paragraph d. 

There is absolutely no problem if the intermittant loads added to your
continuous loads exceed the alternator output.  It's the continuious loads
that you need to be concerned about.  Intermittant loads include the
hydraulic pump, the additional power required when the COM transmit key is
pushed, and aileron/elevator trim.

Continuous loads include things like nav lights, strobe lights, electric
fuel transfer pumps, pitot heat, alternator field, avionics, cockpit
lighting, etc.  You willl be surprised at how fast the continuous loads add
up.  The definition of a continuous load is, "the total electrical load of
devices that a pilot or flight crewmember would normally be expected to use
as the probable continuous load". (section 426 paragraph f)

If the hydraulic pump were to run continiously, you have a dangerious
problem that will cause the battery to discharge and a subsequent total
electrical failure.  It is also possible to melt down of the hydraulic pump
and/or sump.

Generally a 65 amp alternator at 14V will handle the VFR/IFR requirements
but do the load analysis to be sure.   On my Lancair 360 with a rather full
IFR panel, pitot heat, two strobes, two transfer pumps, and a big landing
light, the 65 amp alternator just barely met the 80% rule.

In one of the previous LNNs there was a full load analysis with two
examples.  

Jim Frantz
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