Message-ID: <FDD045B6B83FD311A9880090273053E2CF0C2E@CDIM-PTS-MAIL>
From: "Rumburg, William" <william.rumburg@cdicorp.com>
To: lancair.list@olsusa.com
Subject: 12VDC vs 28VDC 
Date: Thu, 5 Oct 2000 11:13:45 -0400 
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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The following questions(>>) and responses(>) regarding 12VDC vs 28VDC were
posted today by Bob Nuckols on another website:

> >I have an 0-320D2G engine with 28V Alternator & Starter.  Many of the
> >electrical items I already have purchased use 12V (nose lift, mac
> servo's).
> >I have the option of trading for a 12V Alternator & Starter at no
> additional
> >cost.  
> 
>    Please consider taking advantage of this option . . .
> 
>I understand that 2X the voltage = 1/2 the current, meaning the wire
> >size going from the battery to the engine is much small for the 28V
> system
> >(less weight).
> 
>    The 28v option made sense when we were building B-29's and B-52's with
>    as much as several hundred miles of wire in them . . . the weight
> savings
>    was significant. Further, the builders of these machines had pretty
> deep
>    pockets to reach into (yours and mine) to finance the miliatary
> aviation
>    specialty hardware items unique to their task.
> 
>    Later on, folks who built DC-6's and 707's knew that the return on
>    investment for a revenue generating machine was a function of how
>    many pounds of pax/bags could carried. The airplanes were still pretty
>    complex and again, the cash to pay for them would continue to come
>    from the same pockets . . .
> 
>    Now, your airplane isn't going to make you a dime. In fact, it's 
>    going to COST you a lot of dollars. Further, money isn't going
>    to come from any pockets but your own to finance the task. While
>    one might successfully argue a small weight savings it will be quite
>    small. In a simple single it's not over a couple of pounds.
>    
>    However, locking yourself down to aviation specific, 28V hardware
>    shuts you out of all the opportunities to exploit automotive and
>    consumer products that are widely available, high volume, and
>    competitively priced.
> 
>    One exception to consider in this reasoning is unique to
> canard-pushers.
>    Cabin heat.  If you're going to insist on flying in cold weather and/or
>    high altitudes, electric toe warmers might help you survive the
>    experience with a minimum of discomfort. I've had several builders
>    go for 28V, 60A systems. While the weight of the altenrator
>    and battery needed are about the same as for 14V, 60A . . . the energy
>    you can get out of the 28V alternator is twice that of the 14V . . .
>    all of which you will need and wish for more if you're considering
>    electric cabin heat.
> 
> > . . . .  Also, If you mix your loads, what is the best solution?  A
> >converter or two batteries or something else?  Or does someone make a 28V
> >battery with the cell poles exposed with 12V tapping capability?  Any
> >suggestions greatly appreciated.
> 
>    Mixed voltages in a small airplane are almost never practical.
>    The size, weight, and cost of voltage conversion equipment clobbers
>    an otherwise elegant design. Further, you're often forced
>    to run multiple accessories from the voltage converter meaning
>    that it becomes a single point of failure for all the goodies
>    it powers.
> 
>    Tapping down on half of a 28v battery to get 14v is about the
>    best way I know of to kill off an otherwise perfectly good
>    battery.
>      Bob . . .
> 
Bill Rumburg
N403WR  (Sonic bOOm)
>     
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