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rparigor(at)suffolk.lib.n
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 Posted: Fri Jan 20, 2006 11:17 pm Post subject: Battery rear for aft CG? |
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I have heard mixed feelings on mounting battery on foot well or aft to get
a more aft CG.
I ground some numbers tonight.
I used:
****a 62 inch CG, then calculated upset from this point
****used both a PC 545 (12 pounds)and PC680 (15.4 pounds)
****Considered the cables from the footwell to buss and motor needed on
all three scenarios so did not penalize footwell mount
****used three positions on each battery: 1) top of right footwell, 2)
right rear baggage bay, 3) right just aft of baggage bay bulkhead
****weight of wire used was for #4 copper clad aluminium wire, but under
no scenario if #4 pure copper wire was used would it add more than 1 pound
(less actual) The #4 copper clad aluminium wire is larger in diameter than
copper #4:
http://www.periheliondesign.com./fatwires.htm
Bottom line, battery on right footwell and adding lead to forward lower
front face of sternpost to achieve same aft CG as rear mounted battery
adds between 7 and 9 extra pounds for a given CG!
Range from 7 pounds first to 9 pounds:
545 / right rear baggage bay (~ 7 pounds lighter overall)
545 / right just aft of baggage bay bulkhead
680 / right rear baggage bay
680 / right just aft of baggage bay bulkhead ( ~ 9 pounds lighter overall)
Above includes the added weight and longer run of 2 cables and its effect
on CG.
I did not include weight of lugs or battery mounting, if I did it would
make case for aft mounted battery only look better.
Comments please.
Ron Parigoris
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SPurpura(at)AOL.COM
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| Posted: Sat Jan 21, 2006 6:35 am Post subject: Battery rear for aft CG? |
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I mounted the battery below baggage bay shelf on stbd side and my CG is good
nor requiring any ballast.
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topglock(at)cox.net
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| Posted: Sat Jan 21, 2006 8:53 am Post subject: Battery rear for aft CG? |
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The CG will have a lot to do with which prop and which engine you use
and your particular build. My bird, with 912S (lighter engine) and
Airmaster (much heavier than warpdrive), ballances right about 60" with
the 680 battery on top the right footwell. My build is probably a bit
different than most, as I, not being one to trust "miracal" adhesives,
added a few layers of BID to the fuse joint, all around. This, of
course would add some weight to the rear of the plane. It's hard to go
out of CG, with two occupants and enough baggage to get to gross weight.
Even with less fuel, it seems to stay in CG.
Jeff - N55XS
133 hours
SPurpura(at)aol.com wrote:
| Quote: |
I mounted the battery below baggage bay shelf on stbd side and my CG is good
nor requiring any ballast.
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peter.rees01(at)tiscali.c
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| Posted: Sat Jan 21, 2006 9:21 am Post subject: Battery rear for aft CG? |
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I'd think that to keep the resistance down, you'd need very thick cables to
run from the back to the solenoid. If you're going to add this much weight,
you may be better off putting the battery on top of the pax footwell and
adding some dead weight in the back - more reliable electrically and more
adjustable (both position and amount of weight)
Peter
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davidjoyce(at)doctors.org
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| Posted: Sun Jan 22, 2006 9:49 am Post subject: Battery rear for aft CG? |
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Ron, Your maths doesn't work for me. My XS,mono,914,Woodcomp CS prop, empty
wt 839 has CoG at 59" which I think is ideal position. This was achieved
with a 6kg/13.2lb Odyssey battery above R passenger footwell and 3lbs of
lead bolted in front of the stern post. Fixing battery at the front is a
doddle with a small (1" high) retaining wall and a nylon strap between this
wall and a clip on the upper firewall. I would expect a much heavier
construction to be necessary in the rear to give similar security and to
avoid distorting the skin, quite apart from the weight of the extra heavy
duty cable necessary. So I guess that the forward position has cost me no
more than 1 lb. This isn't the whole story either as moving the battery from
station 25 to 85 (my best estimates, rather than measured stations) and
removing the lead would have the effect of shifting my CoG back to 59.65",
and I would have wanted to hang some lead somewhere at the front to get back
to 59" !!
You may wonder why 59" - this allows at two extremes , the carriage
of 400lbs pax with no fuel or baggage, (CoG58.01) or120 pax and full fuel +
80 bags, (62.4 . Now I admit that I weigh a tad more than 120 lbs but
equally there have been occasions when I might have been tempted to take a
bit more than 80 lbs of luggage, (what about along range tank, for
instance?)
For me the smart answer is to build in the possibility of fitting it
in either place, then do a trial weighing and see which makes most sense,
but in any case I don't think you will be looking at very much lead if you
put it in the front unless you are putting an MT prop on, which weighs only
slightly less than a ship's prop!
Regards, David Joyce
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