Why place the battery in the back?

gasman · #1 03-03-2012, 10:59 AM

After reading this http://www.vansairforce.com/communit...ad.php?t=83367 thread, I kept asking myself "why place the battery in the back?"

There was a time when the battery was a very heavy item, and it was worth the effort to place it in the best location for cg control.

The battery has changed, and will continue to get smaller and lighter. My 6A has the battery on the firewall. Easy to service, easy runs to the alt. and starter. Easy run to the main bus.

I would rather have the battery on the firewall and ballast the aircraft with extra weight in the far aft fuselage than deal with the problems of an aft mounted battery and long heavy cable runs.

My 6A is an O320 FP prop so it is light on the nose. Cargo is restricted, but it sure does fly sweet!! Aft cg RV's fly great. So why not build the aircraft to fly great all of the time. Build to the fwd cg chart so you can load up the cargo area, and then use far aft ballast to find the sweet spot for local flights.

scsmith · #2 03-03-2012, 12:28 PM

It totally depends on where the cg range wants to be, and what passive ways to adjust that.

For your 6/6A, the firewall mount is best.

For the RV-8, they tend to be nose heavy, so we want all the heavy things we can put in the back, like strobe power supply, battery, remote compass, etc.
The Odyssey PC-680 still weighs 15 lbs, plus the weight of the battery tray and the master relay. So it is still a healthy chunk of weight to move cg around with. Moving it from the firewall to the rear baggage shelf in the RV-8 moves the empty cg more than an inch.

You said you would rather add ballast in the back than have a heavy cable run.

That makes no sense at all, when you find out that the 2-gage battery cable from the rear battery box to the starter contactor on the firewall weighs 2 lbs. Yes, I weighed it. 2 lbs.

But more important...why would you ever add dead ballast when you can get the same benefit from positioning useful/necessary components where they have the same effect?

Alan Carroll · #3 03-03-2012, 12:39 PM

I estimate that in my -8 moving the PC-680 batter from the firewall to the aft location would shift the CG about 1". To obtain the same CG with the battery in its present location would require adding about 10 pounds to the tail (and to the empty weight). I'm not sure how heavy the longer cable would but I'm guessing 2-3 pounds?

rockwoodrv9 · #4 03-03-2012, 01:17 PM

Quote:

Originally Posted by scsmith

It totally depends on where the cg range wants to be, and what passive ways to adjust that.

But more important...why would you ever add dead ballast when you can get the same benefit from positioning useful/necessary components where they have the same effect?

The reason is I am building to give myself the most options. When I fly with my wife and baggage at gross, it can get the CG close to too far aft. It is much easier to add weight to the back than it is to add it in the front. I am so far away from worrying about that now, that it is still a dream! That CG program that one of the members here posted a couple weeks ago is sure helpful in understanding the limits and where to place things like the battery.

gasman · #5 03-03-2012, 01:55 PM

Quote:

Originally Posted by scsmith

You said you would rather add ballast in the back than have a heavy cable run.

No, I said.......

I would rather have the battery on the firewall and ballast the aircraft with extra weight in the far aft fuselage than deal with the problems of an aft mounted battery and long heavy cable runs.

RVG8tor · #6 03-03-2012, 02:37 PM

I talked with Scott and Van's on this issue and I ended up with my battery up front. On the Demo 8A he said they have no issues with the battery up front. (I am also building an 8A) Now it just may be that the position of the gear closer to the spar makes the 8A less sensitive to weight forward. I did not want the headache of running the fat cables (I stared down that path and did not us the cable I bought). I am thinking I will be fine since I will have the WW prop which saves weight over the Hartzel they run on the demo -8A.

I have not gotten to the Weight and Balance, but I will live with the ballast requirement if it warrants. I too like the idea of max payload and with the -8A you have both forward and rear baggage areas so loading with tow passengers for a trip would be easy it would seem.

Cheers

jthocker · #7 03-03-2012, 03:50 PM

If you're going to do most of your flying with a passenger then a firewall mount is probably best. Very flexible that way. On the other hand, if you're like the rest of us and most of your flying is solo than put the battery in back. Aerobatics and landings are way more fun without that forward CG. If I had it to do over, I'd put the battery in back!
My 2 cents!

airguy · #8 03-03-2012, 06:36 PM

I'm building a 9A with IO360, CS prop and dual alternators - so I know I'm going to be a bit nose-heavy. The RV's in general and the 9A's in particular seem to be allergic to high nosewheel loadings, so I wanted to keep the CG toward the aft end of the acceptable window. It was a bit more hassle to run the large cable forward but worth the effort. I'm also going to run a Whirlwind composite CS prop, trading dollars for pounds up front.

DanH · #9 03-04-2012, 07:03 AM

FWIW.....an Odyssey PC925 fits the standard RV-8 battery tray perfectly when mounted on its side. The Odyssey doesn't care how it is mounted.

Compared to a PC680 weight is 10.6 lbs more, which works out nicely with a IO390/BA Hartzell combination. You'll still run out of pitch trim when solo, slow, and full-flap, but the stick isn't heavy. Feel is excellent with 150 in the rear seat. It still takes 230 in the rear seat, 75 in rear baggage, half fuel, and empty front baggage for me to reach the rear CG limit.

The second benefit is that EFIS/com/xpond/GPS operating time after alternator failure is increased by about 75%, not counting any internal device batteries.

sf3543 · #10 03-04-2012, 09:06 AM

I have built and flown two RV8s, the first with the battery in the back and the second with the battery on the firewall. Both planes had the IO-360 parallel valve engine and Hartzell CS prop. In the first RV8, with rear battery, I noticed good solo flight but with a passenger in the rear, it got squirrelly on landing due to the rearward CG. On the second RV8 with a firewall battery, I noticed a heavier stick on landing, but flying with a passenger was much nicer. I don't really notice any difference in doing aerobatics. Both planes W&B had acceptable CG calculations in all regimes.
For me, I prefer the firewall mounted battery, since it is easier to access, allows shorter wire runs and the plane flies better with a passenger. When I fly solo, which is most of the time, I just carry my tool kit in the back baggage compartment.
These are my observations and opinions only, so you have to decide what you want in your aircraft. I do however, find it interesting to read the opinions and advice of those who have yet to complete and fly their first airplane.