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Tail heavy CG fix

GTWreck

Active Member
When I finished my plane, I was a little disappointed at the initial CG location, which came out at 108.64 (1695 empty). It would be very easy to find myself with the CG too far aft with four people onboard (or even 3 people and lots of bags). I wanted a plane that can haul 4 people and bags. To do that, the CG needed to move forward a good bit.

There's nothing too far out of the ordinary installed in the back of the plane: Odyssey 925 battery, autopilot pitch servo, ELT, strobe pack, and a 3 prong charging/jumper port. While I didn't weigh the plane prior to paint, I'm fairly certain that paint was a significant contributor to the aft CG condition (there's just a lot of paint/clear coat on the plane). Everything else is fairly stock on the plane (IO-540 with Hartzell CS prop) from a weight perspective.

At 25-ish pounds, the battery was the obvious way to attack the problem. For several weeks, I contemplated replacing the battery with a LiFePo battery. That would get me much closer to my target CG, but not quite. Besides, the cost of the LiFePo batteries are pretty steep.

I finally settled on removing the 925 and replacing it with a PC680 on the firewall. This moved my CG to 106.81 (and saved me 5 pounds).

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I've been flying with this configuration for the last month or so, including OSH (where I ferried two of my neighbors and a boatload of bags). I'm much, much happier with the new CG location. Also, no problems with the smaller battery starting the engine (shorter wire run = less resistance), even on hot starts.
 
high temps

I have two thermocouples around that area in free air, one just above your battery and the other above the firewall recess. Temps are usually above 170F on runup, and over 180F after shutdown. In flight it should be no problem.
Keep an eye on that battery.

Lenny
 
A couple of other comments. It appears to me from your photo, that you have bypassed your master solenoid and run your starter solenoid directly from the battery. If this is the case, you should re-think the danger with having a "hot" start switch. I also appears that you do not have a diode installed on the Master relay coil.
 
A couple of other comments. It appears to me from your photo, that you have bypassed your master solenoid and run your starter solenoid directly from the battery. If this is the case, you should re-think the danger with having a "hot" start switch. I also appears that you do not have a diode installed on the Master relay coil.

That's not the case here. The extra wire goes back to a jump plug behind the wing (near where the original battery was located). The original #2 wire that ran from the battery master to the starter solenoid was disconnected from the starter solenoid and connected to this extra wire. This allows me to attach a trickle charger or jump the plane as before.

This arrangement would allow me to re-install the battery in the back again if I choose to.
 
Lenny,
Good deal. I was just concerned. I just went through the W/B calculation for OSH having some friends ride along. It's amazing how easy it is to be to rearward with the CG, especially when the fuel gets low.
Bill
 
Nice airplane

Aaron has actually done a very nice job on this airplane, and received a Bronze Lindy at OSH for it. I am the DAR who licensed it and watched him building it for the last 7 years. I was really disappointed that he finished it as I used him as a source of parts for my RV-10's when I needed them. :)
His battery placement here was also done very well, and really helps in the CG in his particular airplane. It was a nice solution.
I wish the price of the EarthX batteries were more reasonable, as I would rather just replace the battery in the rear of mine and lose 20 lbs, as I have a remote oil filter on my firewall and can not incorporate his solution.

Vic
 
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