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tonyjohnson

Well Known Member
I am building an 8A with a superior IO 360 and a catto 3 blade prop. I had intended to put the battery on the firewall until I heard the opinion, almost universal including guru Jay Pratt, that it should go in the rear.

Van's plans for the aft battery show it placed almost as far as it can go in the rear. It seems possible, unless I am missing something, that the aft battery can be mounted any place in a range of about 20 inches within the rear area. If that is true, it seems a great opportunity to adjust the CG a bit by placing the hunk of weight that is the battery at an optimum point along that range to have the most positive effect on the CG.

I am a lawyer who flunked math twice in high school, and not smart enough to figure out where that would be. Do any of you guys have any ideas, thoughts or observations that would help me?

Thanks,

Tony Johnson
RV8A "Badboy" N12TJ res
Orlando
 
Fine Tuning CG

While I don't have my -8 flying yet, so I don't have any first-hand experience, it's my opinion that you don't (and likely won't) have enough concrete info on how your ship is goign to turn out, with all the variables of equipment placement, to do much in the way of fine-tuning your CG beyond the gross placement of the battery where Van's shows it. I agree 100% in the aft battery placement, based on everything I've been told, but beyond that, I'd say put it where Van's shows, and fine tune with other stuff. In order to move it much forward, you'd have to modify the aft baggage compartment, and aft would be complicated with the elevator bellcrank and pushrod.

I remember back when I was in school, learning how to design airplanes, it was apparent who the students were that had worked on planes, and who hadn't. Those that hadn't put much more faith in analysyis than those of us who knew what actually went into the construction process! There is a lot you don't know when you're at the drawing board....

Paul Dye
 
what about the forward baggage floor?

Paul,

Many builders have opted to put the battery in the floor of the forward baggage compartment. That shifts it just a bit aft of the firewall location and eliminates some of the problem associated with aft placement...long cable run etc. The catto prop is much lighter than most other props which helps move the CG aft.

If you know Randy Morgan who works on the space station stuff at JSC, tell him hello. He is my former brother in law and good friend.

Tony Johnson
 
You're absolutely right Tony - many folks have mounted the battery up there in the forward baggage area. I guess I focused in on the "20 inches" part of the note, so was only thinking in terms of the really aft location.

I think all the guys with real flying experience in their -8's that could relaly advise you are probably at Oshkosh, and not reading the forum this week....

Paul Dye
 
yep

My guess is that you are right Paul. I wondered why there was little activity on this site in the last couple of days...duh. Most of the guys are away from home.

You are right, my first post was exploring the idea of having a varying range of placement to affect CG. I have decided that is way over my head to figure out.

My thought regarding the placement in the baggage compartment is that although it is not as far aft as the rear baggage compartment, therefore will not move the CG as far aft...the catto prop weighs only 18 pounds. That just about makes up for the difference between the 0-320 and my superior IO-360. Moving the battery as far aft as the baggage compartment might actually create more of an aft CG that I would want.

My understanding from reading on the subject, is that an aft CG makes the airplane more fun to fly because it makes the controls lighter, but it also makes in more unstable and less responsive to stall and spin recovery. You can also put a bigger passenger in the back.

The odessey battery is said to weigh only 11 lbs., so it would not be that big a deal in any case. I just want to ensure that I get the CG in the best position to balance the airplane.

An interesting delimma. Any thoughts from rocket guy (or anyone else with experience or knowledge on the subject) would be appreciated.


Tony
 
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