Old 09-25-2011, 05:05 PM
Rockin Robbins
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Join Date: Mar 2009
Location: DeLand, FL
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Chris, the battery location is much less important than where your center of gravity is. The location of the center of gravity is measured from the leading edge of the wing. I use a square to make sure I'm accurate.

Parkzone says the factory center of gravity is 62mm behind the leading edge. Mark that on both wings. Label it 62mm so when you change it later (you will) you can keep track of where you are compared to the factory setting.

The producers of the planes tend to give you a CG that is more toward the stable end of the spectrum than working for ultimate performance. Don't worry about it for now, it will fly great.

I measured my CG by drilling two spaced holes in the styrofoam of the shipping box, and using two pencils with the chisel tipped erasers. Works great and it's part of your box. I used my box for transport too, so everything was always together.

Assemble the entire plane and balance it with the chisels lined up on your 62mm CG line. I used clear packing tape over the line so the erasers didn't bore into my wing. It should balance level or very slightly nose down on those two lines. Move your battery back or forth to make her balance.

You see, there have been some differences in the weighting of individual planes so one may run with the battery position in one spot and another with the battery in a different position, although their CGs were in the same spot. If we just talk about battery position we'll never get anywhere in getting your plane to fly right.

Now, as to ultimate CG position. This is subjective. In order for the plane to have neutral trim and highest responsiveness the CG and center of lift (CL) would have to be coincident: the same spot. Unfortunately at that point the plane loses all inherent stability and you cannot fly it.

But as you approach that point your stall speed decreases and your plane responds more evidently to thermal activity. As a result, you have to make more and more control inputs to keep the thing in the air. It cannot fly itself any longer.

So what we do is, as we become comfortable with the plane, we move the CG back, a little at a time and fly it that way until we're comfortable with that. The further back your CG moves, the greater effect tiny changes in the position will have.

As a result of discussions here, I moved mine back to 72mm. The plane was darned scary, in part because I moved it right back there from stock position, 10 mm forward. I found pitch stability in launches to be terrifying as I was jockeying throttle position continuously and elevator the same way all the way up, never exceeding half throttle. Then when I shut the motor down I found turns were were incredibly scary. The plane just didn't care which direction it flew. It didn't crash but my inexperience could have crashed it if I weren't very careful in flight. Don't repeat my mistake!

Then I moved the CG forward to 70mm, only 2mm! The plane was transformed. NOW it felt directionally stable TO ME (your mileage may vary! Don't just go for 70mm and expect a miracle), I could calm down on the inputs and the plane was mine again. But the stall speed seemed like half of that of the stock 62mm CG position.

When the plane did stall, I didn't get that 20' dive to regain enough airspeed that the elevator could regain enough authority to force the nose level. The plane kind of mushed and came down horizontally. When I blipped in some down elevator, she lost a couple of feet and resumed flying! My sink rate was cut by an amazing amount. It was plastered to the sky, capable of flying at unbelievably slow speed and just didn't want to come down.

I'd recommend flying her absolutely stock until she's YOUR plane, doing exactly what you expect and you know the difference between overcontrolling and "just right." Resist the urge to tinker and just enjoy the plane. It's plenty good right where they recommend.

When you're ready, I'd make marks two mm apart back to 64mm, labeling in a way you can read it easily. Then moving the CG back 2mm to 64mm, fly her that way for awhile. Feel the difference. Get very comfortable with that in multiple flights and days.

Then my movements would be to 66, then one mm at a time, slowly making changes, giving yourself time to get comfortable with each step and knowing for sure what the difference is from your change.

Eventually you'll find a spot where you just aren't having fun flying the plane. That's a danger sign. Move back to the previous comfortable position and fly there for a long time.

Every once in awhile you can try moving back again. As your skills improve you will be able to fly comfortably with a less stable plane.

The pros, who have the ability to fly at insane CG positions, still realize that as you move the CG back you need to make more and more control movements to fly the plane. At some point these induce enough drag to hurt your glide duration. They'll move it forward based on their still air flight duration, not their flying ability. Maybe Ed is at that point, but it's completely out of sight for me. 72mm scared me to death and I'm sure lots of people fly there with no problem.
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