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Solved Correct CG Location?

JasonK

Well-known member
#1
Someone commented that my plane was tail dragging and probably needed the CG moved forward. All 3 photos have the balance sticks sitting were the plans show that the CG should be. Photo A - all the equipment is were I had it in the clip (or as best as I can place it from memory). Photo B & C were me messing with balance points with the battery sitting on top (which ends up being slightly different then in the plane because of the vertical offset apparently. I will say my battery ended up quite a ways farther back then were the build video showed it getting put (you can see about were it ends up by looking at the photos from the top view, I just have it resting on the bottom layer of foam instead of on top of the plane.

So my questions
  1. Does the Full up balance point look correct ok?
  2. Looking at the other photos, is there a better place to put it?

    FullUpAsInFlight.jpg BalanceForward.jpg BallenceBack.jpg
 

Merv

Well-known member
#2
CG location is a matter of personal choice. Most planes will fly well with the CG 25-30% of the mean average cord, MAC. When I maden a new plane, I set the CG at 25%, to improve stability. When I get the plane trimmed out, I will move the CG aft to get the instability I prefer, usually around 30%. I’ll fly through several batteries to get the plane dialed it.

Your plane looks about right to me, although it’s hard to tell from the pictures. You might want to measure where the plane balances in relation to the wing cord.

If you want to increase the stability, move the CG forward. If you want more instability, move the CG aft. Make the plane fly the way you want it to.
 
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Merv

Well-known member
#3
I should add that at FF19, I shared the table with 3 noobs, who all built TT from a kit. When it was time, I was given the honor to maden the planes. All 3 were balanced at recommend CG location & all flew tail heavy, they were flying squirrels. When we moved the CG forward 1/2 inch, they flew great.
 
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JasonK

Well-known member
#4
my cord is 5.25", the wing is almost 100% straight out to the tips, so the 25% would be 1.3125" back, but the marks are at 1.75". Which matchs up with the 1/2" forward and gets the battery located were they show in the built video.
 

Indy durtdigger

Well-known member
#6
I built one of those last summer, first FT build even. That plane flies better a tad nose heavy something like pic 2 in your examples and that's also what they recommend in the build video. That plane also benefits from the extra power a 3s battery pack offers especially when doing aerobatics.
 

JasonK

Well-known member
#7
yah, so I have the balance points were the plan shows the CG markers in those photos, what Merv said the the 'correct' GC is probably closer to almost .5" forward of that (exact math above), which would have the nose on the table at the marked CG points.
 

JasonK

Well-known member
#8
got out with the CG moved forward that ~.5in and even in slightly turbulent ~9mph winds the plane was flying fairly good. Had a few moments where I was wondering what happened to the plane, was rolling into a turn and the plan 'snapped' in the roll direction with almost no roll input.

Ended up cutting my flight time before my batteries were out because the wind was picking up the whole time I was out and when I got above the tree line, the air flow seemed less stable. Right near the end on launch I almost hovered the plane flying into the wind at about 60% throttle... I took that as time to be done until I get some more experiance.
 

Merv

Well-known member
#9
There is not one ‘correct’ CG, there is a CG range 25-30% that most will be happy with.

To increase the stability the first step is to set the CG @ 25%. The next step, turn down the throws. On the control surface, move the rod to a hole further out. OR On the servo move the rod to a hole further in. Both will reduce the throw, pick the one that is easier to change. You could do this just by turning down the rates in theTx. If your rates are something reasonable, like 75% or more, then no need to change the push rod. But if your rates are something like 35%, you’ll be better off adjusting the push rods & make the rates higher. If you don’t change the push rods, you will lose resolution. There are something like 1000 steps in the full servo movement. With rates at 35%, you are only using 350 of the 1000 step available.
 

JasonK

Well-known member
#10
sure, from what I am seeing from various reading, if you get the CG behind the NP, the plane amplifies any disturbances. The NP is behind the primary wings lift point (IIRC the 25% above is that point). So yes, I get it, there is no 'correct' CG, but there is that recommended range.