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finding cg

olds45512

Junior Member
#1
im building a custom trainer style plane for my swappable power pod and the only thing i cant figure out is how to find the cg? generally i build off plans so its not a problem but considering i built this 100% from my head finding the proper cg seems to be a mystery to me. thanks
 

earthsciteach

Moderator
Moderator
#2
A general rule of thumb for a conventional aircraft (horizontal stabilizer) is the cg location should be somewhere between 1/4 to 1/3 the length of the wing chord back from the leading edge. A bit nose heavy is better than tail heavy.
 

squishy

Pirate ParkFlyer
#4
"Finding the CG" -
The center of gravity of an airplane is not something you can lose, it is not something you have to go find like a running fridge, it's always there (lurking under the surface). Just put your fingers under the airplane and move them till the airplane balances. BAMB!! that's the center of gravity...

Now if you asked me "how do I find the proper CG in relation to the center of pressure so my airplane flies optimally"...Then I would have a far different answer, a much more lengthy and educational one...
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#5
I can't disagree more squishy. The CG on my planes tend to get up and move quite often. Just as I think I've got them pinned down, they pack up and go on vacation. They also tend to get offended when you use the finger method for finding them. At least when I try it, they do. On my hands, my middle fingers are the longest, so I tend to use them as the balance points when searching for CG. The problem comes when the CG looks out, and sees this method, and gets offended because it thinks I'm flipping it off. It might not come out where I can find it, for a week afterwards.
 
#6
A good tip that I've read is that after you choose a CG point and balance your plane for that, you should set your control surfaces to neutral and then just give it an unpowered toss to see how well it glides. If it nose dives or tail stands, you should probably move your CG appropriately, favoring the nose dive end of things. I heard David say in one of the videos, "A nose heavy plane flies poorly, a tail heavy plane flies once."
 

squishy

Pirate ParkFlyer
#7
A good tip that I've read is that after you choose a CG point and balance your plane for that, you should set your control surfaces to neutral and then just give it an unpowered toss to see how well it glides. If it nose dives or tail stands, you should probably move your CG appropriately, favoring the nose dive end of things. I heard David say in one of the videos, "A nose heavy plane flies poorly, a tail heavy plane flies once."
Great suggestion! The old glide test...

When I help noobs maiden their planes out at the field, I sometimes say, "lemme trim it out for you" and I just let him throw it and I just glide her in...I can tell right away if the balance is off and which way it needs to go. If it's good to go, I hand the TX back and say, "just kidding dude, READY! Your turn"...It's a quick way to double check the balance without destroying the aircraft. I have seen way way too many tail heavy noob maidens, they never end well. If you do find yourself in that situation (trying to keep a tail heavy airplane in the air), cut the throttle and just land (fall), stop trying to fly it, just land and walk...it's much better than the alternative...
 
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squishy

Pirate ParkFlyer
#8
I can't disagree more squishy. The CG on my planes tend to get up and move quite often. Just as I think I've got them pinned down, they pack up and go on vacation. They also tend to get offended when you use the finger method for finding them. At least when I try it, they do. On my hands, my middle fingers are the longest, so I tend to use them as the balance points when searching for CG. The problem comes when the CG looks out, and sees this method, and gets offended because it thinks I'm flipping it off. It might not come out where I can find it, for a week afterwards.
You guys got some pesky CG's down there dude, that sucks, lol...