How can i find out the gravity center of plane?
 

How can i find out the gravity center of my plane~??? Recently i have put a lot of weight on the tail of my super cub ( 1.5m wing span), but i can't fly this plane after i finish the modification.. so now i want to rebalance my plane by moving the battery to the front ..... has anybody know how to judge the exact position that i can place my battery?

thanks

Re: How can i find out the gravity center of plane?
 

Here is the plane CG calculator ... http://adamone.rchomepage.com/cg_calc.htm

Re: How can i find out the gravity center of plane?
 

Figure out where the CG is now..... where it balances. how much do you have to move it to be in the correct position?

Weight Shifted/Total Weight = ΔCG (change of CG)/Distance weight is shifted

I think this formula will work, someone correct me if im wrong. (no problems expected there!)

For the change of CG, find out where the balance point is now, then measure to where it should be and use that.

example: a/c total weight 10lbs.
battery pack weight 0.5 lbs
cg change needs to be 2cm

= move your battery 20cm foward.

I have never tried this with a model but use it often with full scale airplanes.

Re: How can i find out the gravity center of plane?
 

Actually you can pick the A/C up any where you want and let it hang free. Draw an imaginary line from where you are holding it directly to the center of the earth and it will always run through the C of G. Generally most sport A/C the C of G should be at 26% to 32% of the total wing cord.

Kip

Re: How can i find out the gravity center of plane?
 

That is correct ONLY for straight, constant chord wings. For any swept or tapered (or swept and tapered) wings, or elipticals, etc you first need to find the MAC (mean aerodynamic chord) and then measure 25 to 33% back from the leading edge of the MAC to find the proper balance point (or CG if you will).

AJC

Re: How can i find out the gravity center of plane?
 

That's a cool link, thanks for that.

Re: How can i find out the gravity center of plane?
 

thanks everyone~!!

Re: How can i find out the gravity center of plane?
 

Andrews answer in diagram form. This applies to any wing planform.

Ed S

Re: How can i find out the gravity center of plane?
 

Simplest way is to balance it according to the manual -- that is put your fingers at the recommended position and gently lift the airplane until the wings are off of the ground. BTW, I'm assuming that your Super Cub is the wing-on-top straight-winged airplane and not something else. It's always best to do this at the wing tips, but that requires two people. Doing it an inch or two out from the fuselage sides so there's no rubbing on the fuselage is a good starting point.

If the nose drops, you're nose heavy. Not bad if it's a bit. Bad news if the nose drops quickly and wants to point straight down. Plane should ideally be pointing slightly down (about 30 degrees or so) and stable.

If the tail drops, this can be trouble, seriously trouble. Double-check the measurements and try again. If it happens, start adding more weight as far forward in the plane as you can. RX battery is good for this, then lead tire weights and fishing sinkers. Tail-heavy airplanes can be aerobatic as all get-out (negative stability) but beyond a certain point will have to rely on lots of speed to overcome the tendency to enter a tail-first non-recoverable slide or worse.