Whats the best way to check the CG on a J3 Cub? Is it the balance when the wings are flat and parallel to the ground? Any tips would help as I have heard that the FMS J3 tends to be tail heavy, but it seems ok, but i crashed on first flight plane seemed to dive upon final approach, but could have been my fault as it was pretty windy and I had adrenaline pumping lol Anyway, any tips on CG for this plane would be very helpful and appreciated...

There is no magic about it. Just support the plane on your finger tips at the correct location (as advised in the manual).

To be balanced correctly the fuselage should sit level. If the tail hangs down it's tail heavy, if the nose hangs down it's nose heavy.

Originally Posted by JetPlaneFlyer

There is no magic about it. Just support the plane on your finger tips at the correct location (as advised in the manual).

To be balanced correctly the fuselage should sit level. If the tail hangs down it's tail heavy, if the nose hangs down it's nose heavy.

Thats what Ive been doing. How precise are the measurements? The planes book says that the cg is 45mm aft of the leading edge of the wing. Well, my fingers are 5-7 mm wide so I dont know exactly where the spot is and if i move it slightly one way the taildrops, slightly the other way the nose drops. I may be over analyzing things.

A J3 is a very stable plane so the width of your fingers doesn't really matter.... if it's level when you center your fingers on the exact spot then it's good enough; in reality even if it's slightly tail down or nose down at this spot, you may find that there's no point to added weight since it is still perfectly flyable (slightly nose down is better than tail down). Some people take 1/4" dowels and put pencil erasers on the ends and put the dowels into something solid like a block of wood as a rudimentary balancing machine. There are also commercially available balancers... But, for most planes the finger method is just fine.

Your over analyzing a bit. Use your fingernails to fine tune the point if you like, or some other smaller point that won't poke into the wing. Also some makers manuals don't list the correct CG, it should be from 25% to as far back as 33% of the wing cord, farther forward is more stable and best for a maiden flight, adjust to suit afterwards.

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Awesome replys, thanks for the help!

Originally Posted by jrb

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Thats what im talking about JRB! I like that!http://static.rcgroups.com/forums/im...lies/smile.gif

Get it flying on a calm day, straight and level, at medium cruze speed, and trim it hands off.
Then land it and see if the elevator is slightly up, down or level.
The best C/G is when the elevator is level or very slightly up.

Balance model on your finger tips at the point shown on the instructions with battery in place. It is usually necessary to fine tune the CG after the maiden flight moving battery pack forward or backward. The cg point shown on the plans/instructions is only a good starting point.Best to do the maiden with model balancing somewhat nose down.