All right. I'm stumped. I bought a scale Cessna 150 at a RC auction last year. It's old. Late 70's - early 80's. I've removed the covering, cleaned it up, added some goodies and have began to recover (waiting on fancy navigation lights =) I've come this far and have only just now realized that I have no idea where the center of gravity is supposed to be. Obviously no manual or plans. any formula or rule of thumb? any Ideas? No markings any where except when I replaced the firewall I found a piece of wood that had "Cessna 150 20" painted on it. 54" WS 36" fuselage. Any ideas? send them my way.. much thanks...

zr861:

Measure the chord of the wing.
The model should balance with empty tank, but otherwise ready to fly, when sustained at a distance back from the leading edge of the wing of 0.25 times Chord.
That may be a little nose heavy, but good for a first test flight.

Then, using a trimming chart, find the best position by experimentation, paying attention to the reactions of the model.
Mark that point with something permanent.
The normal range is 25% to 33% of the wind chord; however, some models have the CG beyond that range.

Do not forget to perform a lateral balance as well.

Regards!

For models we can easily go with the CG at around 30% of the wing chord back from the leading edge unless it's an extreme design (the Cessna layout is far from extreme). 25% noted above is used by some as a starting point but other than a few extreme designs it provides more pitch stability than most folks are expecting or comfortable with.

Hey, thanks you guys very much. I'll start there and let you know how it goes.

I purchased a Nitroplanes Ultimate 40 biplane. The instruction book is not clear on the c.g. Can anyone help me in balancing this plane?

455Pilot:

Same process described above, only that the total chord should be considered (from the extreme leading and trailing edges).
This is normally from the leading edge of the top wing, which is more forward, to the trailing edge of the lower wing, which is more aft.

Regards!