Besides using the manufacturer's often Conservative c/g location, is there a formula one can use to help predict the most desirable c/g? I realize that personal preference's come into play but I'd like to start closer to the correct location to cut down on trimming time. Buy the way I am talking about sailplanes in this case. I raise this question because in searching I have seen starting numbers from 35 to 48% of the root cord, compared to the recommended 30% in the case of my plane ( a Sailaire).

There's only the manufacturer's position to start from.
Then it's up to you as to how you evaluate the plane's handling.
Better be safe than sorry.
Tail heavy flies maybe once...

There are several RCU threads that talk about CG location - try a search for "CG location"...

30-35% of the mean aerodynamic chord (MAC) is about right for most models - This may or may not be at 30% of the root chord, depending on how the wing tapers....

HTH

Jim

Sailplane fliers and fun fly model types often use the Dive Test to set the CG very close to the true neutral point. This has been described often in the past in the Gliders and Sailplanes forum. Try doing a search for it.

It's nice and safe because it let's you sneek up on the optimum CG point rather than guessing.

There's also some formulas that relate the Tail Volume coefficient to the wing to determine a good CG starting point in this thread....

http://www.rcuniverse.com/forum/tail...1547609/tm.htm

www.nsrca.org has a trim test chart for Pattern models. (designed as close to 0 wing incidence, 0 decalage, 0 thrust offset as possible with symetrical airfoils) Thier chart is very good for getting the optimum performance from an aerobatic design.

The basic rule of thumb for a "conventional" design is... start at 25 to 27% MAC and work from there. 30 to 35% MAC isn't uncommon on Fun-Fly designs. (The MAC reference is... a line perpendicular to the fuselage with that percentage of the wing area ahead of the line)

Jimmbbo,
Found the thread with the link to the spread sheet that computes c/g's. My plans suggest starting at 3.75" back of the le. And to go no further back than 4.125". The c/g calculator came up with 5.21" back of the le or 45.42% mac. I think to error on the side of caution I am going to start with the plans recommendations. I'll toss it around by hand and see what it looks like.

Does anyone recall a thread on tuning a cg that used a method involving trimming the plane using inverted flight?

By the sounds of it that thread may have been the calculation of the actual NEUTRAL point of the model. Generally you do NOT want to fly with the CG at that point. Coming forward about 7 or 8% would put it at a reasonable point but in this case I'd go with the recomendation for the first flight.