There is, in the FF world, a well known and proof formula for calculating the optimum position of the centre of gravity for a model. This formula has been established many years ago (in the sixties) by the René Jossien, a French free flight's specialist who has also issued many articles on peanuts in the MRA magazine under the nickname "Le Saint" in the seventies.
This empirical formula is the result of statistical analysis carried out on hundreds of successful free flight models of all classes (Peanut to FAI power  See hereafter). It was published for the first time in the French newsletter "Vol Libre" (Association des Amateurs d'Aéromodèles Anciens). It can also be found in some MRA articles (for instance, MRA N°466  September 1978 issue) as well as in 1998 Symposium documents of the National Free Flight Society (NFFS  1998 Symposium Report  pages 52 to 54). René Jossien has slightly adjusted his formula year after year to take into account the evolution of model techniques and FF rules. The version presented hereafter is dated April 2002. It was published by René Jossien in issue #147 (024) of Vol Libre.
This formula is based on the model's specifications : Wing surface (SA), Tail surface (SS), Projected wingspan (EP) and Great lever (GL) which is the distance between the wing leading edge and the stabiliser leading edge. It takes into account flight conditions and model types and can be used not only for all FF categories (and there are many of them  See hereafter) but also for R/C models (slowfliers, parkfliers, etc...). You can use it also with old vintage models for instance to find the COG position when it is not indicated on a plan.
in which we have :
Units: The formula remains valid for any kind of units, provided they are consistent, i.e. :
Wing coefficient KA :
A (Coefficient related to wing's height above fuselage's axis). 
 
B (Coefficient related to wing's airfoil). 
 
C (Coefficient related to model class and usage) :

Stabiliser coefficient KS :
k (Coefficient according to pilot's skills) 

D (Coefficient according to fin design) 

E (Coefficient according to stabiliser airfoil) 

An Excel spreadsheet you can download in a French or an English version will do the calculations for you !
You can also download an .exe software (by Chris Stoddart) also based on this formula, with its documentation in Microsoft Word format.
You will find on this site an historical account on the formula and examples of differences recorded between calculated and actual values for CoG positions on various models. Very often, these differences are less than 1%. Impressive !
Nota :  When the wing is rectangular, the mean
chord has the same length as the root chord.
 When the wing is not rectangular, the mean
chord is the chord running through the centre of gravity of the
surface of the halfwing (can be found by balancing an halfwing
template on a sharp edge parallel to the root chord).
As you can see below, there is a wide range of FF models, from few grams (even less !) to several kilograms (F1E). Therefore, it is not a surprise you can also use René Jossien's formula with all R/C model types.







Wakefield Cup 





Rubber powered models 


automatic steering 





Coupe d'hiver type 





1/2A class 





Indoor  EZB type 


Indoor  "F1D beginner" 








Indoor 


Indoor  semiscale 


Indoor  scale 


Scale 
