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Article by: Dick Tonan ‘94                                                Illustration by: Ed Clayman '99
(Originally published in the Middle Tennessee R/C Society Newsletter)
(Accumulative error correction by: Rh'eal Saulnier, Universit'e de Moncton '99 :^)
Recently I found a technique for determining the most aft CG (center of gravity) location for a model airplane that will provide an "adequate" stability margin. I thought I would put this information together for a wider audience than those that just read "Replica".

Before I go any further, a disclaimer! The word adequate is pretty subjective. An r/c plane with an adequate margin of stability for the r/c pilot with serveral hundred hours on the sticks is going to be a pile of balsa at the bottom of a smoking hole for the r/c pilot that soloed a week ago. Therefore temper the results with your experience! The fewer hours of stick time, the further forward the CG should be from the MOST AFT location determined by this method.

I did not come up with these formulas, therefore I cannot claim credit for them. What caught my interest was that this method was the only one that I have seen that takes into account the aircraft aft of the wing leading edge. All other methods treat tail feathers as if they didn’t exist. The following formula determines the "CG" location expressed in percent of the Mean Aerodynamic Chord as measured from the M.A.C.’s leading edge:

                    CG = [0.17 + 0.3 x ((TMA) x (SH) x HTE))] x 100
                                                      MAC      SW


CG: Aft most center of gravity with adequate stability margin.

T.M.A.: The Mean Aerodynamic Chord

S.H.: The area of the horizonal stabilizer.

S.W.: Wing area including the portion of the wing under or over the fuselage.

H.T.E.: Horizontal Tail Efficiency. Value ranges from 0.5 for a flat tail located in its normal position in the down wash of the wing to 0.9 for a "T-Tail" at the top of the vertical stabilizer. Before we start banging numbers into the calculator, we need to determine 2 critical factors in the formula. They are the M.A.C. and the T.M.A. We
will start with the M.A.C. because we need it to determine the T.M.A.

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