ORIGINAL: hydro-don
Hey guys I have a stick like model that will only spin about 45 degrees nose up. It balances about 35% and snaps out of a loop at 1/2 elevator. It will do a flat turn, rudder, opposite aileron effortlessly, so my question is can this plane be made to flat spin? How much rearward CG can one safely fly with? Is short coupling more desireable or does it not matter?
Length of tail alone cannot tell. Size of rudder and elevators is more important.
You, as the pilot, are in the best position to tell us whether your plane will flat spin or not. Try some of the techniques listed. A common full-scale way to enter a flat spin (because it produces consistent results), is to enter a normal, power-at-idle, upright spin to the left, with ailerons neutral (or negative spin with right rudder), then, after a turn or two, smoothly increase power and apply right aileron simultaneously until the nose comes up and the rotation rate decreases. If this technique doesn't do it, try some others. Factors affecting spins are many and complex, and nobody fully understands them all (otherwise flight testing would not be necessary), so it can seem like an art at times. Sometimes a seemingly very small change can have a dramatic effect on spin characteristics.
There are some generalizations that can be made: Side area ahead of the CG is "pro-spin" (aids spin entry/hinders recovery), or destablizing. Area aft of the CG is anti-spin (hinders spin entry/aids recovery), or stabilizing. Fin/strake/dorsal area below the horizontal stabilizer when upright, or "above" the horizontal stab when inverted, is anti-spin. If a plane has sufficient rudder and elevator authority, and a sufficiently rearward CG, then aileron application is not necessary for spin entry, but it affects spin characteristics, sometimes dramatically. Planes tend to flat spin better to the left, because of gyroscopic precession. Planes with low thrust lines tend to flat spin better upright, and the opposite is true for high thrust lines. All these effects are interdependent and one or more can override others.
A few airplanes that can be made to spin, cannot be made to recover from the spin. And some airplanes that cannot recover from upright flat spins, or can recover with difficulty, can recover easily from inverted spins, like the F-4U. An examination of the empennage in the context of airflow from "below" compared with "above" will reveal the primary reason.