Lately I've been entertaining myself checking out Free Flight Gas designs. Mostly because I like their appearance: graceful polyhedral wings, long thin tailbooms with big tailplanes, and nasty little pressure-feed engines. FAI F1C is an interesting technical accomplishment, especially by some talented Russians who seem to be hybrid engineers/jewelers, but I prefer lower-tech elliptical planforms. I think Billy Hunter's "Satellite" design belongs in an art museum.

OK, check out the aerodynamics. The "Free" in FF has to mean a big margin of inherent stabilty. After all, they fire these things off into hat-sucker thermals out in the California Kitty Litter deserts.

The time-honored FF gas formula seems to be a short nose moment with the engine tucked right up under the leading edge, high aspect wing with a thin (sometimes undercambered) airfoil section, long tailboom with a big lifting stab, and the CG back at 80% of the wing chord or more...

Huh?

CG at 80+%???

I've (mis)-trimmed enough R/C planes to know what happens if the CG gets back anywhere close to where the CP might be.

Could the FF's stab and tail moment be creating enough lift to move the combined wing/tail neutral point behind that rearward CG? Even with airspeed changes from power phase to minimum-sink glide speed? From upwind/downwide circles?

I tried backing the CG up beyond 60% on a flight simulator glider, and pushing the stick forward to control the ballooning. I got wild 180+ degree rotations around the pitch axis. Negative stability.

I can see why a FF designer would want a rearward CG to work. If a contest glider has to carry around the weight and drag of a tailplane, they'd like to get some positive lift out of it (although F3B/Thermal Duration R/C gliders don't try to.) Might even cheat a maximum-wing-area class rule if they don't count tail area.

I watched some AMA A/B Gas Free Flight's fly, and pitch oscillations following that vertical climb always decayed nicely into a stable floating glide. Random distrubances during the glide caused some porposing, but generally it damped out pretty well.

How do they do it?