Perhaps in the good (?) 'ole days, but most of today's trainers with semi-symmetrical wings will perform inverted flight including outside manoeuvres without even a hiccup. Same as + g flight, power is the only consideration. eg: TWM's superb Sky Raider Mach I and World Star 40 et al trainer designs, Kyosho's Calmato (high wing), Seagull's Boomerang 40 (the flat bottom winged 60 isn't as happy with outsides); Phoenix's ubiquitious but tiny Classic ...etc.etc.