A stunt plane turning 5 1/4-ish second laps on 70-foot lines pulls 3g. So if you're not an engineer or a pendant, multiply the airplane's weight by 3.

If you ARE an engineer or a pendant, then multiply the airplane's mass in kg by 29.6m/s^{2} (to get an answer in Newtons), or it's mass in slugs by 187ft/s^{2} (to get an answer in pound-force). Be sure to work out the dimensional analysis to make sure it's right. But then, if you can do that, you don't need me to run the numbers.

12lb for a 4lb plane. Less than 20 pounds, but with all the twisting and turning, and the possibility needing to do fancy footwork if it goes slack on the lines, I'd suggest judging for a few weeks while doing the back exercises recommended by his doc (they really help me, when I hurt my back).