OK, I've seen two of them recently. Like in the last two weeks. And one is mine. And both have a problem with takeoffs.

Mine has had an OS55AX and OS46AX in it. Both are ok for it. Both pull it at MACH 3 if I'm dumb enough to let them. The airplane is the heaviest 40 size ARF I've built in a year and a half. So I figured it'd need the 55AX. It doesn't. It actually hasn't shown one bad habit yet. Except for the takeoffs.

The other guy showed up with his first. I didn't even know he was building it and had maybe a day's worth of work to finish mine when he brought it out for it's maiden flight. He had fits. The sucker wore him out trying to get it off the ground in one piece.

I got mine out a couple of days after watching him deal with his little beauty. I wasn't looking forward to my maidens. So I was extra careful on the first couple of takeoffs. And did a number of touch and gos the first flight. He is a SLAM AND PRAY type of flyer when it comes to takeoffs. And therein is the solution to the mystery. Mine didn't show a single problem on takeoff on the first three or four flights. And I was baffled about why his was so different than mine. And then I did a slam-the-throttle takeoff. Or should I say, I TRIED a slam-the-throttle takeoff. And not being a real fast learner, I immediately tried it again. And both of them gave SUDDEN ground loops.

The airplane has a VERY EFFECTIVE rudder. That is, AFTER it builds enough speed on takeoff for the rudder to get some clean air and enough airspeed to fly. So if you slam-and-pray it's not going to control engine torque one bit until it hits that speed and by then the torque has the airplane by it's ......... And you've probably got max rudder deflection..... and the airplane scares everyone watching.

After my Decathlon did it's little tricks on those takeoffs, I've not once tried the jam-it routine. And not once has it done anything but take off like a kitten.

I got $20 bucks says your son will ground loop the sucker if he's a jam-it-and-pray kind of flyer.