This is the one of the few weaknesses of the S 15. The horizontal and vertical stabilizers are very flimsy. There are reinforcements on the bottom of the horizontal stab., but none visible in the vertical. The vert. stab. is glued into a plastic piece that has two posts the sandwich the horz. stab. to the fuselage. It has about 1 1/2 inches in the front that sits in a groove in the fuselage. I suppose it could be tacked in with a spot of glue but if you wreck the tail section, you could mess up the fuselage trying to remove it.
Of course, the S 15 is designed as a trainer and not meant to be flown very aggressively. That accounts for the flexing in the tail section. That said, I am very impressed with the flight capabilities of this airplane. I have flown in 15mph winds with it and it was very stable considering the conditions. In fact, the winds on the day that I filmed this video were varible between 6-12mph.
I would recommend this plane to anyone learning to fly. I hate to admit it, but I have crashed it numerous times. Parts are reasonably priced and my LHS stocks most items that tend to get broken in a crash. It is large enough to be seen at a good distance or altitude, but still small enough to handle well in winds. My only complaint would be the rudder responsiveness....it has none, or very little. It definitely will not do a hammerhead into the wind. I can get it to drop over if I start adding rudder early.
The only other weak point is the nose steering and ground clearance for the prop. It can be flown from grass but it needs to be cut very short. That was the main reason for the tail dragger conversion. I also added slightly larger wheels and a steerable tail wheel. Taxis like a dream now.