Doubleeight-------Thanks mate I appreciate the kind words!!
That's what sideslips do buddy, place loads on the aerodynamic surfaces that create additional drag to slow the damn plane down!! Extra side loads on the vertical fin and other components.......it's not rocket science surely?
Anyway after some research here It's not the main reason anyway, it's problems of Elevator effectiveness during the Flap down sideslips that can cause problems...
http://www.pprune.org/private-flying...question.htmlb
It's been 35 years since I learnt to fly in GA and a lot has happened since but I can clearly remember being trained NOT to do sideslips with Flaps in C172's ( you shouldn't need to anyway )