A regular control surface does not have a stop near the normal operating range. A flap has a stop right at the end of the up range. If you have a very small setup error, the digital servo may try to drive the flap against the stop at full power, which would burn up the servo and possibly drain your battery. I have seen this once with standard servos on a flap that where way out of rigging. One servo had melted it self down and was locked, the other was in-operable. had it been a small error there would not have been a problem, digitals would have just made it worst. Better to stay with the recomended servos, they work great on my KC.

Steven