I suspect you will need to use lerp or some such fancy mathematical function.
Check out this post for a great explanation: http://www.scirra.com/forum/lerp-opacity_topic55210_page1.html
Based on that, I think you would need to use Sprite.Set angle -> lerp(Sprite.Angle, endAngle, speed), where speed would be some percentage which would define how fast the transition from the current angle to endAngle would proceed.
If I have it wrong, someone will correct me.
@newt is the Master of Lerp.Wastrel2012-08-09 21:06:56