Within a time limit of 7 minutes a number of manoeuvres is flown with high precision and harmony. Judges decide on the score. Extensive freedom of size and design for size and shape of the airplane. Aesthetics is the name of the game. Beautiful airplanes, sophisticated finishes are the norm. Light yet rigid construction is essential for good performance. If only the laquer finish wouldn't be so heavy !      
Nothing can replace volume. Two or four stroke engines up to 15 ccm ( .90 ) capacity . Two stroke engines are run in the traditional 4-2 stroke regime with silencer, or equipped with a "pipe" to control RPM .  
Aerobatic pilots are the dandies of Control Line. They choose their wardrobe to suit the colour of their airplane. White trousers won't hurt! They move slowly and fly precisely and try to have a pleasant relationship to all judges. Practise dry training at home with closed curtains. Make several thousand flights a year ( a month ?).