Complex sentence: At the beginning of July it was one of thirteen small aircraft flying the "Tour de France des avionettes", piloted by van Opstal, though it was forced to withdraw on the Belgian leg when part of the cowling came away and destroyed the propeller.
Split into simpler sentences: At the beginning of July it was one of thirteen small aircraft entered for the "Tour de France des avionettes". Piloted by van Opstal, it was forced to withdraw on the Belgian leg when part of the cowling came away and destroyed the propeller.