The pilot of a Yakovlev Yak-52 reported that he was in the number three position, off the lead airplane’s left wing while in a four-airplane formation flight. The pilot of the lead airplane initiated a left turn and the formation followed. While in the left turn, the pilot of the number three airplane felt “a bump” from underneath and observed the number four airplane pass under his right wing, and ascend toward the lead airplane. The pilot of the number three airplane realized there had been a mid-air collision, broke away from the formation to the left, and executed an emergency landing. The number three airplane’s right wing sustained substantial damage. The pilot of a Ryan Navion reported that, he was in the number four position off of the number three airplane’s left wing in the four-airplane formation flight. The pilot of the lead airplane initiated a left turn and the formation followed. When the pilot of the number four airplane “felt to level off”, his airplane collided with the number three airplane from underneath. He then executed an emergency landing. The fuselage, vertical stabilizer, and rudder were substantially damaged. The pilots of both airplanes reported that there were no preaccident mechanical malfunctions or failures with the airplanes that would have precluded normal operation.