 While the new engines improved performance, the aircraft was found to have handling problems, particularly at high angles of attack, where the aircraft was prone to spinning.To solve these problems the aircraft's horizontal tail was lowered, with other changes including moving the aircraft's airbrakes and deepening the wing fences, with the modifications causing the aircraft to be redesignated SM-2A and then SM-2B.The AM-5F still generated inadequate thrust and so the Mikulin engine design bureau developed a new engine to replace it, the AM-9B (later re-designed the Tumansky RD-9), rated at dry and with reheat.