Complex sentence: The Yak-11 design was based in large part on the successful Yak-3 fighter; it became the Soviet Air Force's most widely used trainer aircraft and has sometimes been equated in importance with the T-6 Texan.
Split into simpler sentences: The Yak-11 design was based in large part on the successful Yak-3 fighter, and became the Soviet Air Force's most widely used trainer aircraft. It has sometimes been equated in importance with the T-6 Texan.