Complex sentence: Despite being designated by the US Navy as a "heavy", the A- 5 was surprisingly agile; without the drag of bombs or missiles, even escorting fighters found that the clean airframe and powerful engines made the Vigilante very fast at high and low altitudes.
Split into simpler sentences: Despite being designated by the US Navy as a "heavy", the A- 5 was surprisingly agile for such a large aircraft. Without the drag of bombs or missiles, even escorting fighters found that the clean airframe and powerful engines made the Vigilante very fast at high and low altitudes.