Final answer:
The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow, due to the higher amount of air displaced, the high angle of attack needed for lift at lower speeds, and minimized drag from a clean configuration.
Step-by-step explanation:
The strength of an aircraft's vortex is influenced by several factors, including the aircraft's weight, configuration (clean or dirty), and speed. The term "dirty" refers to the state of the aircraft when the flaps or gear are extended, which typically increases drag and thus the strength of the vortices created.
When an aircraft is heavy, clean (meaning the flaps and gear are retracted), and moving slowly, it generates the most significant vortex strength. This is because a heavy aircraft displaces more air than a light aircraft, creating stronger wake vortices. Additionally, when flying slowly, an aircraft generates stronger vortices as the wings need a higher angle of attack to produce the same amount of lift. A clean configuration minimizes drag, thus not disrupting the swirl of the vortices as much as a dirty configuration would.
In summary, the greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow.