What factor does NOT affect the strength of a wing-tip vortex?

The strength of a wing-tip vortex is influenced by several physical characteristics and operational conditions of the aircraft, but the altitude does not have a direct impact on its strength. Wing-tip vortices are created due to the pressure difference between the upper and lower surfaces of the wing, leading to a swirling pattern of airflow around the wingtips as air tries to equalize the pressure difference.

Factors such as aircraft weight impact the strength of these vortices because heavier aircraft generate stronger vortices due to their greater mass displacing more air. Likewise, aircraft speed plays a significant role as higher speeds result in a greater lift and subsequently stronger vortices. The shape of the wing also affects vortex strength; for instance, wings with a higher aspect ratio can alter the flow characteristics, thereby affecting vortex strength.

In contrast, altitude, while it can influence air density and performance, does not change the fundamental aerodynamic traits that generate wing-tip vortices. Thus, altitude is not a determining factor in the strength of the wing-tip vortex.