Multi-Engine: Zero-Sideslip
The zero-sideslip is necessary for a pilot to maintain control of an aircraft and have the greatest climb performance during an engine failure in a twin.
Aerodynamics | February 10, 2022
Wingtip vortices are created by flowing air creating a pressure differential between the top and bottom of an aircraft's wing. The air flowing over the top of the wing will create an area of low pressure, and the air flowing across the bottom of a wing will create an area of high pressure.
The high-pressure air under the wing will attempt to meet with the low pressure above the wing. To meet the low pressure on top of the wing, the high-pressure air will travel along the bottom of the wing from the fuselage to the wingtips.
Upon reaching the wingtips, the high-pressure air will flow up and over the wingtips and meet the low-pressure air above the wing. Simultaneously, the low-pressure air above the wing will travel inwards toward the wing root and flow off the trailing edge of the wing.
This motion of air results in four vortices being created. One at each of the wingtips, and one on each side of the aircraft where the wing root meets the fuselage.
Unlike the wingtip vortices, which have little resistance on their path to the wingtips, the vortices at the wing root cannot gain much strength due to the fuselage restricting the inward flow of air. Since these vortices cannot gain much strength, they are disregarded and considered insignificant.
However, the vortex created at the aircraft's wingtips is unobstructed and experiences little to no resistance, and forms strong wingtip vortices, which creates a significant danger to all aircraft in the form of wake turbulence.
Every aircraft in flight generates wingtip vortices and thus wake turbulence. However, the strength of the wingtip vortices and subsequent wake turbulence is determined by three factors:
A heavier aircraft requires its wings to produce more lift to overcome its weight. As more lift is produced, the wingtip vortices and subsequent wake turbulence become stronger. The result is wingtip vortices that are more aggressive and violent than a lighter aircraft.
The strength of the wingtip vortices created by an aircraft is inversely proportional to its speed. What this means is as an aircraft's speed decreases, the strength of its wingtip vortices increases. This is because as an aircraft slows down, it must increase its angle of attack to produce the same amount of lift at the slower airspeed. As the angle of attack increases, a larger difference in pressure is created between the top and bottom of an aircraft's wing. This increase in the difference in pressure results in more air flowing to the wingtips and stronger wingtip vortices.
Changing the wings' shape can decrease a wingtip vortices' strength by disrupting and breaking apart the flow of air towards the wingtips. Flaps accomplish this by resisting the motion of high-pressure air to the wingtips and breaking apart the vortex into smaller, weaker vortexes. Longer wingspan aircraft with a higher aspect ratio, such as a glider, also create weaker wingtip vortices due to air under the wings having a greater chance of escaping from under the wing than traveling to the wingtips.
An aircraft's wingtip vortices travel outward, upward, and around the wingtips when viewed from the front or the back of the aircraft. They will remain spaced slightly less than the wingspan of the generating aircraft once created.
Wingtip vortices will start being created once an aircraft lifts off the ground during the takeoff roll and stop being created once the aircraft touches down on landing.
Close to the ground, within 100-200ft, vortices will tend to move laterally at a speed of 2 to 3 knots at an altitude of half the generating aircraft's wingspan across the ground.
At altitudes greater than the generating aircraft's wingspan above the ground, wingtip vortices will tend to descend at several hundred feet per minute initially and gradually level off with time. Most wingtip vortices will descend about 500-900ft and tend to level off after the generating aircraft has traveled 5nm past the point of generation. Over time, a wingtip vortex will decay. On turbulent days, the decay rate tends to increase, and wingtip vortices will dissipate faster than on smoother days.
An atmosphere with light winds, low turbulence, and a stable atmosphere is the worst-case scenario for wake turbulence avoidance. In these conditions, wingtip vortices from heavy and super aircraft can descend more than 1000ft.
Rarely, wake turbulence can rise when encountering an updraft or by bouncing off a strong inversion layer.
When a helicopter is hovering, the downwash created from the main rotor can contain high wind speeds. To avoid the area of downwash, a pilot should avoid being within a distance of three rotor diameters of any helicopter that is hovering or moving at a slow taxi.
A helicopter traveling foreword generates two trailing vortices similar to those created of a large fixed-wing aircraft. The slow speeds of a helicopter amplify the strength of these vortices, causing strong wake turbulence. Pilots should avoid crossing or flying behind a helicopter to avoid these vortices.
