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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Wingtip Vortices

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Two major aerodynamic factors from the pilot's viewpoint
are lift and velocity because they can be controlled readily
and accurately. Of course, the pilot can also control density by adjusting the altitude and can control wing area if the
aircraft happens to have flaps of the type that enlarge wing
area. However, for most situations, the pilot controls lift and
velocity to maneuver an aircraft. For instance, in straight-and level
flight, cruising along at a constant altitude, altitude is
maintained by adjusting lift to match the aircraft's velocity
or cruise airspeed, while maintaining a state of equilibrium
in which lift equals weight. In an approach to landing, when
the pilot wishes to land as slowly as practical, it is necessary
to increase lift to near maximum to maintain lift equal to the
weight of the aircraft.

Wingtip Vortices

Formation of Vortices

The action of the airfoil that gives an aircraft lift also causes
induced drag. When an airfoil is flown at a positive AOA,
a pressure differential exists between the upper and lower
surfaces of the airfoil. The pressure above the wing is less
than atmospheric pressure and the pressure below the wing
is equal to or greater than atmospheric pressure. Since air
always moves from high pressure toward low pressure,
and the path of least resistance is toward the airfoil's tips,
there is a spanwise movement of air from the bottom of the
airfoil outward from the fuselage around the tips. This flow
of air results in "spillage" over the tips, thereby setting up a
whirlpool of air called a "vortex." [Figure 4-10]

Wingtip vortices.
Figure 4-10. Wingtip vortices.

At the same time, the air on the upper surface has a tendency
to flow in toward the fuselage and off the trailing edge. This
air current forms a similar vortex at the inboard portion of the
trailing edge of the airfoil, but because the fuselage limits the
inward flow, the vortex is insignificant. Consequently, the
deviation in .ow direction is greatest at the outer tips where
the unrestricted lateral .ow is the strongest.

As the air curls upward around the tip, it combines with the
wash to form a fast-spinning trailing vortex. These vortices
increase drag because of energy spent in producing the
turbulence. Whenever an airfoil is producing lift, induced
drag occurs, and wingtip vortices are created.

Just as lift increases with an increase in AOA, induced
drag also increases. This occurs because as the AOA is
increased, there is a greater pressure difference between the
top and bottom of the airfoil, and a greater lateral flow of air;
consequently, this causes more violent vortices to be set up,
resulting in more turbulence and more induced drag.

In Figure 4-10, it is easy to see the formation of wingtip
vortices. The intensity or strength of the vortices is directly
proportional to the weight of the aircraft and inversely
proportional to the wingspan and speed of the aircraft. The
heavier and slower the aircraft, the greater the AOA and the
stronger the wingtip vortices. Thus, an aircraft will create
wingtip vortices with maximum strength occurring during
the takeoff, climb, and landing phases of flight. These
vortices lead to a particularly dangerous hazard to flight,
wake turbulence.

Avoiding Wake Turbulence

Wingtip vortices are greatest when the generating aircraft is
"heavy, clean, and slow." This condition is most commonly
encountered during approaches or departures because an
aircraft's AOA is at the highest to produce the lift necessary
to land or take off. To minimize the chances of flying through
an aircraft's wake turbulence:
• Avoid flying through another aircraft's flightpath
• Rotate prior to the point at which the preceding aircraft
rotated, when taking off behind another aircraft.
• Avoid following another aircraft on a similar flightpath
at an altitude within 1,000 feet. [Figure 4-11]
• Approach the runway above a preceding aircraft's
path when landing behind another aircraft, and touch
down after the point at which the other aircraft wheels
contacted the runway. [Figure 4-12]

A hovering helicopter generates a down wash from its main
rotor(s) similar to the vortices of an airplane. Pilots of small
aircraft should avoid a hovering helicopter by at least three
rotor disc diameters to avoid the effects of this down wash.
In forward flight this energy is transformed into a pair of
strong, high-speed trailing vortices similar to wing-tip
vortices of larger fixed-wing aircraft. Helicopter vortices
should be avoided because helicopter forward flight airspeeds
are often very slow and can generate exceptionally strong
wake turbulence.