Vortices form because of the difference in
pressure between the upper and lower surfaces of a wing that is operating
at a positive lift. Since pressure is a continuous function, the pressures
must become equal at the wing tips. The tendency is for particles of air
to move from the lower wing surface around the wing tip to the upper
surface (from the region of high pressure to the region of low pressure)
so that the pressure becomes equal above and below the wing. In addition,
there exists the oncoming free-stream flow of air approaching the wing. If
these two movements of air are combined, there is an inclined inward flow
of air on the upper wing surface and an inclined outward flow of air on
the lower wing surface. The flow is strongest at the wing tips and
decreases to zero at the midspan point as evidenced by the flow direction
there being parallel to the free-stream direction.
Wing-tip vortices are formed when high-pressure air spills up over the
wing tips into the low-pressure space above the wing.
When the air leaves the trailing edge of
the wing, the air from the upper surface is inclined to that from the
lower surface, and helical paths, or vortices, result. A whole line of
vortices trails back from the wing, the vortex being strongest at the tips
and decreasing rapidly to zero at midspan. A short distance downstream,
the vortices roll up and combine into two distinct cylindrical vortices
that constitute the "tip vortices."
Pressures must become equal at the wing tips since pressure is a
continuous function (figure a). The free stream flow combines with tip
flow, resulting in an inward flow of air on the upper wing surface and an
outward flow of air on the lower wing surface (figure b).
The tip vortices trail back from the wing
tips and they have a tendency to sink and roll toward each other
downstream of the wing. Again, eventually the tip vortices dissipate,
their energy being transformed by viscosity.
Formation of wing-tip vortices.
The tip vortices cause additional downflow
(or downwash) behind the wing within the wingspan. For an observer fixed
in the air, all the air within the vortex system is moving downward
(called downwash) whereas all the air outside the vortex system is moving
upward (called upwash). An aircraft flying perpendicular to the flight
path of the airplane creating the vortex pattern will encounter upwash,
downwash, and upwash in that order. The gradient, or change of downwash to
upwash, can become very large at the tip vortices and cause extreme
motions in the airplane flying through it. An airplane flying into a tip
vortex also has a large tendency to roll over. If the control surfaces of
the airplane are not effective enough to counteract the airplane roll
tendency, the pilot may lose control or, in a violent case, experience
The takeoff and landings of the new generation of jumbo jets compound the
problems of severe tip vortices. During takeoff and landing, the speed of
the airplane is low and the airplane is operating at high lift
coefficients to maintain flight. The Federal Aviation Agency (FAA) has
shown that for a 600 000-pound (2.7 million-kilogram) plane, the tip
vortices may extend back strongly for five miles (eight kilometres) from
the airplane and the downwash may approach 160 meters per minute (500
ft/min). Tests also show that a small light aircraft flying into a vortex
could be rolled over at rates exceeding 90 degrees per second.