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hazard is associated with induced rolling
moments, which can exceed the roll-control
authority of the encountering aircraft. In flight
experiments, aircraft have been intentionally
flown directly up trailing vortex cores of larger
aircraft. It was shown that the capability of an
aircraft to counteract the roll imposed by the
wake vortex primarily depends on the wingspan
and counter-control responsiveness of the
encountering aircraft.
Continuing growth of air traffic has
made "wake vortex" one of the most
challenging technical issues in modern civil
aviation. The requirement for reduced
separation distances on densely flown approach
routes is closely linked to the hazard caused by
wake-generating aircraft and its safety impact
on following aircraft. Great efforts have been
made in recent years to increase the knowledge
base of aircraft-generated wakes. In the light of
a new class of high capacity airliners to enter
service in the next decade, research must
intensify even more to better understand wake
physics, so that vortex-related hazards can be
quantified and means for hazard reduction
implemented. A major role to achieve this goal
is seen in utilizing modern visualization
techniques that became available in recent
years.
The strength of the wake vortex is
governed by the weight, speed, and shape of the
wing of the generating aircraft. However, as the
basic factor is weight, the vortex strength
increases proportionately with increase in
aircraft operating weight. Peak vortex
tangential speeds up to almost 100 meters per
second have been recorded. A lifetime of
several minutes and a length of 30 km behind
large planes have been recorded and are widely
known, though the vortex energy has reached a
very low level. Even bigger aircraft can be
damaged by wake turbulence. This was the case
for a MD-11 airliner during a VFR approach at
Runway 24R of an unspecified U.S. airport9.
The airplane was flying 5.6 km behind a Boeing
747 that was landing at Runway 25L. The
parallel runways were 168 m apart and
staggered; with the threshold of Runway 24L
located 1,312 m beyond of threshold of Runway
24R. The MD-11 was 31 m above the ground
when it rolled left, then right and developed a
high sink rate. The captain initiated a goaround,
but the airplane contacted the runway
and bounced-back into the air. The captain
discontinued the go-around and landed the
airplane on the runway. The MD-11’s aft lower
fuselage and aft pressure bulkhead were
substantially damaged. The accident was
ascribed to improper planning by the MD-11
pilot-in-command. The U.S. Aeronautical
Information Manual recommends that when an
airplane is following a larger airplane on
parallel approaches to runways closer than 763
m, the trailing airplane should remain at or
above the other’s airplane flight path, to avoid
the other’s airplane wake turbulence.
Evidently, the energy necessary to
generate the vortex structures and wing
downwash is driven out from the powerplant. In
other words, a large amount of drag is
generated, called drag due-to-lift or induced
drag. Induced drag represents 30-40 percent of
the total drag of a transport airplane at cruise
condition so it has a big impact on fuel
consumption. The induced drag is directly
proportional to square of the lift coefficient.
Therefore, takeoff, climbing, long-range cruise,
holding are phases of flight where the induced
drag is high because the lift coefficient is also
high.
Airbus undertook a special effort to
keep the A380 wake vortex no stronger than the
747 so other aircraft wouldn't require extra intrail
separation from it. Engineers reviewed
NASA, European and Russian TsAGI studies.
They noticed that the two-engine Airbus A330
and four-engine A340 have different vortex
patterns even though they have the same wing,
owing to changes in location of the flaps and
engines. They also observed that the A320 and
A321 have different patterns, apparently
because one model has single-slotted and the
other, double-slotted flaps. Airbus reports the
location of the flaps, ailerons and engines on the
A380 was adjusted to minimize the wake
vortex, and it is estimated to be a few percent
stronger than the 747-400's.
Winglet benefits
Winglets belong to the class of wingtip devices
aimed to reduce induced drag. Selection of the
wingtip device depends on the specific situation
and the airplane model. In the case of winglets,
　
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