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then there is no point in spending money on
winglets, where the useful life remaining in the
airframe time is less than would be required for
the fuel savings to offset the original
investment. On the other hand, if this is the
deciding factor, then owners of older aircraft
should make sure their aircraft have sufficient
life remaining for the winglet’s fuel burn
reductions to payback the installation cost.
Furthermore, all this assumes that the winglet
never gets damaged during this period. If,
however, it does become damaged, for example
by collision with an errant ground-support
truck, then the originally economic justification
could fade.
In the early days of the winglet era,
only business jets adopted winglets, mostly due
to aesthetic reasons. Fig. 4 shows the drag
reduction for the Gulfstream III, one of the
pioneer corporate aircraft to adopt winglets. The
flight test conducted at Mach number of 0.75
indicates a greater drag reduction than the windtunnel
test had indicated. Here, the deformation
of the winglet due to the high loads on the windtunnel
model can contribute to blanket part of
the benefits of the winglet installation due to the
departure of an optimal designed geometry. The
Flight test conducted at a higher Mach number,
in this case 0.775, shows the degradation of the
winglet contribution to the overall performance
caused by the presence of shock waves on the
winglet planform.
Fig. 4 – Drag reduction provided by winglets for
the Gulfstream III business jet5.
Fig. 5 - Effect of winglets on takeoff field length
of the Boeing 737-800 (Source: Boeing Aero
Magazine).
Currently, a large number of recent
commercial and military aircraft projects were
already designed with winglets. There are some
companies offering winglet retrofits for existing
products. Boeing Co. contracted a third
company to design, test and manufacture the
winglets of its large-capacity business jet
family, known as Boeing Business Jet (BBJ).
However, for mainline 737 operators operating
the aircraft on short sectors, where most of time
is spent climbing and descending, and less time
at the cruise condition, the extra weight of the
installation, the added wetted-area and the
parasitic drag, could possibly negate
aerodynamic benefits in the cruise condition,
remaining only the advantages for takeoff and
climb flight phases. In any case, Boeing reports
that a B737-800 equipped with blended winglets
would be able to fly further burning 3 to 5% less
block fuel. Besides these benefits, the 737-800
is also able to carry up to 6,000 lb more
payload. According to Boeing Co., derived
benefits include a reduction in noise near
airports (-0.5 to -0.7 EPNdB at cutback, sea
level), lower engine maintenance costs, and
improved takeoff performance at high-altitude
airports and in hot climate conditions. Fig. 5
shows the impact on field length due to the
addition of winglets. The effect is more
noticeable on takeoffs at airports located at
higher altitudes.
Fig. 6 – AEW&C aircraft based on the ERJ 145
regional jet (Photo by Guilherme Weigert).
Embraer developed an Early Warning
& Airspace Control aircraft based on the ERJ
145 platform (Fig. 6). This kind of aircraft has a
mission marked by a large-endurance flight at
high lift coefficients. Thus, induced drag is
responsible for a large portion of the total drag
and winglets could provide significant drag
reductions. Fig. 7 shows the percentual drag
reduction for the clean-wing configuration at a
certain flight condition according to data
acquired during an extensive flight-test
campaign.
Fig. 7 – Winglet drag reduction for the AEW&C
variant of ERJ 145.
Design highlights
Despite the benefits of winglets, there are some
drawbacks that need to be addressed. For
example, the bending moment at the wing root
is higher and may require additional structural
wing reinforcements. This especially is the case
when an airplane model has been designed and
certified without winglets. The magnitude of the
winglet-induced load increases and its
distribution along the wing can significantly
affect the cost of modifying the wing structure.
The winglet also generates viscous and induced
drag, which should be minimized and obviously
avoiding offsetting the induced-drag reduction
caused by the winglet itself on the
configuration.
Winglets could also contribute to slightly
worsening of the aircraft Dutch-roll. Too much
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