STRUCTURAL DESIGN CONSIDERATIONS
The aerodynamic benefits
of a winglet application are determined in part by the extent of
the wing modifications made to accommodate the winglet. This especially
is the case when an airplane model has been designed and certified
without winglets. The magnitude of the winglet-induced load increase
and its distribution along the wing can significantly affect the
cost of modifying the wing structure. From the perspective of loads
and dynamics, the three areas that affect structural change are
static loads, dynamic flight loads, and flutter.
Static loads are determined by Boeing and U.S. Federal Aviation
Administration (FAA) design requirements, such as a symmetric 2.5-g
maneuver, a roll maneuver, or an abrupt rudder input that results
in a sideslip maneuver. Although these maneuvers all contribute
to the wingbox design, most of the wingbox is designed for 2.5-g
maneuvers. The highest loads on the mid- to outboard part of the
wing occur when speed brakes are extended. The inboard portion of
the wing reaches its highest loads in the clean wing configuration
(i.e., with speed brakes retracted).
The outboard tip of the
wing generally is designed for roll maneuvers. However, when winglets
are added, the high loads on the winglets during sideslip maneuvers
cause the wingtip area to be more highly loaded. Therefore, sideslip
maneuvers became the design case for the wingtip and winglet.
Dynamic flight loads also contribute to the maximum load envelope
of the outboard wing. The response of the airframe to gusts or turbulence
creates dynamic flight loads on the wing and winglet. During turbulence,
the airframe responds at different frequencies depending on its
aerodynamics, inertia, and stiffness. Modifications to these parameters
change how the airframe responds to turbulence, which in turn changes
the loads. In addition to the winglet-induced increase in air load,
the weight of the winglet itself and its extreme outboard location
also increase the loads for the outboard wing. The heavier the winglets
are, the higher the dynamic loads.
The flutter characteristics of an airplane are evident at high speed
when the combined structural and aerodynamic interaction can produce
a destabilizing or divergent condition. Under such circumstances,
an airplane with winglets is sensitive to the weight and center
of gravity (CG) of the winglets and associated structural wing changes.
Additional weight near the wingtip, either higher than or aft of
the wing structural neutral axis, will adversely affect flutter.