496
P. Liu
Due to the pressure difference between the upper and lower surfaces of
the wing tip, the air tends to flow along the lower surface to the upper
surface. After the winglet is added, it will have an end plate effect on the
spanwise flow, and the winglet vortex will diffuse the wing tip vortex, which
will weaken the downwash effect of the wing tail vortex, lower the downwash angle, and reduce the induced drag. At present, the winglets used in
the civil aircraft include ordinary winglets, wing tip vortex diffusers, shark fin
winglets, wing tip sail, etc.
The main features of the winglet include (1) end plate effect, blocking the
flow from the lower surface to the upper surface, weakening the wing tip
vortex strength, thereby increasing the effective aspect ratio of the wing; (2)
Dissipating the wing tip vortex of the main wing, because the winglet itself
is also a small wing, it can also generate a wing tip vortex, its direction is
the same as the wing tip vortex of the main wing, but because the distance
is very close, at the intersection of the two vortices the shearing effect is very
strong, casing a large viscous dissipation, preventing the winding of the main
vortex. The wing tip vortex plays the role of diffusing the main vortex and
also reducing the induced drag (as shown in Fig. 7.65); (3) Increasing the
wing lift and forward thrust. The upper winglet can generate lift and thrust
components using the ternary distortion flow field (as shown in Fig. 7.66);
(4) Delay the premature separation of the wing tip flow and increase the stall
angle of attack. Generally, the wing tip of the swept wing is a ternary effect
zone, the flow tube contracts, the airflow firstly accelerates rapidly, the pressure decreases, and then experiences an intense pressure gradient recovery,
enters a steep reverse pressure gradient zone, causing the wing tip boundary
layer separate prematurely, resulting in stall. However, the winglet mounted
on the wing tip can use its forward pressure field to counteract the partial
wing tip reverse pressure field, so that the pressure distribution is gentle and
the reverse pressure gradient is reduced. If properly designed, the airflow separation at the wing tip can be delayed, and the aircraft stall angle of attack and
buffeting lift coefficient can be improved.
The effect of the winglet: the winglet has single upper winglet, upper and
lower winglet and many other forms. The single upper winglet is used more
because of its simple structure. The induced drag of the aircraft accounts
for about 30% of the cruise drag. Reducing the induced drag is important
for improving cruise economy. The larger the aspect ratio of the wing, the
smaller the induced drag. However, an excessively large aspect ratio will make
the wing too heavy, thus there is a limit to increasing the wing aspect ratio. In
addition to being a wing tip end plate to increase the effective aspect ratio of
the wing, the winglet also utilizes the “pull effect” produced by the deflection
P. Liu
Due to the pressure difference between the upper and lower surfaces of
the wing tip, the air tends to flow along the lower surface to the upper
surface. After the winglet is added, it will have an end plate effect on the
spanwise flow, and the winglet vortex will diffuse the wing tip vortex, which
will weaken the downwash effect of the wing tail vortex, lower the downwash angle, and reduce the induced drag. At present, the winglets used in
the civil aircraft include ordinary winglets, wing tip vortex diffusers, shark fin
winglets, wing tip sail, etc.
The main features of the winglet include (1) end plate effect, blocking the
flow from the lower surface to the upper surface, weakening the wing tip
vortex strength, thereby increasing the effective aspect ratio of the wing; (2)
Dissipating the wing tip vortex of the main wing, because the winglet itself
is also a small wing, it can also generate a wing tip vortex, its direction is
the same as the wing tip vortex of the main wing, but because the distance
is very close, at the intersection of the two vortices the shearing effect is very
strong, casing a large viscous dissipation, preventing the winding of the main
vortex. The wing tip vortex plays the role of diffusing the main vortex and
also reducing the induced drag (as shown in Fig. 7.65); (3) Increasing the
wing lift and forward thrust. The upper winglet can generate lift and thrust
components using the ternary distortion flow field (as shown in Fig. 7.66);
(4) Delay the premature separation of the wing tip flow and increase the stall
angle of attack. Generally, the wing tip of the swept wing is a ternary effect
zone, the flow tube contracts, the airflow firstly accelerates rapidly, the pressure decreases, and then experiences an intense pressure gradient recovery,
enters a steep reverse pressure gradient zone, causing the wing tip boundary
layer separate prematurely, resulting in stall. However, the winglet mounted
on the wing tip can use its forward pressure field to counteract the partial
wing tip reverse pressure field, so that the pressure distribution is gentle and
the reverse pressure gradient is reduced. If properly designed, the airflow separation at the wing tip can be delayed, and the aircraft stall angle of attack and
buffeting lift coefficient can be improved.
The effect of the winglet: the winglet has single upper winglet, upper and
lower winglet and many other forms. The single upper winglet is used more
because of its simple structure. The induced drag of the aircraft accounts
for about 30% of the cruise drag. Reducing the induced drag is important
for improving cruise economy. The larger the aspect ratio of the wing, the
smaller the induced drag. However, an excessively large aspect ratio will make
the wing too heavy, thus there is a limit to increasing the wing aspect ratio. In
addition to being a wing tip end plate to increase the effective aspect ratio of
the wing, the winglet also utilizes the “pull effect” produced by the deflection