Wing Tip Devices
Summary
- Tip Vortices
- Winglets
- Endplates
- Hoerner Tips
- Tip Tanks
- Tip Sails
- Selected References
Tip devices have become a popular technique to increase the aerodynamic performances of lifting wings, short and slender alike.
Tip Vortices
The idea behind all the devices described below is to diffuse the strong vortices released at the tip and optimize the spanwise lift distribution, while maintaining the additional moments on the wing within certain limits. A visualization of tip vortices is shown in the photo below, that is a manoeuvering canard-wing Eurofighter EF-2000 on a demo flight over Le Bourget (France).
Figure 1: Eurofighter EF-2000
Similar vortices are visible on commercial aiplanes when landing and taking off in high humidity weather conditions.
The Winglet
Several theoretical investigations (for ex. Weber, 1954), and later experiments, indicated that the use of vertical lifting surfaces placed at the wing tips produce a beneficial effect on both lift and drag characteristics. This is found at the cost of increased bending moment.
The increase in root bending moment is found to be lower than for an equivalent tip
extension. Fig. 2 shows a complicated, yet common, winglet design, with one large
tapered upper winglet, and a smaller lower winglet (Whitcomb, 1977). The winglet
sections can be airfoils with their own design.
Winglets can be used to produce extra lift, besides lower drag. The winglets must be mounted on the rear part of the wing (region of lowest pressure), to minimize interference effects. Drag reduction rates are of the order of 5 %.
Winglets are applied in the latest generation of Boeing 747, MD 11, Airbus, and most
executive jets, besides sail planes. Data available for the Boeing 747-400 indicate
that without winglets the aircraft suffers about 2.5 % drag losses, which corresponds
to +9.5 tons at take-off.
Wing Endplates
The endplates are vertical surfaces (generally rectangular) added to a wing to redistribute the lift along the span. The effect is an strongly increased lift coefficients, against a drag coefficient that decreases by a tiny amount.
The resulting efficiency L/D is generally greatly improved. The effect of the
endplates is generally described as an increased wing aspect-ratio, that is proved to
be effective. Endplates are generally considered a more simple engineering solution than
the winglets.
Typical applications of endplates are in racing cars, where the aspect-ratios are
necessary small, and enormous efforts are made by the engineers to produce a few
percent of extra lift, while not penalizing the drag. A typical example of lift
distribution for a short wing (AR=2) is shown in Fig. 3. The computations were
performed with the panel method VSAERO.
Figure 3: Spanwise Cl distribution w/ and w/o endplates
Hoerner tips are crescent-shaped geometries with a slight upward feathering.
They are proved to be slighly better than conventional round tips, since they
promote a better diffusion of the tip vortex.
In the figure below two geometries (Hoerner and round tip) are being tested in the
low speed wind tunnel of the University of Illinois. The two geometries are designed
as to not change the effective aspect-ratio of the wing.
Tip tanks are devices that extend beyond the physical range of the wings and
protrude from both the leading-edge and the trailing-edge. They are generally
cylindrical bodies that modify the tip vortex structure and work as endplates,
although they are not as effective. On fighter aircraft they can be disposable bodies.
Tip sails are more complicated devices, as shown in the figure below,
consisting of several tapered fins (or smaller winglets), placed radially with an
axial gap between two elements (Spillman, 1978). They also have the leading edge
protuberance similar to the tip tanks.
Figure 5: tip sail
For best performance is it suggested that the number of vanes be no more than 4, at
angles 15-20 deg between 2 vanes; each vane should have a chord no larger than 30 %
of the tip tank chord.
Applications
Hoerner Tips
Figure: 4 Hoerner tips (at left) and round tips (at left) before wind tunnel
testing
Tip Tanks
Tip Sails
Selected References