Figure 3.45. Wing flow modifier and vortex generators

winglet and Whitcomb types are is seen in high-subsonic aircraft. The Hoerner type and sharp-raked winglet are used in lower-speed aircraft.

The Whitcomb-type wing tip and its variants without the lower extension are popular with high-subsonic turbofan aircraft. Extensive analyses and tests indicate that approximately 1% of induced-drag reduction may be possible with a carefully designed winglet. Until the 1970s and 1980s, the winglet was not prominent in aircraft. In this book, no credit is taken for the use of the winglet. Coursework can incorporate the winglet in project work.

Wing flow modifier devices (Figure 3.45) are intended to improve the flow quality over the wing. In the figure, a fence is positioned at about half the distance of the wingspan. The devices are carefully aligned to prevent airflow that tends to move spanwise (i.e., outward) on swept wings.

Figure 3.45 also shows examples of vortex generators, which are stub wings carefully placed in a row to generate vortex tubes that energize flow at the aft wing. This enables the flow to remain attached; however, additional drag increase due to vortex generators must be tolerated to gain this benefit.

Vortex generators and/or a fence also can be installed on a nacelle to prevent separation.

3.21 Finalizing Wing Design Parameters

Sections 3.11 through 3.20 cover a wide range of wing design features. This section describes the considerations necessary to finalize the wing design. Selection of the aerofoil is the most important initial task. The wing aerofoil t/c ratio is established for the maximum cruise speed by the choice of aerofoil and sweep. It can vary along the span, with the root demanding the thickest section to withstand the bending moment. Once the aerofoil is selected, six parameters must be established for wing design: (1) wing planform area, (2) wing aspect ratio, (3) wing span, (4) wing sweep,

(5)wing dihedral, and (6) wing twist.

1.Establish the wing reference area (see Chapter 11).

2.Establish the wing planform geometry (i.e., the maximum aspect ratio permitted by the structural technology). The statistics provided previously are a good guide. A new design should have higher aspect ratios compared to current designs.